truss-image 0.11.5

//! On-disk transform and origin caching.
//!
//! # Design decisions
//!
//! **Sharded directory layout:** Cache entries are stored under
//! `<root>/ab/cd/ef/<sha256>`, splitting the first three byte-pairs of the hex key
//! into nested directories. This creates up to 4,096 intermediate directories and
//! prevents filesystem inode exhaustion or performance degradation when millions of
//! entries accumulate in a flat directory.
//!
//! **Atomic writes:** Cache entries are written to a temporary file (with a unique
//! PID + counter suffix) then renamed atomically. This ensures readers never see
//! partial data. Corrupted entries (detected during reads) are cleaned up
//! automatically.
//!
//! **mtime-based TTL:** Staleness is determined by file modification time rather
//! than an embedded timestamp. This keeps the on-disk format simple (media-type
//! header + raw bytes) and allows operators to touch files to extend their lifetime.
//!
//! **Optional size-based eviction:** When `TRUSS_CACHE_MAX_BYTES` is set to a
//! positive value, the cache performs LRU-style eviction after writes. Eviction
//! scans are throttled to at most once per 60 seconds to avoid expensive
//! directory walks on every cache write. When the variable is unset or `0`, no
//! size-based eviction is performed and operators should use external tools
//! (e.g., `tmpwatch`, `tmpreaper`, cron) for disk management.

use super::LogHandler;
use crate::MediaType;
use sha2::{Digest, Sha256};
use std::fs;
use std::io::{self, Write};
use std::path::{Path, PathBuf};
use std::str::FromStr;
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::Duration;

use super::ServerConfig;
use super::http_parse::HttpRequest;
use super::metrics::CACHE_HITS_TOTAL;
use super::negotiate::{
    CacheHitStatus, ImageResponsePolicy, build_image_etag, build_image_response_headers,
    if_none_match_matches,
};
use super::response::HttpResponse;
use crate::core::default_lossy_target_quality;
use crate::{Fit, Position, Rotation, TransformOptions};

pub(super) const DEFAULT_CACHE_TTL_SECONDS: u64 = 3600;

/// Monotonically increasing counter used to generate unique temp-file suffixes
/// for cache writes.  Combined with the process ID this avoids collisions from
/// concurrent writers within the same process.
pub(super) static CACHE_TEMP_COUNTER: AtomicU64 = AtomicU64::new(0);

/// On-disk transform cache using a sharded directory layout.
///
/// The cache stores transformed image bytes under `<root>/ab/cd/ef/<sha256_hex>`, where
/// `ab`, `cd`, `ef` are the first three byte-pairs of the hex-encoded cache key. Each file
/// starts with a media-type header line (e.g. `"jpeg\n"`) followed by the raw output bytes.
///
/// Staleness is determined by file modification time. Entries older than
/// [`DEFAULT_CACHE_TTL_SECONDS`] are treated as misses and overwritten on the next transform.
///
/// When `max_bytes` is set to a positive value, the cache performs LRU-style eviction
/// after each write to keep the total on-disk size under the configured limit.
/// Minimum interval between eviction scans (seconds).
const EVICTION_INTERVAL_SECS: u64 = 60;

pub(super) struct TransformCache {
    pub(super) root: PathBuf,
    pub(super) ttl: Duration,
    pub(super) log_handler: Option<LogHandler>,
    /// Maximum total cache size in bytes. `0` means unlimited (no eviction).
    pub(super) max_bytes: u64,
    /// Unix timestamp of the last eviction scan, used to throttle scans.
    last_eviction_secs: AtomicU64,
}

/// The result of a cache lookup.
#[derive(Debug)]
pub(super) enum CacheLookup {
    /// The entry was found and is still fresh.
    Hit {
        media_type: MediaType,
        body: Vec<u8>,
        age: Duration,
    },
    /// The entry was not found or is stale.
    Miss,
}

impl TransformCache {
    /// Creates a new transform cache rooted at the given directory.
    pub(super) fn new(root: PathBuf) -> Self {
        Self {
            root,
            ttl: Duration::from_secs(DEFAULT_CACHE_TTL_SECONDS),
            log_handler: None,
            max_bytes: 0,
            last_eviction_secs: AtomicU64::new(0),
        }
    }

    pub(super) fn with_log_handler(mut self, handler: Option<LogHandler>) -> Self {
        self.log_handler = handler;
        self
    }

    pub(super) fn with_max_bytes(mut self, max_bytes: u64) -> Self {
        self.max_bytes = max_bytes;
        self
    }

    pub(super) fn log(&self, msg: &str) {
        if let Some(handler) = &self.log_handler {
            handler(msg);
        } else {
            eprintln!("{msg}");
        }
    }

    /// Returns the sharded file path for the given cache key.
    ///
    /// # Panics
    ///
    /// Debug-asserts that `key` is a 64-character hex string (SHA-256 output).
    pub(super) fn entry_path(&self, key: &str) -> PathBuf {
        debug_assert!(
            key.len() == 64 && key.bytes().all(|b| b.is_ascii_hexdigit()),
            "cache key must be a 64-character hex string"
        );
        // Layout: <root>/ab/cd/ef/<key>
        // where ab, cd, ef are the first 6 hex characters split into pairs.
        let a = &key[0..2];
        let b = &key[2..4];
        let c = &key[4..6];
        self.root.join(a).join(b).join(c).join(key)
    }

    /// Looks up a cached transform result.
    ///
    /// Returns [`CacheLookup::Hit`] if the file exists, is readable, and its modification
    /// time is within the TTL. Returns [`CacheLookup::Miss`] otherwise.
    pub(super) fn get(&self, key: &str) -> CacheLookup {
        let path = self.entry_path(key);

        // Open a single file handle to avoid TOCTOU between read and metadata.
        let file = match fs::File::open(&path) {
            Ok(f) => f,
            Err(_) => return CacheLookup::Miss,
        };

        // Check staleness via mtime on the same file handle.
        let age = match file
            .metadata()
            .and_then(|m| m.modified())
            .and_then(|mtime| mtime.elapsed().map_err(io::Error::other))
        {
            Ok(age) => age,
            Err(_) => return CacheLookup::Miss,
        };

        if age > self.ttl {
            return CacheLookup::Miss;
        }

        let mut data = Vec::new();
        if io::Read::read_to_end(&mut &file, &mut data).is_err() {
            self.remove_corrupted(&path, "read failed");
            return CacheLookup::Miss;
        }

        // Parse the header line: "<media_type>\n<body>"
        let newline_pos = match data.iter().position(|&b| b == b'\n') {
            Some(pos) => pos,
            None => {
                self.remove_corrupted(&path, "missing header newline");
                return CacheLookup::Miss;
            }
        };
        let media_type_str = match std::str::from_utf8(&data[..newline_pos]) {
            Ok(s) => s,
            Err(_) => {
                self.remove_corrupted(&path, "invalid UTF-8 in header");
                return CacheLookup::Miss;
            }
        };
        let media_type = match MediaType::from_str(media_type_str) {
            Ok(mt) => mt,
            Err(_) => {
                self.remove_corrupted(&path, "unrecognized media type");
                return CacheLookup::Miss;
            }
        };

        // Remove the header in-place to avoid a second allocation.
        data.drain(..=newline_pos);

        CacheLookup::Hit {
            media_type,
            body: data,
            age,
        }
    }

    /// Removes a corrupted cache entry and logs the reason.
    fn remove_corrupted(&self, path: &Path, reason: &str) {
        self.log(&format!(
            "truss: removing corrupted cache entry ({reason}): {}",
            path.display()
        ));
        let _ = fs::remove_file(path);
    }

    /// Writes a transform result to the cache.
    ///
    /// Uses write-to-tempfile-then-rename for atomic writes, preventing readers from seeing
    /// partial data.
    pub(super) fn put(&self, key: &str, media_type: MediaType, body: &[u8]) {
        let path = self.entry_path(key);
        if let Some(parent) = path.parent()
            && let Err(err) = fs::create_dir_all(parent)
        {
            self.log(&format!("truss: cache mkdir failed: {err}"));
            return;
        }

        // Write to a temp file with a unique suffix, then rename atomically.
        let tmp_path = path.with_extension(unique_tmp_suffix());
        let mut header = media_type.as_name().as_bytes().to_vec();
        header.push(b'\n');

        let result = (|| -> io::Result<()> {
            let mut file = fs::File::create(&tmp_path)?;
            file.write_all(&header)?;
            file.write_all(body)?;
            drop(file);
            fs::rename(&tmp_path, &path)?;
            Ok(())
        })();

        if let Err(err) = result {
            self.log(&format!("truss: cache write failed: {err}"));
            // Clean up the temp file if it exists.
            let _ = fs::remove_file(&tmp_path);
        } else {
            self.maybe_evict();
        }
    }

    /// Runs LRU-style eviction if a maximum cache size is configured and exceeded.
    ///
    /// Scans all files under the cache root, sorts them by modification time
    /// (oldest first), and removes entries until the total size drops below
    /// `self.max_bytes`. Eviction scans are throttled to at most once per
    /// [`EVICTION_INTERVAL_SECS`]. Errors on individual files (e.g. concurrent
    /// deletion) are silently ignored.
    fn maybe_evict(&self) {
        if self.max_bytes == 0 {
            return;
        }
        // Throttle eviction scans to at most once per EVICTION_INTERVAL_SECS
        // to avoid O(n log n) directory walks on every cache write.
        let now = std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .unwrap_or_default()
            .as_secs();
        let last = self.last_eviction_secs.load(Ordering::Relaxed);
        if now.saturating_sub(last) < EVICTION_INTERVAL_SECS {
            return;
        }
        if self
            .last_eviction_secs
            .compare_exchange(last, now, Ordering::Relaxed, Ordering::Relaxed)
            .is_err()
        {
            return; // another thread won the race
        }
        if let Err(err) = self.evict_to_limit() {
            self.log(&format!("truss: cache eviction scan failed: {err}"));
        }
    }

    /// Performs the eviction scan and removal. Returns an error only if the
    /// top-level directory walk cannot be started.
    fn evict_to_limit(&self) -> io::Result<()> {
        let mut entries = collect_cache_entries(&self.root)?;
        let total_size: u64 = entries.iter().map(|e| e.size).sum();
        if total_size <= self.max_bytes {
            return Ok(());
        }

        // Sort oldest-first (largest mtime = most recently modified = keep).
        entries.sort_by_key(|e| e.mtime);

        let mut current_size = total_size;
        for entry in &entries {
            if current_size <= self.max_bytes {
                break;
            }
            if fs::remove_file(&entry.path).is_ok() {
                self.log(&format!(
                    "truss: cache eviction: removed {} ({} bytes)",
                    entry.path.display(),
                    entry.size
                ));
                current_size = current_size.saturating_sub(entry.size);
            }
        }
        Ok(())
    }
}

/// A cache file entry used during eviction scanning.
struct CacheEntry {
    path: PathBuf,
    size: u64,
    /// Modification time as duration since `UNIX_EPOCH`. Entries with smaller
    /// values are older and evicted first.
    mtime: Duration,
}

/// Recursively collects all regular files under `root` with their size and
/// modification time. Temp files (containing `.tmp.`) are skipped.
fn collect_cache_entries(root: &Path) -> io::Result<Vec<CacheEntry>> {
    let mut entries = Vec::new();
    collect_entries_recursive(root, &mut entries);
    Ok(entries)
}

fn collect_entries_recursive(dir: &Path, entries: &mut Vec<CacheEntry>) {
    let read_dir = match fs::read_dir(dir) {
        Ok(rd) => rd,
        Err(_) => return,
    };
    for entry in read_dir.flatten() {
        let path = entry.path();
        if path.is_dir() {
            collect_entries_recursive(&path, entries);
        } else if path.is_file() {
            // Skip temp files from in-flight writes.
            if let Some(name) = path.file_name().and_then(|n| n.to_str())
                && name.contains(".tmp.")
            {
                continue;
            }
            if let Ok(meta) = fs::metadata(&path) {
                let size = meta.len();
                let mtime = meta
                    .modified()
                    .ok()
                    .and_then(|t| t.duration_since(std::time::UNIX_EPOCH).ok())
                    .unwrap_or(Duration::ZERO);
                entries.push(CacheEntry { path, size, mtime });
            }
        }
    }
}

/// On-disk origin response cache for remote URL fetches.
///
/// Caches raw source bytes fetched from remote URLs so repeated requests for the same
/// remote source avoid redundant HTTP round-trips. This sits in front of the transform
/// cache in the cache hierarchy (design doc section 8.1).
///
/// The cache key is the SHA-256 of the canonical URL string. The stored value is the
/// raw source bytes with no header. Staleness uses the same mtime-based TTL as the
/// transform cache.
pub(super) struct OriginCache {
    root: PathBuf,
    pub(super) ttl: Duration,
    log_handler: Option<LogHandler>,
}

impl OriginCache {
    /// Creates a new origin cache rooted at `<cache_root>/origin/`.
    pub(super) fn new(cache_root: &Path) -> Self {
        Self {
            root: cache_root.join("origin"),
            ttl: Duration::from_secs(DEFAULT_CACHE_TTL_SECONDS),
            log_handler: None,
        }
    }

    pub(super) fn with_log_handler(mut self, handler: Option<LogHandler>) -> Self {
        self.log_handler = handler;
        self
    }

    fn log(&self, msg: &str) {
        if let Some(handler) = &self.log_handler {
            handler(msg);
        } else {
            eprintln!("{msg}");
        }
    }

    /// Returns the sharded file path for the given URL and namespace.
    fn entry_path(&self, namespace: &str, url: &str) -> PathBuf {
        let mut hasher = Sha256::new();
        hasher.update(namespace.as_bytes());
        hasher.update(b":");
        hasher.update(url.as_bytes());
        let key = hex::encode(hasher.finalize());
        let a = &key[0..2];
        let b = &key[2..4];
        let c = &key[4..6];
        self.root.join(a).join(b).join(c).join(&key)
    }

    /// Looks up cached source bytes for a remote URL within the given namespace.
    pub(super) fn get(&self, namespace: &str, url: &str) -> Option<Vec<u8>> {
        let path = self.entry_path(namespace, url);
        let file = fs::File::open(&path).ok()?;

        let age = file
            .metadata()
            .and_then(|m| m.modified())
            .and_then(|mtime| mtime.elapsed().map_err(io::Error::other))
            .ok()?;

        if age > self.ttl {
            return None;
        }

        let mut data = Vec::new();
        io::Read::read_to_end(&mut &file, &mut data).ok()?;
        Some(data)
    }

    /// Writes fetched source bytes to the origin cache within the given namespace.
    pub(super) fn put(&self, namespace: &str, url: &str, body: &[u8]) {
        let path = self.entry_path(namespace, url);
        if let Some(parent) = path.parent()
            && let Err(err) = fs::create_dir_all(parent)
        {
            self.log(&format!("truss: origin cache mkdir failed: {err}"));
            return;
        }

        let tmp_path = path.with_extension(unique_tmp_suffix());
        let result = (|| -> io::Result<()> {
            let mut file = fs::File::create(&tmp_path)?;
            file.write_all(body)?;
            drop(file);
            fs::rename(&tmp_path, &path)?;
            Ok(())
        })();

        if let Err(err) = result {
            self.log(&format!("truss: origin cache write failed: {err}"));
            let _ = fs::remove_file(&tmp_path);
        }
    }
}

/// Returns a unique temporary-file suffix for cache writes.
///
/// The suffix combines the process ID with a monotonically increasing counter
/// so that concurrent writers within the same process never collide on the
/// same temp path (the previous PID-only scheme could).
pub(super) fn unique_tmp_suffix() -> String {
    let seq = CACHE_TEMP_COUNTER.fetch_add(1, Ordering::Relaxed);
    format!("tmp.{}.{seq}", std::process::id())
}

/// Computes a SHA-256 cache key from the source identifier, transform options, and
/// optionally the negotiated Accept value.
///
/// The canonical form follows the design specification (section 8.2):
/// ```text
/// SHA256(
///   canonical_source_identifier + "\n" +
///   canonical_transform_parameters + "\n" +
///   normalized_accept_if_negotiation_enabled_and_format_absent
/// )
/// ```
///
/// Auth-related parameters (`keyId`, `expires`, `signature`) are excluded. The `deadline`
/// field is excluded because it is an adapter concern, not a transform identity.
pub(super) fn compute_cache_key(
    source_identifier: &str,
    options: &TransformOptions,
    negotiated_accept: Option<&str>,
    watermark_identity: Option<&str>,
) -> String {
    use std::fmt::Write;

    let mut canonical = String::new();
    canonical.push_str(source_identifier);
    canonical.push('\n');

    // Build sorted canonical transform parameters.
    //
    // Where the core `TransformOptions::normalize()` method fills in defaults
    // (e.g. fit -> Contain, position -> Center when width+height are set), we
    // replicate the same defaults here so that the omitted-vs-explicit-default
    // distinction does not produce different cache keys for identical transforms.
    let has_bounded_resize = options.width.is_some() && options.height.is_some();

    let mut first = true;
    let mut push_param = |canonical: &mut String, k: &str, v: &str| {
        if !first {
            canonical.push('&');
        }
        first = false;
        canonical.push_str(k);
        canonical.push('=');
        canonical.push_str(v);
    };

    if options.auto_orient {
        push_param(&mut canonical, "autoOrient", "true");
    }
    if let Some(bg) = &options.background {
        let mut buf = String::new();
        let _ = write!(buf, "{:02x}{:02x}{:02x}{:02x}", bg.r, bg.g, bg.b, bg.a);
        push_param(&mut canonical, "background", &buf);
    }
    if let Some(blur) = options.blur {
        let mut buf = String::new();
        let _ = write!(buf, "{blur}");
        push_param(&mut canonical, "blur", &buf);
    }
    if let Some(crop) = options.crop {
        let buf = crop.to_string();
        push_param(&mut canonical, "crop", &buf);
    }
    if has_bounded_resize {
        let fit = options.fit.unwrap_or(Fit::Contain);
        push_param(&mut canonical, "fit", fit.as_name());
    }
    if let Some(format) = options.format {
        push_param(&mut canonical, "format", format.as_name());
    }
    if let Some(h) = options.height {
        let buf = h.to_string();
        push_param(&mut canonical, "height", &buf);
    }
    if options.optimize != crate::OptimizeMode::None {
        push_param(&mut canonical, "optimize", options.optimize.as_name());
    }
    if has_bounded_resize {
        let pos = options.position.unwrap_or(Position::Center);
        push_param(&mut canonical, "position", pos.as_name());
    }
    if options.preserve_exif {
        push_param(&mut canonical, "preserveExif", "true");
    }
    if let Some(q) = options.quality {
        let buf = q.to_string();
        push_param(&mut canonical, "quality", &buf);
    }
    if let Some(target_quality) = options.target_quality {
        let buf = target_quality.to_string();
        push_param(&mut canonical, "targetQuality", &buf);
    } else if matches!(
        options.optimize,
        crate::OptimizeMode::Auto | crate::OptimizeMode::Lossy
    ) && options.quality.is_none()
        && let Some(format) = options.format
        && let Some(target_quality) = default_lossy_target_quality(format)
    {
        let buf = target_quality.to_string();
        push_param(&mut canonical, "targetQuality", &buf);
    }
    if options.rotate != Rotation::Deg0 {
        let buf = options.rotate.as_degrees().to_string();
        push_param(&mut canonical, "rotate", &buf);
    }
    if let Some(sharpen) = options.sharpen {
        let mut buf = String::new();
        let _ = write!(buf, "{sharpen}");
        push_param(&mut canonical, "sharpen", &buf);
    }
    if options.strip_metadata {
        push_param(&mut canonical, "stripMetadata", "true");
    }
    if let Some(w) = options.width {
        let buf = w.to_string();
        push_param(&mut canonical, "width", &buf);
    }

    canonical.push('\n');
    if let Some(accept) = negotiated_accept {
        canonical.push_str(accept);
    }
    canonical.push('\n');
    if let Some(wm) = watermark_identity {
        canonical.push_str(wm);
    }

    let digest = Sha256::digest(canonical.as_bytes());
    hex::encode(digest)
}

/// Computes a stable identity string for watermark parameters that can be
/// included in cache key computation. The identity is a SHA-256 hex digest
/// of the watermark URL, position, opacity, and margin concatenated with
/// newline separators.
pub(super) fn compute_watermark_identity(
    url: &str,
    position: &str,
    opacity: u8,
    margin: u32,
) -> String {
    let mut hasher = Sha256::new();
    hasher.update(b"watermark\n");
    hasher.update(url.as_bytes());
    hasher.update(b"\n");
    hasher.update(position.as_bytes());
    hasher.update(b"\n");
    hasher.update(opacity.to_string().as_bytes());
    hasher.update(b"\n");
    hasher.update(margin.to_string().as_bytes());
    hex::encode(hasher.finalize())
}

pub(super) fn compute_watermark_content_identity(
    content_hash: &str,
    position: &str,
    opacity: u8,
    margin: u32,
) -> String {
    let mut hasher = Sha256::new();
    hasher.update(b"watermark-content\n");
    hasher.update(content_hash.as_bytes());
    hasher.update(b"\n");
    hasher.update(position.as_bytes());
    hasher.update(b"\n");
    hasher.update(opacity.to_string().as_bytes());
    hasher.update(b"\n");
    hasher.update(margin.to_string().as_bytes());
    hex::encode(hasher.finalize())
}

/// Attempts a cache lookup using a version-based source hash, which avoids reading
/// the full source bytes. Returns `Some(response)` on a cache hit (including `304`
/// for conditional requests). Returns `None` on miss or when a version-based lookup
/// is not possible (no version, no cache, or format not yet known).
pub(super) fn try_versioned_cache_lookup(
    versioned_hash: Option<&str>,
    options: &TransformOptions,
    request: &HttpRequest,
    response_policy: ImageResponsePolicy,
    config: &ServerConfig,
    watermark_identity: Option<&str>,
) -> Option<HttpResponse> {
    let source_hash = versioned_hash?;
    let cache_root = config.cache_root.as_ref()?;
    // We can only do a pre-lookup when the output format is already set, because
    // Accept negotiation requires sniffing the source to know the input type.
    options.format?;

    let cache =
        TransformCache::new(cache_root.clone()).with_log_handler(config.log_handler.clone());
    let cache_key = compute_cache_key(source_hash, options, None, watermark_identity);
    if let CacheLookup::Hit {
        media_type,
        body,
        age,
    } = cache.get(&cache_key)
    {
        CACHE_HITS_TOTAL.fetch_add(1, Ordering::Relaxed);
        let etag = build_image_etag(&body);
        let mut headers = build_image_response_headers(
            media_type,
            &etag,
            response_policy,
            false,
            CacheHitStatus::Hit,
            config.public_max_age_seconds,
            config.public_stale_while_revalidate_seconds,
            &config.custom_response_headers,
        );
        headers.push(("Age".to_string(), age.as_secs().to_string()));
        if matches!(response_policy, ImageResponsePolicy::PublicGet)
            && if_none_match_matches(request.header("if-none-match"), &etag)
        {
            return Some(HttpResponse::empty("304 Not Modified", headers));
        }
        return Some(HttpResponse::binary_with_headers(
            "200 OK",
            media_type.as_mime(),
            headers,
            body,
        ));
    }
    None
}

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

    #[test]
    fn cache_key_blur_full_precision() {
        let opts_a = TransformOptions {
            blur: Some(0.11),
            ..TransformOptions::default()
        };
        let opts_b = TransformOptions {
            blur: Some(0.14),
            ..TransformOptions::default()
        };
        let key_a = compute_cache_key("img.png", &opts_a, None, None);
        let key_b = compute_cache_key("img.png", &opts_b, None, None);
        assert_ne!(
            key_a, key_b,
            "blur=0.11 and blur=0.14 must produce different cache keys"
        );
    }

    #[test]
    fn cache_key_differs_by_optimize_mode() {
        let base = TransformOptions::default();
        let optimized = TransformOptions {
            optimize: crate::OptimizeMode::Auto,
            ..TransformOptions::default()
        };

        assert_ne!(
            compute_cache_key("img.png", &base, None, None),
            compute_cache_key("img.png", &optimized, None, None)
        );
    }

    #[test]
    fn cache_key_differs_by_target_quality() {
        let a = TransformOptions {
            format: Some(MediaType::Jpeg),
            optimize: crate::OptimizeMode::Lossy,
            target_quality: Some(crate::TargetQuality {
                metric: crate::QualityMetric::Ssim,
                value: 0.98,
            }),
            ..TransformOptions::default()
        };
        let b = TransformOptions {
            format: Some(MediaType::Jpeg),
            optimize: crate::OptimizeMode::Lossy,
            target_quality: Some(crate::TargetQuality {
                metric: crate::QualityMetric::Ssim,
                value: 0.99,
            }),
            ..TransformOptions::default()
        };

        assert_ne!(
            compute_cache_key("img.png", &a, None, None),
            compute_cache_key("img.png", &b, None, None)
        );
    }

    #[test]
    fn cache_key_matches_explicit_default_target_quality() {
        let implicit = TransformOptions {
            format: Some(MediaType::Jpeg),
            optimize: crate::OptimizeMode::Lossy,
            ..TransformOptions::default()
        };
        let explicit = TransformOptions {
            format: Some(MediaType::Jpeg),
            optimize: crate::OptimizeMode::Lossy,
            target_quality: default_lossy_target_quality(MediaType::Jpeg),
            ..TransformOptions::default()
        };

        assert_eq!(
            compute_cache_key("img.png", &implicit, None, None),
            compute_cache_key("img.png", &explicit, None, None)
        );
    }

    /// Helper to generate a deterministic 64-char hex key for testing.
    fn test_key(index: u8) -> String {
        let digest = Sha256::digest([index]);
        hex::encode(digest)
    }

    #[test]
    fn eviction_removes_oldest_entries_when_over_limit() {
        let dir = tempfile::tempdir().unwrap();
        let cache = TransformCache::new(dir.path().to_path_buf()).with_max_bytes(200);

        // Write four entries, each ~60 bytes on disk (header + body).
        // Total will exceed 200 bytes after all writes.
        let body = vec![0u8; 50];
        cache.put(&test_key(0), MediaType::Jpeg, &body);
        // Ensure distinct mtimes by touching the file timestamps manually.
        std::thread::sleep(std::time::Duration::from_millis(50));
        cache.put(&test_key(1), MediaType::Jpeg, &body);
        std::thread::sleep(std::time::Duration::from_millis(50));
        cache.put(&test_key(2), MediaType::Jpeg, &body);
        std::thread::sleep(std::time::Duration::from_millis(50));
        cache.put(&test_key(3), MediaType::Jpeg, &body);

        // Directly trigger eviction (maybe_evict is throttled in production).
        let _ = cache.evict_to_limit();

        // Collect remaining files.
        let remaining: Vec<_> = collect_cache_entries(dir.path())
            .unwrap()
            .into_iter()
            .map(|e| e.path)
            .collect();

        let total_size: u64 = collect_cache_entries(dir.path())
            .unwrap()
            .iter()
            .map(|e| e.size)
            .sum();

        assert!(
            total_size <= 200,
            "cache size {total_size} should be <= 200 after eviction"
        );
        // The most recent entry must survive.
        assert!(
            remaining.contains(&cache.entry_path(&test_key(3))),
            "newest entry should survive eviction"
        );
    }

    #[test]
    fn no_eviction_when_max_bytes_is_zero() {
        let dir = tempfile::tempdir().unwrap();
        let cache = TransformCache::new(dir.path().to_path_buf()).with_max_bytes(0);

        let body = vec![0u8; 100];
        for i in 0..5 {
            cache.put(&test_key(i), MediaType::Jpeg, &body);
        }

        let entries = collect_cache_entries(dir.path()).unwrap();
        assert_eq!(
            entries.len(),
            5,
            "all entries should survive when max_bytes is 0"
        );
    }

    #[test]
    fn no_eviction_when_under_limit() {
        let dir = tempfile::tempdir().unwrap();
        // Set a very generous limit.
        let cache = TransformCache::new(dir.path().to_path_buf()).with_max_bytes(1_000_000);

        let body = vec![0u8; 50];
        for i in 0..3 {
            cache.put(&test_key(i), MediaType::Jpeg, &body);
        }

        let entries = collect_cache_entries(dir.path()).unwrap();
        assert_eq!(
            entries.len(),
            3,
            "all entries should survive when under limit"
        );
    }

    #[test]
    fn collect_cache_entries_skips_temp_files() {
        let dir = tempfile::tempdir().unwrap();
        let cache = TransformCache::new(dir.path().to_path_buf());

        // Write a normal entry.
        cache.put(&test_key(0), MediaType::Jpeg, b"data");

        // Create a temp file that should be skipped.
        let key = test_key(1);
        let path = cache.entry_path(&key);
        if let Some(parent) = path.parent() {
            fs::create_dir_all(parent).unwrap();
        }
        let tmp_path = path.with_extension("tmp.12345.0");
        fs::write(&tmp_path, b"partial").unwrap();

        let entries = collect_cache_entries(dir.path()).unwrap();
        assert_eq!(entries.len(), 1, "temp files should be excluded from scan");
    }
}