irontide_session/

settings.rs

//! Unified settings pack for session configuration.
//!
//! Replaces the old `SessionConfig` with a single strongly-typed struct that
//! consolidates all configurable knobs. Supports presets, validation, and
//! serde serialization (bencode + JSON).

use std::net::IpAddr;
use std::path::PathBuf;

use serde::{Deserialize, Serialize};

use irontide_core::StorageMode;
use irontide_wire::mse::EncryptionMode;

use crate::alert::AlertCategory;
use crate::choker::{ChokingAlgorithm, SeedChokingAlgorithm};
use crate::proxy::ProxyConfig;
use crate::rate_limiter::MixedModeAlgorithm;

/// M171: Action taken when a torrent's seed ratio reaches its configured limit.
///
/// Wire format is qBt's `snake_case` string (`"pause"` / `"remove"` /
/// `"enable_super_seeding"`). Pause keeps the torrent in the session in a
/// user-stopped state, Remove deletes the torrent record (files remain),
/// and `EnableSuperSeeding` flips the torrent into BEP 16 super-seed mode
/// without stopping.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum MaxRatioAction {
    /// Pause the torrent — keep it in the session but stop all activity.
    #[default]
    Pause,
    /// Remove the torrent record. Files on disk are left untouched.
    Remove,
    /// Enable BEP 16 super seeding on the torrent rather than stopping it.
    EnableSuperSeeding,
}

// ── Serde default helpers ────────────────────────────────────────────

fn default_true() -> bool {
    true
}
fn default_listen_port() -> u16 {
    42020
}
fn default_download_dir() -> PathBuf {
    PathBuf::from(".")
}
fn default_max_torrents() -> usize {
    100
}
fn default_encryption() -> EncryptionMode {
    EncryptionMode::Disabled
}
fn default_auto_upload_slots_min() -> usize {
    2
}
fn default_auto_upload_slots_max() -> usize {
    20
}
fn default_active_downloads() -> i32 {
    3
}
fn default_active_seeds() -> i32 {
    5
}
fn default_active_limit() -> i32 {
    500
}
fn default_active_checking() -> i32 {
    3
}
fn default_inactive_rate() -> u64 {
    2048
}
fn default_auto_manage_interval() -> u64 {
    30
}
fn default_auto_manage_startup() -> u64 {
    60
}
fn default_queue_rate_ewma_alpha() -> f64 {
    0.3
}
fn default_seed_queue_min_active_secs() -> u64 {
    1800
}
fn default_alert_mask() -> AlertCategory {
    AlertCategory::ALL
}
fn default_alert_channel_size() -> usize {
    1024
}
fn default_smart_ban_max_failures() -> u32 {
    3
}
fn default_disk_io_threads() -> usize {
    let cores = std::thread::available_parallelism().map_or(4, std::num::NonZero::get);
    (cores / 2).clamp(4, 16)
}
fn default_max_blocking_threads() -> usize {
    std::thread::available_parallelism().map_or(4, std::num::NonZero::get)
}
fn default_storage_mode() -> StorageMode {
    StorageMode::Auto
}
fn default_disk_cache_size() -> usize {
    16 * 1024 * 1024
}
fn default_disk_write_cache_ratio() -> f32 {
    0.5
}
fn default_buffer_pool_capacity() -> usize {
    64 * 1024 * 1024
}
fn default_enable_mlock() -> bool {
    cfg!(unix)
}
fn default_io_uring_sq_depth() -> u32 {
    256
}
fn default_io_uring_batch_threshold() -> usize {
    4
}
fn default_disk_channel_capacity() -> usize {
    512
}
fn default_hashing_threads() -> usize {
    let cores = std::thread::available_parallelism().map_or(4, std::num::NonZero::get);
    (cores / 4).clamp(2, 8)
}
fn default_max_request_queue_depth() -> usize {
    250
}
fn default_initial_queue_depth() -> usize {
    128
}
fn default_request_queue_time() -> f64 {
    3.0
}
fn default_block_request_timeout() -> u32 {
    60
}
fn default_max_concurrent_streams() -> usize {
    8
}
fn default_dht_qps() -> usize {
    50
}
fn default_dht_timeout() -> u64 {
    5
}
fn default_upnp_lease() -> u32 {
    3600
}
fn default_natpmp_lifetime() -> u32 {
    7200
}
fn default_utp_max_conns() -> usize {
    256
}
fn default_dht_max_items() -> usize {
    700
}
fn default_dht_item_lifetime() -> u64 {
    7200
}
fn default_dht_sample_interval() -> u64 {
    0
}
fn default_suggest_mode() -> bool {
    true
}
fn default_max_suggest_pieces() -> usize {
    16
}
fn default_predictive_piece_announce_ms() -> u64 {
    0
}
fn default_ssl_listen_port() -> u16 {
    0 // 0 = disabled
}
fn default_seed_choking_algorithm() -> SeedChokingAlgorithm {
    SeedChokingAlgorithm::FastestUpload
}
fn default_choking_algorithm() -> ChokingAlgorithm {
    ChokingAlgorithm::FixedSlots
}
fn default_mixed_mode() -> MixedModeAlgorithm {
    MixedModeAlgorithm::PeerProportional
}
fn default_steal_threshold_ratio() -> f64 {
    10.0
}
fn default_use_block_stealing() -> bool {
    true
}
fn default_peer_connect_timeout() -> u64 {
    10 // M139: match rqbit — longer timeout produces more natural connect failures for cycling
}
fn default_peer_dscp() -> u8 {
    0x08 // CS1 (scavenger/low-priority)
}
fn default_max_peers_per_torrent() -> usize {
    128
}
// v0.187.3: eviction-policy tunables.
fn default_pass0_grace_secs() -> u64 {
    60 // Per OV2/12A: full minute of post-handshake slow-start before Pass 0 fires.
}
fn default_proactive_evictions_per_minute_limit() -> u32 {
    30 // Sliding-window cap that prevents the 130→20-50 churn observed in dogfood.
}
fn default_eviction_ban_duration_secs() -> u64 {
    600 // 10 min (was 1800/30 min). Long enough to break churn loops, short enough
    // that a legitimately slow peer can rejoin after warming up.
}
fn default_eviction_ban_set_cap() -> usize {
    1024 // FIFO cap on the banned-peer set (raised from the legacy 256).
}
fn default_stats_report_interval() -> u64 {
    1000
}
fn default_strict_end_game() -> bool {
    true
}
fn default_max_web_seeds() -> usize {
    4
}
fn default_web_seed_retry_base_secs() -> u64 {
    10
}
fn default_web_seed_retry_factor() -> u64 {
    6
}
fn default_web_seed_retry_cap_secs() -> u64 {
    3600
}
fn default_web_seed_max_failures() -> u32 {
    10
}
fn default_initial_picker_threshold() -> u32 {
    4
}
fn default_whole_pieces_threshold() -> u32 {
    20
}
fn default_snub_timeout_secs() -> u32 {
    15
}
fn default_readahead_pieces() -> u32 {
    8
}
fn default_max_metadata_size() -> u64 {
    4 * 1024 * 1024 // 4 MiB — libtorrent default
}
fn default_max_message_size() -> usize {
    16 * 1024 * 1024 // 16 MiB — matches wire codec constant
}
fn default_max_piece_length() -> u64 {
    32 * 1024 * 1024 // 32 MiB — largest reasonable piece size
}
fn default_max_outstanding_requests() -> usize {
    500
}
fn default_max_in_flight_pieces() -> usize {
    512
}
fn default_fixed_pipeline_depth() -> usize {
    128
}
fn default_i2p_hostname() -> String {
    "127.0.0.1".into()
}
fn default_i2p_port() -> u16 {
    7656
}
fn default_i2p_tunnel_quantity() -> u8 {
    3
}
fn default_i2p_tunnel_length() -> u8 {
    3
}
fn default_runtime_worker_threads() -> usize {
    std::thread::available_parallelism().map_or(4, |n| n.get().min(8))
}
fn default_lock_warn_threshold_ms() -> u64 {
    50
}
fn default_steal_stale_piece_secs() -> u64 {
    2
}
fn default_steal_threshold_endgame() -> f64 {
    3.0
}
fn default_peer_read_timeout_secs() -> u64 {
    10
}
fn default_peer_write_timeout_secs() -> u64 {
    10
}
fn default_data_contribution_timeout() -> u64 {
    0 // M139: disabled by default — rqbit doesn't evict for no data
}
fn default_choke_rotation_max_evictions() -> u32 {
    0 // M139: disabled by default — rqbit doesn't proactively rotate choked peers
}
fn default_max_concurrent_connects() -> u16 {
    128 // M147: ConnectPool — gates connection attempts, released on handshake
}
fn default_connect_soft_timeout() -> u64 {
    3 // M147: seconds without TCP SYN-ACK before soft reap disconnects
}
fn default_dispatch_backlog_cap() -> usize {
    8 // M182: dispatch_tx reader-side spill cap (constant pre-perf-harness)
}
fn default_event_backlog_cap() -> usize {
    32 // M182: event_tx reader-side spill cap (constant pre-perf-harness)
}
fn default_web_seed_progress_throttle_ms() -> u64 {
    250 // M178: per-URL minimum interval for PeerEvent::WebSeedProgress (0 = disabled)
}
fn default_save_resume_interval() -> u64 {
    300 // M161: 5 minutes between periodic resume file saves
}
fn default_max_upload_slots_global() -> i32 {
    -1
}
fn default_max_upload_slots_per_torrent() -> i32 {
    4
}
fn default_max_connections_global() -> i32 {
    -1
}
fn default_max_uploads_per_torrent() -> i32 {
    -1
}

// ── Settings ─────────────────────────────────────────────────────────

/// Unified session settings (replaces `SessionConfig`).
///
/// All 56 configurable fields in a single strongly-typed struct.
/// Supports presets via factory functions and runtime mutation via
/// `SessionHandle::apply_settings()`.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct Settings {
    // ── General ──
    /// TCP listen port for incoming peer connections (default: 42020).
    #[serde(default = "default_listen_port")]
    pub listen_port: u16,
    /// Randomize the listen port each time the session starts. Default: false.
    #[serde(default)]
    pub randomize_port_on_startup: bool,
    /// Default download directory for new torrents (default: ".").
    #[serde(default = "default_download_dir")]
    pub download_dir: PathBuf,
    /// Maximum number of concurrent torrents (default: 100).
    #[serde(default = "default_max_torrents")]
    pub max_torrents: usize,
    /// Directory for fast-resume data files. If `None`, resume data is not persisted.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub resume_data_dir: Option<PathBuf>,
    /// Interval in seconds between periodic resume file saves (0 = disabled).
    /// Default: 300 (5 minutes).
    #[serde(default = "default_save_resume_interval")]
    pub save_resume_interval_secs: u64,

    // ── Protocol features ──
    /// Enable Kademlia DHT peer discovery (BEP 5). Default: true.
    #[serde(default = "default_true")]
    pub enable_dht: bool,
    /// Enable Peer Exchange (BEP 11). Default: true.
    #[serde(default = "default_true")]
    pub enable_pex: bool,
    /// Enable Local Service Discovery via multicast (BEP 14). Default: true.
    #[serde(default = "default_true")]
    pub enable_lsd: bool,
    /// Enable BEP 6 Fast Extension (`AllowedFast`, `HaveAll`, `HaveNone`, Reject,
    /// `SuggestPiece`). Default: true.
    #[serde(default = "default_true")]
    pub enable_fast_extension: bool,
    /// Enable uTP (BEP 29) micro transport protocol. When enabled, outbound
    /// connections try uTP first with a 5-second timeout before falling back
    /// to TCP. Default: true.
    #[serde(default = "default_true")]
    pub enable_utp: bool,
    /// Enable `UPnP` IGD port mapping (last resort after PCP and NAT-PMP).
    /// Default: true.
    #[serde(default = "default_true")]
    pub enable_upnp: bool,
    /// Enable NAT-PMP (RFC 6886) and PCP (RFC 6887) port mapping.
    /// PCP is tried first, then NAT-PMP as fallback. Default: true.
    #[serde(default = "default_true")]
    pub enable_natpmp: bool,
    /// Enable IPv6 dual-stack support (BEP 7, 24). Binds listeners on both
    /// IPv4 and IPv6, starts a second DHT instance, and processes IPv6 peers
    /// in PEX and tracker responses. Default: true.
    #[serde(default = "default_true")]
    pub enable_ipv6: bool,
    /// Enable HTTP/web seeding (BEP 19 `GetRight`, BEP 17 Hoffman). Torrents
    /// with `url-list` or `httpseeds` download pieces from HTTP servers
    /// alongside peer-to-peer transfers. Default: true.
    #[serde(default = "default_true")]
    pub enable_web_seed: bool,
    /// Enable BEP 55 holepunch extension for NAT traversal. Advertises
    /// `ut_holepunch` in the extension handshake and can act as initiator,
    /// relay, or target for holepunch connections. Default: true.
    #[serde(default = "default_true")]
    pub enable_holepunch: bool,
    /// Enable BEP 40 canonical peer priority for connection eviction.
    /// When at capacity, incoming peers with higher deterministic priority
    /// can displace lower-priority ones. Default: true.
    #[serde(default = "default_true")]
    pub enable_bep40_eviction: bool,
    /// Enable diagnostic counters (dispatch timing, backpressure high-water,
    /// peer unchoke/choke/lifetime telemetry). Default: false — enable for
    /// benchmarking or troubleshooting via `--diagnostics` or config.
    #[serde(default)]
    pub enable_diagnostic_counters: bool,
    /// Connection encryption mode (MSE/PE). Default: Disabled.
    #[serde(default = "default_encryption")]
    pub encryption_mode: EncryptionMode,
    /// Suppress identifying information (client version in BEP 10 handshake)
    /// and disable DHT, LSD, `UPnP`, and NAT-PMP. Default: false.
    #[serde(default)]
    pub anonymous_mode: bool,
    /// Manually configured external IP for BEP 40 peer priority.
    /// If not set, discovered automatically via NAT traversal.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub external_ip: Option<IpAddr>,

    // ── Seeding ──
    /// Stop seeding when this upload/download ratio is reached. `None` = unlimited.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub seed_ratio_limit: Option<f64>,
    /// M171: Stop seeding after this many cumulative seeding seconds.
    /// `None` = no limit. Mirrors qBt's "Maximum seeding time" preference,
    /// which is exposed in minutes on the wire but stored here in seconds
    /// to match the other duration-typed fields.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub seed_time_limit_secs: Option<u64>,
    /// M171: Stop seeding after this many seconds of inactivity while in the
    /// Seeding state (no outgoing Piece data). `None` = no limit. Mirrors
    /// qBt's "Maximum inactive seeding time" preference.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub inactive_seed_time_limit_secs: Option<u64>,
    /// M171: What to do when `seed_ratio_limit` is reached.
    /// Mirrors qBt's `max_ratio_act` wire enum. Default: `Pause`.
    #[serde(default)]
    pub max_ratio_action: MaxRatioAction,
    /// M171: Create a subfolder named after the torrent when adding a
    /// multi-file torrent. Mirrors qBt's `create_subfolder_enabled`.
    /// Default: `true` (qBt factory default).
    #[serde(default = "default_true")]
    pub create_subfolder: bool,
    /// M171: Automatically manage torrent resources via the queueing
    /// subsystem (start/stop/recheck order). Mirrors qBt's
    /// `auto_tmm_enabled`. Default: `false`.
    #[serde(default)]
    pub auto_manage_torrents: bool,
    /// M171: Enable the download/upload queueing subsystem. When `false`,
    /// no queueing is applied and torrents run concurrently up to per-torrent
    /// limits. Mirrors qBt's `queueing_enabled`. Default: `false`.
    #[serde(default)]
    pub queueing_enabled: bool,
    /// Enable BEP 16 super seeding for new torrents. Reveals pieces one-per-peer
    /// to maximize piece diversity across the swarm. Default: false.
    #[serde(default)]
    pub default_super_seeding: bool,
    /// Default share mode for new torrents. When true, torrents relay pieces
    /// in memory without writing to disk. Requires fast extension (BEP 6).
    #[serde(default)]
    pub default_share_mode: bool,
    /// Advertise upload-only status via extension handshake when a torrent
    /// transitions to seeding (BEP 21). Default: true.
    #[serde(default = "default_true")]
    pub upload_only_announce: bool,
    // ── Rate limiting ──
    /// Global upload rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub upload_rate_limit: u64,
    /// Global download rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub download_rate_limit: u64,
    /// TCP upload rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub tcp_upload_rate_limit: u64,
    /// TCP download rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub tcp_download_rate_limit: u64,
    /// uTP upload rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub utp_upload_rate_limit: u64,
    /// uTP download rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub utp_download_rate_limit: u64,
    /// Automatically adjust the number of upload slots based on bandwidth. Default: true.
    #[serde(default = "default_true")]
    pub auto_upload_slots: bool,
    /// Minimum number of automatic upload slots (default: 2).
    #[serde(default = "default_auto_upload_slots_min")]
    pub auto_upload_slots_min: usize,
    /// Maximum number of automatic upload slots (default: 20).
    #[serde(default = "default_auto_upload_slots_max")]
    pub auto_upload_slots_max: usize,
    /// Maximum upload slots across all torrents (-1 = unlimited). Default: -1.
    #[serde(default = "default_max_upload_slots_global")]
    pub max_upload_slots_global: i32,
    /// Maximum upload slots per torrent. Default: 4.
    #[serde(default = "default_max_upload_slots_per_torrent")]
    pub max_upload_slots_per_torrent: i32,
    /// Maximum peer connections across all torrents (-1 = unlimited). Default: -1.
    #[serde(default = "default_max_connections_global")]
    pub max_connections_global: i32,
    /// Maximum unchoked upload slots per torrent (-1 = unlimited). Default: -1.
    /// M224: qBt wire `max_uploads_per_torrent`. `-1` is unlimited; `n >= 1`
    /// caps the choker's unchoke set; `0` is explicitly rejected by
    /// [`Settings::validate`] (choking every peer is almost certainly a
    /// wire-format mistake, not user intent).
    #[serde(default = "default_max_uploads_per_torrent")]
    pub max_uploads_per_torrent: i32,
    /// Alternative download rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub alt_download_rate_limit: u64,
    /// Alternative upload rate limit in bytes/sec (0 = unlimited).
    #[serde(default)]
    pub alt_upload_rate_limit: u64,
    /// Whether alternative speed limits are currently active. Default: false.
    #[serde(default)]
    pub alt_speed_enabled: bool,
    /// Whether the alternative speed schedule is enabled. Default: false.
    #[serde(default)]
    pub alt_speed_schedule_enabled: bool,
    /// Schedule start time in minutes-of-day (0-1439). Default: 0.
    #[serde(default)]
    pub alt_speed_schedule_from: u16,
    /// Schedule end time in minutes-of-day (0-1439). Default: 0.
    #[serde(default)]
    pub alt_speed_schedule_to: u16,
    /// Schedule active days as a bitmask (bit 0 = Mon .. bit 6 = Sun). Default: 0.
    #[serde(default)]
    pub alt_speed_schedule_days: u8,
    /// Include protocol overhead in rate limit calculations. Default: true.
    #[serde(default = "default_true")]
    pub rate_limit_includes_overhead: bool,
    /// Apply rate limits to uTP connections. Default: true.
    #[serde(default = "default_true")]
    pub rate_limit_utp: bool,
    /// Apply rate limits to LAN connections. Default: false.
    #[serde(default)]
    pub rate_limit_lan: bool,
    /// Mixed-mode TCP/uTP bandwidth allocation algorithm.
    #[serde(default = "default_mixed_mode")]
    pub mixed_mode_algorithm: MixedModeAlgorithm,

    // ── Queue management ──
    /// Maximum concurrent auto-managed downloading torrents (-1 = unlimited, default: 3).
    #[serde(default = "default_active_downloads")]
    pub active_downloads: i32,
    /// Maximum concurrent auto-managed seeding torrents (-1 = unlimited, default: 5).
    #[serde(default = "default_active_seeds")]
    pub active_seeds: i32,
    /// Hard cap on all active auto-managed torrents (-1 = unlimited, default: 500).
    #[serde(default = "default_active_limit")]
    pub active_limit: i32,
    /// Maximum concurrent hash-check operations (default: 1).
    #[serde(default = "default_active_checking")]
    pub active_checking: i32,
    /// Exempt inactive torrents from download/seed limits. A torrent is inactive
    /// if its rate is below `inactive_down_rate` / `inactive_up_rate`. Default: true.
    #[serde(default = "default_true")]
    pub dont_count_slow_torrents: bool,
    /// Download rate threshold (bytes/sec) below which a torrent is considered
    /// inactive for queue management purposes (default: 2048).
    #[serde(default = "default_inactive_rate")]
    pub inactive_down_rate: u64,
    /// Upload rate threshold (bytes/sec) below which a torrent is considered
    /// inactive for queue management purposes (default: 2048).
    #[serde(default = "default_inactive_rate")]
    pub inactive_up_rate: u64,
    /// Interval in seconds between queue evaluations (default: 30).
    #[serde(default = "default_auto_manage_interval")]
    pub auto_manage_interval: u64,
    /// Grace period in seconds where a torrent is considered active regardless
    /// of speed after being started (default: 60).
    #[serde(default = "default_auto_manage_startup")]
    pub auto_manage_startup: u64,
    /// Allocate seeding slots before download slots. Default: false.
    #[serde(default)]
    pub auto_manage_prefer_seeds: bool,
    /// EWMA smoothing factor for queue rate classification (0.0–1.0).
    /// 0.0 = pure history (never adapts), 1.0 = no smoothing (raw rate).
    /// Default: 0.3.
    #[serde(default = "default_queue_rate_ewma_alpha")]
    pub queue_rate_ewma_alpha: f64,
    /// Anti-flap grace period for seeding torrents, in seconds.
    /// Seeding torrents are exempt from queue-pause for this duration after
    /// starting (default: 1800 = 30 minutes, matching libtorrent).
    #[serde(default = "default_seed_queue_min_active_secs")]
    pub seed_queue_min_active_secs: u64,

    // ── Alerts ──
    /// Bitmask of alert categories to receive (default: ALL).
    #[serde(default = "default_alert_mask")]
    pub alert_mask: AlertCategory,
    /// Capacity of the alert broadcast channel (default: 1024).
    #[serde(default = "default_alert_channel_size")]
    pub alert_channel_size: usize,

    // ── Smart banning ──
    /// Number of hash-failure involvements before a peer is auto-banned.
    /// Lower values ban faster but risk false positives (default: 3).
    #[serde(default = "default_smart_ban_max_failures")]
    pub smart_ban_max_failures: u32,
    /// Enable parole mode: re-download a failed piece from a single uninvolved
    /// peer to definitively attribute fault before striking. Default: true.
    #[serde(default = "default_true")]
    pub smart_ban_parole: bool,

    // ── Disk I/O ──
    /// Number of concurrent disk I/O threads (default: 4).
    #[serde(default = "default_disk_io_threads")]
    pub disk_io_threads: usize,
    /// Maximum number of concurrent blocking I/O operations dispatched via
    /// `block_in_place`. Defaults to the number of available CPU cores.
    #[serde(default = "default_max_blocking_threads")]
    pub max_blocking_threads: usize,
    /// Storage allocation mode: Auto, `FullPreallocate`, or `SparseFile` (default: Auto).
    #[serde(default = "default_storage_mode")]
    pub storage_mode: StorageMode,
    /// Override pre-allocation strategy (None/Sparse/Full). When `None` (default),
    /// derived from `storage_mode`: Full → `PreallocateMode::Full`, else → `PreallocateMode::None`.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub preallocate_mode: Option<irontide_storage::PreallocateMode>,
    /// Total ARC disk cache size in bytes (default: 16 MiB, minimum: 1 MiB).
    #[serde(default = "default_disk_cache_size")]
    pub disk_cache_size: usize,
    /// Fraction of disk cache reserved for write buffering (0.0–1.0, default: 0.5).
    #[serde(default = "default_disk_write_cache_ratio")]
    pub disk_write_cache_ratio: f32,
    /// Capacity of the async disk I/O command channel (default: 512).
    #[serde(default = "default_disk_channel_capacity")]
    pub disk_channel_capacity: usize,
    /// Unified buffer pool capacity in bytes (default: 64 MiB).
    /// Replaces `disk_cache_size` when set. Covers both write buffering and read cache.
    #[serde(default = "default_buffer_pool_capacity")]
    pub buffer_pool_capacity: usize,
    /// Lock cached piece data in physical memory (default: true on Unix).
    /// Prevents the OS from swapping out hot cache entries. Silently ignored
    /// if `RLIMIT_MEMLOCK` is exceeded.
    #[serde(default = "default_enable_mlock")]
    pub enable_mlock: bool,
    /// `io_uring` submission queue depth (number of SQEs). Only used when
    /// `storage_mode` is `IoUring`. Default: 256.
    #[serde(default = "default_io_uring_sq_depth")]
    pub io_uring_sq_depth: u32,
    /// Enable `O_DIRECT` for `io_uring` writes, bypassing the kernel page cache.
    /// Unaligned writes fall back to regular pwritev. Default: false.
    #[serde(default)]
    pub io_uring_direct_io: bool,
    /// Enable direct I/O for filesystem storage (bypasses kernel page cache).
    /// Linux/FreeBSD: `O_DIRECT`, macOS: `F_NOCACHE`. Windows: use `--iocp`
    /// with `--direct-io`. Default: false.
    #[serde(default)]
    pub filesystem_direct_io: bool,
    /// Minimum number of file segments to batch before using `io_uring`.
    /// Below this threshold, pwritev may be cheaper. Default: 4.
    #[serde(default = "default_io_uring_batch_threshold")]
    pub io_uring_batch_threshold: usize,
    /// IOCP concurrent thread count (0 = system default). Only used when
    /// `storage_mode` is `Iocp`. Default: 0.
    #[serde(default)]
    pub iocp_concurrent_threads: u32,
    /// Enable `FILE_FLAG_NO_BUFFERING` for IOCP I/O, bypassing the OS page cache.
    /// Requires sector-aligned writes. Default: false.
    #[serde(default)]
    pub iocp_direct_io: bool,
    // ── Hashing & piece picking ──
    /// Number of concurrent piece hash verification threads (default: 2).
    #[serde(default = "default_hashing_threads")]
    pub hashing_threads: usize,
    /// Maximum per-peer request queue depth (default: 250).
    #[serde(default = "default_max_request_queue_depth")]
    pub max_request_queue_depth: usize,
    /// Initial per-peer request queue depth (default: 128). Higher values let
    /// peers reach full throughput faster by skipping slow-start ramp-up.
    #[serde(default = "default_initial_queue_depth")]
    pub initial_queue_depth: usize,
    /// Request queue time multiplier in seconds (default: 3.0).
    ///
    /// **Deprecated**: This field is retained for backward compatibility with
    /// existing config files. The pipeline now uses a fixed-depth model where
    /// queue depth equals `initial_queue_depth` for the lifetime of the
    /// connection; this value is no longer used in depth computation.
    #[serde(default = "default_request_queue_time")]
    pub request_queue_time: f64,
    /// Block request timeout in seconds before the request is considered
    /// lost and re-issued (default: 60).
    #[serde(default = "default_block_request_timeout")]
    pub block_request_timeout_secs: u32,
    /// Maximum concurrent `FileStream` readers. Controls how many simultaneous
    /// file-streaming reads can proceed (default: 8).
    #[serde(default = "default_max_concurrent_streams")]
    pub max_concurrent_stream_reads: usize,
    /// Automatically switch to sequential piece picking when too many partial
    /// pieces accumulate. Uses hysteresis (1.6x activate / 1.3x deactivate).
    #[serde(default = "default_true")]
    pub auto_sequential: bool,
    /// In end-game mode, cancel duplicate requests when a piece completes.
    /// When false, both copies download — wastes bandwidth but finishes faster
    /// on unreliable peers. Default: true.
    #[serde(default = "default_strict_end_game")]
    pub strict_end_game: bool,
    /// Maximum concurrent web seed connections per torrent (default: 4).
    #[serde(default = "default_max_web_seeds")]
    pub max_web_seeds: usize,
    /// M186: Base delay (seconds) for web seed exponential backoff. Default: 10.
    #[serde(default = "default_web_seed_retry_base_secs")]
    pub web_seed_retry_base_secs: u64,
    /// M186: Multiplier for web seed exponential backoff. Default: 6.
    #[serde(default = "default_web_seed_retry_factor")]
    pub web_seed_retry_factor: u64,
    /// M186: Maximum backoff (seconds) for web seed retry. Default: 3600.
    #[serde(default = "default_web_seed_retry_cap_secs")]
    pub web_seed_retry_cap_secs: u64,
    /// M186: Consecutive failures before permanently banning a web seed. Default: 10.
    #[serde(default = "default_web_seed_max_failures")]
    pub web_seed_max_failures: u32,
    /// Completed piece count below which the picker uses random selection
    /// to promote piece diversity in the swarm. Default: 4.
    #[serde(default = "default_initial_picker_threshold")]
    pub initial_picker_threshold: u32,
    /// Seconds to download a piece — if a peer is faster, it gets exclusive
    /// assignment (no block splitting). Default: 20.
    #[serde(default = "default_whole_pieces_threshold")]
    pub whole_pieces_threshold: u32,
    /// Seconds without data from a peer before marking it as snubbed.
    /// Snubbed peers get queue depth clamped to 1. Default: 60.
    #[serde(default = "default_snub_timeout_secs")]
    pub snub_timeout_secs: u32,
    /// Number of pieces ahead of the streaming cursor to prioritize (default: 8).
    #[serde(default = "default_readahead_pieces")]
    pub readahead_pieces: u32,
    /// Escalate streaming piece requests that exceed the mean RTT. Default: true.
    #[serde(default = "default_true")]
    pub streaming_timeout_escalation: bool,
    /// Steal blocks from peers this many times slower than the requesting peer (default: 10.0).
    /// Set to 0.0 to disable stealing.
    #[serde(default = "default_steal_threshold_ratio")]
    pub steal_threshold_ratio: f64,
    /// Enable per-block stealing: fast peers can steal individual unrequested
    /// blocks from pieces reserved by slower peers (default: true).
    #[serde(default = "default_use_block_stealing")]
    pub use_block_stealing: bool,
    /// Seconds between steal-queue population scans. Every N seconds, all
    /// in-flight pieces are pushed into the steal queue so fast peers can
    /// steal blocks mid-download (not just at endgame). 0 = disabled.
    /// Default: 2.
    #[serde(default = "default_steal_stale_piece_secs")]
    pub steal_stale_piece_secs: u64,
    /// M149: Steal threshold multiplier when >90% complete (endgame).
    /// Pieces taking longer than `swarm_avg` * this value are stolen. Default: 3.0.
    #[serde(default = "default_steal_threshold_endgame")]
    pub steal_threshold_endgame: f64,
    /// Fixed per-peer pipeline depth (number of concurrent requests per peer).
    /// Replaces the old AIMD dynamic depth system. rqbit uses a fixed
    /// `Semaphore(128)` per peer — simpler and faster. This setting allows
    /// benchmarking different fixed depths. Default: 128.
    #[serde(default = "default_fixed_pipeline_depth")]
    pub fixed_pipeline_depth: usize,

    // ── Piece picker enhancements (M44) ──
    /// Prefer pieces adjacent to those already downloaded for improved sequential
    /// disk access patterns (4 MiB extent groups). Default: true.
    #[serde(default = "default_true")]
    pub piece_extent_affinity: bool,
    /// Enable BEP 6 `SuggestPiece`: suggest newly verified pieces to peers that
    /// Send `SuggestPiece` messages for cached pieces so peers can request what they
    /// don't have them, improving piece diversity in the swarm. Default: true.
    #[serde(default = "default_suggest_mode")]
    pub suggest_mode: bool,
    /// Maximum `SuggestPiece` messages per peer to avoid flooding (default: 10).
    #[serde(default = "default_max_suggest_pieces")]
    pub max_suggest_pieces: usize,
    /// Predictive piece announce delay in milliseconds. When > 0, a Have message
    /// is sent before hash verification completes, reducing piece availability
    /// latency at the cost of a possible false announce. Default: 0 (disabled).
    #[serde(default = "default_predictive_piece_announce_ms")]
    pub predictive_piece_announce_ms: u64,

    // ── Proxy ──
    /// Proxy configuration for peer and tracker connections. Default: no proxy.
    #[serde(default)]
    pub proxy: ProxyConfig,
    /// Force all connections through the configured proxy. Disables listen
    /// sockets, `UPnP`, NAT-PMP, DHT, and LSD. Default: false.
    #[serde(default)]
    pub force_proxy: bool,

    // ── IP Filtering ──
    /// Enable the IP filter (blocklist). Default: false.
    #[serde(default)]
    pub ip_filter_enabled: bool,
    /// Path to the IP filter file (e.g. `ipfilter.dat`). Default: empty.
    #[serde(default)]
    pub ip_filter_path: String,
    /// Automatically refresh the IP filter when the file changes. Default: false.
    #[serde(default)]
    pub ip_filter_auto_refresh: bool,

    /// Check tracker IP addresses against the IP filter. When false, trackers
    /// are exempt from IP filtering. Default: true.
    #[serde(default = "default_true")]
    pub apply_ip_filter_to_trackers: bool,

    // ── DHT tuning ──
    /// Maximum DHT queries per second to control network traffic (default: 50).
    #[serde(default = "default_dht_qps")]
    pub dht_queries_per_second: usize,
    /// Timeout in seconds for a single DHT query before it is abandoned (default: 5).
    #[serde(default = "default_dht_timeout")]
    pub dht_query_timeout_secs: u64,
    /// BEP 42: Enforce node ID verification in DHT routing table.
    /// Disabled by default: too many real DHT nodes lack BEP 42-compliant IDs.
    #[serde(default)]
    pub dht_enforce_node_id: bool,
    /// BEP 42: Restrict DHT routing table to one node per IP.
    #[serde(default = "default_true")]
    pub dht_restrict_routing_ips: bool,
    /// Maximum number of BEP 44 items stored in the DHT (immutable + mutable).
    #[serde(default = "default_dht_max_items")]
    pub dht_max_items: usize,
    /// Lifetime of BEP 44 DHT items in seconds before expiry (default: 7200 = 2 hours).
    #[serde(default = "default_dht_item_lifetime")]
    pub dht_item_lifetime_secs: u64,
    /// Interval in seconds for periodic `sample_infohashes` queries (BEP 51).
    /// 0 = disabled (default). Non-zero enables background DHT indexing.
    #[serde(default = "default_dht_sample_interval")]
    pub dht_sample_infohashes_interval: u64,
    /// BEP 43: Run DHT in read-only mode. Read-only nodes can query the DHT
    /// but do not store data or announce. Other nodes should not add us to
    /// their routing tables. Useful for resource-constrained clients.
    #[serde(default)]
    pub dht_read_only: bool,

    // ── NAT tuning ──
    /// `UPnP` lease duration in seconds (default: 3600).
    #[serde(default = "default_upnp_lease")]
    pub upnp_lease_duration: u32,
    /// NAT-PMP mapping lifetime in seconds (default: 7200).
    #[serde(default = "default_natpmp_lifetime")]
    pub natpmp_lifetime: u32,

    // ── uTP tuning ──
    /// Maximum concurrent uTP connections (default: 256).
    #[serde(default = "default_utp_max_conns")]
    pub utp_max_connections: usize,

    // ── I2P ──
    /// Enable I2P anonymous network support (requires SAM bridge).
    #[serde(default)]
    pub enable_i2p: bool,
    /// SAM bridge hostname (default: "127.0.0.1").
    #[serde(default = "default_i2p_hostname")]
    pub i2p_hostname: String,
    /// SAM bridge port (default: 7656).
    #[serde(default = "default_i2p_port")]
    pub i2p_port: u16,
    /// Number of inbound I2P tunnels (1-16, default: 3).
    #[serde(default = "default_i2p_tunnel_quantity")]
    pub i2p_inbound_quantity: u8,
    /// Number of outbound I2P tunnels (1-16, default: 3).
    #[serde(default = "default_i2p_tunnel_quantity")]
    pub i2p_outbound_quantity: u8,
    /// Number of hops in inbound I2P tunnels (0-7, default: 3).
    #[serde(default = "default_i2p_tunnel_length")]
    pub i2p_inbound_length: u8,
    /// Number of hops in outbound I2P tunnels (0-7, default: 3).
    #[serde(default = "default_i2p_tunnel_length")]
    pub i2p_outbound_length: u8,
    /// Allow mixing I2P and clearnet peers in the same torrent.
    /// When false (default), I2P-enabled torrents only connect to I2P peers.
    #[serde(default)]
    pub allow_i2p_mixed: bool,

    // ── SSL torrents (M42) ──
    /// SSL listen port for SSL torrent incoming connections.
    /// 0 = disabled (no SSL listener). When set, a TLS listener is bound
    /// on this port for torrents with `ssl-cert` in their info dict.
    #[serde(default = "default_ssl_listen_port")]
    pub ssl_listen_port: u16,
    /// Path to the PEM-encoded certificate file for SSL torrent connections.
    /// If not set, a self-signed certificate is auto-generated on first use
    /// and stored in `resume_data_dir` (or a temp directory).
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub ssl_cert_path: Option<PathBuf>,
    /// Path to the PEM-encoded private key file for SSL torrent connections.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub ssl_key_path: Option<PathBuf>,

    // ── Choking algorithms (M43) ──
    /// Algorithm for ranking peers during seed-mode choking.
    #[serde(default = "default_seed_choking_algorithm")]
    pub seed_choking_algorithm: SeedChokingAlgorithm,
    /// Algorithm for determining the number of unchoke slots.
    #[serde(default = "default_choking_algorithm")]
    pub choking_algorithm: ChokingAlgorithm,

    // ── Peer connections ──
    /// Maximum peer connections per torrent (default: 128). When `0`, falls back
    /// to `HARD_PEER_CEILING` (4096) — there is no "unlimited" mode, see Bug 7
    /// fix in v0.187.3.
    #[serde(default = "default_max_peers_per_torrent")]
    pub max_peers_per_torrent: usize,

    /// v0.187.3 / OV2 / 12A: seconds after a peer goes Live before Pass 0
    /// (zero-throughput) eviction can fire against it. Default 60. Prevents
    /// the proactive-eviction loop from culling peers still in `BitTorrent`
    /// slow-start. 0 = disable grace (legacy v0.187.2 behaviour).
    #[serde(default = "default_pass0_grace_secs")]
    pub pass0_grace_secs: u64,

    /// v0.187.3 / 3A: sliding-window cap on proactive evictions in any
    /// rolling 60s window. Default 30. The churn from the dogfood report
    /// ("130 → 20-50 every few seconds") shows what 0 looks like; this is
    /// the upper bound on how aggressive the eviction loop can be.
    #[serde(default = "default_proactive_evictions_per_minute_limit")]
    pub proactive_evictions_per_minute_limit: u32,

    /// v0.187.3: how long a Pass 0 eviction victim is blocked from
    /// reconnection. Default 600 (10 min). Was effectively 1800 (30 min)
    /// pre-v0.187.3 — shorter ban duration lets slow peers warm up and
    /// rejoin without forcing the user to restart.
    #[serde(default = "default_eviction_ban_duration_secs")]
    pub eviction_ban_duration_secs: u64,

    /// v0.187.3 / OV4: FIFO cap on the banned-peer set. Default 1024 (was
    /// 256). With the previous cap, busy swarms thrashed the ban set —
    /// peers fell off the back faster than ban duration could elapse.
    #[serde(default = "default_eviction_ban_set_cap")]
    pub eviction_ban_set_cap: usize,

    /// M133: Seconds without any wire message before disconnecting a peer.
    /// Matches rqbit's 10s read timeout. 0 = disabled. Default: 10.
    #[serde(default = "default_peer_read_timeout_secs")]
    pub peer_read_timeout_secs: u64,
    /// M133: Seconds before a stalled outgoing write disconnects a peer.
    /// 0 = disabled. Default: 10.
    #[serde(default = "default_peer_write_timeout_secs")]
    pub peer_write_timeout_secs: u64,

    /// M137: Data contribution timeout — seconds without receiving a Piece
    /// message before disconnecting. Set to 0 to disable. Default: 60.
    #[serde(default = "default_data_contribution_timeout")]
    pub data_contribution_timeout_secs: u64,

    /// M138: Maximum peers to evict per choke rotation tick (0 = disabled).
    #[serde(default = "default_choke_rotation_max_evictions")]
    pub choke_rotation_max_evictions: u32,

    /// M138: Maximum concurrent outbound peer connections (throttles connect ramp).
    #[serde(default = "default_max_concurrent_connects")]
    pub max_concurrent_connects: u16,

    /// M147: Seconds without TCP SYN-ACK before soft reap disconnects a connecting
    /// peer. Peers that have received SYN-ACK get the full `peer_connect_timeout`.
    #[serde(default = "default_connect_soft_timeout")]
    pub connect_soft_timeout: u64,

    /// M182: dispatch-channel backlog cap. The reader's `BackpressureQueue`
    /// spills up to this many items if `dispatch_tx` is full; on overflow
    /// the peer is disconnected. Lowering this value forces overflow under
    /// less load — the would-have-caught harness uses `cap = 2` to
    /// reproduce the M182 backlog-too-small regression class.
    #[serde(default = "default_dispatch_backlog_cap")]
    pub dispatch_backlog_cap: usize,

    /// M182: event-channel backlog cap. Same role as
    /// [`Self::dispatch_backlog_cap`] for the `event_tx` queue carrying
    /// `PeerEvent::*` from reader to `TorrentActor`.
    #[serde(default = "default_event_backlog_cap")]
    pub event_backlog_cap: usize,

    /// M187 A/B: use actor-centralised dispatch (true) or per-peer CAS dispatch (false).
    #[serde(default = "default_true")]
    pub use_actor_dispatch: bool,

    /// M178: Minimum milliseconds between `PeerEvent::WebSeedProgress` emissions
    /// per URL. Coalesces stat updates from `WebSeedTask` so the actor channel
    /// stays bounded under fast piece-fetch loops. Cold-start (first event for
    /// a URL) and error events bypass the throttle. `0` disables coalescing.
    #[serde(default = "default_web_seed_progress_throttle_ms")]
    pub web_seed_progress_throttle_ms: u64,

    // ── Security ──
    /// Enable SSRF mitigation: restrict localhost tracker paths, block
    /// public-to-private redirects, and reject query strings on local web seeds.
    #[serde(default = "default_true")]
    pub ssrf_mitigation: bool,
    /// Allow internationalised (non-ASCII) domain names in tracker/web seed URLs.
    #[serde(default)]
    pub allow_idna: bool,
    /// Require HTTPS for HTTP tracker announces (UDP trackers are unaffected).
    #[serde(default = "default_true")]
    pub validate_https_trackers: bool,
    /// Maximum BEP 9 metadata size in bytes that will be accepted from peers.
    /// Protects against OOM from peers claiming enormous metadata. Default: 4 MiB.
    #[serde(default = "default_max_metadata_size")]
    pub max_metadata_size: u64,
    /// Maximum wire protocol message size in bytes. Messages exceeding this are
    /// rejected by the codec. Default: 16 MiB.
    #[serde(default = "default_max_message_size")]
    pub max_message_size: usize,
    /// Maximum accepted piece length when adding a torrent. Rejects torrents
    /// with piece sizes above this limit. Default: 32 MiB.
    #[serde(default = "default_max_piece_length")]
    pub max_piece_length: u64,
    /// Maximum outstanding incoming requests per peer. When a peer sends more
    /// Request messages than this without them being served, excess requests
    /// are dropped. Default: 500.
    #[serde(default = "default_max_outstanding_requests")]
    pub max_outstanding_requests: usize,
    /// Maximum number of pieces simultaneously in-flight (downloaded but not
    /// yet verified). Caps memory usage for in-progress pieces. When the cap
    /// is reached, the piece selector only returns blocks from already-in-flight
    /// pieces. Default: 512.
    #[serde(default = "default_max_in_flight_pieces")]
    pub max_in_flight_pieces: usize,
    /// Timeout in seconds for outbound TCP peer connections.
    /// Default 10. Set to 0 to use the OS default (~2 minutes on Linux).
    #[serde(default = "default_peer_connect_timeout")]
    pub peer_connect_timeout: u64,
    /// DSCP (Differentiated Services Code Point) value for peer traffic sockets.
    /// Applied to TCP listeners, outbound TCP connections, uTP sockets, and UDP tracker sockets.
    /// Default 0x08 (CS1/scavenger — low-priority background). Set to 0 to disable DSCP marking.
    #[serde(default = "default_peer_dscp")]
    pub peer_dscp: u8,

    // ── Session Stats (M50) ──
    /// Interval in milliseconds between `SessionStatsAlert` emissions.
    /// Default 1000 (1 second). Set to 0 to disable periodic stats alerts.
    #[serde(default = "default_stats_report_interval")]
    pub stats_report_interval: u64,

    // ── Runtime tuning (M95) ──
    /// Number of tokio worker threads. Default: min(available cores, 8).
    /// Set to 0 to use tokio's default (= `available_parallelism()`).
    #[serde(default = "default_runtime_worker_threads")]
    pub runtime_worker_threads: usize,
    /// Pin tokio worker threads to CPU cores for cache locality. Default: true.
    #[serde(default = "default_true")]
    pub pin_cores: bool,

    // ── Lock diagnostics (M120) ──
    /// Warning threshold in milliseconds for lock hold duration.
    /// When a hot-path lock is held longer than this, a tracing warning is
    /// emitted. Set to 0 to disable timing entirely (zero overhead).
    /// Default: 50.
    #[serde(default = "default_lock_warn_threshold_ms")]
    pub lock_warn_threshold_ms: u64,

    // ── DHT bootstrap (M56) ──
    /// Previously saved DHT routing table nodes for fast bootstrap.
    /// These are prepended to the bootstrap node list on startup so that
    /// peer discovery starts instantly instead of bootstrapping from scratch.
    /// Runtime-injected, not serialized.
    #[serde(skip)]
    pub dht_saved_nodes: Vec<String>,
    /// BEP 42-compliant DHT node ID from previous session.
    /// Reusing the same ID avoids routing table regeneration on every startup.
    /// Runtime-injected, not serialized.
    #[serde(skip)]
    pub dht_node_id: Option<irontide_core::Id20>,

    /// qBittorrent `WebUI` v2 compatibility layer (M168).
    /// Opt-in; disabled by default. Enables *arr integration via qBt's de-facto
    /// API protocol. See `QbtCompatSettings` for full field documentation.
    #[serde(default)]
    pub qbt_compat: QbtCompatSettings,

    /// M170: Path to the qBt-compat category registry TOML file. When
    /// `None`, the default `$XDG_CONFIG_HOME/irontide/categories.toml`
    /// resolution is used (matching the `config.toml` convention).
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub category_registry_path: Option<PathBuf>,

    /// M171: Path to the qBt-compat tag registry TOML file. When `None`,
    /// the default `$XDG_CONFIG_HOME/irontide/tags.toml` resolution is
    /// used (matching the `category_registry_path` convention).
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub tag_registry_path: Option<PathBuf>,

    // ── M226: Notifications / paths / watched folder / network ──────────
    /// M226: Fire an OS desktop notification when a torrent finishes
    /// downloading. Wired through `NotificationSink` in `notification.rs`.
    /// Default: false.
    #[serde(default)]
    pub notify_on_complete: bool,
    /// M226: Fire an OS desktop notification when a torrent enters an error
    /// state. Default: false.
    #[serde(default)]
    pub notify_on_error: bool,
    /// M226: Path to a program to run on torrent completion (qBt parity
    /// field). STORED ONLY — subprocess spawning is deferred to a future
    /// engine milestone (child-reaper, env scrubbing, exec safety audit
    /// pending). Default: None.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub on_complete_program: Option<PathBuf>,
    /// M226: Whether in-progress downloads use a separate directory before
    /// being moved to `download_dir` on completion. STORED ONLY — storage
    /// layer wiring deferred. Default: false.
    #[serde(default)]
    pub use_incomplete_dir: bool,
    /// M226: Directory for in-progress downloads (paired with
    /// `use_incomplete_dir`). STORED ONLY — storage layer wiring deferred.
    /// Must be absolute when `Some`. Default: None.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub incomplete_dir: Option<PathBuf>,
    /// M226: Default value for `AddTorrentParams.skip_checking` when the
    /// caller does not specify. STORED ONLY — add-torrent flow wiring is
    /// deferred to M227's GUI "Skip hash check" toggle. Default: false.
    #[serde(default)]
    pub default_skip_hash_check: bool,
    /// M226: Append `.!ut` to filenames during download (qBt convention to
    /// signal partial files to file managers). STORED ONLY — storage layer
    /// wiring deferred. Default: true (qBt parity).
    #[serde(default = "default_true")]
    pub incomplete_extension_enabled: bool,
    /// M226: Path to a folder to watch for new `.torrent` files; on detection
    /// the file is auto-added to the session. Wired through `watched_folder.rs`.
    /// Must be absolute when `Some`. Default: None.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub watched_folder: Option<PathBuf>,
    /// M226: After successfully adding a `.torrent` file from `watched_folder`,
    /// delete the source file. When false, the file is renamed to
    /// `<name>.duplicate` to prevent infinite-rescan (see plan §G2). Default:
    /// false (dry-run safe).
    #[serde(default)]
    pub delete_torrent_after_add: bool,
    /// M226: Whether to move completed torrents to `move_completed_to`.
    /// STORED ONLY — on-completion move logic deferred to a future engine
    /// milestone. Default: false.
    #[serde(default)]
    pub move_completed_enabled: bool,
    /// M226: Destination for completed torrents (paired with
    /// `move_completed_enabled`). STORED ONLY — move logic deferred. Must be
    /// absolute when `Some`. Default: None.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub move_completed_to: Option<PathBuf>,
    /// M226: Enable `HTTPS` for the qBt v2 `WebUI` listener. STORED ONLY —
    /// rustls integration deferred to a Phase O follow-on or Phase P
    /// milestone. Default: false.
    #[serde(default)]
    pub web_ui_https_enabled: bool,
    /// M226: Bind peer listeners to a specific network interface (qBt
    /// `current_network_interface`). STORED ONLY — `SO_BINDTODEVICE` wiring
    /// deferred. Default: None (use 0.0.0.0 / [::]).
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub network_interface: Option<String>,
    /// M226: When `AddTorrentParams.paused` is `None`, this default decides
    /// whether new torrents start paused. Surfaced through the qBt v2
    /// `preferences.rs` GET projection as `start_paused_enabled`. Default:
    /// false.
    #[serde(default)]
    pub default_add_paused: bool,
}

/// qBittorrent `WebUI` v2 compatibility layer configuration.
///
/// # Security note (M172a)
/// As of M172a passwords are stored in PHC-format argon2id hashes in
/// [`Self::password_hash`]. The legacy [`Self::password`] field is retained
/// as a one-shot upgrade path — on daemon startup, if `password_hash` is
/// empty and `password` is non-empty, the daemon hashes the plaintext,
/// writes it back to [`Self::password_hash`] (zeroing the plaintext), and
/// atomically rewrites the config file via
/// [`crate::migrate_qbt_credentials`]. Fresh installs ship with
/// `password = ""` and a pre-hashed `password_hash` so no migration WARN
/// ever fires on a clean daemon.
///
/// File permissions (`0o600`) are still enforced by
/// [`irontide-config`'s `save_config_atomic`] as defence-in-depth — the PHC
/// hash is not directly reversible, but password-cracking dictionaries
/// remain feasible for weak passwords.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
#[serde(default)]
pub struct QbtCompatSettings {
    /// Master enable flag. When `false`, all `/api/v2/*` routes return 404
    /// (not 403) — the route must appear non-existent. Default: `true`
    /// (v0.172.1 flip, inverted from v0.168.0's security-through-invisibility
    /// default). Set `enabled = false` in `config.toml` under `[qbt_compat]`
    /// to opt out — the argon2id hash + brute-force ban + CSRF middleware
    /// (M172a) remain the primary defences; 404-on-disabled is defence-in-
    /// depth, not primary security.
    pub enabled: bool,
    /// Username required for qBt v2 login. Default: `"admin"` (qBt factory
    /// default). Must be non-empty when `enabled`.
    pub username: String,
    /// Argon2id PHC-format password hash (M172a). Example:
    /// `"$argon2id$v=19$m=19456,t=2,p=1$<salt>$<hash>"`. OWASP-recommended
    /// parameters (m=19456 KiB, t=2, p=1).
    ///
    /// Fresh installs ship a non-empty default pre-hashing the factory
    /// "adminadmin" password so no migration WARN ever fires. The daemon
    /// rejects a malformed hash at validate-time.
    pub password_hash: String,
    /// Legacy plaintext password — **deprecated, migration-only**. Populated
    /// on config files written before M172a; the daemon rehashes and
    /// zeroizes this on next startup, leaving it permanently empty
    /// afterwards. New configs must ship with `password = ""` and a
    /// non-empty `password_hash`. Default: `""`.
    #[serde(default)]
    pub password: String,
    /// Version string returned by `GET /api/v2/app/version`. Must match the
    /// regex `^v\d+\.\d+(\.\d+)?(-\w+)?$`. Default: `"v5.1.4"`.
    pub spoof_app_version: String,
    /// Version string returned by `GET /api/v2/app/webapiVersion`. Must match
    /// the regex `^\d+\.\d+(\.\d+)?$` (no leading `v`). Default: `"2.11.4"`.
    pub spoof_webapi_version: String,
    /// Session cookie TTL in seconds. Bounds: `[60, 604_800]` (1 minute to
    /// 1 week). Default: `86_400` (24 hours).
    pub session_ttl_secs: u64,
    /// Maximum concurrent sessions. Prevents unbounded growth on login
    /// storms. Must be > 0. Default: `1024`.
    pub max_sessions: usize,
    /// Optional override for the global argon2 verification semaphore size
    /// (M172a G2). `None` means use the computed default
    /// `num_cpus::get() * 2`, clamped to `[2, 16]`. Rejects `Some(0)`. Peak
    /// memory under flood is bounded by `permits * 19 MiB`.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub max_concurrent_argon2_ops: Option<u32>,

    /// v0.187.3 / 2A: TCP port the Web UI listens on. Single source of truth
    /// for the listen port; the legacy `[api].port` field is deprecated and
    /// auto-migrates to this value on config load with a one-time warning.
    /// Default: `9080`. Validated `> 0` when `enabled` is true.
    #[serde(default = "default_qbt_port")]
    pub port: u16,

    /// v0.187.3 / 2A: bind address the Web UI listens on. Single source of
    /// truth; the legacy `[api].bind` field is deprecated and auto-migrates.
    /// Default: `"127.0.0.1"`. Use `"0.0.0.0"` to expose on all interfaces
    /// (behind a reverse proxy strongly recommended).
    #[serde(default = "default_qbt_bind_address")]
    pub bind_address: String,

    // M172a Lane B (CSRF + reverse-proxy). Fields appended at the end of the
    // struct to keep merge conflicts minimal with parallel Lane C.
    /// M172a Lane B: enable Origin/Referer CSRF checks on mutating requests
    /// (POST/PATCH/PUT/DELETE) against `/webui/*` and `/api/v2/*`. When both
    /// headers are absent the request is allowed (server-to-server case;
    /// matches qBt and is what `*arr` clients need). Default: `true`.
    #[serde(default = "default_csrf_protection_enabled")]
    pub csrf_protection_enabled: bool,
    /// M172a Lane B: enable Host-header validation against Origin/Referer.
    /// When `csrf_protection_enabled` is `true` but this flag is `false`, the
    /// middleware short-circuits to allow. Useful for reverse-proxy setups
    /// whose proxy strips/rewrites the Host header in a non-trivial way.
    /// Default: `true`.
    #[serde(default = "default_host_header_validation_enabled")]
    pub host_header_validation_enabled: bool,
    /// M172a Lane B: when true, the CSRF middleware resolves the real client
    /// IP via the XFF trust-hop algorithm and validates Host against
    /// `X-Forwarded-Host` + `X-Forwarded-Proto` *only when* the peer matches
    /// one of the CIDRs in [`Self::web_ui_reverse_proxies_list`]. Untrusted
    /// peers fall back to direct Host validation — defence-in-depth against
    /// an attacker spoofing XFH from outside the proxy layer. Default:
    /// `false`.
    #[serde(default)]
    pub web_ui_reverse_proxy_enabled: bool,
    /// M172a Lane B: list of CIDRs trusted to supply `X-Forwarded-For` and
    /// `X-Forwarded-Host` headers. Each entry must parse as
    /// [`ipnet::IpNet`] (validated in [`Settings::validate`]). Empty list
    /// is valid but degrades reverse-proxy mode to "trust nobody" — the
    /// middleware falls back to direct Host validation in that case.
    ///
    /// **Narrow is safer.** Prefer single-host CIDRs like `172.20.0.5/32`
    /// over block-wide `172.16.0.0/12`. A too-wide mask means any client
    /// inside a trusted subnet can spoof `X-Forwarded-Host` and defeat
    /// CSRF protection; a `/32` binds trust to the exact proxy IP.
    #[serde(default)]
    pub web_ui_reverse_proxies_list: Vec<String>,

    // ── M172a Lane C: brute-force ban ──────────────────────────────────
    /// Maximum number of failed `auth/login` attempts from a single source
    /// IP before the IP is banned. Must be `> 0` unless
    /// [`Self::bypass_local_auth`] is `true`. Default: `5`.
    ///
    /// The counter resets on a successful login and on ban expiry.
    #[serde(default = "default_max_failed_auth_count")]
    pub max_failed_auth_count: u32,
    /// Ban duration (seconds) after hitting [`Self::max_failed_auth_count`].
    /// Bounds: `[60, 86_400]` (1 minute to 1 day). Default: `3_600` (1 hour).
    #[serde(default = "default_ban_duration_secs")]
    pub ban_duration_secs: u64,
    /// When `true`, any request whose resolved client IP is loopback
    /// (`127.0.0.0/8`, `::1`) bypasses authentication entirely and
    /// receives a valid SID cookie. Default: `false`.
    ///
    /// Combined with [`Self::bypass_auth_subnet_whitelist`] these provide
    /// the qBt-parity "local auth off" and "whitelisted subnets" escape
    /// hatches that `*arr` clients rely on.
    #[serde(default)]
    pub bypass_local_auth: bool,
    /// CIDR strings whose resolved client IP bypasses authentication
    /// entirely. Each string must parse as an [`ipnet::IpNet`]. Default:
    /// `vec![]`.
    ///
    /// Interpreted at router construction time and re-parsed on
    /// `setPreferences` apply so runtime reconfiguration flows through the
    /// shared `QbtState::bypass_auth_subnet_whitelist` `RwLock`.
    #[serde(default)]
    pub bypass_auth_subnet_whitelist: Vec<String>,
    /// Optional override for the brute-force-ban registry's LRU capacity.
    /// `None` means use the internal default of `10_000`. Rejects values
    /// `< 100`. Default: `None`.
    ///
    /// The registry retains its initial capacity until daemon restart —
    /// runtime changes only affect NEW entries admitted afterwards (see
    /// `FIXME` in the `classify_immediate` handler).
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub brute_force_registry_capacity: Option<usize>,
}

fn default_csrf_protection_enabled() -> bool {
    true
}

fn default_host_header_validation_enabled() -> bool {
    true
}

// v0.187.3 / 2A: Web UI bind + port defaults. Match what previously lived
// on `[api]`.
fn default_qbt_port() -> u16 {
    9080
}

fn default_qbt_bind_address() -> String {
    "127.0.0.1".to_owned()
}

/// Default for [`QbtCompatSettings::max_failed_auth_count`].
#[must_use]
pub const fn default_max_failed_auth_count() -> u32 {
    5
}

/// Default for [`QbtCompatSettings::ban_duration_secs`].
#[must_use]
pub const fn default_ban_duration_secs() -> u64 {
    3_600
}

/// Argon2id PHC hash of the default "adminadmin" password (M172a A3).
///
/// Pre-computed once using the OWASP-recommended parameters with a
/// deterministic salt so round-tripping the default config across installs
/// is stable. The salt literal below is not a secret — it's supposed to be
/// recognisably the shipped default so operators know to rotate it.
///
/// The hash is regenerated by the [`tests::default_hash_roundtrips_admin_admin`]
/// test, which will fail with a suggested replacement value if parameters
/// or salt change.
pub const DEFAULT_ADMINADMIN_HASH: &str = "$argon2id$v=19$m=19456,t=2,p=1$u3doPIM7ab7NlbMfhMFm6A$ctIAjFfl70eUfUsThdGcXICr0lcD6bEUilRujvnXLPg";

impl Default for QbtCompatSettings {
    fn default() -> Self {
        Self {
            enabled: true,
            username: "admin".into(),
            password_hash: DEFAULT_ADMINADMIN_HASH.into(),
            password: String::new(),
            spoof_app_version: "v5.1.4".into(),
            spoof_webapi_version: "2.11.4".into(),
            session_ttl_secs: 86_400,
            max_sessions: 1024,
            max_concurrent_argon2_ops: None,
            // v0.187.3 / 2A: Web UI listen socket — single source of truth.
            port: default_qbt_port(),
            bind_address: default_qbt_bind_address(),
            // M172a Lane B defaults — CSRF on, host validation on, no proxy.
            csrf_protection_enabled: true,
            host_header_validation_enabled: true,
            web_ui_reverse_proxy_enabled: false,
            web_ui_reverse_proxies_list: Vec::new(),
            // M172a Lane C: brute-force ban defaults.
            max_failed_auth_count: default_max_failed_auth_count(),
            ban_duration_secs: default_ban_duration_secs(),
            bypass_local_auth: false,
            bypass_auth_subnet_whitelist: Vec::new(),
            brute_force_registry_capacity: None,
        }
    }
}

/// Outcome of a legacy-plaintext migration pass (M172a A3 / C2).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum QbtCredentialMigration {
    /// `password_hash` already present or nothing to migrate — no change.
    NoOp,
    /// Plaintext was hashed and written back into `password_hash`; the
    /// in-memory `password` was zeroed. The in-memory settings are mutated
    /// in place; callers should persist the mutation.
    Upgraded,
}

/// Hash `plaintext` with OWASP-recommended argon2id parameters and return the
/// PHC-format encoded string (M172a).
///
/// Pure CPU work — callers on async stacks should wrap in
/// `tokio::task::spawn_blocking` for anything other than a one-shot startup
/// migration. Login-time verification has its own concurrency limiter.
///
/// # Errors
///
/// Returns an error when the `argon2` crate's own hashing fails (empty
/// plaintext, OS entropy failure, internal parameter error).
pub fn hash_qbt_password(plaintext: &str) -> Result<String, QbtMigrationError> {
    use argon2::password_hash::{PasswordHasher, SaltString};
    use argon2::{Algorithm, Argon2, Params, Version};

    // `rand_core::OsRng` + its `getrandom` feature is our entropy source —
    // pulled in directly rather than via argon2's `rand` feature flag so the
    // hash path is decoupled from argon2 feature churn.
    let salt = SaltString::generate(&mut rand_core::OsRng);
    // OWASP cheat-sheet (argon2id): m=19_456 KiB, t=2, p=1. Output length
    // 32 bytes = 256 bits of key material.
    let params = Params::new(19_456, 2, 1, Some(32))
        .map_err(|e| QbtMigrationError::Hash(format!("argon2 params: {e}")))?;
    let argon2 = Argon2::new(Algorithm::Argon2id, Version::V0x13, params);
    let hash = argon2
        .hash_password(plaintext.as_bytes(), &salt)
        .map_err(|e| QbtMigrationError::Hash(format!("argon2 hash: {e}")))?;
    Ok(hash.to_string())
}

/// Errors raised by [`migrate_qbt_credentials`].
#[derive(Debug, thiserror::Error)]
pub enum QbtMigrationError {
    /// argon2 hashing failure — returned by [`hash_qbt_password`].
    #[error("argon2 hash: {0}")]
    Hash(String),
}

/// One-shot legacy-plaintext → argon2id migration for
/// [`QbtCompatSettings`].
///
/// Semantics:
///
/// * `password_hash` non-empty → [`QbtCredentialMigration::NoOp`] (most
///   common path: fresh install, already-migrated install).
/// * `password_hash` empty **and** `password` non-empty → compute a fresh
///   PHC-format hash, assign it to `password_hash`, zero the plaintext via
///   [`zeroize::Zeroizing`] + `std::mem::take`, return
///   [`QbtCredentialMigration::Upgraded`].
/// * Both empty → [`QbtCredentialMigration::NoOp`]. Validation elsewhere
///   rejects an "enabled and both-empty" combo so we never authenticate an
///   unconfigured daemon.
///
/// This helper does *not* touch disk — callers pair it with
/// [`irontide_config::save_config_atomic`] (or a hand-rolled rewrite) to
/// persist the rewritten `Settings`. On migration failure the plaintext is
/// left untouched in memory so the daemon can still authenticate during
/// this boot; the migration will retry on the next startup.
///
/// # Errors
///
/// Propagates [`QbtMigrationError::Hash`] if argon2 hashing itself fails.
/// On `Err` the input is left unmodified so the caller's session-startup
/// path can continue with the plaintext still in memory.
pub fn migrate_qbt_credentials(
    qbt: &mut QbtCompatSettings,
) -> Result<QbtCredentialMigration, QbtMigrationError> {
    if !qbt.password_hash.is_empty() {
        return Ok(QbtCredentialMigration::NoOp);
    }
    if qbt.password.is_empty() {
        return Ok(QbtCredentialMigration::NoOp);
    }

    let hash = hash_qbt_password(&qbt.password)?;
    qbt.password_hash = hash;
    // C7: substantive zeroize — the Settings struct has a longer in-memory
    // lifetime than the request path, so scrubbing the plaintext here
    // actually removes a residual copy rather than theatre.
    let _drain = zeroize::Zeroizing::new(std::mem::take(&mut qbt.password));
    Ok(QbtCredentialMigration::Upgraded)
}

/// Validate an app-version string (e.g. `v5.1.4` or `v4.6.4-rc1`).
fn is_valid_app_version(s: &str) -> bool {
    let Some(rest) = s.strip_prefix('v') else {
        return false;
    };
    // Split optional "-suffix" (pre-release tag like rc1, beta2) from the numeric core.
    let (core, suffix_ok) = match rest.split_once('-') {
        Some((core, suffix)) => (
            core,
            !suffix.is_empty() && suffix.chars().all(|c| c.is_ascii_alphanumeric()),
        ),
        None => (rest, true),
    };
    if !suffix_ok {
        return false;
    }
    is_valid_dotted_numeric(core)
}

/// Validate a webapi-version string (e.g. `2.11.4`).
fn is_valid_webapi_version(s: &str) -> bool {
    is_valid_dotted_numeric(s)
}

/// Accepts two or three non-empty dot-separated numeric segments.
fn is_valid_dotted_numeric(s: &str) -> bool {
    let parts: Vec<&str> = s.split('.').collect();
    if !(2..=3).contains(&parts.len()) {
        return false;
    }
    parts
        .iter()
        .all(|p| !p.is_empty() && p.chars().all(|c| c.is_ascii_digit()))
}

impl Default for Settings {
    fn default() -> Self {
        Self {
            // General
            listen_port: 42020,
            randomize_port_on_startup: false,
            download_dir: PathBuf::from("."),
            max_torrents: 100,
            resume_data_dir: None,
            save_resume_interval_secs: 300,
            // Protocol features
            enable_dht: true,
            enable_pex: true,
            enable_lsd: true,
            enable_fast_extension: true,
            enable_utp: true,
            enable_upnp: true,
            enable_natpmp: true,
            enable_ipv6: true,
            enable_web_seed: true,
            enable_holepunch: true,
            enable_bep40_eviction: true,
            enable_diagnostic_counters: false,
            encryption_mode: EncryptionMode::Disabled,
            anonymous_mode: false,
            external_ip: None,
            // Seeding
            seed_ratio_limit: None,
            seed_time_limit_secs: None,
            inactive_seed_time_limit_secs: None,
            max_ratio_action: MaxRatioAction::Pause,
            create_subfolder: true,
            auto_manage_torrents: false,
            queueing_enabled: false,
            default_super_seeding: false,
            default_share_mode: false,
            upload_only_announce: true,
            // Rate limiting
            upload_rate_limit: 0,
            download_rate_limit: 0,
            tcp_upload_rate_limit: 0,
            tcp_download_rate_limit: 0,
            utp_upload_rate_limit: 0,
            utp_download_rate_limit: 0,
            auto_upload_slots: true,
            auto_upload_slots_min: 2,
            auto_upload_slots_max: 20,
            max_upload_slots_global: -1,
            max_upload_slots_per_torrent: 4,
            max_connections_global: -1,
            max_uploads_per_torrent: -1,
            alt_download_rate_limit: 0,
            alt_upload_rate_limit: 0,
            alt_speed_enabled: false,
            alt_speed_schedule_enabled: false,
            alt_speed_schedule_from: 0,
            alt_speed_schedule_to: 0,
            alt_speed_schedule_days: 0,
            rate_limit_includes_overhead: true,
            rate_limit_utp: true,
            rate_limit_lan: false,
            mixed_mode_algorithm: MixedModeAlgorithm::PeerProportional,
            // Queue management
            active_downloads: 3,
            active_seeds: 5,
            active_limit: 500,
            active_checking: 3,
            dont_count_slow_torrents: true,
            inactive_down_rate: 2048,
            inactive_up_rate: 2048,
            auto_manage_interval: 30,
            auto_manage_startup: 60,
            auto_manage_prefer_seeds: false,
            queue_rate_ewma_alpha: 0.3,
            seed_queue_min_active_secs: 1800,
            // Alerts
            alert_mask: AlertCategory::ALL,
            alert_channel_size: 1024,
            // Smart banning
            smart_ban_max_failures: 3,
            smart_ban_parole: true,
            // Disk I/O
            disk_io_threads: default_disk_io_threads(),
            max_blocking_threads: default_max_blocking_threads(),
            storage_mode: StorageMode::Auto,
            preallocate_mode: None,
            disk_cache_size: 16 * 1024 * 1024,
            disk_write_cache_ratio: 0.5,
            disk_channel_capacity: 512,
            buffer_pool_capacity: 64 * 1024 * 1024,
            enable_mlock: cfg!(unix),
            io_uring_sq_depth: 256,
            io_uring_direct_io: false,
            filesystem_direct_io: false,
            io_uring_batch_threshold: 4,
            iocp_concurrent_threads: 0,
            iocp_direct_io: false,
            // Hashing & piece picking
            hashing_threads: default_hashing_threads(),
            max_request_queue_depth: 250,
            initial_queue_depth: 128,
            request_queue_time: 3.0,
            block_request_timeout_secs: 60,
            max_concurrent_stream_reads: 8,
            auto_sequential: true,
            steal_threshold_ratio: 10.0,
            use_block_stealing: true,
            steal_stale_piece_secs: 2,
            steal_threshold_endgame: 3.0,
            fixed_pipeline_depth: 128,
            strict_end_game: true,
            max_web_seeds: 4,
            web_seed_retry_base_secs: 10,
            web_seed_retry_factor: 6,
            web_seed_retry_cap_secs: 3600,
            web_seed_max_failures: 10,
            initial_picker_threshold: 4,
            whole_pieces_threshold: 20,
            snub_timeout_secs: 15,
            readahead_pieces: 8,
            streaming_timeout_escalation: true,
            // Piece picker enhancements (M44)
            piece_extent_affinity: true,
            suggest_mode: true,
            max_suggest_pieces: 16,
            predictive_piece_announce_ms: 0,
            // Proxy
            proxy: ProxyConfig::default(),
            force_proxy: false,
            // IP Filtering
            ip_filter_enabled: false,
            ip_filter_path: String::new(),
            ip_filter_auto_refresh: false,
            apply_ip_filter_to_trackers: true,
            // DHT tuning
            dht_queries_per_second: 50,
            dht_query_timeout_secs: 5,
            dht_enforce_node_id: false,
            dht_restrict_routing_ips: true,
            dht_max_items: 700,
            dht_item_lifetime_secs: 7200,
            dht_sample_infohashes_interval: 0,
            dht_read_only: false,
            // NAT tuning
            upnp_lease_duration: 3600,
            natpmp_lifetime: 7200,
            // uTP tuning
            utp_max_connections: 256,
            // I2P
            enable_i2p: false,
            i2p_hostname: "127.0.0.1".into(),
            i2p_port: 7656,
            i2p_inbound_quantity: 3,
            i2p_outbound_quantity: 3,
            i2p_inbound_length: 3,
            i2p_outbound_length: 3,
            allow_i2p_mixed: false,
            // SSL torrents
            ssl_listen_port: 0,
            ssl_cert_path: None,
            ssl_key_path: None,
            // Choking algorithms
            seed_choking_algorithm: SeedChokingAlgorithm::FastestUpload,
            choking_algorithm: ChokingAlgorithm::FixedSlots,
            // Peer connections
            max_peers_per_torrent: 128,
            // v0.187.3 eviction policy (see field doc-comments)
            pass0_grace_secs: 60,
            proactive_evictions_per_minute_limit: 30,
            eviction_ban_duration_secs: 600,
            eviction_ban_set_cap: 1024,
            peer_read_timeout_secs: 10,
            peer_write_timeout_secs: 10,
            data_contribution_timeout_secs: 0,
            choke_rotation_max_evictions: 0,
            max_concurrent_connects: 128,
            connect_soft_timeout: 3,
            dispatch_backlog_cap: 8,
            event_backlog_cap: 32,
            use_actor_dispatch: true,
            web_seed_progress_throttle_ms: 250,
            // Security
            ssrf_mitigation: true,
            allow_idna: false,
            validate_https_trackers: true,
            max_metadata_size: 4 * 1024 * 1024,
            max_message_size: 16 * 1024 * 1024,
            max_piece_length: 32 * 1024 * 1024,
            max_outstanding_requests: 500,
            max_in_flight_pieces: 512,
            peer_connect_timeout: 10,
            peer_dscp: 0x08,
            // Session Stats (M50)
            stats_report_interval: 1000,
            // Runtime tuning (M95)
            runtime_worker_threads: default_runtime_worker_threads(),
            pin_cores: true,
            // Lock diagnostics (M120)
            lock_warn_threshold_ms: 50,
            // DHT bootstrap (M56)
            dht_saved_nodes: Vec::new(),
            dht_node_id: None,
            // qBt v2 compatibility (M168)
            qbt_compat: QbtCompatSettings::default(),
            // M170: default to None → resolved via XDG to
            // `$XDG_CONFIG_HOME/irontide/categories.toml` at registry load time.
            category_registry_path: None,
            // M171: default to None → resolved via XDG to
            // `$XDG_CONFIG_HOME/irontide/tags.toml` at registry load time.
            tag_registry_path: None,
            // M226: notifications + paths + watched folder + network
            notify_on_complete: false,
            notify_on_error: false,
            on_complete_program: None,
            use_incomplete_dir: false,
            incomplete_dir: None,
            default_skip_hash_check: false,
            incomplete_extension_enabled: true,
            watched_folder: None,
            delete_torrent_after_add: false,
            move_completed_enabled: false,
            move_completed_to: None,
            web_ui_https_enabled: false,
            network_interface: None,
            default_add_paused: false,
        }
    }
}

impl Settings {
    /// Preset for constrained/embedded environments.
    #[must_use]
    pub fn min_memory() -> Self {
        Self {
            disk_cache_size: 8 * 1024 * 1024,
            buffer_pool_capacity: 16 * 1024 * 1024,
            max_torrents: 20,
            max_peers_per_torrent: 30,
            active_downloads: 1,
            active_seeds: 2,
            active_limit: 10,
            alert_channel_size: 256,
            utp_max_connections: 64,
            max_request_queue_depth: 50,
            initial_queue_depth: 16,
            max_concurrent_stream_reads: 2,
            hashing_threads: 1,
            disk_io_threads: 1,
            dht_max_items: 100,
            max_in_flight_pieces: 32,
            fixed_pipeline_depth: 32,
            ..Self::default()
        }
    }

    /// Preset for desktop/server environments with ample resources.
    #[must_use]
    pub fn high_performance() -> Self {
        Self {
            disk_cache_size: 256 * 1024 * 1024,
            buffer_pool_capacity: 256 * 1024 * 1024,
            max_torrents: 2000,
            max_peers_per_torrent: 200,
            active_downloads: 30,
            active_seeds: 100,
            active_limit: 2000,
            alert_channel_size: 4096,
            utp_max_connections: 1024,
            max_request_queue_depth: 1000,
            initial_queue_depth: 256,
            max_concurrent_stream_reads: 32,
            hashing_threads: 4,
            disk_io_threads: 8,
            auto_upload_slots_max: 100,
            suggest_mode: true,
            steal_threshold_ratio: 5.0,
            steal_threshold_endgame: 2.0,
            use_block_stealing: true,
            max_in_flight_pieces: 512,
            ..Self::default()
        }
    }

    /// Validate settings. Returns error on the first invalid combination found.
    ///
    /// # Errors
    ///
    /// Returns an error if validation fails.
    pub fn validate(&self) -> crate::Result<()> {
        use crate::proxy::ProxyType;

        if self.force_proxy && self.proxy.proxy_type == ProxyType::None {
            return Err(crate::Error::InvalidSettings(
                "force_proxy is enabled but no proxy type is configured".into(),
            ));
        }

        if self.active_downloads > 0
            && self.active_limit > 0
            && self.active_downloads > self.active_limit
        {
            return Err(crate::Error::InvalidSettings(
                "active_downloads exceeds active_limit".into(),
            ));
        }

        if self.active_seeds > 0 && self.active_limit > 0 && self.active_seeds > self.active_limit {
            return Err(crate::Error::InvalidSettings(
                "active_seeds exceeds active_limit".into(),
            ));
        }

        if !(0.0..=1.0).contains(&self.disk_write_cache_ratio) {
            return Err(crate::Error::InvalidSettings(
                "disk_write_cache_ratio must be between 0.0 and 1.0".into(),
            ));
        }

        if self.disk_cache_size < 1024 * 1024 {
            return Err(crate::Error::InvalidSettings(
                "disk_cache_size must be at least 1 MiB".into(),
            ));
        }

        if self.hashing_threads == 0 {
            return Err(crate::Error::InvalidSettings(
                "hashing_threads must be at least 1".into(),
            ));
        }

        // M226: paired-field validation + path-shape validation.
        if self.use_incomplete_dir && self.incomplete_dir.is_none() {
            return Err(crate::Error::InvalidSettings(
                "incomplete_dir must be set when use_incomplete_dir=true".into(),
            ));
        }
        if self.move_completed_enabled && self.move_completed_to.is_none() {
            return Err(crate::Error::InvalidSettings(
                "move_completed_to must be set when move_completed_enabled=true".into(),
            ));
        }
        // M226 F11: every Option<PathBuf> path-field must be absolute when Some
        // so silent breakage at runtime is replaced with a config-load error.
        for (name, opt) in [
            ("watched_folder", self.watched_folder.as_ref()),
            ("incomplete_dir", self.incomplete_dir.as_ref()),
            ("move_completed_to", self.move_completed_to.as_ref()),
        ] {
            if let Some(p) = opt
                && !p.is_absolute()
            {
                return Err(crate::Error::InvalidSettings(format!(
                    "{name} must be an absolute path, got {}",
                    p.display()
                )));
            }
        }
        // M226 H6: reject obviously-dangerous watched_folder paths. When
        // delete_torrent_after_add=true a typo here could shred system files.
        if let Some(p) = self.watched_folder.as_ref() {
            const DENY: &[&str] = &[
                "/", "/etc", "/usr", "/bin", "/sbin", "/lib", "/lib64", "/boot", "/sys",
                "/proc", "/dev", "/run", "/var/lib", "/var/log",
            ];
            let s = p.to_string_lossy();
            if DENY.iter().any(|d| s == *d) {
                return Err(crate::Error::InvalidSettings(format!(
                    "watched_folder rejected: {} is a system path (would risk shredding system files if delete_torrent_after_add=true)",
                    p.display()
                )));
            }
            if let Some(home) = std::env::var_os("HOME") {
                let home_path = PathBuf::from(home);
                if p == &home_path {
                    return Err(crate::Error::InvalidSettings(format!(
                        "watched_folder cannot be $HOME ({}) — too broad to be a torrent dropbox; pick a dedicated subdirectory",
                        p.display()
                    )));
                }
            }
        }

        // M224: max_uploads_per_torrent uses `-1` sentinel for unlimited
        // (matches max_connections_global precedent). `0` is rejected — qBt's
        // wire format accepts `0` on some GET paths to mean unlimited, but
        // qBt's setPreferences accepts `-1`; we mirror the `-1` convention
        // for input and reject `0` as a likely wire-format mistake.
        if self.max_uploads_per_torrent == 0 || self.max_uploads_per_torrent < -1 {
            return Err(crate::Error::InvalidSettings(
                "max_uploads_per_torrent must be -1 (unlimited) or >= 1".into(),
            ));
        }

        if self.disk_io_threads == 0 {
            return Err(crate::Error::InvalidSettings(
                "disk_io_threads must be at least 1".into(),
            ));
        }

        if self.max_blocking_threads == 0 {
            return Err(crate::Error::InvalidSettings(
                "max_blocking_threads must be at least 1".into(),
            ));
        }

        if self.default_share_mode && !self.enable_fast_extension {
            return Err(crate::Error::InvalidSettings(
                "share_mode requires enable_fast_extension for RejectRequest messages".into(),
            ));
        }

        // SSL cert/key must both be set or both absent
        if self.ssl_cert_path.is_some() != self.ssl_key_path.is_some() {
            return Err(crate::Error::InvalidSettings(
                "ssl_cert_path and ssl_key_path must both be set or both absent".into(),
            ));
        }

        if self.enable_i2p {
            if self.i2p_inbound_quantity == 0 || self.i2p_inbound_quantity > 16 {
                return Err(crate::Error::InvalidSettings(
                    "i2p_inbound_quantity must be 1-16".into(),
                ));
            }
            if self.i2p_outbound_quantity == 0 || self.i2p_outbound_quantity > 16 {
                return Err(crate::Error::InvalidSettings(
                    "i2p_outbound_quantity must be 1-16".into(),
                ));
            }
            if self.i2p_inbound_length > 7 {
                return Err(crate::Error::InvalidSettings(
                    "i2p_inbound_length must be 0-7".into(),
                ));
            }
            if self.i2p_outbound_length > 7 {
                return Err(crate::Error::InvalidSettings(
                    "i2p_outbound_length must be 0-7".into(),
                ));
            }
        }

        if self.runtime_worker_threads > 256 {
            return Err(crate::Error::InvalidSettings(
                "runtime_worker_threads must be at most 256".into(),
            ));
        }

        // qBt v2 compatibility settings (M168, extended M172a) — only validated
        // when enabled, so projects with qbt_compat disabled can leave bogus
        // defaults in place.
        if self.qbt_compat.enabled {
            if self.qbt_compat.username.is_empty() {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.username must not be empty when enabled".into(),
                ));
            }
            // M172a: either the PHC hash is present, or the legacy plaintext
            // is set to a non-trivial value (for the upcoming migration).
            // Forbidding both-empty prevents an "anyone can log in" misconfig.
            if self.qbt_compat.password_hash.is_empty() {
                if self.qbt_compat.password.len() < 8 {
                    return Err(crate::Error::InvalidSettings(
                        "qbt_compat: either password_hash must be set OR \
                         password must be at least 8 characters (legacy upgrade path)"
                            .into(),
                    ));
                }
            } else if !self.qbt_compat.password_hash.starts_with("$argon2id$") {
                // M172a: reject unknown-scheme hashes early so an operator
                // copy-pasting a bcrypt or plaintext into `password_hash`
                // doesn't silently authenticate every request.
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.password_hash must be an argon2id PHC string \
                     starting with `$argon2id$`"
                        .into(),
                ));
            }
            if let Some(0) = self.qbt_compat.max_concurrent_argon2_ops {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.max_concurrent_argon2_ops must be > 0 when set".into(),
                ));
            }
            if !is_valid_app_version(&self.qbt_compat.spoof_app_version) {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.spoof_app_version must match vN.N[.N][-suffix] (e.g. v5.1.4)"
                        .into(),
                ));
            }
            if !is_valid_webapi_version(&self.qbt_compat.spoof_webapi_version) {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.spoof_webapi_version must match N.N[.N] (e.g. 2.11.4)".into(),
                ));
            }
            if !(60..=604_800).contains(&self.qbt_compat.session_ttl_secs) {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.session_ttl_secs must be in [60, 604800]".into(),
                ));
            }
            if self.qbt_compat.max_sessions == 0 {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.max_sessions must be at least 1".into(),
                ));
            }
            // M172a Lane B: every CIDR in the reverse-proxy list must parse as
            // an `ipnet::IpNet`. An empty list is valid (middleware falls back
            // to direct-Host validation). The parse failure mode names the
            // offending entry so operators can fix the config without a diff.
            for entry in &self.qbt_compat.web_ui_reverse_proxies_list {
                if entry.parse::<ipnet::IpNet>().is_err() {
                    return Err(crate::Error::InvalidSettings(format!(
                        "qbt_compat.web_ui_reverse_proxies_list: invalid CIDR '{entry}'"
                    )));
                }
            }

            // M172a Lane C: brute-force ban validation.
            // max_failed_auth_count must be > 0 unless the operator has
            // explicitly enabled bypass_local_auth (in which case loopback
            // requests skip the check entirely and the counter is inert for
            // the only caller class that could trip it).
            if self.qbt_compat.max_failed_auth_count == 0 && !self.qbt_compat.bypass_local_auth {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.max_failed_auth_count must be > 0 when bypass_local_auth is false"
                        .into(),
                ));
            }
            if !(60..=86_400).contains(&self.qbt_compat.ban_duration_secs) {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.ban_duration_secs must be in [60, 86400]".into(),
                ));
            }
            for cidr in &self.qbt_compat.bypass_auth_subnet_whitelist {
                if cidr.parse::<ipnet::IpNet>().is_err() {
                    return Err(crate::Error::InvalidSettings(format!(
                        "qbt_compat.bypass_auth_subnet_whitelist: invalid CIDR `{cidr}`"
                    )));
                }
            }
            if let Some(cap) = self.qbt_compat.brute_force_registry_capacity
                && cap < 100
            {
                return Err(crate::Error::InvalidSettings(
                    "qbt_compat.brute_force_registry_capacity must be at least 100".into(),
                ));
            }
        }

        Ok(())
    }
}

// ── Sub-config conversions ───────────────────────────────────────────

impl From<&Settings> for crate::disk::DiskConfig {
    fn from(s: &Settings) -> Self {
        Self {
            io_threads: s.disk_io_threads,
            storage_mode: s.storage_mode,
            cache_size: s.disk_cache_size,
            write_cache_ratio: s.disk_write_cache_ratio,
            channel_capacity: s.disk_channel_capacity,
            buffer_pool_capacity: s.buffer_pool_capacity,
            enable_mlock: s.enable_mlock,
            lock_warn_threshold_ms: s.lock_warn_threshold_ms,
            io_uring_sq_depth: s.io_uring_sq_depth,
            io_uring_direct_io: s.io_uring_direct_io,
            filesystem_direct_io: s.filesystem_direct_io,
            io_uring_batch_threshold: s.io_uring_batch_threshold,
            iocp_concurrent_threads: s.iocp_concurrent_threads,
            iocp_direct_io: s.iocp_direct_io,
        }
    }
}

impl From<&Settings> for crate::ban::BanConfig {
    fn from(s: &Settings) -> Self {
        Self {
            max_failures: s.smart_ban_max_failures,
            use_parole: s.smart_ban_parole,
        }
    }
}

impl Settings {
    pub(crate) fn to_dht_config(&self) -> irontide_dht::DhtConfig {
        let default = irontide_dht::DhtConfig::default();
        let mut bootstrap = self.dht_saved_nodes.clone();
        bootstrap.extend(default.bootstrap_nodes.iter().cloned());
        irontide_dht::DhtConfig {
            bootstrap_nodes: bootstrap,
            own_id: self.dht_node_id,
            queries_per_second: self.dht_queries_per_second,
            query_timeout: std::time::Duration::from_secs(self.dht_query_timeout_secs),
            enforce_node_id: self.dht_enforce_node_id,
            restrict_routing_ips: self.dht_restrict_routing_ips,
            dht_max_items: self.dht_max_items,
            dht_item_lifetime_secs: self.dht_item_lifetime_secs,
            state_dir: self.resume_data_dir.clone(),
            read_only_mode: self.dht_read_only,
            ..default
        }
    }

    pub(crate) fn to_dht_config_v6(&self) -> irontide_dht::DhtConfig {
        let default = irontide_dht::DhtConfig::default_v6();
        let mut bootstrap = self.dht_saved_nodes.clone();
        bootstrap.extend(default.bootstrap_nodes.iter().cloned());
        irontide_dht::DhtConfig {
            bootstrap_nodes: bootstrap,
            queries_per_second: self.dht_queries_per_second,
            query_timeout: std::time::Duration::from_secs(self.dht_query_timeout_secs),
            enforce_node_id: self.dht_enforce_node_id,
            restrict_routing_ips: self.dht_restrict_routing_ips,
            dht_max_items: self.dht_max_items,
            dht_item_lifetime_secs: self.dht_item_lifetime_secs,
            state_dir: self.resume_data_dir.clone(),
            read_only_mode: self.dht_read_only,
            ..default
        }
    }

    pub(crate) fn to_nat_config(&self) -> irontide_nat::NatConfig {
        irontide_nat::NatConfig {
            enable_upnp: self.enable_upnp,
            enable_natpmp: self.enable_natpmp,
            upnp_lease_duration: self.upnp_lease_duration,
            natpmp_lifetime: self.natpmp_lifetime,
        }
    }

    pub(crate) fn to_utp_config(&self, port: u16) -> irontide_utp::UtpConfig {
        irontide_utp::UtpConfig {
            bind_addr: std::net::SocketAddr::from(([0, 0, 0, 0], port)),
            max_connections: self.utp_max_connections,
            dscp: self.peer_dscp,
        }
    }

    pub(crate) fn to_utp_config_v6(&self, port: u16) -> irontide_utp::UtpConfig {
        irontide_utp::UtpConfig {
            bind_addr: std::net::SocketAddr::from((std::net::Ipv6Addr::UNSPECIFIED, port)),
            max_connections: self.utp_max_connections,
            dscp: self.peer_dscp,
        }
    }

    /// Build a `SamTunnelConfig` from the I2P-related settings.
    pub(crate) fn to_sam_tunnel_config(&self) -> crate::i2p::SamTunnelConfig {
        crate::i2p::SamTunnelConfig {
            inbound_quantity: self.i2p_inbound_quantity,
            outbound_quantity: self.i2p_outbound_quantity,
            inbound_length: self.i2p_inbound_length,
            outbound_length: self.i2p_outbound_length,
        }
    }
}

// ── PartialEq (manual — f32/f64 fields need special handling) ────────


// ── Tests ────────────────────────────────────────────────────────────

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn default_settings_values() {
        let s = Settings::default();
        assert_eq!(s.listen_port, 42020);
        assert_eq!(s.download_dir, PathBuf::from("."));
        assert_eq!(s.max_torrents, 100);
        assert!(s.resume_data_dir.is_none());
        assert_eq!(s.save_resume_interval_secs, 300);
        assert!(s.enable_dht);
        assert!(s.enable_pex);
        assert!(s.enable_lsd);
        assert!(s.enable_fast_extension);
        assert!(s.enable_utp);
        assert!(s.enable_upnp);
        assert!(s.enable_natpmp);
        assert!(s.enable_ipv6);
        assert!(s.enable_web_seed);
        assert_eq!(s.encryption_mode, EncryptionMode::Disabled);
        assert!(!s.anonymous_mode);
        assert!(s.seed_ratio_limit.is_none());
        assert!(!s.default_super_seeding);
        assert!(!s.default_share_mode);
        assert!(s.upload_only_announce);
        assert_eq!(s.upload_rate_limit, 0);
        assert_eq!(s.download_rate_limit, 0);
        assert!(s.auto_upload_slots);
        assert_eq!(s.active_downloads, 3);
        assert_eq!(s.active_seeds, 5);
        assert_eq!(s.active_limit, 500);
        assert_eq!(s.active_checking, 3);
        assert!(s.dont_count_slow_torrents);
        assert_eq!(s.alert_mask, AlertCategory::ALL);
        assert_eq!(s.alert_channel_size, 1024);
        assert_eq!(s.smart_ban_max_failures, 3);
        assert!(s.smart_ban_parole);
        assert_eq!(s.disk_io_threads, default_disk_io_threads());
        assert_eq!(s.max_blocking_threads, default_max_blocking_threads());
        assert_eq!(s.storage_mode, StorageMode::Auto);
        assert_eq!(s.disk_cache_size, 16 * 1024 * 1024);
        assert!((s.disk_write_cache_ratio - 0.5).abs() < f32::EPSILON);
        assert_eq!(s.disk_channel_capacity, 512);
        assert_eq!(s.hashing_threads, default_hashing_threads());
        assert_eq!(s.max_request_queue_depth, 250);
        assert_eq!(s.initial_queue_depth, 128);
        assert!((s.request_queue_time - 3.0).abs() < f64::EPSILON);
        assert_eq!(s.block_request_timeout_secs, 60);
        assert_eq!(s.max_concurrent_stream_reads, 8);
        assert!(!s.force_proxy);
        assert!(s.apply_ip_filter_to_trackers);
        assert_eq!(s.dht_queries_per_second, 50);
        assert_eq!(s.dht_query_timeout_secs, 5);
        assert!(!s.dht_enforce_node_id);
        assert!(s.dht_restrict_routing_ips);
        assert_eq!(s.upnp_lease_duration, 3600);
        assert_eq!(s.natpmp_lifetime, 7200);
        assert_eq!(s.utp_max_connections, 256);
        assert_eq!(s.mixed_mode_algorithm, MixedModeAlgorithm::PeerProportional);
        assert!(s.auto_sequential);
        assert!(s.strict_end_game);
        assert_eq!(s.max_web_seeds, 4);
        assert_eq!(s.initial_picker_threshold, 4);
        assert_eq!(s.whole_pieces_threshold, 20);
        assert_eq!(s.snub_timeout_secs, 15);
        assert_eq!(s.readahead_pieces, 8);
        assert!(s.streaming_timeout_escalation);
        assert_eq!(s.max_peers_per_torrent, 128);
        assert_eq!(s.runtime_worker_threads, default_runtime_worker_threads());
        assert!(s.pin_cores);
    }

    #[test]
    fn min_memory_preset() {
        let s = Settings::min_memory();
        assert_eq!(s.disk_cache_size, 8 * 1024 * 1024);
        assert_eq!(s.max_torrents, 20);
        assert_eq!(s.max_peers_per_torrent, 30);
        assert_eq!(s.active_downloads, 1);
        assert_eq!(s.active_seeds, 2);
        assert_eq!(s.active_limit, 10);
        assert_eq!(s.alert_channel_size, 256);
        assert_eq!(s.utp_max_connections, 64);
        assert_eq!(s.max_request_queue_depth, 50);
        assert_eq!(s.initial_queue_depth, 16);
        assert_eq!(s.max_concurrent_stream_reads, 2);
        assert_eq!(s.hashing_threads, 1);
        assert_eq!(s.disk_io_threads, 1);
    }

    #[test]
    fn high_performance_preset() {
        let s = Settings::high_performance();
        assert_eq!(s.disk_cache_size, 256 * 1024 * 1024);
        assert_eq!(s.max_torrents, 2000);
        assert_eq!(s.max_peers_per_torrent, 200);
        assert_eq!(s.active_downloads, 30);
        assert_eq!(s.active_seeds, 100);
        assert_eq!(s.active_limit, 2000);
        assert_eq!(s.alert_channel_size, 4096);
        assert_eq!(s.utp_max_connections, 1024);
        assert_eq!(s.max_request_queue_depth, 1000);
        assert_eq!(s.initial_queue_depth, 256);
        assert_eq!(s.max_concurrent_stream_reads, 32);
        assert_eq!(s.hashing_threads, 4);
        assert_eq!(s.disk_io_threads, 8);
        assert_eq!(s.auto_upload_slots_max, 100);
    }

    #[test]
    fn json_round_trip() {
        let original = Settings::default();
        let json = serde_json::to_string(&original).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(original, decoded);
    }

    #[test]
    fn json_round_trip_presets() {
        // Verify all presets survive JSON serialization
        for original in [Settings::min_memory(), Settings::high_performance()] {
            let json = serde_json::to_string(&original).unwrap();
            let decoded: Settings = serde_json::from_str(&json).unwrap();
            assert_eq!(original, decoded);
        }
    }

    #[test]
    fn json_missing_fields_use_defaults() {
        // An empty JSON object should deserialize to defaults (via serde(default))
        let decoded: Settings = serde_json::from_str("{}").unwrap();
        assert_eq!(decoded, Settings::default());
    }

    // M171 D1a — seed-time limit Settings fields
    #[test]
    fn seed_time_limits_default_none() {
        let s = Settings::default();
        assert!(s.seed_time_limit_secs.is_none());
        assert!(s.inactive_seed_time_limit_secs.is_none());
    }

    #[test]
    fn seed_time_limits_round_trip_json() {
        let s = Settings {
            seed_time_limit_secs: Some(3600),
            inactive_seed_time_limit_secs: Some(1800),
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(decoded.seed_time_limit_secs, Some(3600));
        assert_eq!(decoded.inactive_seed_time_limit_secs, Some(1800));
    }

    #[test]
    fn seed_time_limits_skipped_when_none() {
        // `skip_serializing_if` keeps the wire format small when no limit is set.
        let s = Settings::default();
        let json = serde_json::to_string(&s).unwrap();
        assert!(
            !json.contains("seed_time_limit_secs"),
            "None should not be serialised: {json}"
        );
        assert!(
            !json.contains("inactive_seed_time_limit_secs"),
            "None should not be serialised: {json}"
        );
    }

    #[test]
    fn seed_time_limits_flow_to_torrent_config() {
        let s = Settings {
            seed_time_limit_secs: Some(7200),
            inactive_seed_time_limit_secs: Some(900),
            ..Settings::default()
        };
        let tc = crate::types::TorrentConfig::from(&s);
        assert_eq!(tc.seed_time_limit_secs, Some(7200));
        assert_eq!(tc.inactive_seed_time_limit_secs, Some(900));
    }

    // M171 D1 — max_ratio_action + create_subfolder + auto_manage_torrents + queueing_enabled
    #[test]
    fn m171_settings_defaults_pause_true_false_false() {
        let s = Settings::default();
        assert_eq!(s.max_ratio_action, MaxRatioAction::Pause);
        assert!(
            s.create_subfolder,
            "create_subfolder defaults true (qBt factory default)"
        );
        assert!(!s.auto_manage_torrents);
        assert!(!s.queueing_enabled);
    }

    #[test]
    fn m171_settings_round_trip_preserves_all_four() {
        let s = Settings {
            max_ratio_action: MaxRatioAction::EnableSuperSeeding,
            create_subfolder: false,
            auto_manage_torrents: true,
            queueing_enabled: true,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(decoded, s);
    }

    #[test]
    fn max_ratio_action_wire_snake_case() {
        // Critical for qBt compat: the wire format must be snake_case.
        let pause = serde_json::to_string(&MaxRatioAction::Pause).unwrap();
        let remove = serde_json::to_string(&MaxRatioAction::Remove).unwrap();
        let super_seed = serde_json::to_string(&MaxRatioAction::EnableSuperSeeding).unwrap();
        assert_eq!(pause, "\"pause\"");
        assert_eq!(remove, "\"remove\"");
        assert_eq!(super_seed, "\"enable_super_seeding\"");
    }

    #[test]
    fn max_ratio_action_wire_snake_case_round_trip() {
        // Deserialisation from snake_case works too.
        let pause: MaxRatioAction = serde_json::from_str("\"pause\"").unwrap();
        let remove: MaxRatioAction = serde_json::from_str("\"remove\"").unwrap();
        let super_seed: MaxRatioAction = serde_json::from_str("\"enable_super_seeding\"").unwrap();
        assert_eq!(pause, MaxRatioAction::Pause);
        assert_eq!(remove, MaxRatioAction::Remove);
        assert_eq!(super_seed, MaxRatioAction::EnableSuperSeeding);
    }

    #[test]
    fn validation_force_proxy_no_proxy() {
        let s = Settings {
            force_proxy: true,
            ..Settings::default()
        };
        // proxy_type defaults to None
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("force_proxy"));
    }

    #[test]
    fn validation_valid_defaults() {
        Settings::default().validate().unwrap();
        Settings::min_memory().validate().unwrap();
        Settings::high_performance().validate().unwrap();
    }

    #[test]
    fn disk_config_from_settings() {
        let s = Settings::default();
        let dc = crate::disk::DiskConfig::from(&s);
        assert_eq!(dc.io_threads, default_disk_io_threads());
        assert_eq!(dc.storage_mode, StorageMode::Auto);
        assert_eq!(dc.cache_size, 16 * 1024 * 1024);
        assert!((dc.write_cache_ratio - 0.5).abs() < f32::EPSILON);
        assert_eq!(dc.channel_capacity, 512);
    }

    #[test]
    fn torrent_config_from_settings() {
        let s = Settings::default();
        let tc = crate::types::TorrentConfig::from(&s);
        assert_eq!(tc.listen_port, 0); // random per-torrent
        assert_eq!(tc.max_peers, s.max_peers_per_torrent);
        assert_eq!(tc.download_dir, s.download_dir);
        assert_eq!(tc.enable_dht, s.enable_dht);
        assert_eq!(tc.enable_pex, s.enable_pex);
        assert_eq!(tc.encryption_mode, s.encryption_mode);
        assert_eq!(tc.enable_utp, s.enable_utp);
        assert_eq!(tc.enable_web_seed, s.enable_web_seed);
        assert_eq!(tc.hashing_threads, s.hashing_threads);
        assert_eq!(
            tc.max_concurrent_stream_reads,
            s.max_concurrent_stream_reads
        );
        assert_eq!(tc.anonymous_mode, s.anonymous_mode);
        assert_eq!(tc.enable_i2p, s.enable_i2p);
        assert_eq!(tc.allow_i2p_mixed, s.allow_i2p_mixed);
        // Previously hardcoded — now wired from Settings
        assert_eq!(tc.strict_end_game, s.strict_end_game);
        assert_eq!(tc.upload_rate_limit, s.upload_rate_limit);
        assert_eq!(tc.download_rate_limit, s.download_rate_limit);
        assert_eq!(tc.max_web_seeds, s.max_web_seeds);
        assert_eq!(tc.initial_picker_threshold, s.initial_picker_threshold);
        assert_eq!(tc.whole_pieces_threshold, s.whole_pieces_threshold);
        assert_eq!(tc.snub_timeout_secs, s.snub_timeout_secs);
        assert_eq!(tc.readahead_pieces, s.readahead_pieces);
        assert_eq!(
            tc.streaming_timeout_escalation,
            s.streaming_timeout_escalation
        );
        // New fields
        assert_eq!(tc.storage_mode, s.storage_mode);
        assert_eq!(tc.block_request_timeout_secs, s.block_request_timeout_secs);
        assert_eq!(tc.enable_lsd, s.enable_lsd);
        assert_eq!(tc.force_proxy, s.force_proxy);
        // M132: steal-queue population interval
        assert_eq!(tc.steal_stale_piece_secs, 2);
        assert_eq!(tc.steal_stale_piece_secs, s.steal_stale_piece_secs);
    }

    #[test]
    fn torrent_config_from_nondefault_settings() {
        // Verify non-default values flow through (catches re-hardcoding regressions)
        let mut s = Settings {
            strict_end_game: false,
            upload_rate_limit: 1_000_000,
            download_rate_limit: 2_000_000,
            max_web_seeds: 8,
            initial_picker_threshold: 10,
            whole_pieces_threshold: 50,
            snub_timeout_secs: 120,
            readahead_pieces: 16,
            streaming_timeout_escalation: false,
            storage_mode: StorageMode::Full,
            block_request_timeout_secs: 30,
            enable_lsd: false,
            force_proxy: true,
            ..Settings::default()
        };
        s.proxy.proxy_type = crate::proxy::ProxyType::Socks5;

        let tc = crate::types::TorrentConfig::from(&s);
        assert!(!tc.strict_end_game);
        assert_eq!(tc.upload_rate_limit, 1_000_000);
        assert_eq!(tc.download_rate_limit, 2_000_000);
        assert_eq!(tc.max_web_seeds, 8);
        assert_eq!(tc.initial_picker_threshold, 10);
        assert_eq!(tc.whole_pieces_threshold, 50);
        assert_eq!(tc.snub_timeout_secs, 120);
        assert_eq!(tc.readahead_pieces, 16);
        assert!(!tc.streaming_timeout_escalation);
        assert_eq!(tc.storage_mode, StorageMode::Full);
        assert_eq!(tc.block_request_timeout_secs, 30);
        assert!(!tc.enable_lsd);
        assert!(tc.force_proxy);
    }

    #[test]
    fn external_ip_default_and_json() {
        let s = Settings::default();
        assert!(s.external_ip.is_none());

        // JSON with external_ip set
        let json = r#"{"external_ip": "203.0.113.5"}"#;
        let decoded: Settings = serde_json::from_str(json).unwrap();
        assert_eq!(
            decoded.external_ip,
            Some(std::net::IpAddr::V4(std::net::Ipv4Addr::new(
                203, 0, 113, 5
            )))
        );

        // Round-trip preserves external_ip
        let encoded = serde_json::to_string(&decoded).unwrap();
        let roundtrip: Settings = serde_json::from_str(&encoded).unwrap();
        assert_eq!(roundtrip.external_ip, decoded.external_ip);
    }

    #[test]
    fn validation_zero_threads() {
        let s = Settings {
            hashing_threads: 0,
            ..Settings::default()
        };
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("hashing_threads"));

        let s = Settings {
            disk_io_threads: 0,
            ..Settings::default()
        };
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("disk_io_threads"));

        let s = Settings {
            max_blocking_threads: 0,
            ..Settings::default()
        };
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("max_blocking_threads"));
    }

    #[test]
    fn share_mode_requires_fast_extension() {
        let mut s = Settings {
            default_share_mode: true,
            enable_fast_extension: false,
            ..Settings::default()
        };
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("share_mode"));

        // With fast extension enabled, share mode is valid
        s.enable_fast_extension = true;
        s.validate().unwrap();
    }

    #[test]
    fn share_mode_default_false() {
        let cfg = crate::types::TorrentConfig::default();
        assert!(!cfg.share_mode);
    }

    #[test]
    fn dht_storage_settings_defaults() {
        let s = Settings::default();
        assert_eq!(s.dht_max_items, 700);
        assert_eq!(s.dht_item_lifetime_secs, 7200);
    }

    #[test]
    fn dht_sample_interval_default_disabled() {
        let s = Settings::default();
        assert_eq!(s.dht_sample_infohashes_interval, 0);
    }

    #[test]
    fn dht_sample_interval_json_round_trip() {
        let json = r#"{"dht_sample_infohashes_interval": 300}"#;
        let decoded: Settings = serde_json::from_str(json).unwrap();
        assert_eq!(decoded.dht_sample_infohashes_interval, 300);

        let encoded = serde_json::to_string(&decoded).unwrap();
        let roundtrip: Settings = serde_json::from_str(&encoded).unwrap();
        assert_eq!(roundtrip.dht_sample_infohashes_interval, 300);
    }

    #[test]
    fn min_memory_restricts_dht_items() {
        let s = Settings::min_memory();
        assert_eq!(s.dht_max_items, 100);
    }

    #[test]
    fn dht_config_inherits_security_settings() {
        let s = Settings {
            dht_enforce_node_id: false,
            ..Settings::default()
        };
        let dht = s.to_dht_config();
        assert!(!dht.enforce_node_id);
        assert!(dht.restrict_routing_ips);

        let dht_v6 = s.to_dht_config_v6();
        assert!(!dht_v6.enforce_node_id);
        assert!(dht_v6.restrict_routing_ips);
    }

    #[test]
    fn enable_holepunch_default_true() {
        let s = Settings::default();
        assert!(s.enable_holepunch);
    }

    #[test]
    fn enable_holepunch_json_round_trip() {
        let json = r#"{"enable_holepunch": false}"#;
        let decoded: Settings = serde_json::from_str(json).unwrap();
        assert!(!decoded.enable_holepunch);

        let encoded = serde_json::to_string(&decoded).unwrap();
        let roundtrip: Settings = serde_json::from_str(&encoded).unwrap();
        assert!(!roundtrip.enable_holepunch);
    }

    #[test]
    fn i2p_settings_defaults() {
        let s = Settings::default();
        assert!(!s.enable_i2p);
        assert_eq!(s.i2p_hostname, "127.0.0.1");
        assert_eq!(s.i2p_port, 7656);
        assert_eq!(s.i2p_inbound_quantity, 3);
        assert_eq!(s.i2p_outbound_quantity, 3);
        assert_eq!(s.i2p_inbound_length, 3);
        assert_eq!(s.i2p_outbound_length, 3);
        assert!(!s.allow_i2p_mixed);
    }

    #[test]
    fn i2p_settings_json_roundtrip() {
        let s = Settings {
            enable_i2p: true,
            i2p_hostname: "10.0.0.1".into(),
            i2p_port: 7700,
            i2p_inbound_quantity: 5,
            i2p_outbound_quantity: 4,
            i2p_inbound_length: 2,
            i2p_outbound_length: 1,
            allow_i2p_mixed: true,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(s, decoded);
    }

    #[test]
    fn i2p_validation_quantity_zero() {
        let s = Settings {
            enable_i2p: true,
            i2p_inbound_quantity: 0,
            ..Settings::default()
        };
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("i2p_inbound_quantity"));
    }

    #[test]
    fn i2p_validation_quantity_too_high() {
        let s = Settings {
            enable_i2p: true,
            i2p_outbound_quantity: 17,
            ..Settings::default()
        };
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("i2p_outbound_quantity"));
    }

    #[test]
    fn i2p_validation_length_too_high() {
        let s = Settings {
            enable_i2p: true,
            i2p_inbound_length: 8,
            ..Settings::default()
        };
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("i2p_inbound_length"));
    }

    #[test]
    fn i2p_validation_passes_when_disabled() {
        // Invalid values should not trigger errors when I2P is disabled
        let mut s = Settings {
            enable_i2p: false,
            ..Settings::default()
        };
        s.i2p_inbound_quantity = 0; // would be invalid if enabled
        s.validate().unwrap(); // should pass
    }

    #[test]
    fn i2p_validation_valid_config() {
        let s = Settings {
            enable_i2p: true,
            i2p_inbound_quantity: 1,
            i2p_outbound_quantity: 16,
            i2p_inbound_length: 0,
            i2p_outbound_length: 7,
            ..Settings::default()
        };
        s.validate().unwrap();
    }

    #[test]
    fn ssl_settings_defaults() {
        let s = Settings::default();
        assert_eq!(s.ssl_listen_port, 0);
        assert!(s.ssl_cert_path.is_none());
        assert!(s.ssl_key_path.is_none());
    }

    #[test]
    fn ssl_settings_json_round_trip() {
        let s = Settings {
            ssl_listen_port: 4433,
            ssl_cert_path: Some(PathBuf::from("/etc/ssl/cert.pem")),
            ssl_key_path: Some(PathBuf::from("/etc/ssl/key.pem")),
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(s, decoded);
    }

    #[test]
    fn ssl_validation_cert_without_key() {
        let s = Settings {
            ssl_cert_path: Some(PathBuf::from("/tmp/cert.pem")),
            ..Settings::default()
        };
        // ssl_key_path is None
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("ssl_cert_path"));
    }

    #[test]
    fn ssl_validation_key_without_cert() {
        let s = Settings {
            ssl_key_path: Some(PathBuf::from("/tmp/key.pem")),
            ..Settings::default()
        };
        // ssl_cert_path is None
        let err = s.validate().unwrap_err();
        assert!(err.to_string().contains("ssl_cert_path"));
    }

    #[test]
    fn ssl_validation_both_set_passes() {
        let s = Settings {
            ssl_cert_path: Some(PathBuf::from("/tmp/cert.pem")),
            ssl_key_path: Some(PathBuf::from("/tmp/key.pem")),
            ..Settings::default()
        };
        s.validate().unwrap();
    }

    #[test]
    fn ssl_validation_both_absent_passes() {
        let s = Settings::default();
        // Both are None by default
        s.validate().unwrap();
    }

    #[test]
    fn default_choking_algorithms() {
        let s = Settings::default();
        assert_eq!(
            s.seed_choking_algorithm,
            SeedChokingAlgorithm::FastestUpload
        );
        assert_eq!(s.choking_algorithm, ChokingAlgorithm::FixedSlots);
    }

    #[test]
    fn choking_algorithm_json_round_trip() {
        let s = Settings {
            seed_choking_algorithm: SeedChokingAlgorithm::AntiLeech,
            choking_algorithm: ChokingAlgorithm::RateBased,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(
            decoded.seed_choking_algorithm,
            SeedChokingAlgorithm::AntiLeech
        );
        assert_eq!(decoded.choking_algorithm, ChokingAlgorithm::RateBased);
    }

    #[test]
    fn m44_settings_defaults() {
        let s = Settings::default();
        assert!(s.piece_extent_affinity);
        assert!(s.suggest_mode);
        assert_eq!(s.max_suggest_pieces, 16);
        assert_eq!(s.predictive_piece_announce_ms, 0);
    }

    #[test]
    fn m44_high_performance_enables_suggest() {
        let s = Settings::high_performance();
        assert!(s.suggest_mode);
    }

    #[test]
    fn m44_json_round_trip() {
        let s = Settings {
            piece_extent_affinity: false,
            suggest_mode: true,
            max_suggest_pieces: 5,
            predictive_piece_announce_ms: 50,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(s, decoded);
    }

    #[test]
    fn security_settings_defaults() {
        let s = Settings::default();
        assert!(s.ssrf_mitigation);
        assert!(!s.allow_idna);
        assert!(s.validate_https_trackers);
    }

    #[test]
    fn security_settings_json_round_trip() {
        let s = Settings {
            ssrf_mitigation: false,
            allow_idna: true,
            validate_https_trackers: false,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(s, decoded);
    }

    #[test]
    fn security_settings_missing_use_defaults() {
        // An empty JSON object should deserialize security fields to defaults.
        let decoded: Settings = serde_json::from_str("{}").unwrap();
        assert!(decoded.ssrf_mitigation);
        assert!(!decoded.allow_idna);
        assert!(decoded.validate_https_trackers);
    }

    #[test]
    fn url_security_config_from_settings() {
        let s = Settings {
            ssrf_mitigation: false,
            allow_idna: true,
            validate_https_trackers: false,
            ..Settings::default()
        };
        let cfg = crate::url_guard::UrlSecurityConfig::from(&s);
        assert!(!cfg.ssrf_mitigation);
        assert!(cfg.allow_idna);
        assert!(!cfg.validate_https_trackers);
    }

    #[test]
    fn default_peer_dscp_value() {
        let s = Settings::default();
        assert_eq!(s.peer_dscp, 0x08);
    }

    #[test]
    fn peer_dscp_json_round_trip() {
        let s = Settings {
            peer_dscp: 0x2E, // EF
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(decoded.peer_dscp, 0x2E);
    }

    #[test]
    fn peer_dscp_zero_disables() {
        let s = Settings {
            peer_dscp: 0,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(decoded.peer_dscp, 0);
    }

    #[test]
    fn utp_config_includes_dscp() {
        let s = Settings {
            peer_dscp: 0x0A,
            ..Settings::default()
        };
        let utp = s.to_utp_config(6881);
        assert_eq!(utp.dscp, 0x0A);

        let utp_v6 = s.to_utp_config_v6(6881);
        assert_eq!(utp_v6.dscp, 0x0A);
    }

    #[test]
    fn default_stats_report_interval() {
        let s = Settings::default();
        assert_eq!(s.stats_report_interval, 1000);
    }

    #[test]
    fn stats_report_interval_json_round_trip() {
        let s = Settings {
            stats_report_interval: 5000,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(decoded.stats_report_interval, 5000);
    }

    #[test]
    fn stats_report_interval_zero_disables() {
        let s = Settings {
            stats_report_interval: 0,
            ..Settings::default()
        };
        let json = serde_json::to_string(&s).unwrap();
        let decoded: Settings = serde_json::from_str(&json).unwrap();
        assert_eq!(decoded.stats_report_interval, 0);
    }

    #[test]
    fn settings_runtime_worker_threads_and_pin_cores() {
        // Defaults
        let s = Settings::default();
        assert_eq!(s.runtime_worker_threads, default_runtime_worker_threads());
        assert!(s.pin_cores);

        // 0 is valid (means auto-detect)
        let mut s = Settings {
            runtime_worker_threads: 0,
            ..Settings::default()
        };
        assert!(s.validate().is_ok());

        // 256 is valid (boundary)
        s.runtime_worker_threads = 256;
        assert!(s.validate().is_ok());

        // 257 is invalid
        s.runtime_worker_threads = 257;
        assert!(s.validate().is_err());
    }

    #[test]
    fn max_in_flight_512_default() {
        let s = Settings::default();
        assert_eq!(s.max_in_flight_pieces, 512);
        assert_eq!(s.fixed_pipeline_depth, 128);

        // Presets
        let mm = Settings::min_memory();
        assert_eq!(mm.max_in_flight_pieces, 32);
        assert_eq!(mm.fixed_pipeline_depth, 32);

        let hp = Settings::high_performance();
        assert_eq!(hp.max_in_flight_pieces, 512);
        assert_eq!(hp.fixed_pipeline_depth, 128); // inherits default
    }

    #[test]
    fn recalc_max_in_flight_formula() {
        // M104: The formula in torrent.rs: max(512, connected * 4), clamped to
        // num_pieces / 2, floored at 512. Validate the logic here.
        let base = 512_usize;

        // Few peers: floor dominates
        let connected = 10;
        let num_pieces = 2000_u32;
        let calculated = base.max(connected * 4);
        let result = calculated.min(num_pieces as usize / 2).max(base);
        assert_eq!(result, 512); // max(512, 40) = 512, min(512, 1000) = 512

        // Many peers: peer count drives it up
        let connected = 200;
        let calculated = base.max(connected * 4);
        let result = calculated.min(num_pieces as usize / 2).max(base);
        assert_eq!(result, 800); // max(512, 800) = 800, min(800, 1000) = 800

        // Small torrent: piece clamp wins
        let connected = 200;
        let num_pieces = 100_u32;
        let calculated = base.max(connected * 4);
        let result = calculated.min(num_pieces as usize / 2).max(base);
        assert_eq!(result, 512); // max(512, 800) = 800, min(800, 50) = 50, max(50, 512) = 512

        // Exact boundary: connected * 4 == base
        let connected = 129; // 129 * 4 = 516, just above 512
        let num_pieces = 10000_u32;
        let calculated = base.max(connected * 4);
        let result = calculated.min(num_pieces as usize / 2).max(base);
        assert_eq!(result, 516); // max(512, 516) = 516, min(516, 5000) = 516
    }

    // ── M168: qBt v2 compatibility settings tests ────────────────────

    #[test]
    fn settings_default_enables_qbt_compat_v0_172_1() {
        // v0.172.1: default flipped from false (M168 security-through-
        // invisibility) to true so *arr clients work out of the box. The
        // real defences (argon2id hash, brute-force ban, CSRF middleware)
        // all ship in M172a. Operators opt out via [qbt_compat] enabled = false.
        let s = Settings::default();
        assert!(s.qbt_compat.enabled);
        assert_eq!(s.qbt_compat.username, "admin");
        // M172a: plaintext `password` ships empty by default; `password_hash`
        // ships a pre-hashed "adminadmin" so fresh installs never run the
        // legacy-migration path.
        assert_eq!(s.qbt_compat.password, "");
        assert!(
            s.qbt_compat
                .password_hash
                .starts_with("$argon2id$v=19$m=19456,t=2,p=1$")
        );
        assert_eq!(s.qbt_compat.spoof_app_version, "v5.1.4");
        assert_eq!(s.qbt_compat.spoof_webapi_version, "2.11.4");
        assert_eq!(s.qbt_compat.session_ttl_secs, 86_400);
        assert_eq!(s.qbt_compat.max_sessions, 1024);
        assert!(s.qbt_compat.max_concurrent_argon2_ops.is_none());
    }

    #[test]
    fn validate_rejects_empty_username() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.username = String::new();
        let err = s.validate().expect_err("empty username must fail");
        let msg = format!("{err}");
        assert!(msg.contains("username"), "error was: {msg}");
    }

    #[test]
    fn validate_rejects_short_legacy_password_lt_8_when_hash_empty() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        // Simulate a pre-M172a config: hash is empty, plaintext is too short.
        s.qbt_compat.password_hash.clear();
        s.qbt_compat.password = "short".into();
        let err = s.validate().expect_err("short password must fail");
        let msg = format!("{err}");
        assert!(
            msg.contains("password") && msg.contains("hash"),
            "error was: {msg}"
        );
    }

    #[test]
    fn validate_rejects_bad_app_version_format() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.spoof_app_version = "garbage".into();
        let err = s.validate().expect_err("bad app version must fail");
        let msg = format!("{err}");
        assert!(msg.contains("spoof_app_version"), "error was: {msg}");
    }

    #[test]
    fn validate_rejects_bad_webapi_version_format() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.spoof_webapi_version = "v2.11".into(); // leading v is wrong for webapi
        let err = s.validate().expect_err("bad webapi version must fail");
        let msg = format!("{err}");
        assert!(msg.contains("spoof_webapi_version"), "error was: {msg}");
    }

    #[test]
    fn validate_rejects_ttl_out_of_bounds() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.session_ttl_secs = 10; // below 60
        let err = s.validate().expect_err("ttl too small must fail");
        assert!(format!("{err}").contains("session_ttl_secs"));

        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.session_ttl_secs = 604_801; // above 604800
        let err = s.validate().expect_err("ttl too large must fail");
        assert!(format!("{err}").contains("session_ttl_secs"));
    }

    // ── M172a Lane A: argon2 PHC + migration ──────────────────────────

    #[test]
    fn default_hash_roundtrips_admin_admin() {
        use argon2::Argon2;
        use argon2::password_hash::{PasswordHash, PasswordVerifier};

        // If this test fails because someone changed the default salt or
        // Argon2 parameters, regenerate `DEFAULT_ADMINADMIN_HASH` with:
        //
        //   cargo run --example regen_qbt_default_hash
        //
        // and paste the output back into the constant. The asymmetry matters:
        // production verification uses the same crate + params, so this test
        // is the canary for a bad paste. We do not regenerate the hash here
        // (non-deterministic salt would break cross-install round-tripping).
        let hash = PasswordHash::new(DEFAULT_ADMINADMIN_HASH)
            .expect("DEFAULT_ADMINADMIN_HASH must be a valid PHC string");
        Argon2::default()
            .verify_password(b"adminadmin", &hash)
            .expect("default hash must verify the 'adminadmin' plaintext");
    }

    #[test]
    fn validate_rejects_password_hash_not_starting_with_argon2id() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        // bcrypt-style hash — wrong scheme.
        s.qbt_compat.password_hash =
            "$2b$12$KIXQ5.pHJN3iLz9H6CfQEe2/6rFv1h4jdXWv.0eoGzJ6w7L4Yj7vi".into();
        let err = s.validate().expect_err("non-argon2id hash must fail");
        let msg = format!("{err}");
        assert!(msg.contains("argon2id"), "error was: {msg}");
    }

    #[test]
    fn validate_rejects_zero_max_concurrent_argon2_ops() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.max_concurrent_argon2_ops = Some(0);
        let err = s.validate().expect_err("zero argon2 semaphore must fail");
        assert!(format!("{err}").contains("max_concurrent_argon2_ops"));
    }

    #[test]
    fn default_settings_ship_pre_hashed_no_migration_needed() {
        let s = Settings::default();
        assert!(s.qbt_compat.password_hash.starts_with("$argon2id$"));
        assert!(s.qbt_compat.password.is_empty());
    }

    #[test]
    fn hash_qbt_password_roundtrips() {
        let h = hash_qbt_password("correct horse battery staple")
            .expect("hash must succeed for a simple plaintext");
        assert!(h.starts_with("$argon2id$v=19$m=19456,t=2,p=1$"));
        // Every call produces a fresh salt → different PHC output.
        let h2 =
            hash_qbt_password("correct horse battery staple").expect("second hash must succeed");
        assert_ne!(h, h2, "argon2 must use a fresh salt per call");
    }

    #[test]
    fn migrate_qbt_credentials_noop_when_hash_present() {
        let mut qbt = QbtCompatSettings {
            password_hash: DEFAULT_ADMINADMIN_HASH.into(),
            password: String::new(),
            ..Default::default()
        };
        let outcome = migrate_qbt_credentials(&mut qbt).expect("noop");
        assert_eq!(outcome, QbtCredentialMigration::NoOp);
        assert_eq!(qbt.password_hash, DEFAULT_ADMINADMIN_HASH);
        assert!(qbt.password.is_empty());
    }

    #[test]
    fn migrate_qbt_credentials_upgrades_legacy_plaintext() {
        use argon2::Argon2;
        use argon2::password_hash::{PasswordHash, PasswordVerifier};

        let mut qbt = QbtCompatSettings {
            password_hash: String::new(),
            password: "legacy-plaintext-pw".into(),
            ..Default::default()
        };
        let outcome = migrate_qbt_credentials(&mut qbt).expect("upgrade");
        assert_eq!(outcome, QbtCredentialMigration::Upgraded);
        assert!(qbt.password_hash.starts_with("$argon2id$"));
        assert!(
            qbt.password.is_empty(),
            "plaintext must be zeroed after migration"
        );

        let parsed =
            PasswordHash::new(&qbt.password_hash).expect("migration wrote a valid PHC string");
        Argon2::default()
            .verify_password(b"legacy-plaintext-pw", &parsed)
            .expect("migrated hash must verify the original plaintext");
    }

    #[test]
    fn migrate_qbt_credentials_noop_when_both_empty() {
        let mut qbt = QbtCompatSettings {
            password_hash: String::new(),
            password: String::new(),
            ..Default::default()
        };
        let outcome = migrate_qbt_credentials(&mut qbt).expect("noop on empty");
        assert_eq!(outcome, QbtCredentialMigration::NoOp);
    }

    // ── M172a Lane C: brute-force ban settings ────────────────────────

    #[test]
    fn brute_force_defaults_are_5_attempts_and_one_hour_ban() {
        let s = Settings::default();
        assert_eq!(s.qbt_compat.max_failed_auth_count, 5);
        assert_eq!(s.qbt_compat.ban_duration_secs, 3_600);
        assert!(!s.qbt_compat.bypass_local_auth);
        assert!(s.qbt_compat.bypass_auth_subnet_whitelist.is_empty());
        assert!(s.qbt_compat.brute_force_registry_capacity.is_none());
    }

    #[test]
    fn validate_rejects_zero_max_failed_auth_count_without_bypass() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.max_failed_auth_count = 0;
        s.qbt_compat.bypass_local_auth = false;
        let err = s
            .validate()
            .expect_err("zero attempts without bypass must fail");
        assert!(format!("{err}").contains("max_failed_auth_count"));
    }

    #[test]
    fn validate_accepts_zero_max_failed_auth_count_when_bypass_local() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.max_failed_auth_count = 0;
        s.qbt_compat.bypass_local_auth = true;
        s.validate().expect("bypass_local_auth disarms the check");
    }

    #[test]
    fn validate_rejects_ban_duration_out_of_bounds() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.ban_duration_secs = 59;
        let err = s.validate().expect_err("too short ban must fail");
        assert!(format!("{err}").contains("ban_duration_secs"));

        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.ban_duration_secs = 86_401;
        let err = s.validate().expect_err("too long ban must fail");
        assert!(format!("{err}").contains("ban_duration_secs"));
    }

    #[test]
    fn validate_rejects_malformed_bypass_whitelist_cidr() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.bypass_auth_subnet_whitelist = vec!["not-a-cidr".into()];
        let err = s.validate().expect_err("bad cidr must fail");
        let msg = format!("{err}");
        assert!(msg.contains("bypass_auth_subnet_whitelist"));
        assert!(msg.contains("not-a-cidr"));
    }

    #[test]
    fn validate_accepts_valid_bypass_whitelist_cidrs() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.bypass_auth_subnet_whitelist = vec![
            "10.0.0.0/8".into(),
            "192.168.1.0/24".into(),
            "::1/128".into(),
        ];
        s.validate().expect("valid cidrs pass");
    }

    #[test]
    fn validate_rejects_registry_capacity_below_floor() {
        let mut s = Settings::default();
        s.qbt_compat.enabled = true;
        s.qbt_compat.brute_force_registry_capacity = Some(99);
        let err = s
            .validate()
            .expect_err("capacity < 100 must fail sanity floor");
        assert!(format!("{err}").contains("brute_force_registry_capacity"));
    }

    // ── M224: max_uploads_per_torrent validate + serde-default ──────────

    #[test]
    fn validate_rejects_zero_max_uploads_per_torrent() {
        let s = Settings {
            max_uploads_per_torrent: 0,
            ..Settings::default()
        };
        let err = s
            .validate()
            .expect_err("max_uploads_per_torrent = 0 must fail");
        let msg = format!("{err}");
        assert!(
            msg.contains("max_uploads_per_torrent"),
            "error was: {msg}"
        );
    }

    #[test]
    fn validate_rejects_negative_below_minus_one_max_uploads_per_torrent() {
        let s = Settings {
            max_uploads_per_torrent: -2,
            ..Settings::default()
        };
        let err = s
            .validate()
            .expect_err("max_uploads_per_torrent < -1 must fail");
        let msg = format!("{err}");
        assert!(
            msg.contains("max_uploads_per_torrent"),
            "error was: {msg}"
        );
    }

    #[test]
    fn validate_accepts_minus_one_max_uploads_per_torrent() {
        let s = Settings::default();
        assert_eq!(s.max_uploads_per_torrent, -1);
        s.validate().expect("default -1 must validate");
    }

    #[test]
    fn validate_accepts_positive_max_uploads_per_torrent() {
        let s = Settings {
            max_uploads_per_torrent: 4,
            ..Settings::default()
        };
        s.validate().expect("n >= 1 must validate");
    }

    #[test]
    fn max_uploads_per_torrent_default_deserialize_without_field() {
        // R6 guard: existing Settings JSON files without the new field must
        // deserialize cleanly to -1 (the default-fn sentinel), not 0 (which
        // would then fail validate() and break every existing config).
        let s = Settings::default();
        let mut value = serde_json::to_value(&s).expect("serialise");
        let obj = value.as_object_mut().expect("Settings is a JSON object");
        assert!(
            obj.remove("max_uploads_per_torrent").is_some(),
            "field should have been present in serialised default"
        );
        let decoded: Settings = serde_json::from_value(value).expect("deserialise without field");
        assert_eq!(decoded.max_uploads_per_torrent, -1);
        decoded.validate().expect("default-via-serde must validate");
    }

    #[test]
    fn brute_force_settings_json_round_trip() {
        let mut s = Settings::default();
        s.qbt_compat.max_failed_auth_count = 7;
        s.qbt_compat.ban_duration_secs = 1_800;
        s.qbt_compat.bypass_local_auth = true;
        s.qbt_compat.bypass_auth_subnet_whitelist = vec!["10.0.0.0/8".into()];
        s.qbt_compat.brute_force_registry_capacity = Some(5_000);

        let json = serde_json::to_string(&s).expect("serialise");
        let decoded: Settings = serde_json::from_str(&json).expect("deserialise");
        assert_eq!(decoded.qbt_compat.max_failed_auth_count, 7);
        assert_eq!(decoded.qbt_compat.ban_duration_secs, 1_800);
        assert!(decoded.qbt_compat.bypass_local_auth);
        assert_eq!(
            decoded.qbt_compat.bypass_auth_subnet_whitelist,
            vec!["10.0.0.0/8".to_string()]
        );
        assert_eq!(
            decoded.qbt_compat.brute_force_registry_capacity,
            Some(5_000)
        );
    }

    // ── M226: Notifications / paths / watched folder / network — defaults ────

    #[test]
    fn settings_default_notify_on_complete_is_false() {
        assert!(!Settings::default().notify_on_complete);
    }

    #[test]
    fn settings_default_notify_on_error_is_false() {
        assert!(!Settings::default().notify_on_error);
    }

    #[test]
    fn settings_default_on_complete_program_is_none() {
        assert!(Settings::default().on_complete_program.is_none());
    }

    #[test]
    fn settings_default_use_incomplete_dir_is_false() {
        assert!(!Settings::default().use_incomplete_dir);
    }

    #[test]
    fn settings_default_incomplete_dir_is_none() {
        assert!(Settings::default().incomplete_dir.is_none());
    }

    #[test]
    fn settings_default_default_skip_hash_check_is_false() {
        assert!(!Settings::default().default_skip_hash_check);
    }

    #[test]
    fn settings_default_incomplete_extension_enabled_is_true() {
        assert!(Settings::default().incomplete_extension_enabled);
    }

    #[test]
    fn settings_default_watched_folder_is_none() {
        assert!(Settings::default().watched_folder.is_none());
    }

    #[test]
    fn settings_default_delete_torrent_after_add_is_false() {
        assert!(!Settings::default().delete_torrent_after_add);
    }

    #[test]
    fn settings_default_move_completed_enabled_is_false() {
        assert!(!Settings::default().move_completed_enabled);
    }

    #[test]
    fn settings_default_move_completed_to_is_none() {
        assert!(Settings::default().move_completed_to.is_none());
    }

    #[test]
    fn settings_default_web_ui_https_enabled_is_false() {
        assert!(!Settings::default().web_ui_https_enabled);
    }

    #[test]
    fn settings_default_network_interface_is_none() {
        assert!(Settings::default().network_interface.is_none());
    }

    #[test]
    fn settings_default_default_add_paused_is_false() {
        assert!(!Settings::default().default_add_paused);
    }

    // ── M226: validation rules ────

    #[test]
    fn validate_rejects_use_incomplete_dir_without_incomplete_dir() {
        let s = Settings {
            use_incomplete_dir: true,
            incomplete_dir: None,
            ..Settings::default()
        };
        let err = s.validate().expect_err("must require incomplete_dir");
        assert!(format!("{err}").contains("incomplete_dir"));
    }

    #[test]
    fn validate_accepts_use_incomplete_dir_with_incomplete_dir() {
        let s = Settings {
            use_incomplete_dir: true,
            incomplete_dir: Some(PathBuf::from("/tmp/irontide-incomplete")),
            ..Settings::default()
        };
        s.validate().expect("paired fields valid");
    }

    #[test]
    fn validate_rejects_move_completed_without_move_completed_to() {
        let s = Settings {
            move_completed_enabled: true,
            move_completed_to: None,
            ..Settings::default()
        };
        let err = s.validate().expect_err("must require move_completed_to");
        assert!(format!("{err}").contains("move_completed_to"));
    }

    #[test]
    fn validate_rejects_relative_watched_folder() {
        let s = Settings {
            watched_folder: Some(PathBuf::from("relative/path")),
            ..Settings::default()
        };
        let err = s.validate().expect_err("relative path must fail");
        assert!(format!("{err}").contains("absolute"));
    }

    #[test]
    fn validate_rejects_relative_incomplete_dir() {
        let s = Settings {
            incomplete_dir: Some(PathBuf::from("inc")),
            ..Settings::default()
        };
        let err = s.validate().expect_err("relative path must fail");
        assert!(format!("{err}").contains("absolute"));
    }

    #[test]
    fn validate_rejects_relative_move_completed_to() {
        let s = Settings {
            move_completed_to: Some(PathBuf::from("done")),
            ..Settings::default()
        };
        let err = s.validate().expect_err("relative path must fail");
        assert!(format!("{err}").contains("absolute"));
    }

    #[test]
    fn validate_rejects_system_path_as_watched_folder() {
        for sys in ["/", "/etc", "/usr", "/bin", "/sys", "/proc"] {
            let s = Settings {
                watched_folder: Some(PathBuf::from(sys)),
                ..Settings::default()
            };
            let err = s
                .validate()
                .expect_err("system path must be rejected");
            assert!(
                format!("{err}").contains("system path"),
                "{sys}: error must mention 'system path', got: {err}"
            );
        }
    }

    #[test]
    fn validate_accepts_safe_watched_folder() {
        let s = Settings {
            watched_folder: Some(PathBuf::from("/tmp/irontide-watched")),
            ..Settings::default()
        };
        s.validate().expect("safe path must validate");
    }
}