limnifs_write/config/

mod.rs

//! User-facing write configuration.
//!
//! [`WriteConfig`] captures all user-tunable writer settings in one
//! place. The configuration is model-driven: each sub-config is a
//! distinct type that owns its own validation, serialization, and
//! defaults. This eliminates the v0.1 pattern of hardcoded
//! constants scattered across `limnifs-write/src/lib.rs`.
//!
//! ## Architecture
//!
//! ```text
//! WriteConfig (top-level)
//!   ├── Defaults           (codecs, qualities, inline threshold)
//!   ├── CategorizerConfig[] (extension/magic → codec routing)
//!   ├── ChunkingConfig      (FastCDC parameters)
//!   ├── TournamentConfig   (which codecs + min sizes)
//!   ├── EncryptionConfig    (AEAD + key wrap)
//!   └── DictionaryConfig    (ZSTD dictionary training)
//! ```
//!
//! ## OCP
//!
//! Adding a new sub-config = adding a new struct + wiring into
//! [`WriteConfig::default_v0_1`] + adding a TOML section. No
//! existing code changes.

pub mod defaults;
pub mod error;
pub mod profile;
pub mod toml;

use std::collections::BTreeMap;

use serde::{Deserialize, Serialize};

use crate::config::error::ConfigError;

/// Default codec for text/code/sparse content.
/// Matches v0.1 behavior: Brotli.
pub const DEFAULT_TEXT_CODEC: &str = "brotli";
/// Default codec for binary content.
/// Matches v0.1 behavior: LZ4.
pub const DEFAULT_BINARY_CODEC: &str = "lz4";
/// Default codec for the metadata blob.
/// Matches v0.1 behavior: Brotli.
pub const DEFAULT_METADATA_CODEC: &str = "brotli";
/// Default Brotli quality for small metadata blobs.
pub const DEFAULT_METADATA_QUALITY: u8 = 5;
/// Default inline-data threshold (bytes).
pub const DEFAULT_INLINE_THRESHOLD: u16 = 4096;

/// Default cap on a single whole-file drop's plaintext (4 MiB).
/// Bounds the decompressed unit behind every random access by
/// construction (EROFS fixed-output pclusters): a file above the cap
/// falls back to FastCDC chunking + tournament. 0 disables the cap.
pub const DEFAULT_MAX_DROP_SIZE: u32 = 4 * 1024 * 1024;
/// Default `FastCDC` average chunk size.
pub const DEFAULT_AVG_CHUNK_SIZE: u32 = 262_144;
/// Default `FastCDC` minimum chunk size.
pub const DEFAULT_MIN_CHUNK_SIZE: u32 = 65_536;
/// Default `FastCDC` maximum chunk size.
pub const DEFAULT_MAX_CHUNK_SIZE: u32 = 1_048_576;
/// Default minimum size for the tournament to try a codec.
pub const DEFAULT_TOURNAMENT_MIN_SIZE: u32 = 256;
/// Default: skip tournament for binary class.
pub const DEFAULT_TOURNAMENT_SKIP_BINARY: bool = true;
/// Default AEAD algorithm.
pub const DEFAULT_AEAD: &str = "chacha20-poly1305";
/// Default key wrap algorithm.
pub const DEFAULT_KEY_WRAP: &str = "x25519-hkdf";
/// Default: enable dictionary training.
pub const DEFAULT_DICT_ENABLED: bool = true;
/// Default minimum drops per class to train a dict.
pub const DEFAULT_DICT_MIN_CLASS_SIZE: u32 = 100;
/// Default maximum dictionary size in bytes.
pub const DEFAULT_DICT_MAX_SIZE: u32 = 65_536;

/// Top-level write configuration. All fields are public so the
/// TOML loader can construct values directly; runtime validation
/// lives in [`WriteConfig::validate`].
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct WriteConfig {
    /// Default codec selection + inline threshold.
    pub defaults: Defaults,
    /// File categorizer rules (extension/magic → codec).
    #[serde(default, rename = "categorizer")]
    pub categorizers: Vec<CategorizerConfig>,
    /// `FastCDC` chunking parameters.
    pub chunking: ChunkingConfig,
    /// Compression tournament settings.
    pub tournament: TournamentConfig,
    /// Per-codec tunable parameters (memory budgets, quality levels).
    #[serde(default)]
    pub codec_tunables: CodecTunables,
    /// Image mode: read-only archive or read-write filesystem.
    #[serde(default)]
    pub mode: ImageMode,
    /// Codec for incremental writes (RW mode only). Defaults to LZ4.
    /// During turnover, `defaults.text_codec` is used for re-compression.
    #[serde(default = "default_write_codec")]
    pub write_codec: String,
    /// Turnover threshold: number of history entries before automatic
    /// compaction triggers (RW mode only). 0 = manual turnover only.
    #[serde(default)]
    pub turnover_threshold: u32,
    /// Skip FastCDC chunking; compress each file as a single drop.
    /// Trades dedup granularity for create speed. Recommended for
    /// `max-write` profile where speed >> ratio.
    #[serde(default)]
    pub skip_chunking: bool,
    /// Encryption configuration.
    pub encryption: EncryptionConfig,
    /// ZSTD dictionary configuration.
    pub dictionaries: DictionaryConfig,
}

fn default_write_codec() -> String {
    "lz4".into()
}

fn default_metadata_externalize_threshold() -> usize {
    crate::METADATA_EXTERNALIZE_THRESHOLD
}

/// Default codec + quality settings.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct Defaults {
    pub text_codec: String,
    pub binary_codec: String,
    pub metadata_codec: String,
    pub metadata_quality: u8,
    /// Compressed-metadata size (bytes) above which the blob is
    /// externalized to a `metadata.bin` sidecar instead of inlined
    /// in the manifest. Default: just under the reader's 1 MiB
    /// inline ceiling (`DEFAULT_INLINE_METADATA_MAX_BYTES`); raise to
    /// that ceiling for maximally self-contained images, lower it to
    /// keep manifests small. See limnifs#187.
    #[serde(default = "default_metadata_externalize_threshold")]
    pub metadata_externalize_threshold: usize,
    /// Dedup identical inline file contents into the shared-inline
    /// table (issue #189). Default ; set  to emit plain
    /// inline inodes readable by pre-#186 readers (cached tebako
    /// runtimes whose reserved mask rejects the SHARED_INLINE flag).
    #[serde(default = "default_true")]
    pub shared_inline: bool,
    /// Emit large general-codec drops (> 1 MiB plaintext) as seekable
    /// containers (256 KiB independent frames; bounded random reads).
    /// Default true. Set false for maximum compression ratio — frames
    /// give up cross-frame context (~1-3%) and windowed reads pay a
    /// full-drop decode again.
    #[serde(default = "default_true")]
    pub seekable_drops: bool,
    /// Maximum plaintext size (bytes) of a drop the writer may emit
    /// from a whole-file path (categorizer claims and the whole-file
    /// fallback). Files larger than the cap fall back to `FastCDC`
    /// chunking + tournament so every drop's decode cost is bounded
    /// by construction (EROFS fixed-output pclusters). Default
    /// 4 MiB; `0` = unlimited (pre-knob behavior). The
    /// `skip_chunking` (max-write) profile is exempt — whole-file IS
    /// its speed contract.
    #[serde(default = "default_max_drop_size")]
    pub max_drop_size: u32,
    /// Inline data threshold (bytes).
    pub inline_threshold: u16,
}

/// One file categorizer rule.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct CategorizerConfig {
    /// Human-readable name (e.g. "dna", "json").
    pub name: String,
    /// File extensions that trigger this rule (lowercase, no dot).
    #[serde(default)]
    pub extensions: Vec<String>,
    /// Magic bytes at offset 0 that trigger this rule.
    #[serde(default)]
    pub magic_bytes: Vec<u8>,
    /// Codec identifier (string name or numeric id).
    pub codec: String,
    /// Maximum file size to apply this rule to.
    #[serde(default)]
    pub max_size: Option<u32>,
    /// Whether this rule is active.
    #[serde(default = "default_true")]
    pub enabled: bool,
}

fn default_true() -> bool {
    true
}

fn default_max_drop_size() -> u32 {
    DEFAULT_MAX_DROP_SIZE
}

/// `FastCDC` parameters.
#[derive(Clone, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
pub struct ChunkingConfig {
    /// Algorithm name. Defaults to `"fastcdc"`. Reserved for future
    /// chunkers (`"gear-simd"`, `"leap-cdc"`, etc.) — today only
    /// `FastCDC` is wired. The writer ignores unknown values today;
    /// a `chunker_from_config` factory lands with the second chunker.
    #[serde(default = "default_chunker_name")]
    pub name: String,
    #[serde(default)]
    pub avg_chunk_size: u32,
    #[serde(default)]
    pub min_chunk_size: u32,
    #[serde(default)]
    pub max_chunk_size: u32,
}

fn default_chunker_name() -> String {
    "fastcdc".into()
}

/// Compression tournament settings.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct TournamentConfig {
    /// Codecs to try in the tournament (ordered fast → slow).
    /// The writer iterates these in order and stops early when
    /// `short_circuit_threshold` is met.
    pub codecs: Vec<String>,
    /// Minimum chunk size for the tournament to try a codec.
    pub min_size_threshold: u32,
    /// Skip tournament for binary class (use `binary_codec` directly).
    pub skip_for_binary: bool,
    /// Short-circuit the tournament once a codec achieves this ratio
    /// or better. Stored as per-mille (parts per 1000) so it serialises
    /// as an integer — 250 means "accept the moment a codec reaches
    /// 25% of original size". 0 disables short-circuit (try every codec).
    ///
    /// For example, on a highly-compressible CSV chunk, LZ4 typically
    /// reaches ~10% ratio in microseconds; the short-circuit lets us
    /// accept that and skip the much-slower Brotli pass we would
    /// otherwise run for ratio parity. On hard-to-compress text where
    /// LZ4 only reaches ~40%, the tournament continues to Brotli to
    /// preserve ratio.
    #[serde(default = "default_short_circuit_threshold")]
    pub short_circuit_threshold: u32,
}

fn default_short_circuit_threshold() -> u32 {
    250
}

/// Encryption configuration.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct EncryptionConfig {
    /// AEAD algorithm name.
    pub aead: String,
    /// Key wrap algorithm name.
    pub key_wrap: String,
}

/// ZSTD dictionary training configuration.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct DictionaryConfig {
    pub enabled: bool,
    pub min_class_size: u32,
    pub max_dict_size: u32,
    /// Trainer algorithm: `"frequency"` (default — top-K substrings
    /// by frequency × length) or `"fastcover"` (dmer-frequency
    /// scoring per FastCover, Facebook 2018). FastCover tends to
    /// win on corpora with distributed redundancy (mixed JSON,
    /// source files, log lines); FrequencyTrainer wins on corpora
    /// with strong common substrings.
    #[serde(default = "default_trainer")]
    pub trainer: String,
}

fn default_trainer() -> String {
    "frequency".into()
}

/// Image mode: read-only (one-shot archive) or read-write (live filesystem).
///
/// LimniFS's key differentiator vs SquashFS/DwarFS is RW support —
/// images can be updated incrementally without full rebuilds.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, Default)]
pub enum ImageMode {
    /// Read-only archive. Created once, read many times. All data
    /// is available at creation time — aggressive compression and
    /// full dedup are worthwhile.
    #[default]
    ReadOnly,
    /// Read-write image supporting incremental updates.
    ReadWrite(RWMode),
}

/// Read-write sub-mode controlling how updates are applied.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, Default)]
pub enum RWMode {
    /// Append-only: files can be added but never modified or deleted.
    /// No history tracking needed. Best for archival, data lakes.
    AppendOnly,
    /// Update-in-place: files can be modified and deleted. Old versions
    /// are kept as history entries. Best for dev directories, config mgmt.
    #[default]
    UpdateInPlace,
    /// Copy-on-write: modifications create new drops; old drops are
    /// unreferenced and reclaimed during turnover. Best for container
    /// layers, VM disk images.
    CopyOnWrite,
}

/// Per-codec tunable parameters. Each sub-struct has serde defaults
/// so the TOML can omit any codec the user doesn't want to customise.
///
/// ```toml
/// [codec_tunables.ppmd7]
/// order = 4
/// memory_budget_mb = 80
///
/// [codec_tunables.brotli]
/// quality = 11
/// window = 22
/// ```
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, Default)]
pub struct CodecTunables {
    #[serde(default)]
    pub ppmd7: Ppmd7Tunables,
    #[serde(default)]
    pub ppmd8: Ppmd8Tunables,
    #[serde(default)]
    pub brotli: BrotliTunables,
    #[serde(default)]
    pub zstd: ZstdTunables,
    #[serde(default)]
    pub lzma: LzmaTunables,
    #[serde(default)]
    pub bzip2: Bzip2Tunables,
}

/// PPMd7 tunables: context order + memory budget.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct Ppmd7Tunables {
    pub order: u8,
    pub memory_budget_mb: u32,
}

impl Default for Ppmd7Tunables {
    fn default() -> Self {
        Self {
            order: 4,
            memory_budget_mb: 80,
        }
    }
}

/// PPMd8 tunables: context order + memory budget.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct Ppmd8Tunables {
    pub order: u8,
    pub memory_budget_mb: u32,
}

impl Default for Ppmd8Tunables {
    fn default() -> Self {
        Self {
            order: 6,
            memory_budget_mb: 64,
        }
    }
}

/// Brotli tunables: quality (0..=11) + window log2 (10..=24).
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct BrotliTunables {
    pub quality: u8,
    pub window: u8,
}

impl Default for BrotliTunables {
    fn default() -> Self {
        Self {
            quality: 11,
            window: 22,
        }
    }
}

/// ZSTD tunables: quality (level proxy). Default 2 (Fastest) —
/// since omnizip 0.21.12+ every level >= L3 runs the optimal parser
/// (~16x slower at our chunk sizes), so the default deliberately
/// sits in the only remaining fast tier. Independent of the brotli
/// quality knob; the two scales diverged and were decoupled.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct ZstdTunables {
    pub quality: u8,
}

impl Default for ZstdTunables {
    fn default() -> Self {
        Self { quality: 2 }
    }
}

/// LZMA tunables: lc/lp/pb + dictionary size in MiB + optimal parser.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct LzmaTunables {
    pub lc: u8,
    pub lp: u8,
    pub pb: u8,
    pub dict_size_mb: u32,
    pub use_optimal_parser: bool,
}

impl Default for LzmaTunables {
    fn default() -> Self {
        Self {
            lc: 3,
            lp: 0,
            pb: 2,
            dict_size_mb: 16,
            use_optimal_parser: false,
        }
    }
}

/// BZip2 tunables: block size in KB (100..=900, must be multiple of 100).
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub struct Bzip2Tunables {
    pub block_size_kb: u32,
}

impl Default for Bzip2Tunables {
    fn default() -> Self {
        Self { block_size_kb: 900 }
    }
}

impl WriteConfig {
    /// Create the v0.1-compatible default configuration.
    /// All fields match the behavior of `limnifs-write` before
    /// this config was introduced.
    #[must_use]
    pub fn default_v0_1() -> Self {
        Self {
            defaults: Defaults {
                text_codec: DEFAULT_TEXT_CODEC.to_string(),
                binary_codec: DEFAULT_BINARY_CODEC.to_string(),
                metadata_codec: DEFAULT_METADATA_CODEC.to_string(),
                metadata_quality: DEFAULT_METADATA_QUALITY,
                metadata_externalize_threshold: crate::METADATA_EXTERNALIZE_THRESHOLD,
                shared_inline: true,
                seekable_drops: true,
                max_drop_size: DEFAULT_MAX_DROP_SIZE,
                inline_threshold: DEFAULT_INLINE_THRESHOLD,
            },
            categorizers: Vec::new(),
            chunking: ChunkingConfig {
                name: "fastcdc".into(),
                avg_chunk_size: 262_144,
                min_chunk_size: 65_536,
                max_chunk_size: 1_048_576,
            },
            tournament: TournamentConfig {
                codecs: vec![
                    "store".to_string(),
                    "lz4".to_string(),
                    "zstd".to_string(),
                    "brotli".to_string(),
                ],
                min_size_threshold: DEFAULT_TOURNAMENT_MIN_SIZE,
                skip_for_binary: DEFAULT_TOURNAMENT_SKIP_BINARY,
                short_circuit_threshold: default_short_circuit_threshold(),
            },
            encryption: EncryptionConfig {
                aead: DEFAULT_AEAD.to_string(),
                key_wrap: DEFAULT_KEY_WRAP.to_string(),
            },
            dictionaries: DictionaryConfig {
                enabled: DEFAULT_DICT_ENABLED,
                min_class_size: DEFAULT_DICT_MIN_CLASS_SIZE,
                max_dict_size: DEFAULT_DICT_MAX_SIZE,
                trainer: "frequency".into(),
            },
            codec_tunables: CodecTunables::default(),
            mode: ImageMode::ReadOnly,
            write_codec: default_write_codec(),
            turnover_threshold: 0,
            skip_chunking: false,
        }
    }

    /// Load a built-in profile by name, then override fields via
    /// builder methods.
    #[must_use]
    pub fn from_profile(name: &str) -> Option<Self> {
        profile::select(name)
    }

    /// Override the text codec.
    #[must_use]
    pub fn with_text_codec(mut self, codec: &str) -> Self {
        self.defaults.text_codec = codec.into();
        self
    }

    /// Override the binary codec.
    #[must_use]
    pub fn with_binary_codec(mut self, codec: &str) -> Self {
        self.defaults.binary_codec = codec.into();
        self
    }

    /// Override the average chunk size.
    #[must_use]
    pub fn with_chunk_size(mut self, size: u32) -> Self {
        self.chunking.avg_chunk_size = size;
        self
    }

    /// Override the image mode (RO vs RW).
    #[must_use]
    pub fn with_mode(mut self, mode: ImageMode) -> Self {
        self.mode = mode;
        self
    }

    /// Override Brotli quality.
    #[must_use]
    pub fn with_brotli_quality(mut self, quality: u8) -> Self {
        self.codec_tunables.brotli.quality = quality;
        self
    }

    /// Finalize (validate and return).
    /// # Errors
    /// Returns [`ConfigError`] on invalid configuration.
    pub fn build(self) -> Result<Self, ConfigError> {
        self.validate()?;
        Ok(self)
    }

    /// Validate field relationships and range constraints.
    /// # Errors
    /// Returns a [`ConfigError`] on any invalid value.
    pub fn validate(&self) -> Result<(), ConfigError> {
        if self.chunking.min_chunk_size > self.chunking.avg_chunk_size {
            return Err(ConfigError::InvalidValue {
                field: "chunking.min_chunk_size".into(),
                reason: format!(
                    "min_chunk_size ({}) > avg_chunk_size ({})",
                    self.chunking.min_chunk_size, self.chunking.avg_chunk_size
                ),
            });
        }
        if self.chunking.avg_chunk_size > self.chunking.max_chunk_size {
            return Err(ConfigError::InvalidValue {
                field: "chunking.avg_chunk_size".into(),
                reason: format!(
                    "avg_chunk_size ({}) > max_chunk_size ({})",
                    self.chunking.avg_chunk_size, self.chunking.max_chunk_size
                ),
            });
        }
        if self.defaults.metadata_quality < 1 || self.defaults.metadata_quality > 11 {
            return Err(ConfigError::InvalidValue {
                field: "defaults.metadata_quality".into(),
                reason: format!(
                    "metadata_quality ({}) out of range 1..=11",
                    self.defaults.metadata_quality
                ),
            });
        }
        let ceiling = limnifs_core::metadata_reference::DEFAULT_INLINE_METADATA_MAX_BYTES as usize;
        if self.defaults.metadata_externalize_threshold == 0
            || self.defaults.metadata_externalize_threshold > ceiling
        {
            return Err(ConfigError::InvalidValue {
                field: "defaults.metadata_externalize_threshold".into(),
                reason: format!(
                    "metadata_externalize_threshold ({}) must be within 1..={ceiling}                      (the reader inline ceiling; larger inline metadata is unreadable)",
                    self.defaults.metadata_externalize_threshold
                ),
            });
        }
        if self.defaults.max_drop_size != 0 && self.defaults.max_drop_size < 1024 {
            return Err(ConfigError::InvalidValue {
                field: "defaults.max_drop_size".into(),
                reason: format!(
                    "max_drop_size ({}) is nonzero but below 1 KiB — no whole-file \
                     drop can form under the cap; raise it or set 0 (unlimited)",
                    self.defaults.max_drop_size
                ),
            });
        }
        if self.tournament.short_circuit_threshold > 1000 {
            return Err(ConfigError::InvalidValue {
                field: "tournament.short_circuit_threshold".into(),
                reason: format!(
                    "short_circuit_threshold ({}) out of range 0..=1000 (per-mille)",
                    self.tournament.short_circuit_threshold
                ),
            });
        }
        // Validate unique categorizer names.
        let mut names_seen: BTreeMap<&str, ()> = BTreeMap::new();
        for rule in &self.categorizers {
            if !names_seen
                .insert(rule.name.as_str(), ())
                .map_or(true, |()| false)
            {
                return Err(ConfigError::DuplicateCategorizer(rule.name.clone()));
            }
        }
        Ok(())
    }

    /// Build the codec registry to use for this config.
    /// Maps codec names to numeric ids.
    pub fn codec_registry(&self) -> Result<CodecRegistry, ConfigError> {
        let mut registry = CodecRegistry::default();
        registry.insert("store", 0x00);
        registry.insert("lz4", 0x01);
        registry.insert("lz4-hc", 0x13);
        registry.insert("zstd", 0x02);
        registry.insert("xz", 0x03);
        registry.insert("brotli", 0x04);
        registry.insert("deflate", 0x05);
        registry.insert("snappy", 0x06);
        registry.insert("flac", 0x07);
        registry.insert("ricepp", 0x08);
        registry.insert("fsst+brotli", 0x09);
        registry.insert("shuffle+lz4", 0x0A);
        registry.insert("zpaq", 0x0B);
        registry.insert("ppmd", 0x0C);
        registry.insert("glza", 0x0D);
        registry.insert("shuffle+zstd", 0x0E);
        registry.insert("bitshuffle+lz4", 0x0F);
        registry.insert("bzip2", 0x10);
        registry.insert("deflate64", 0x11);
        registry.insert("libdeflate", 0x14);
        registry.insert("bcj-x86-lz4", 0x20);
        registry.insert("bcj-x86-zstd", 0x21);
        registry.insert("bcj-arm64-lz4", 0x23);
        registry.insert("bcj-arm64-zstd", 0x24);

        if !registry.contains_name(&self.defaults.text_codec) {
            return Err(ConfigError::UnknownCodec(self.defaults.text_codec.clone()));
        }
        if !registry.contains_name(&self.defaults.binary_codec) {
            return Err(ConfigError::UnknownCodec(
                self.defaults.binary_codec.clone(),
            ));
        }
        if !registry.contains_name(&self.defaults.metadata_codec) {
            return Err(ConfigError::UnknownCodec(
                self.defaults.metadata_codec.clone(),
            ));
        }
        for rule in &self.categorizers {
            if !registry.contains_name(&rule.codec) {
                return Err(ConfigError::UnknownCodec(rule.codec.clone()));
            }
        }
        for codec in &self.tournament.codecs {
            if !registry.contains_name(codec) {
                return Err(ConfigError::UnknownCodec(codec.clone()));
            }
        }
        Ok(registry)
    }

    /// Resolve the default text codec id.
    /// # Errors
    /// Returns [`ConfigError`] if the codec name is unknown.
    pub fn text_codec_id(&self) -> Result<u8, ConfigError> {
        let registry = self.codec_registry()?;
        Ok(registry
            .lookup_by_name(&self.defaults.text_codec)
            .unwrap_or(0x04))
    }

    /// Resolve the default binary codec id.
    /// # Errors
    /// Returns [`ConfigError`] if the codec name is unknown.
    pub fn binary_codec_id(&self) -> Result<u8, ConfigError> {
        let registry = self.codec_registry()?;
        Ok(registry
            .lookup_by_name(&self.defaults.binary_codec)
            .unwrap_or(0x01))
    }

    /// Build the codec-agnostic [`limnifs_core::codec::CodecTunables`]
    /// view of this config's per-codec knobs. Used by the parallel
    /// writer to honour PPMd order/budget, Brotli quality, ZSTD
    /// level, Bzip2 block size — anything else falls back to codec
    /// defaults.
    #[must_use]
    pub fn to_core_tunables(&self) -> limnifs_core::codec::CodecTunables {
        limnifs_core::codec::CodecTunables {
            quality: self.codec_tunables.brotli.quality,
            zstd_quality: self.codec_tunables.zstd.quality,
            xz_level: 0,
            ppmd_order: self
                .codec_tunables
                .ppmd7
                .order
                .max(self.codec_tunables.ppmd8.order),
            ppmd7_budget: (self.codec_tunables.ppmd7.memory_budget_mb as usize)
                .saturating_mul(1024 * 1024),
            ppmd8_budget: (self.codec_tunables.ppmd8.memory_budget_mb as usize)
                .saturating_mul(1024 * 1024),
            bzip2_block_kb: self.codec_tunables.bzip2.block_size_kb,
            lzma_dict_mb: self.codec_tunables.lzma.dict_size_mb,
        }
    }

    /// Resolve the default metadata codec id.
    /// # Errors
    /// Returns [`ConfigError`] if the codec name is unknown.
    pub fn metadata_codec_id(&self) -> Result<u8, ConfigError> {
        let registry = self.codec_registry()?;
        Ok(registry
            .lookup_by_name(&self.defaults.metadata_codec)
            .unwrap_or(0x04))
    }
}

/// Bidirectional map between codec names and numeric ids.
#[derive(Clone, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
pub struct CodecRegistry {
    by_name: BTreeMap<String, u8>,
    by_id: BTreeMap<u8, String>,
}

impl CodecRegistry {
    /// Insert a new (name, id) mapping.
    pub fn insert(&mut self, name: &str, id: u8) {
        self.by_name.insert(name.to_string(), id);
        self.by_id.insert(id, name.to_string());
    }

    /// Look up a codec id by name.
    #[must_use]
    pub fn lookup_by_name(&self, name: &str) -> Option<u8> {
        self.by_name.get(name).copied()
    }

    /// Look up a codec name by id.
    #[must_use]
    pub fn lookup_by_id(&self, id: u8) -> Option<&str> {
        self.by_id.get(&id).map(String::as_str)
    }

    /// Returns true if the name is registered.
    #[must_use]
    pub fn contains_name(&self, name: &str) -> bool {
        self.by_name.contains_key(name)
    }
}

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

    #[test]
    fn default_v0_1_validates() {
        let config = WriteConfig::default_v0_1();
        config.validate().expect("v0.1 default should validate");
    }

    #[test]
    fn default_v0_1_codec_ids() {
        let config = WriteConfig::default_v0_1();
        assert_eq!(config.text_codec_id().unwrap(), 0x04); // brotli
        assert_eq!(config.binary_codec_id().unwrap(), 0x01); // lz4
        assert_eq!(config.metadata_codec_id().unwrap(), 0x04); // brotli
    }

    #[test]
    fn rejects_invalid_chunking() {
        let mut config = WriteConfig::default_v0_1();
        config.chunking.min_chunk_size = 16_000;
        config.chunking.avg_chunk_size = 8_000;
        assert!(config.validate().is_err());
    }

    #[test]
    fn rejects_invalid_quality() {
        let mut config = WriteConfig::default_v0_1();
        config.defaults.metadata_quality = 12;
        assert!(config.validate().is_err());
    }

    #[test]
    fn rejects_duplicate_categorizer_names() {
        let mut config = WriteConfig::default_v0_1();
        config.categorizers.push(CategorizerConfig {
            name: "dna".into(),
            extensions: vec!["fasta".into()],
            magic_bytes: vec![],
            codec: "glza".into(),
            max_size: None,
            enabled: true,
        });
        config.categorizers.push(CategorizerConfig {
            name: "dna".into(),
            extensions: vec!["fa".into()],
            magic_bytes: vec![],
            codec: "glza".into(),
            max_size: None,
            enabled: true,
        });
        assert!(config.validate().is_err());
    }

    #[test]
    fn rejects_unknown_codec() {
        let mut config = WriteConfig::default_v0_1();
        config.defaults.text_codec = "does-not-exist".into();
        assert!(config.codec_registry().is_err());
    }
}