//! Native-image backend harness.
//!
//!   zig build image-harness                  fake a Kitty terminal and check
//!                                            real transmit + placement APCs
//!   zig build image-harness -Dplatform=gui   run the real SDL GPU renderer,
//!                                            capture a PPM, inspect pixels
//!   zig build pdf-harness                    render a generated multi-page PDF
//!   zig build pdf-harness -Dplatform=gui
//!
//! Unlike the text snapshot suite, both arms observe the thing the native
//! backend actually emits. The TTY arm completes vaxis's asynchronous
//! capability handshake over a PTY; the GUI arm consumes PARDES_TEST's GPU
//! readback, so a PETSCII fallback cannot accidentally satisfy either one.
const std = @import("std");
const libc = std.c;
const eh = @import("e2e_harness.zig");

pub const std_options: std.Options = .{ .log_level = .err };

extern "c" fn setenv(name: [*:0]const u8, value: [*:0]const u8, overwrite: c_int) c_int;
extern "c" fn unsetenv(name: [*:0]const u8) c_int;

const gpa = std.heap.page_allocator;
const fixture_rgb = [3]u8{ 13, 77, 231 };
const pdf_red = [3]u8{ 255, 0, 0 };
const pdf_green = [3]u8{ 0, 255, 0 };

pub fn main(init: std.process.Init) !void {
    var arena_state: std.heap.ArenaAllocator = .init(gpa);
    defer arena_state.deinit();
    const arena = arena_state.allocator();
    const args = try init.minimal.args.toSlice(arena);
    if (args.len != 3 and args.len != 4) return error.BadArgs;

    const exe = try arena.dupeZ(u8, args[1]);
    const backend = args[2];
    const pdf_mode = args.len == 4 and std.mem.eql(u8, args[3], "pdf");
    if (args.len == 4 and !pdf_mode) return error.BadArgs;
    const base = try std.fmt.allocPrintSentinel(arena, "/tmp/pardes-image-harness-{d}", .{libc.getpid()}, 0);
    if (libc.mkdir(base, 0o755) != 0 and libc.errno(-1) != .EXIST) return error.MkdirFailed;
    defer cleanupBase(arena, base);
    _ = setenv("HOME", base, 1);
    // Do not let an exported developer XDG_CONFIG_HOME style or mutate the
    // native process under test; this nonexistent directory is test-owned.
    const config_home = try std.fmt.allocPrintSentinel(arena, "{s}/.config", .{base}, 0);
    _ = setenv("XDG_CONFIG_HOME", config_home, 1);
    _ = setenv("HISTFILE", "/dev/null", 1);
    _ = setenv("TERM", "xterm-256color", 1);
    _ = setenv("LC_ALL", "C", 1);

    if (std.mem.eql(u8, backend, "tty")) {
        if (pdf_mode)
            try runKittyPdf(arena, exe, base)
        else
            try runKitty(arena, exe, base);
    } else if (std.mem.eql(u8, backend, "gui")) {
        if (pdf_mode)
            try runGuiPdf(arena, exe, base)
        else
            try runGui(arena, exe, base);
    } else return error.BadArgs;
}

fn cleanupBase(arena: std.mem.Allocator, base: [:0]const u8) void {
    const names = [_][]const u8{ "native.ppm", "pages.pdf", "latest.ppm", "latest.ppm.tmp", ".bash_history" };
    for (names) |name| {
        const path = std.fmt.allocPrintSentinel(arena, "{s}/{s}", .{ base, name }, 0) catch continue;
        _ = libc.unlink(path);
    }
    _ = libc.rmdir(base);
}

fn appendFmt(arena: std.mem.Allocator, out: *std.ArrayList(u8), comptime fmt: []const u8, args: anytype) !void {
    try out.appendSlice(arena, try std.fmt.allocPrint(arena, fmt, args));
}

/// A self-contained, valid PDF fixture. The first two pages retain the red /
/// green and search-text contract exercised below; two differently-sized
/// trailing pages make skipped native renders visible in Kitty's transmit
/// headers. Building it here keeps the backend test independent of mutool,
/// fonts, and checked-in binary fixtures while the xref offsets remain exact.
fn writePdf(arena: std.mem.Allocator, path: [:0]const u8) !void {
    const first_stream =
        \\1 0 0 rg
        \\0 0 72 48 re
        \\f
        \\0 0 0 rg
        \\BT
        \\/F1 10 Tf
        \\4 20 Td
        \\(FIRST NEEDLE) Tj
        \\ET
    ;
    const second_stream =
        \\0 1 0 rg
        \\0 0 72 48 re
        \\f
        \\0 0 0 rg
        \\BT
        \\/F1 10 Tf
        \\4 20 Td
        \\(SECOND TARGET) Tj
        \\ET
    ;
    const third_stream =
        \\0 0 1 rg
        \\0 0 72 50 re
        \\f
        \\0 0 0 rg
        \\BT
        \\/F1 10 Tf
        \\4 20 Td
        \\(THIRD PAGE) Tj
        \\ET
    ;
    const fourth_stream =
        \\1 1 0 rg
        \\0 0 72 52 re
        \\f
        \\0 0 0 rg
        \\BT
        \\/F1 10 Tf
        \\4 20 Td
        \\(FOURTH PAGE) Tj
        \\ET
    ;

    var out: std.ArrayList(u8) = .empty;
    var offsets: [12]usize = @splat(0);
    try out.appendSlice(arena, "%PDF-1.4\n%\xE2\xE3\xCF\xD3\n");

    offsets[1] = out.items.len;
    try out.appendSlice(arena, "1 0 obj\n<< /Type /Catalog /Pages 2 0 R >>\nendobj\n");
    offsets[2] = out.items.len;
    try out.appendSlice(arena, "2 0 obj\n<< /Type /Pages /Count 4 /Kids [3 0 R 5 0 R 7 0 R 9 0 R] >>\nendobj\n");
    offsets[3] = out.items.len;
    try out.appendSlice(arena, "3 0 obj\n<< /Type /Page /Parent 2 0 R /MediaBox [0 0 72 48] /Resources << /Font << /F1 11 0 R >> >> /Contents 4 0 R >>\nendobj\n");
    offsets[4] = out.items.len;
    try appendFmt(arena, &out, "4 0 obj\n<< /Length {d} >>\nstream\n{s}endstream\nendobj\n", .{ first_stream.len, first_stream });
    offsets[5] = out.items.len;
    try out.appendSlice(arena, "5 0 obj\n<< /Type /Page /Parent 2 0 R /MediaBox [0 0 72 48] /Resources << /Font << /F1 11 0 R >> >> /Contents 6 0 R >>\nendobj\n");
    offsets[6] = out.items.len;
    try appendFmt(arena, &out, "6 0 obj\n<< /Length {d} >>\nstream\n{s}endstream\nendobj\n", .{ second_stream.len, second_stream });
    offsets[7] = out.items.len;
    try out.appendSlice(arena, "7 0 obj\n<< /Type /Page /Parent 2 0 R /MediaBox [0 0 72 50] /Resources << /Font << /F1 11 0 R >> >> /Contents 8 0 R >>\nendobj\n");
    offsets[8] = out.items.len;
    try appendFmt(arena, &out, "8 0 obj\n<< /Length {d} >>\nstream\n{s}endstream\nendobj\n", .{ third_stream.len, third_stream });
    offsets[9] = out.items.len;
    try out.appendSlice(arena, "9 0 obj\n<< /Type /Page /Parent 2 0 R /MediaBox [0 0 72 52] /Resources << /Font << /F1 11 0 R >> >> /Contents 10 0 R >>\nendobj\n");
    offsets[10] = out.items.len;
    try appendFmt(arena, &out, "10 0 obj\n<< /Length {d} >>\nstream\n{s}endstream\nendobj\n", .{ fourth_stream.len, fourth_stream });
    offsets[11] = out.items.len;
    try out.appendSlice(arena, "11 0 obj\n<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>\nendobj\n");

    const xref = out.items.len;
    try out.appendSlice(arena, "xref\n0 12\n0000000000 65535 f \n");
    for (offsets[1..]) |offset|
        try appendFmt(arena, &out, "{d:0>10} 00000 n \n", .{offset});
    try appendFmt(arena, &out, "trailer\n<< /Size 12 /Root 1 0 R >>\nstartxref\n{d}\n%%EOF\n", .{xref});
    try eh.writeFile(path, out.items);
}

fn writePpm(arena: std.mem.Allocator, path: [:0]const u8, width: usize, height: usize) !void {
    var out: std.ArrayList(u8) = .empty;
    try out.appendSlice(arena, try std.fmt.allocPrint(arena, "P6\n{d} {d}\n255\n", .{ width, height }));
    for (0..width * height) |_| try out.appendSlice(arena, &fixture_rgb);
    try eh.writeFile(path, out.items);
}

fn runKitty(arena: std.mem.Allocator, exe: [:0]const u8, base: [:0]const u8) !void {
    _ = unsetenv("PARDES_TEST");
    _ = unsetenv("PARDES_TEST_CAPTURE_DIR");
    const image_path = try std.fmt.allocPrintSentinel(arena, "{s}/native.ppm", .{base}, 0);
    try writePpm(arena, image_path, 2, 2);

    var h = try eh.Harness.initArgs(gpa, exe, 12, 30, image_path);
    defer h.deinit();
    try h.pump(150);
    try h.expectRawContains("\x1b_Gi=1,a=q\x1b\\", "app did not issue the Kitty graphics capability query");

    // APC says graphics is present; DA1 is the last capability response and
    // closes the asynchronous handshake. Arrow-up is an ordinary queued event
    // that wakes the main loop even on a terminal which supplied no in-band
    // resize report, matching the documented idle-handshake edge case.
    try h.send("\x1b_Gi=1;OK\x1b\\\x1b[?1;2c\x1b[A");
    const deadline = eh.nowMs() + 3000;
    while (eh.nowMs() < deadline) {
        _ = try h.pumpOnce(50);
        const sent = std.mem.indexOf(u8, h.raw.items, "\x1b_Gf=32,s=2,v=2,i=") != null;
        const placed = std.mem.indexOf(u8, h.raw.items, "\x1b_Ga=p,i=") != null;
        if (sent and placed) break;
    }
    try h.expectRawContains("\x1b_Gf=32,s=2,v=2,i=", "decoded RGBA was not transmitted to the Kitty terminal");
    try h.expectRawContains("\x1b_Ga=p,i=", "transmitted Kitty image was not placed in the pane body");

    // SPC t p toggles the attachment off and back on. The first transition
    // must release terminal-side storage; the second must transmit again
    // instead of resurrecting a stale pane-slot handle.
    try h.send(" tp");
    const delete_deadline = eh.nowMs() + 2000;
    while (eh.nowMs() < delete_deadline and std.mem.indexOf(u8, h.raw.items, "\x1b_Ga=d,d=I,i=") == null)
        _ = try h.pumpOnce(50);
    try h.expectRawContains("\x1b_Ga=d,d=I,i=", "PETSCII toggle did not release the cached Kitty image");
    try h.send(" tp");
    const retransmit_deadline = eh.nowMs() + 2000;
    while (eh.nowMs() < retransmit_deadline and std.mem.count(u8, h.raw.items, "\x1b_Gf=32,s=2,v=2,i=") < 2)
        _ = try h.pumpOnce(50);
    if (std.mem.count(u8, h.raw.items, "\x1b_Gf=32,s=2,v=2,i=") < 2)
        return error.KittyRetransmitMissing;
    std.debug.print("native image harness ok: kitty transmit + placement\n", .{});
}

fn runKittyPdf(arena: std.mem.Allocator, exe: [:0]const u8, base: [:0]const u8) !void {
    _ = unsetenv("PARDES_TEST");
    _ = unsetenv("PARDES_TEST_CAPTURE_DIR");
    const pdf_path = try std.fmt.allocPrintSentinel(arena, "{s}/pages.pdf", .{base}, 0);
    try writePdf(arena, pdf_path);

    var h = try eh.Harness.initArgs(gpa, exe, 16, 80, pdf_path);
    defer h.deinit();
    try h.pump(150);
    try h.expectRawContains("\x1b_Gi=1,a=q\x1b\\", "app did not issue the Kitty graphics capability query");

    try h.send("\x1b_Gi=1;OK\x1b\\\x1b[?1;2c\x1b[A");
    const transmit = "\x1b_Gf=32,s=96,v=64,i=";
    const first_deadline = eh.nowMs() + 5000;
    while (eh.nowMs() < first_deadline) {
        _ = try h.pumpOnce(50);
        const sent = std.mem.indexOf(u8, h.raw.items, transmit) != null;
        const placed = std.mem.indexOf(u8, h.raw.items, "\x1b_Ga=p,i=") != null;
        if (sent and placed and h.contains("pdf 1/4 width PdfFit")) break;
    }
    try h.expectRawContains(transmit, "first PDF page was not transmitted as the 96x64 Kitty raster");
    try h.expectRawContains("\x1b_Ga=p,i=", "first PDF page was not placed in the pane body");
    try h.expectContains("pdf 1/4 width PdfFit", "PDF pane did not expose its initial width fit");

    // Fit-width is the initial PDF policy. The short fixture pages make the
    // first boundary visible immediately, so both page rasters must coexist.
    // Page one fills its own document box instead of being letterboxed inside
    // the whole pane viewport.
    const initial_transmits = std.mem.count(u8, h.raw.items, transmit);
    if (initial_transmits < 2) return error.KittyPdfBoundaryPageMissing;
    const width_apc = kittyPlacementApc(h.raw.items, 0) orelse return error.KittyPdfWidthPlacementMissing;
    const width_clip_h = kittyParam(width_apc, 'h') orelse return error.KittyPdfWidthCropWrong;
    if (kittyParam(width_apc, 'x') != 0 or kittyParam(width_apc, 'y') != 0 or
        kittyParam(width_apc, 'w') != 96 or width_clip_h == 0 or width_clip_h > 64 or
        kittyParam(width_apc, 'c') == null)
    {
        std.debug.print("unexpected Kitty fit-width placement: {s}\n", .{width_apc});
        return error.KittyPdfWidthCropWrong;
    }
    const initial_aspect_h = (38 * 8 * width_clip_h + 95) / 96;
    if (((initial_aspect_h + 15) / 16) * 16 > 202)
        return error.KittyPdfWidthPlacementOverdraw;

    // A one-row document scroll changes placements, not pixel generations or
    // the active page. This is the cache-reuse seam the old page-at-a-time
    // harness could not exercise.
    const placements_before = std.mem.count(u8, h.raw.items, "\x1b_Ga=p,i=");
    const placement_start = h.raw.items.len;
    try h.send("j");
    const placement_deadline = eh.nowMs() + 2000;
    while (eh.nowMs() < placement_deadline and
        std.mem.count(u8, h.raw.items, "\x1b_Ga=p,i=") <= placements_before)
        _ = try h.pumpOnce(50);
    if (std.mem.count(u8, h.raw.items, transmit) != initial_transmits)
        return error.KittyPdfPlacementScrollRetransmitted;
    try h.expectContains("pdf 1/4 width PdfFit", "one-row scroll snapped to another PDF page");

    // The newly exposed page-two head is 29px tall and begins three pixels
    // into a cell. Kitty infers an integer row count when only c is supplied;
    // the crop must therefore stay within one 16px row, not ceil into the gap.
    const tail_deadline = eh.nowMs() + 2000;
    while (eh.nowMs() < tail_deadline and
        kittyPlacementApcWithParam(h.raw.items, placement_start, 'Y') == null)
        _ = try h.pumpOnce(50);
    const tail_apc = kittyPlacementApcWithParam(h.raw.items, placement_start, 'Y') orelse
        return error.KittyPdfSafeTailPlacementMissing;
    const tail_cols = kittyParam(tail_apc, 'c') orelse return error.KittyPdfSafeTailPlacementWrong;
    const tail_w = kittyParam(tail_apc, 'w') orelse return error.KittyPdfSafeTailPlacementWrong;
    const tail_h = kittyParam(tail_apc, 'h') orelse return error.KittyPdfSafeTailPlacementWrong;
    if (kittyParam(tail_apc, 'Y') != 3 or tail_cols != 38 or tail_w != 96)
        return error.KittyPdfSafeTailPlacementWrong;
    const tail_aspect_h = (tail_cols * 8 * tail_h + tail_w - 1) / tail_w;
    const tail_declared_h = ((tail_aspect_h + 15) / 16) * 16;
    if (tail_declared_h > 29) return error.KittyPdfTailOverpaintedGap;

    // A viewport step crosses the boundary continuously: page one leaves and
    // is deleted, page two remains cached, and newly intersecting page three
    // is transmitted. Distinct fixture heights identify the latter on wire.
    const page_step_start = h.raw.items.len;
    const page_three_transmit = "\x1b_Gf=32,s=96,v=67,i=";
    try h.send("\x06");
    try h.expectWaitFor("pdf 2/4 width PdfFit", 5000, "continuous page-down did not enter PDF page two");
    const page_step_deadline = eh.nowMs() + 5000;
    while (eh.nowMs() < page_step_deadline and
        std.mem.indexOf(u8, h.raw.items[page_step_start..], page_three_transmit) == null)
        _ = try h.pumpOnce(50);
    if (std.mem.indexOf(u8, h.raw.items[page_step_start..], "\x1b_Ga=d,d=I,i=") == null)
        return error.KittyPdfOffscreenPageNotReleased;
    if (std.mem.indexOf(u8, h.raw.items[page_step_start..], page_three_transmit) == null)
        return error.KittyPdfNewVisiblePageMissing;
    if (std.mem.count(u8, h.raw.items, transmit) != initial_transmits)
        return error.KittyPdfRetainedNeighborRetransmitted;

    // Search still addresses semantic pages inside the strip. Returning one
    // viewport, then revealing page two's hit, creates a decorated revision.
    try h.send("\x02");
    try h.expectWaitFor("pdf 1/4 width PdfFit", 5000, "continuous page-up did not return to PDF page one");
    try h.send("/SECOND\r");
    try h.expectWaitFor("pages.pdf:2:1 SECOND", 5000, "PDF search did not preserve the exact page-two query occurrence");
    const before_look = std.mem.count(u8, h.raw.items, transmit);
    try h.send("n");
    const look_deadline = eh.nowMs() + 5000;
    while (eh.nowMs() < look_deadline) {
        _ = try h.pumpOnce(50);
        if (std.mem.count(u8, h.raw.items, transmit) > before_look and h.contains("pdf 2/4 width PdfFit")) break;
    }
    try h.expectContains("pdf 2/4 width PdfFit", "n did not reveal the PDF search result at page two");
    if (std.mem.count(u8, h.raw.items, transmit) <= before_look) return error.KittyPdfLookRetransmitMissing;

    // PdfFit changes only placement. The active decorated raster remains the
    // same terminal image while height fit supplies an explicit row extent.
    const before_fit = std.mem.count(u8, h.raw.items, transmit);
    const fit_raw_start = h.raw.items.len;
    try h.send(" tz");
    try h.expectWaitFor("pdf 2/4 height PdfFit", 5000, "PdfFit did not expose height fit");
    const fit_deadline = eh.nowMs() + 3000;
    while (eh.nowMs() < fit_deadline and
        kittyPlacementApcWithParam(h.raw.items, fit_raw_start, 'r') == null)
        _ = try h.pumpOnce(50);
    const height_apc = kittyPlacementApcWithParam(h.raw.items, fit_raw_start, 'r') orelse
        return error.KittyPdfHeightPlacementMissing;
    const height_crop_w = kittyParam(height_apc, 'w') orelse return error.KittyPdfHeightPlacementWrong;
    if (height_crop_w == 0 or height_crop_w > 96 or kittyParam(height_apc, 'h') != 64)
        return error.KittyPdfHeightPlacementWrong;
    if (std.mem.count(u8, h.raw.items, transmit) != before_fit)
        return error.KittyPdfFitRetransmitted;

    std.debug.print("PDF harness ok: Kitty continuous boundary + exact cache + search/fit\n", .{});
}

fn kittyPlacementApc(raw: []const u8, start: usize) ?[]const u8 {
    if (start > raw.len) return null;
    const relative = std.mem.indexOf(u8, raw[start..], "\x1b_Ga=p,i=") orelse return null;
    const begin = start + relative;
    const end = std.mem.indexOf(u8, raw[begin..], "\x1b\\") orelse return null;
    return raw[begin .. begin + end + 2];
}

fn kittyPlacementApcWithParam(raw: []const u8, start: usize, key: u8) ?[]const u8 {
    var at = start;
    while (kittyPlacementApc(raw, at)) |apc| {
        if (kittyParam(apc, key) != null) return apc;
        const next = @intFromPtr(apc.ptr) - @intFromPtr(raw.ptr) + apc.len;
        if (next <= at or next > raw.len) return null;
        at = next;
    }
    return null;
}

fn kittyParam(apc: []const u8, key: u8) ?u32 {
    var i: usize = 0;
    while (i + 3 <= apc.len) : (i += 1) {
        if (apc[i] != ',' or apc[i + 1] != key or apc[i + 2] != '=') continue;
        const begin = i + 3;
        var end = begin;
        while (end < apc.len and std.ascii.isDigit(apc[end])) : (end += 1) {}
        if (end == begin) return null;
        return std.fmt.parseUnsigned(u32, apc[begin..end], 10) catch null;
    }
    return null;
}

fn runGui(arena: std.mem.Allocator, exe: [:0]const u8, base: [:0]const u8) !void {
    const image_path = try std.fmt.allocPrintSentinel(arena, "{s}/native.ppm", .{base}, 0);
    const capture_path = try std.fmt.allocPrintSentinel(arena, "{s}/latest.ppm", .{base}, 0);
    try writePpm(arena, image_path, 32, 16);
    _ = libc.unlink(capture_path);
    _ = setenv("PARDES_TEST", "1", 1);
    _ = setenv("PARDES_TEST_CAPTURE_DIR", base, 1);
    _ = setenv("PARDES_TEST_COLS", "20", 1);
    _ = setenv("PARDES_TEST_ROWS", "10", 1);

    var h = try eh.Harness.initArgs(gpa, exe, 10, 20, image_path);
    defer h.deinit();
    try h.pump(1200);

    const capture = readFile(gpa, capture_path) catch |err| {
        std.debug.print("SDL image harness produced no capture ({t}); raw child output: {s}\n", .{ err, h.raw.items });
        return err;
    };
    defer gpa.free(capture);
    const pixels = ppmPixels(capture) orelse return error.BadCapture;
    var exact: usize = 0;
    var i: usize = 0;
    while (i + 2 < pixels.len) : (i += 3) {
        if (std.mem.eql(u8, pixels[i .. i + 3], &fixture_rgb)) exact += 1;
    }
    // The source contributes 512 aligned pixels. Leave ample room for driver
    // filtering while still excluding glyph-art fallback and incidental UI
    // colors.
    if (exact < 200) {
        std.debug.print("SDL image harness found only {d} native-color pixels in the GPU capture\n", .{exact});
        return error.NativePixelsMissing;
    }
    std.debug.print("native image harness ok: SDL GPU capture ({d} source pixels)\n", .{exact});
}

fn runGuiPdf(arena: std.mem.Allocator, exe: [:0]const u8, base: [:0]const u8) !void {
    const pdf_path = try std.fmt.allocPrintSentinel(arena, "{s}/pages.pdf", .{base}, 0);
    const capture_path = try std.fmt.allocPrintSentinel(arena, "{s}/latest.ppm", .{base}, 0);
    try writePdf(arena, pdf_path);
    _ = libc.unlink(capture_path);
    _ = setenv("PARDES_TEST", "1", 1);
    _ = setenv("PARDES_TEST_CAPTURE_DIR", base, 1);
    _ = setenv("PARDES_TEST_COLS", "20", 1);
    _ = setenv("PARDES_TEST_ROWS", "10", 1);

    var h = try eh.Harness.initArgs(gpa, exe, 10, 20, pdf_path);
    defer h.deinit();
    const fit_width = try waitForCaptureBounds(&h, capture_path, pdf_red, 500, 5000);
    const initial_green = try waitForCaptureBounds(&h, capture_path, pdf_green, 500, 5000);
    try expectThemeGap(capture_path, fit_width, initial_green);

    // One row moves both native quads by the exact document distance. They
    // remain together on screen, separated by the same theme-colored gap; a
    // page-at-a-time implementation would replace red with green here.
    _ = libc.unlink(capture_path);
    try h.send("j");
    const moved_red = try waitForCaptureBounds(&h, capture_path, pdf_red, 500, 5000);
    const moved_green = try waitForCaptureBounds(&h, capture_path, pdf_green, 500, 5000);
    if (moved_red.max_y >= fit_width.max_y or moved_green.min_y >= initial_green.min_y)
        return error.SdlPdfContinuousStepDidNotMove;
    try expectThemeGap(capture_path, moved_red, moved_green);

    // Fit-height fills/crops the full body height. These are GPU readback
    // extents, so this exercises SDL's UV crop and destination rectangle
    // rather than merely observing the page color.
    _ = libc.unlink(capture_path);
    try h.send(" tz");
    const fit_height = try waitForCaptureBounds(&h, capture_path, pdf_red, 500, 5000);
    const width_delta = if (fit_width.width() > fit_height.width())
        fit_width.width() - fit_height.width()
    else
        fit_height.width() - fit_width.width();
    if (fit_height.height() < fit_width.height() * 2 or width_delta > 8) {
        std.debug.print("SDL PDF fit extents did not change as expected: width={any}, height={any}\n", .{ fit_width, fit_height });
        return error.SdlPdfFitGeometryWrong;
    }

    // In height fit one row exposes page two at the bottom but keeps page one
    // active. Enough additional rows then move page one wholly offscreen and
    // leave page two at the top, proving continuous traversal in GPU output.
    _ = libc.unlink(capture_path);
    try h.send("j");
    const height_red = try waitForCaptureBounds(&h, capture_path, pdf_red, 500, 5000);
    const height_green = try waitForCaptureBounds(&h, capture_path, pdf_green, 200, 5000);
    try expectThemeGap(capture_path, height_red, height_green);

    _ = libc.unlink(capture_path);
    try h.send("jjjjjjjj");
    const green = try waitForCaptureBounds(&h, capture_path, pdf_green, 500, 5000);
    const final_capture = try readFile(gpa, capture_path);
    defer gpa.free(final_capture);
    const final_pixels = ppmPixels(final_capture) orelse return error.BadCapture;
    if (countColor(final_pixels, pdf_red) != 0) return error.SdlPdfOffscreenPageStillDrawn;
    std.debug.print("PDF harness ok: SDL continuous strip + theme gap + red->green ({d}->{d} pixels)\n", .{ fit_width.count, green.count });
}

const ColorBounds = struct {
    count: usize,
    min_x: usize,
    min_y: usize,
    max_x: usize,
    max_y: usize,

    fn width(bounds: ColorBounds) usize {
        return bounds.max_x - bounds.min_x + 1;
    }

    fn height(bounds: ColorBounds) usize {
        return bounds.max_y - bounds.min_y + 1;
    }
};

fn waitForCaptureBounds(
    h: *eh.Harness,
    path: [:0]const u8,
    color: [3]u8,
    minimum: usize,
    timeout_ms: i64,
) !ColorBounds {
    const deadline = eh.nowMs() + timeout_ms;
    var best: usize = 0;
    while (eh.nowMs() < deadline) {
        if (readFile(gpa, path)) |capture| {
            defer gpa.free(capture);
            if (ppmView(capture)) |ppm| {
                if (colorBounds(ppm, color)) |bounds| {
                    best = @max(best, bounds.count);
                    if (bounds.count >= minimum) return bounds;
                }
            }
        } else |_| {}
        _ = try h.pumpOnce(50);
    }
    std.debug.print("SDL PDF harness found only {d} pixels of color {any}\n", .{ best, color });
    return error.PdfPageColorMissing;
}

fn expectThemeGap(path: [:0]const u8, upper: ColorBounds, lower: ColorBounds) !void {
    if (lower.min_y <= upper.max_y + 1) return error.SdlPdfGapMissing;
    const gap = lower.min_y - upper.max_y - 1;
    if (gap < 7 or gap > 9) return error.SdlPdfGapWrongSize;
    const capture = try readFile(gpa, path);
    defer gpa.free(capture);
    const ppm = ppmView(capture) orelse return error.BadCapture;
    const y = upper.max_y + 1 + gap / 2;
    const x0 = upper.min_x + 2;
    const x1 = @min(upper.max_x, lower.max_x) -| 2;
    if (x0 > x1 or y >= ppm.height) return error.SdlPdfGapOutOfBounds;
    const first = ppm.pixels[(y * ppm.width + x0) * 3 ..][0..3];
    if (std.mem.eql(u8, first, &pdf_red) or std.mem.eql(u8, first, &pdf_green))
        return error.SdlPdfGapCovered;
    var x = x0 + 1;
    while (x <= x1) : (x += 1) {
        const pixel = ppm.pixels[(y * ppm.width + x) * 3 ..][0..3];
        if (!std.mem.eql(u8, pixel, first)) return error.SdlPdfGapNotThemeBackground;
    }
}

const PpmView = struct { width: usize, height: usize, pixels: []const u8 };

fn colorBounds(ppm: PpmView, color: [3]u8) ?ColorBounds {
    var bounds = ColorBounds{
        .count = 0,
        .min_x = std.math.maxInt(usize),
        .min_y = std.math.maxInt(usize),
        .max_x = 0,
        .max_y = 0,
    };
    var pixel: usize = 0;
    while (pixel < ppm.width * ppm.height) : (pixel += 1) {
        const i = pixel * 3;
        if (!std.mem.eql(u8, ppm.pixels[i .. i + 3], &color)) continue;
        const x = pixel % ppm.width;
        const y = pixel / ppm.width;
        bounds.count += 1;
        bounds.min_x = @min(bounds.min_x, x);
        bounds.min_y = @min(bounds.min_y, y);
        bounds.max_x = @max(bounds.max_x, x);
        bounds.max_y = @max(bounds.max_y, y);
    }
    return if (bounds.count == 0) null else bounds;
}

fn countColor(pixels: []const u8, color: [3]u8) usize {
    var count: usize = 0;
    var i: usize = 0;
    while (i + 2 < pixels.len) : (i += 3)
        count += @intFromBool(std.mem.eql(u8, pixels[i .. i + 3], &color));
    return count;
}

fn readFile(allocator: std.mem.Allocator, path: [:0]const u8) ![]u8 {
    const fd = libc.open(path, .{ .ACCMODE = .RDONLY }, @as(libc.mode_t, 0));
    if (fd < 0) return error.OpenFailed;
    defer _ = libc.close(fd);
    var out: std.ArrayList(u8) = .empty;
    errdefer out.deinit(allocator);
    var buf: [8192]u8 = undefined;
    while (true) {
        const n = libc.read(fd, &buf, buf.len);
        if (n < 0) return error.ReadFailed;
        if (n == 0) break;
        try out.appendSlice(allocator, buf[0..@intCast(n)]);
    }
    return out.toOwnedSlice(allocator);
}

fn ppmPixels(bytes: []const u8) ?[]const u8 {
    return (ppmView(bytes) orelse return null).pixels;
}

fn ppmView(bytes: []const u8) ?PpmView {
    if (!std.mem.startsWith(u8, bytes, "P6\n")) return null;
    const dimensions_end = std.mem.indexOfScalarPos(u8, bytes, 3, '\n') orelse return null;
    var dimensions = std.mem.splitScalar(u8, bytes[3..dimensions_end], ' ');
    const width = std.fmt.parseUnsigned(usize, dimensions.next() orelse return null, 10) catch return null;
    const height = std.fmt.parseUnsigned(usize, dimensions.next() orelse return null, 10) catch return null;
    if (dimensions.next() != null) return null;
    const max_end = std.mem.indexOfScalarPos(u8, bytes, dimensions_end + 1, '\n') orelse return null;
    if (!std.mem.eql(u8, bytes[dimensions_end + 1 .. max_end], "255")) return null;
    const expected = std.math.mul(usize, width, height) catch return null;
    const byte_len = std.math.mul(usize, expected, 3) catch return null;
    if (bytes.len -| (max_end + 1) < byte_len) return null;
    return .{ .width = width, .height = height, .pixels = bytes[max_end + 1 ..][0..byte_len] };
}