otf_pixels_core/

codec.rs

//! Container formats and the codec traits every format plugs in behind.
//!
//! Every format is implemented in this workspace (ADR-0004, then ADR-0013,
//! ADR-0014 and ADR-0018), and that is invisible here on purpose: everything
//! sits behind [`Decoder`] and [`Encoder`], so a format can be rewritten
//! without an API change.

use crate::{
    ImageDescriptor, Orientation, PixelFormat, PixelsError, Region, Result, Sink, TileMut,
};
use core::fmt;

/// A container format.
///
/// Format is **data**, not a type parameter: `output(format, options)` is the
/// single encode terminal, which keeps the API surface stable as formats are
/// added and maps cleanly onto host bindings (ADR-0006). Formats not yet
/// implemented are still nameable, so an unsupported request is a catchable
/// [`PixelsError::Unsupported`] rather than a compile error in a host binding.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Format {
    /// Uncompressed pixels described entirely by the caller.
    Raw,
    /// Portable Network Graphics.
    Png,
    /// JPEG.
    Jpeg,
    /// Graphics Interchange Format.
    Gif,
    /// Tagged Image File Format.
    Tiff,
    /// WebP.
    WebP,
    /// AV1 Image File Format.
    Avif,
    /// JPEG XL.
    Jxl,
}

impl Format {
    /// A short, stable, lowercase name for this format.
    #[must_use]
    pub const fn as_str(self) -> &'static str {
        match self {
            Self::Raw => "raw",
            Self::Png => "png",
            Self::Jpeg => "jpeg",
            Self::Gif => "gif",
            Self::Tiff => "tiff",
            Self::WebP => "webp",
            Self::Avif => "avif",
            Self::Jxl => "jxl",
        }
    }
}

impl fmt::Display for Format {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.as_str())
    }
}

/// What a decoder can produce without a full decode.
///
/// The scheduler uses this to decide whether it may request arbitrary regions
/// from a source or must pull rows in order (ARCHITECTURE §Layer 2).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum DecodeCapability {
    /// Emits rows in top-to-bottom order as bytes arrive.
    ///
    /// Out-of-order region requests are not supported; the engine reads
    /// forward. This covers non-interlaced PNG, baseline JPEG, GIF frames,
    /// strip TIFF and raw.
    Sequential,
    /// Can produce an arbitrary region without decoding the whole image.
    ///
    /// This covers tiled TIFF, and is what makes the giant-image thumbnail
    /// path constant-memory.
    ///
    /// A JPEG decoding at a reduced `M/8` scale is *not* this, though an
    /// earlier version of this comment said it was: scaled decode lowers the
    /// resolution but still emits rows in order, and every coefficient is
    /// still entropy-decoded. It is a decoder configuration, not a capability,
    /// and claiming otherwise would be a lie the scheduler acts on.
    Regions,
}

/// How an animated image plays: its frames and their timing.
///
/// Reported by [`Decoder::animation`] for a file with more than one frame.
/// The pipeline processes the first frame; this describes the source, so a
/// caller can see it is animated and decide what to do (pass it through,
/// refuse it, or take the still).
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub struct Animation {
    /// Frames in the animation, always two or more.
    pub frame_count: u32,
    /// How many times it plays; 0 means forever, as in GIF and WebP.
    pub loop_count: u32,
    /// How long each frame shows, in milliseconds, in order.
    pub frame_durations_ms: Vec<u32>,
}

impl Animation {
    /// An animation of the given frame durations, playing `loop_count`
    /// times (0 for forever). `None` for fewer than two frames, which is a
    /// still image.
    #[must_use]
    pub fn new(frame_durations_ms: Vec<u32>, loop_count: u32) -> Option<Self> {
        let frame_count = u32::try_from(frame_durations_ms.len()).ok()?;
        (frame_count >= 2).then_some(Self {
            frame_count,
            loop_count,
            frame_durations_ms,
        })
    }

    /// The total time one play-through takes, in milliseconds.
    #[must_use]
    pub fn duration_ms(&self) -> u64 {
        self.frame_durations_ms.iter().map(|&d| u64::from(d)).sum()
    }
}

/// Header-only facts about an image.
///
/// Answering this must not decode pixels (SPEC §Guarantees 3).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub struct Metadata {
    /// Width in pixels.
    pub width: u32,
    /// Height in pixels.
    pub height: u32,
    /// The container format the image was read from.
    pub format: Format,
    /// The pixel format the decoder will produce.
    pub pixel: PixelFormat,
}

impl Metadata {
    /// Build metadata from a decoded header descriptor and its container.
    #[must_use]
    pub const fn new(desc: &ImageDescriptor, format: Format) -> Self {
        Self {
            width: desc.width,
            height: desc.height,
            format,
            pixel: desc.pixel,
        }
    }
}

/// A resolution's unit, as the file states it.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum ResolutionUnit {
    /// Pixels per inch.
    Inch,
    /// Pixels per centimetre.
    Centimetre,
}

/// How densely the image is meant to be printed or shown (sharp's
/// `density`, `resolutionUnit`).
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Density {
    /// Horizontal pixels per `unit`.
    pub x: f64,
    /// Vertical pixels per `unit`.
    pub y: f64,
    /// The unit the file states.
    pub unit: ResolutionUnit,
}

impl Density {
    /// A density of `dpi` both ways, per inch.
    #[must_use]
    pub const fn dpi(dpi: f64) -> Self {
        Self {
            x: dpi,
            y: dpi,
            unit: ResolutionUnit::Inch,
        }
    }

    /// The density an EXIF block states in IFD0 (`XResolution`,
    /// `YResolution`, `ResolutionUnit`), or `None` if it states none.
    #[must_use]
    pub fn from_exif_block(exif: &[u8]) -> Option<Self> {
        let tree = crate::exif::Exif::parse(exif)?;
        let x = tree.rational(0x011A)?;
        let y = tree.rational(0x011B).unwrap_or(x);
        let unit = match tree.integer(0x0128).unwrap_or(2) {
            3 => ResolutionUnit::Centimetre,
            2 => ResolutionUnit::Inch,
            _ => return None,
        };
        (x > 0.0 && y > 0.0).then_some(Self { x, y, unit })
    }

    /// The horizontal density in pixels per inch, sharp's `density`.
    #[must_use]
    pub fn x_dpi(&self) -> f64 {
        match self.unit {
            ResolutionUnit::Inch => self.x,
            ResolutionUnit::Centimetre => self.x * 2.54,
        }
    }

    /// The vertical density in pixels per inch.
    #[must_use]
    pub fn y_dpi(&self) -> f64 {
        match self.unit {
            ResolutionUnit::Inch => self.y,
            ResolutionUnit::Centimetre => self.y * 2.54,
        }
    }
}

/// A text entry a file carries, such as PNG's `tEXt` (sharp's `comments`).
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct Comment {
    /// The entry's keyword, empty where the format has none.
    pub keyword: String,
    /// Its text.
    pub text: String,
}

/// The colour a file names as its background, in its own samples.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Background {
    /// A grey level.
    Grey(u16),
    /// Red, green and blue.
    Rgb(u16, u16, u16),
}

/// What a decoder learned about its file from the header, beyond the pixel
/// layout: the facts sharp's `metadata()` reports per format. Every field is
/// empty for a format with no notion of it.
#[derive(Debug, Clone, Default, PartialEq)]
#[non_exhaustive]
pub struct SourceInfo {
    /// The stated resolution, if any (JPEG's JFIF or EXIF, PNG's `pHYs`,
    /// TIFF's tags).
    pub density: Option<Density>,
    /// Progressive JPEG, or interlaced PNG and GIF.
    pub progressive: bool,
    /// Samples are indices into a palette (PNG colour type 3, GIF, TIFF
    /// palette).
    pub palette: bool,
    /// Bits per stored sample, which may be fewer than the output's (a
    /// 10-bit AVIF, a 4-bit PNG).
    pub bits_per_sample: Option<u8>,
    /// Chroma subsampling as sharp spells it (`"4:2:0"`, `"4:4:4"`, ...),
    /// for formats that subsample.
    pub chroma_subsampling: Option<&'static str>,
    /// The background colour the file names (PNG `bKGD`, GIF's).
    pub background: Option<Background>,
    /// The compression scheme, where a format has several (TIFF, AVIF).
    pub compression: Option<&'static str>,
    /// Text entries (PNG `tEXt`, `zTXt`, `iTXt`; GIF and JPEG comments).
    pub comments: Vec<Comment>,
}

/// How an encoder samples chroma (sharp's `chromaSubsampling`).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum ChromaSubsampling {
    /// Chroma at half resolution both ways (`'4:2:0'`): smaller files, and
    /// a loss photographs rarely show.
    Yuv420,
    /// Chroma at full resolution (`'4:4:4'`): edges between colours, as in
    /// screenshots and text, stay sharp.
    Yuv444,
}

/// Encoder tuning shared across formats.
///
/// Fields that a format has no notion of are ignored by that format rather
/// than rejected, so the same options value can be handed to any encoder.
/// Every field's default leaves a format's output as it was before the field
/// existed.
#[derive(Debug, Clone, Copy, PartialEq)]
#[non_exhaustive]
pub struct EncodeOptions {
    /// Lossy quality, 1–100. Ignored by lossless formats, and by a format
    /// with both modes when `lossless` is set.
    pub quality: u8,
    /// For a format that has both a lossy and a lossless mode (WebP), choose
    /// lossless. Off by default: such a format encodes lossy at `quality`,
    /// as `cwebp` and sharp do. Formats with one mode ignore it.
    pub lossless: bool,
    /// CPU spent for a smaller file, 0 (fastest) to 10, in each format's
    /// own range as sharp defines it: WebP 0–6 and AVIF 0–9, both with a
    /// default of 4. `None` is the format's default.
    pub effort: Option<u8>,
    /// PNG's DEFLATE level, 0 (store) to 9. `None` derives it from
    /// `quality`, as before this option existed.
    pub compression_level: Option<u8>,
    /// Write PNG as a palette image of at most `colours` entries (sharp's
    /// `palette`). Setting `colours` or `dither` implies it, as in sharp.
    pub palette: bool,
    /// The palette size for palette PNG and GIF, 2–256. `None` is 256 for
    /// PNG, and for GIF the size `quality` implies.
    pub colours: Option<u16>,
    /// How much of the quantization error a palette image diffuses, 0 (none)
    /// to 1 (full Floyd-Steinberg, the default). `None` is 1.
    pub dither: Option<f32>,
    /// Quality of lossy WebP's alpha channel, 0–100; 100, the default, keeps
    /// it lossless.
    pub alpha_quality: u8,
    /// Lossless WebP that may change pixels a little where it saves bytes,
    /// by as much as `quality` allows (sharp's `nearLossless`; 100 changes
    /// nothing). Implies lossless.
    pub near_lossless: bool,
    /// Chroma sampling for JPEG and AVIF. `None` is each format's default:
    /// for JPEG 4:2:0 below quality 90 and 4:4:4 from it, for AVIF 4:2:0.
    pub chroma_subsampling: Option<ChromaSubsampling>,
    /// Copy the source's EXIF block into the output (sharp's `keepExif`),
    /// where the format has a place for one: JPEG, PNG, WebP and AVIF. Off
    /// by default, which strips camera data and location from uploads.
    pub keep_exif: bool,
    /// Copy the source's XMP packet into the output (sharp's `keepXmp`),
    /// where the format has a place for one: JPEG, PNG, WebP, TIFF, AVIF
    /// and JPEG XL. Off by default, as for EXIF.
    pub keep_xmp: bool,
    /// Write JPEG as progressive (sharp's `progressive`): a coarse picture
    /// first, sharpened by later scans, and usually a little smaller. The
    /// encoder then holds the image's coefficients rather than streaming.
    pub progressive: bool,
}

/// Check `value` lies in `range`, naming `name` if not.
fn within(name: &'static str, value: u16, range: core::ops::RangeInclusive<u16>) -> Result<()> {
    if range.contains(&value) {
        Ok(())
    } else {
        Err(PixelsError::invalid_argument(
            name,
            format!(
                "must be in {}..={}, got {value}",
                range.start(),
                range.end()
            ),
        ))
    }
}

impl EncodeOptions {
    /// The default quality used when none is given.
    pub const DEFAULT_QUALITY: u8 = 80;

    /// Options with the given quality.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] unless `quality` is in 1..=100.
    pub fn with_quality(quality: u8) -> Result<Self> {
        within("quality", quality.into(), 1..=100)?;
        Ok(Self {
            quality,
            ..Self::default()
        })
    }

    /// These options with `lossless` replaced.
    #[must_use]
    pub const fn with_lossless(mut self, lossless: bool) -> Self {
        self.lossless = lossless;
        self
    }

    /// These options with an encoding effort, 0..=10 (see
    /// [`EncodeOptions::effort`]). A value past a format's own range is
    /// clamped to it.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] above 10.
    pub fn with_effort(mut self, effort: u8) -> Result<Self> {
        within("effort", effort.into(), 0..=10)?;
        self.effort = Some(effort);
        Ok(self)
    }

    /// These options with a PNG DEFLATE level, 0..=9.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] above 9.
    pub fn with_compression_level(mut self, level: u8) -> Result<Self> {
        within("compression_level", level.into(), 0..=9)?;
        self.compression_level = Some(level);
        Ok(self)
    }

    /// These options with palette PNG on or off.
    #[must_use]
    pub const fn with_palette(mut self, palette: bool) -> Self {
        self.palette = palette;
        self
    }

    /// These options with a palette size, 2..=256, which for PNG implies a
    /// palette.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] outside 2..=256.
    pub fn with_colours(mut self, colours: u16) -> Result<Self> {
        within("colours", colours, 2..=256)?;
        self.colours = Some(colours);
        Ok(self)
    }

    /// These options with a dithering level, 0..=1, which for PNG implies a
    /// palette.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] outside 0..=1.
    pub fn with_dither(mut self, dither: f32) -> Result<Self> {
        if !(0.0..=1.0).contains(&dither) {
            return Err(PixelsError::invalid_argument(
                "dither",
                format!("must be in 0..=1, got {dither}"),
            ));
        }
        self.dither = Some(dither);
        Ok(self)
    }

    /// These options with a lossy WebP alpha quality, 0..=100.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] above 100.
    pub fn with_alpha_quality(mut self, quality: u8) -> Result<Self> {
        within("alpha_quality", quality.into(), 0..=100)?;
        self.alpha_quality = quality;
        Ok(self)
    }

    /// These options with near-lossless WebP on or off.
    #[must_use]
    pub const fn with_near_lossless(mut self, near_lossless: bool) -> Self {
        self.near_lossless = near_lossless;
        self
    }

    /// These options with an explicit chroma subsampling.
    #[must_use]
    pub const fn with_chroma_subsampling(mut self, subsampling: ChromaSubsampling) -> Self {
        self.chroma_subsampling = Some(subsampling);
        self
    }

    /// These options keeping the source's EXIF, or not.
    #[must_use]
    pub const fn with_keep_exif(mut self, keep: bool) -> Self {
        self.keep_exif = keep;
        self
    }

    /// These options with `keep_xmp` replaced.
    #[must_use]
    pub const fn with_keep_xmp(mut self, keep: bool) -> Self {
        self.keep_xmp = keep;
        self
    }

    /// These options keeping EXIF and XMP both (sharp's `keepMetadata`;
    /// the ICC profile is always kept).
    #[must_use]
    pub const fn with_keep_metadata(mut self, keep: bool) -> Self {
        self.keep_exif = keep;
        self.keep_xmp = keep;
        self
    }

    /// These options with progressive JPEG on or off.
    #[must_use]
    pub const fn with_progressive(mut self, progressive: bool) -> Self {
        self.progressive = progressive;
        self
    }

    /// Whether a PNG should be written as a palette image.
    #[must_use]
    pub const fn wants_palette(&self) -> bool {
        self.palette || self.colours.is_some() || self.dither.is_some()
    }
}

impl Default for EncodeOptions {
    fn default() -> Self {
        Self {
            quality: Self::DEFAULT_QUALITY,
            lossless: false,
            effort: None,
            compression_level: None,
            palette: false,
            colours: None,
            dither: None,
            alpha_quality: 100,
            near_lossless: false,
            chroma_subsampling: None,
            keep_exif: false,
            keep_xmp: false,
            progressive: false,
        }
    }
}

/// Format sniffing.
///
/// Detection is by **magic bytes only** — extensions and MIME types are
/// ignored (SPEC §Formats), because they are attacker-controlled hints rather
/// than facts about the bytes.
pub trait Codec {
    /// The format this codec handles.
    fn format(&self) -> Format;

    /// The number of leading bytes [`Codec::probe`] needs to decide.
    fn magic_len(&self) -> usize;

    /// Whether `prefix` looks like this format.
    ///
    /// `prefix` may be shorter than [`Codec::magic_len`] if the source ended
    /// early; an implementation must then return `false`, never index blindly.
    fn probe(&self, prefix: &[u8]) -> bool;
}

/// Decodes a byte stream into rows of pixels.
///
/// Implementations must return [`PixelsError::Malformed`] for **every** input
/// they cannot parse. Panicking on hostile bytes is a defect, not a caller
/// error (ARCHITECTURE §Failure model); every parser is fuzzed in CI from the
/// first codec onward.
///
/// Dimension limits are enforced at header parse, before any pixel buffer is
/// allocated, so a header claiming enormous dimensions costs nothing.
pub trait Decoder: Send + fmt::Debug {
    /// The shape of the image, known after the header is parsed.
    fn descriptor(&self) -> ImageDescriptor;

    /// What this decoder can produce without a full decode.
    fn capability(&self) -> DecodeCapability {
        DecodeCapability::Sequential
    }

    /// How the stored image must be turned to display upright, as its
    /// metadata declares.
    ///
    /// Reported, never applied: rows come out in stored order whatever this
    /// says. Applying it is the pipeline's `auto_orient` decision (SPEC
    /// §Safety and limits), which a decoder that rotated its own output would
    /// take away from the caller. Known after the header is parsed, like
    /// [`Decoder::descriptor`].
    fn orientation(&self) -> Orientation {
        Orientation::Normal
    }

    /// The source's animation, if it has more than one frame.
    ///
    /// Reported, like [`Decoder::orientation`]: rows are always the first
    /// frame's. Known once the decoder is constructed; a format that must
    /// read further to count its frames does so up front.
    fn animation(&self) -> Option<Animation> {
        None
    }

    /// How many pages the source document holds, at least 1.
    ///
    /// A page is a separate image in one file, as in a multi-page TIFF: a
    /// stack of scans or faxes, not the frames of an animation, which
    /// [`Decoder::animation`] reports. Rows are always the page the decoder
    /// was opened on. Known once the decoder is constructed, without decoding
    /// any page.
    fn pages(&self) -> u32 {
        1
    }

    /// The embedded ICC colour profile, if the file carries one.
    ///
    /// Reported, never applied, like [`Decoder::orientation`]: samples come
    /// out as stored, and converting them is the pipeline's decision. Known
    /// after the header is parsed; a profile stored after the image data is
    /// not seen.
    fn icc_profile(&self) -> Option<&[u8]> {
        None
    }

    /// The embedded EXIF block, if the file carries one: the TIFF-structured
    /// bytes from the byte-order mark (`II` or `MM`), without the `Exif\0\0`
    /// identifier a JPEG wraps it in.
    ///
    /// Reported for an encoder to copy, like [`Decoder::icc_profile`]; its
    /// orientation is what [`Decoder::orientation`] reports.
    fn exif(&self) -> Option<&[u8]> {
        None
    }

    /// The embedded XMP packet, if the file carries one: the XML as stored,
    /// from `<?xpacket` or `<x:xmpmeta`.
    ///
    /// Reported for an encoder to copy, like [`Decoder::exif`].
    fn xmp(&self) -> Option<&[u8]> {
        None
    }

    /// What the header said beyond the pixel layout. Known after the header
    /// is parsed, like [`Decoder::descriptor`].
    fn source_info(&self) -> SourceInfo {
        SourceInfo::default()
    }

    /// Decode the next row, top to bottom, into `out`.
    ///
    /// `out` is exactly [`ImageDescriptor::row_bytes`] long. Each call advances
    /// the cursor by one row; the caller reads `descriptor().height` rows.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::Malformed`] on invalid or truncated input,
    /// [`PixelsError::Io`] on source failure, or
    /// [`PixelsError::InvalidArgument`] if `out` is the wrong length or every
    /// row has already been read.
    fn read_row(&mut self, out: &mut [u8]) -> Result<()>;

    /// Decode an arbitrary region into `out`.
    ///
    /// Only meaningful when [`Decoder::capability`] is
    /// [`DecodeCapability::Regions`]; the default implementation reports that
    /// this decoder is sequential.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::Unsupported`] for sequential decoders, and
    /// otherwise as [`Decoder::read_row`].
    fn read_region(&mut self, region: Region, out: &mut TileMut<'_>) -> Result<()> {
        let _ = (region, out);
        Err(PixelsError::unsupported(
            "this decoder is sequential; region decode requires DecodeCapability::Regions",
        ))
    }

    /// What this decoder would produce if asked for `target` or larger, when
    /// the format lets it reach that size more cheaply than a full decode.
    ///
    /// JPEG is the motivating case — the low-frequency corner of a DCT block
    /// is a smaller version of that block, so 1/8, 1/4 and 1/2 come almost
    /// free — but nothing here is JPEG-specific: a pyramidal TIFF or a WebP
    /// with scaled decode fits the same shape.
    ///
    /// **Pure**: nothing is committed. The planner asks before it knows
    /// whether the reduction is legal for the pipeline as a whole.
    ///
    /// The returned descriptor is never smaller than `target` in either axis.
    fn reduced_descriptor(&self, target: (u32, u32)) -> Option<ImageDescriptor> {
        let _ = target;
        None
    }

    /// Commit to producing `descriptor`, which
    /// [`Decoder::reduced_descriptor`] must have returned.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::Unsupported`] if this decoder has one
    /// resolution, or [`PixelsError::InvalidArgument`] if any row has already
    /// been read — the resolution is fixed from the first row onward.
    fn reduce_to(&mut self, descriptor: ImageDescriptor) -> Result<()> {
        let _ = descriptor;
        Err(PixelsError::unsupported(
            "this decoder has only one resolution",
        ))
    }
}

/// Encodes rows of pixels into a byte stream.
///
/// The sink is passed to each call rather than held by the encoder, which
/// keeps [`Encoder`] object-safe and lets the caller retain ownership of the
/// destination. Encoders write incrementally as rows arrive; they must not
/// buffer the whole image unless the format leaves no choice (ADR-0005).
pub trait Encoder: Send {
    /// Embed `profile` as the image's ICC colour profile, or write none.
    ///
    /// Called before [`Encoder::write_header`], whose header usually holds
    /// it. The default ignores it, which is right for a format with nowhere
    /// to put one (raw pixels, GIF): the pixels are written as given either
    /// way.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] if the header has already
    /// been written.
    fn set_icc_profile(&mut self, profile: Option<&[u8]>) -> Result<()> {
        let _ = profile;
        Ok(())
    }

    /// Embed `exif` (TIFF-structured bytes, as [`Decoder::exif`] gives
    /// them) as the image's EXIF block, or write none.
    ///
    /// Called before [`Encoder::write_header`]. The default ignores it, which
    /// is right for a format with nowhere to put one.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] if the header has already
    /// been written.
    fn set_exif(&mut self, exif: Option<&[u8]>) -> Result<()> {
        let _ = exif;
        Ok(())
    }

    /// Embed `xmp`, an XMP packet as [`Decoder::xmp`] gives it, or write
    /// none.
    ///
    /// Called before [`Encoder::write_header`]. The default ignores it, which
    /// is right for a format with nowhere to put one.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] if the header has already
    /// been written.
    fn set_xmp(&mut self, xmp: Option<&[u8]>) -> Result<()> {
        let _ = xmp;
        Ok(())
    }

    /// Record `density` as the image's resolution, or write the format's
    /// default.
    ///
    /// Called before [`Encoder::write_header`]. The default ignores it, for
    /// a format with no resolution field.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::InvalidArgument`] if the header has already
    /// been written.
    fn set_density(&mut self, density: Option<Density>) -> Result<()> {
        let _ = density;
        Ok(())
    }

    /// Begin an image, writing any container header.
    ///
    /// Must be called exactly once, before any [`Encoder::write_row`].
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::Io`] on sink failure, or
    /// [`PixelsError::Unsupported`] if the encoder cannot represent `desc`
    /// (for example an alpha channel in a format without one).
    fn write_header(&mut self, desc: &ImageDescriptor, sink: &mut dyn Sink) -> Result<()>;

    /// Write the next row, top to bottom.
    ///
    /// `row` is exactly [`ImageDescriptor::row_bytes`] long.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::Io`] on sink failure, or
    /// [`PixelsError::InvalidArgument`] if `row` is the wrong length or more
    /// rows are written than the header declared.
    fn write_row(&mut self, row: &[u8], sink: &mut dyn Sink) -> Result<()>;

    /// Finish the image, writing any trailer and flushing the sink.
    ///
    /// # Errors
    ///
    /// Returns [`PixelsError::Io`] on sink failure, or
    /// [`PixelsError::Malformed`] if fewer rows were written than the header
    /// declared — a partial image is never silently emitted
    /// (ARCHITECTURE §Failure model).
    fn finish(&mut self, sink: &mut dyn Sink) -> Result<()>;
}

#[cfg(test)]
#[allow(
    clippy::unwrap_used,
    clippy::indexing_slicing,
    reason = "tests operate on known-good values and assert shapes directly"
)]
mod tests {
    use super::*;
    use crate::ErrorCode;

    #[test]
    fn quality_is_validated_at_the_boundary() {
        assert_eq!(EncodeOptions::with_quality(1).unwrap().quality, 1);
        assert_eq!(EncodeOptions::with_quality(100).unwrap().quality, 100);
        assert_eq!(
            EncodeOptions::with_quality(0).unwrap_err().code(),
            ErrorCode::InvalidArgument
        );
        assert_eq!(
            EncodeOptions::with_quality(101).unwrap_err().code(),
            ErrorCode::InvalidArgument
        );
        assert_eq!(EncodeOptions::default().quality, 80);
    }

    #[test]
    fn format_names_are_stable_and_unique() {
        let all = [
            Format::Raw,
            Format::Png,
            Format::Jpeg,
            Format::Gif,
            Format::Tiff,
            Format::WebP,
            Format::Avif,
            Format::Jxl,
        ];
        let mut seen = std::collections::HashSet::new();
        for f in all {
            assert!(seen.insert(f.as_str()), "duplicate name {f}");
        }
        assert_eq!(Format::WebP.as_str(), "webp");
    }

    #[test]
    fn metadata_mirrors_the_descriptor() {
        let desc = ImageDescriptor::new(7, 5, PixelFormat::Rgba8).unwrap();
        let meta = Metadata::new(&desc, Format::Png);
        assert_eq!((meta.width, meta.height), (7, 5));
        assert_eq!(meta.pixel, PixelFormat::Rgba8);
        assert_eq!(meta.format, Format::Png);
    }

    #[test]
    fn sequential_decoders_decline_region_decode() {
        #[derive(Debug)]
        struct Seq;
        impl Decoder for Seq {
            fn descriptor(&self) -> ImageDescriptor {
                ImageDescriptor::new(1, 1, PixelFormat::Gray8).unwrap_or(ImageDescriptor {
                    width: 1,
                    height: 1,
                    pixel: PixelFormat::Gray8,
                    color: crate::ColorModel::Srgb,
                })
            }
            fn read_row(&mut self, _: &mut [u8]) -> Result<()> {
                Ok(())
            }
        }
        let mut buf = crate::TileBuf::zeroed(Region::from_size(1, 1), PixelFormat::Gray8).unwrap();
        let mut tile = buf.as_tile_mut().unwrap();
        let err = Seq
            .read_region(Region::from_size(1, 1), &mut tile)
            .unwrap_err();
        assert_eq!(err.code(), ErrorCode::Unsupported);
        assert_eq!(Seq.capability(), DecodeCapability::Sequential);
    }
}