path: root/libpanto/src/image.zig

//! Native image processing for tool-returned attachments.
//!
//! Two responsibilities:
//!
//!   1. `detectMediaType` — identify an attachment's MIME type from its
//!      leading bytes (magic numbers), not its file extension.
//!   2. `maybeResize` — bound raster images to `max_dim` on each side so a
//!      single screenshot can't blow out the model's context. PDFs and
//!      already-small images pass through untouched.
//!
//! Raster codecs go through the vendored stb single-header trio
//! (decode -> Mitchell resize -> re-encode in the *same* codec). WEBP is
//! decode-only (jebp), so a resized WEBP is re-encoded as JPEG.

const std = @import("std");
const Allocator = std.mem.Allocator;

const c = @cImport({
    @cInclude("stb_image.h");
    @cInclude("stb_image_resize2.h");
    @cInclude("stb_image_write.h");
    @cInclude("jebp.h");
});

/// Longest side (px) allowed before we resize. pi uses 2000x2000.
pub const max_dim: u32 = 2000;
/// JPEG quality used when re-encoding (WEBP path, and JPEG inputs).
const jpeg_quality: c_int = 80;

pub const Codec = enum { png, jpeg, gif, bmp, webp, pdf };

/// Detect a supported media type from leading bytes. Returns null when the
/// bytes don't match any known signature.
pub fn detectMediaType(bytes: []const u8) ?[]const u8 {
    return switch (detectCodec(bytes) orelse return null) {
        .png => "image/png",
        .jpeg => "image/jpeg",
        .gif => "image/gif",
        .bmp => "image/bmp",
        .webp => "image/webp",
        .pdf => "application/pdf",
    };
}

/// Detect a supported codec from leading bytes (magic numbers).
pub fn detectCodec(bytes: []const u8) ?Codec {
    if (bytes.len >= 8 and std.mem.eql(u8, bytes[0..8], &.{ 0x89, 'P', 'N', 'G', 0x0D, 0x0A, 0x1A, 0x0A }))
        return .png;
    if (bytes.len >= 3 and bytes[0] == 0xFF and bytes[1] == 0xD8 and bytes[2] == 0xFF)
        return .jpeg;
    if (bytes.len >= 6 and (std.mem.eql(u8, bytes[0..6], "GIF87a") or std.mem.eql(u8, bytes[0..6], "GIF89a")))
        return .gif;
    if (bytes.len >= 2 and bytes[0] == 'B' and bytes[1] == 'M')
        return .bmp;
    // WEBP: "RIFF"????"WEBP"
    if (bytes.len >= 12 and std.mem.eql(u8, bytes[0..4], "RIFF") and std.mem.eql(u8, bytes[8..12], "WEBP"))
        return .webp;
    if (bytes.len >= 5 and std.mem.eql(u8, bytes[0..5], "%PDF-"))
        return .pdf;
    return null;
}

/// Map a media-type string back to a codec (for callers that already have
/// the MIME string). Returns null for unsupported types.
pub fn codecForMediaType(media_type: []const u8) ?Codec {
    if (std.mem.eql(u8, media_type, "image/png")) return .png;
    if (std.mem.eql(u8, media_type, "image/jpeg")) return .jpeg;
    if (std.mem.eql(u8, media_type, "image/gif")) return .gif;
    if (std.mem.eql(u8, media_type, "image/bmp")) return .bmp;
    if (std.mem.eql(u8, media_type, "image/webp")) return .webp;
    if (std.mem.eql(u8, media_type, "application/pdf")) return .pdf;
    return null;
}

/// The result of `maybeResize`. `media_type` is the MIME type of `data`
/// (may differ from the input when a WEBP was re-encoded as JPEG).
/// `data` is always an owned slice the caller must free.
pub const Processed = struct {
    media_type: []const u8, // static string, do not free
    data: []u8, // owned by `allocator`
};

/// Full attachment pipeline for a tool-returned media part: resolve the
/// media type (detecting from magic bytes when `hint` is null), then
/// resize. Returns owned raw bytes + the resolved media type.
///
/// Errors `error.UnknownMediaType` when neither the hint nor magic-byte
/// detection recognizes the bytes — the caller decides how to surface
/// that (e.g. drop the attachment, or fall back to text).
pub fn process(allocator: Allocator, bytes: []const u8, hint: ?[]const u8) !Processed {
    const media_type = blk: {
        if (hint) |h| {
            if (codecForMediaType(h) != null) break :blk h;
        }
        break :blk detectMediaType(bytes) orelse return error.UnknownMediaType;
    };
    return maybeResize(allocator, bytes, media_type);
}

/// Resize `bytes` so neither dimension exceeds `max_dim`, preserving the
/// input codec where possible. Returns an owned copy of the (possibly
/// unchanged) bytes plus the resulting media type.
///
///   - PDF or unknown: returned verbatim (a copy), media type unchanged.
///   - raster <= max_dim on both sides: returned verbatim (a copy) — we
///     skip the decode/encode round-trip to avoid quality loss + CPU.
///   - stb-supported raster larger than max_dim: decode -> resize -> same
///     codec.
///   - WEBP larger than max_dim: jebp decode -> resize -> JPEG.
pub fn maybeResize(allocator: Allocator, bytes: []const u8, media_type: []const u8) !Processed {
    const codec = codecForMediaType(media_type) orelse
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };

    if (codec == .pdf)
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };

    if (codec == .webp) return resizeWebp(allocator, bytes, media_type);
    return resizeStb(allocator, bytes, media_type, codec);
}

/// Probe a raster image's dimensions without fully decoding. Returns null
/// on failure.
fn probeDims(bytes: []const u8) ?struct { w: u32, h: u32 } {
    var w: c_int = 0;
    var h: c_int = 0;
    var comp: c_int = 0;
    const ok = c.stbi_info_from_memory(bytes.ptr, @intCast(bytes.len), &w, &h, &comp);
    if (ok == 0 or w <= 0 or h <= 0) return null;
    return .{ .w = @intCast(w), .h = @intCast(h) };
}

/// Compute target dimensions that fit within `max_dim` x `max_dim` while
/// preserving aspect ratio. Returns null when no resize is needed.
fn targetDims(w: u32, h: u32) ?struct { w: u32, h: u32 } {
    if (w <= max_dim and h <= max_dim) return null;
    const wf: f64 = @floatFromInt(w);
    const hf: f64 = @floatFromInt(h);
    const scale = @min(@as(f64, @floatFromInt(max_dim)) / wf, @as(f64, @floatFromInt(max_dim)) / hf);
    const nw: u32 = @max(1, @as(u32, @intFromFloat(@round(wf * scale))));
    const nh: u32 = @max(1, @as(u32, @intFromFloat(@round(hf * scale))));
    return .{ .w = nw, .h = nh };
}

const StbWriteCtx = struct {
    list: *std.ArrayList(u8),
    allocator: Allocator,
    failed: bool = false,
};

fn stbWriteCb(ctx_opaque: ?*anyopaque, data: ?*anyopaque, size: c_int) callconv(.c) void {
    const ctx: *StbWriteCtx = @ptrCast(@alignCast(ctx_opaque.?));
    if (ctx.failed or size <= 0) return;
    const bytes: [*]const u8 = @ptrCast(data.?);
    ctx.list.appendSlice(ctx.allocator, bytes[0..@intCast(size)]) catch {
        ctx.failed = true;
    };
}

fn resizeStb(allocator: Allocator, bytes: []const u8, media_type: []const u8, codec: Codec) !Processed {
    const dims = probeDims(bytes) orelse
        // Can't parse it; pass through rather than fail the read.
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };

    const target = targetDims(dims.w, dims.h) orelse
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };

    // Decode to RGBA (4 channels) for a uniform resize path.
    var w: c_int = 0;
    var h: c_int = 0;
    var comp: c_int = 0;
    const pixels = c.stbi_load_from_memory(bytes.ptr, @intCast(bytes.len), &w, &h, &comp, 4);
    if (pixels == null)
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };
    defer c.stbi_image_free(pixels);

    const out_pixels = try allocator.alloc(u8, @as(usize, target.w) * @as(usize, target.h) * 4);
    defer allocator.free(out_pixels);

    const res = c.stbir_resize_uint8_srgb(
        pixels,
        w,
        h,
        0,
        out_pixels.ptr,
        @intCast(target.w),
        @intCast(target.h),
        0,
        c.STBIR_RGBA,
    );
    if (res == null)
        return error.ResizeFailed;

    var out: std.ArrayList(u8) = .empty;
    errdefer out.deinit(allocator);
    var ctx = StbWriteCtx{ .list = &out, .allocator = allocator };

    const tw: c_int = @intCast(target.w);
    const th: c_int = @intCast(target.h);
    const ok = switch (codec) {
        .png => c.stbi_write_png_to_func(stbWriteCb, &ctx, tw, th, 4, out_pixels.ptr, tw * 4),
        .bmp => c.stbi_write_bmp_to_func(stbWriteCb, &ctx, tw, th, 4, out_pixels.ptr),
        // stb has no GIF encoder; re-encode resized GIFs as PNG (lossless).
        .gif => c.stbi_write_png_to_func(stbWriteCb, &ctx, tw, th, 4, out_pixels.ptr, tw * 4),
        .jpeg => c.stbi_write_jpg_to_func(stbWriteCb, &ctx, tw, th, 4, out_pixels.ptr, jpeg_quality),
        else => unreachable,
    };
    if (ok == 0 or ctx.failed) return error.EncodeFailed;

    const result_media: []const u8 = switch (codec) {
        .gif => "image/png", // re-encoded
        else => media_type,
    };
    return .{ .media_type = result_media, .data = try out.toOwnedSlice(allocator) };
}

fn resizeWebp(allocator: Allocator, bytes: []const u8, media_type: []const u8) !Processed {
    var img: c.jebp_image_t = std.mem.zeroes(c.jebp_image_t);
    // Peek at the header first to learn dimensions cheaply.
    if (c.jebp_decode_size(&img, bytes.len, bytes.ptr) != c.JEBP_OK)
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };

    const w: u32 = @intCast(img.width);
    const h: u32 = @intCast(img.height);
    const target = targetDims(w, h) orelse
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };

    // Full decode to RGBA (jebp_color_t is 4 contiguous bytes per pixel).
    if (c.jebp_decode(&img, bytes.len, bytes.ptr) != c.JEBP_OK)
        return .{ .media_type = media_type, .data = try allocator.dupe(u8, bytes) };
    defer c.jebp_free_image(&img);

    const src: [*]const u8 = @ptrCast(img.pixels);
    const out_pixels = try allocator.alloc(u8, @as(usize, target.w) * @as(usize, target.h) * 4);
    defer allocator.free(out_pixels);

    const res = c.stbir_resize_uint8_srgb(
        src,
        @intCast(w),
        @intCast(h),
        0,
        out_pixels.ptr,
        @intCast(target.w),
        @intCast(target.h),
        0,
        c.STBIR_RGBA,
    );
    if (res == null) return error.ResizeFailed;

    // TODO: when the source WEBP has an alpha layer, re-encoding to JPEG
    // flattens transparency, which can look wrong for screenshots and
    // diagrams. Consider re-encoding to PNG when alpha is present. For now
    // we always emit JPEG: there is no small single-header WEBP encoder,
    // and token size matters more than fidelity for LLM input.
    var out: std.ArrayList(u8) = .empty;
    errdefer out.deinit(allocator);
    var ctx = StbWriteCtx{ .list = &out, .allocator = allocator };
    const tw: c_int = @intCast(target.w);
    const th: c_int = @intCast(target.h);
    const ok = c.stbi_write_jpg_to_func(stbWriteCb, &ctx, tw, th, 4, out_pixels.ptr, jpeg_quality);
    if (ok == 0 or ctx.failed) return error.EncodeFailed;

    return .{ .media_type = "image/jpeg", .data = try out.toOwnedSlice(allocator) };
}

// -----------------------------------------------------------------------------
// Tests
// -----------------------------------------------------------------------------

const testing = std.testing;

test "detectMediaType - magic bytes" {
    try testing.expectEqualStrings("image/png", detectMediaType(&.{ 0x89, 'P', 'N', 'G', 0x0D, 0x0A, 0x1A, 0x0A }).?);
    try testing.expectEqualStrings("image/jpeg", detectMediaType(&.{ 0xFF, 0xD8, 0xFF, 0xE0 }).?);
    try testing.expectEqualStrings("image/gif", detectMediaType("GIF89a....").?);
    try testing.expectEqualStrings("image/bmp", detectMediaType("BM....").?);
    try testing.expectEqualStrings("application/pdf", detectMediaType("%PDF-1.7").?);
    const webp = "RIFF" ++ &[_]u8{ 0, 0, 0, 0 } ++ "WEBP";
    try testing.expectEqualStrings("image/webp", detectMediaType(webp).?);
    try testing.expect(detectMediaType("not an image") == null);
    try testing.expect(detectMediaType(&.{0x89}) == null);
}

test "targetDims - skip when small, scale when large preserving aspect" {
    try testing.expect(targetDims(100, 100) == null);
    try testing.expect(targetDims(max_dim, max_dim) == null);
    const t = targetDims(4000, 2000).?;
    try testing.expectEqual(@as(u32, 2000), t.w);
    try testing.expectEqual(@as(u32, 1000), t.h);
    const t2 = targetDims(1000, 8000).?;
    try testing.expectEqual(@as(u32, 250), t2.w);
    try testing.expectEqual(@as(u32, 2000), t2.h);
}

test "process - detects type from raw bytes when hint absent" {
    const a = testing.allocator;
    // A tiny PNG (header + IHDR enough for stbi_info) — but simplest is to
    // round-trip an stb-encoded small PNG and feed it with no hint.
    const w: c_int = 4;
    const h: c_int = 4;
    var px: [4 * 4 * 4]u8 = undefined;
    @memset(&px, 0x40);
    var png: std.ArrayList(u8) = .empty;
    defer png.deinit(a);
    var ctx = StbWriteCtx{ .list = &png, .allocator = a };
    try testing.expect(c.stbi_write_png_to_func(stbWriteCb, &ctx, w, h, 4, &px, w * 4) != 0);

    const out = try process(a, png.items, null);
    defer a.free(out.data);
    try testing.expectEqualStrings("image/png", out.media_type);

    // Unknown bytes -> error so the caller can fall back / drop.
    try testing.expectError(error.UnknownMediaType, process(a, "totally not an image", null));
}

test "maybeResize - PDF passes through unchanged" {
    const a = testing.allocator;
    const pdf = "%PDF-1.7\nfake pdf body";
    const out = try maybeResize(a, pdf, "application/pdf");
    defer a.free(out.data);
    try testing.expectEqualStrings("application/pdf", out.media_type);
    try testing.expectEqualStrings(pdf, out.data);
}

test "maybeResize - small PNG round-trips, large PNG shrinks and stays PNG" {
    const a = testing.allocator;

    // Build a small (8x8) RGBA PNG via stb and confirm pass-through.
    const small_w: c_int = 8;
    const small_h: c_int = 8;
    var small_px: [8 * 8 * 4]u8 = undefined;
    for (&small_px, 0..) |*b, i| b.* = @truncate(i);
    var small_png: std.ArrayList(u8) = .empty;
    defer small_png.deinit(a);
    var sctx = StbWriteCtx{ .list = &small_png, .allocator = a };
    try testing.expect(c.stbi_write_png_to_func(stbWriteCb, &sctx, small_w, small_h, 4, &small_px, small_w * 4) != 0);

    const small_out = try maybeResize(a, small_png.items, "image/png");
    defer a.free(small_out.data);
    try testing.expectEqualStrings("image/png", small_out.media_type);
    // Small image is returned verbatim (byte-identical copy).
    try testing.expectEqualSlices(u8, small_png.items, small_out.data);

    // Build a large (2400x100) PNG and confirm it shrinks to <= max_dim.
    const big_w: c_int = 2400;
    const big_h: c_int = 100;
    const big_px = try a.alloc(u8, @as(usize, @intCast(big_w * big_h * 4)));
    defer a.free(big_px);
    @memset(big_px, 0x7F);
    var big_png: std.ArrayList(u8) = .empty;
    defer big_png.deinit(a);
    var bctx = StbWriteCtx{ .list = &big_png, .allocator = a };
    try testing.expect(c.stbi_write_png_to_func(stbWriteCb, &bctx, big_w, big_h, 4, big_px.ptr, big_w * 4) != 0);

    const big_out = try maybeResize(a, big_png.items, "image/png");
    defer a.free(big_out.data);
    try testing.expectEqualStrings("image/png", big_out.media_type);
    const dims = probeDims(big_out.data).?;
    try testing.expectEqual(@as(u32, 2000), dims.w);
    try testing.expect(dims.w <= max_dim and dims.h <= max_dim);
}