path: root/petscii.zig

// PETSCII image rendering — a fallback when the terminal has no kitty graphics,
// and a per-pane toggle. Ported in spirit from caioluders/petsciinator: split the
// image into character cells and match each cell to the Unicode block/sextant glyph
// + a C64 foreground/background color pair that best reproduces the cell's pixels
// (minimum per-pixel color error). Pure: only std + the raw RGBA bytes (no vaxis),
// so the matcher is unit-testable on its own.
const std = @import("std");

// The match palette is supplied by the caller (mode-dependent): the Commodore 64
// colors below by default, or the terminal's own ANSI palette. Cells store fg/bg as
// palette indices; image.draw paints them as C64 RGB or as indexed colors per mode.
// This is Pepto's canonical C64 palette.
pub const commodore = [16][3]u8{
    .{ 0x00, 0x00, 0x00 }, // 0  black
    .{ 0xff, 0xff, 0xff }, // 1  white
    .{ 0x68, 0x37, 0x2b }, // 2  red
    .{ 0x70, 0xa4, 0xb2 }, // 3  cyan
    .{ 0x6f, 0x3d, 0x86 }, // 4  purple
    .{ 0x58, 0x8d, 0x43 }, // 5  green
    .{ 0x35, 0x28, 0x79 }, // 6  blue
    .{ 0xb8, 0xc7, 0x6f }, // 7  yellow
    .{ 0x6f, 0x4f, 0x25 }, // 8  orange
    .{ 0x43, 0x39, 0x00 }, // 9  brown
    .{ 0x9a, 0x67, 0x59 }, // 10 light red
    .{ 0x44, 0x44, 0x44 }, // 11 dark grey
    .{ 0x6c, 0x6c, 0x6c }, // 12 grey
    .{ 0x9a, 0xd2, 0x84 }, // 13 light green
    .{ 0x6c, 0x5e, 0xb5 }, // 14 light blue
    .{ 0x95, 0x95, 0x95 }, // 15 light grey
};

const Palette = [16][3]u8;

// One rendered character cell: the UTF-8 bytes of the chosen glyph (stored inline
// so the slice handed to the retained vaxis screen outlives the frame) + palette
// fg/bg indices.
pub const Cell = struct {
    glyph: [4]u8 = .{ ' ', 0, 0, 0 },
    glen: u3 = 1,
    fg: u4 = 1,
    bg: u4 = 0,
};

pub const Grid = struct { cells: []Cell, gw: usize, gh: usize };

fn dist2(a: [3]u8, b: [3]u8) u32 {
    const dr = @as(i32, a[0]) - b[0];
    const dg = @as(i32, a[1]) - b[1];
    const db = @as(i32, a[2]) - b[2];
    return @intCast(dr * dr + dg * dg + db * db);
}

fn nearest(px: [3]u8, pal: Palette) u4 {
    var best: u4 = 0;
    var bestd: u32 = std.math.maxInt(u32);
    for (pal, 0..) |c, i| {
        const d = dist2(px, c);
        if (d < bestd) {
            bestd = d;
            best = @intCast(i);
        }
    }
    return best;
}

// ---- glyph set: each is a codepoint + an 8x8 ink bitmap (bit y*8+x set = fg) ----
const Glyph = struct { cp: u21, bits: u64 };

// The Unicode codepoint for a 2x3 sextant pattern. Bits: 1=upper-left, 2=upper-right,
// 4=mid-left, 8=mid-right, 16=lower-left, 32=lower-right. The four patterns that
// coincide with existing block characters are mapped to those instead.
fn sextantCp(p: u6) u21 {
    return switch (p) {
        0 => ' ',
        21 => 0x258C, // left half  ▌
        42 => 0x2590, // right half ▐
        63 => 0x2588, // full block █
        else => blk: {
            var off: u21 = @as(u21, p) - 1;
            if (p > 21) off -= 1;
            if (p > 42) off -= 1;
            break :blk 0x1FB00 + off; // Symbols for Legacy Computing sextants
        },
    };
}

// the 8x8 ink bitmap for a sextant pattern (2 cols x 3 rows of subcells).
fn sextantBits(p: u6) u64 {
    var bits: u64 = 0;
    var y: usize = 0;
    while (y < 8) : (y += 1) {
        const band = (y * 3) / 8; // 0,0,0,1,1,1,2,2
        var x: usize = 0;
        while (x < 8) : (x += 1) {
            const col: usize = if (x < 4) 0 else 1;
            const sub: u6 = @intCast(band * 2 + col);
            if ((p >> sub) & 1 != 0) bits |= @as(u64, 1) << @intCast(y * 8 + x);
        }
    }
    return bits;
}

// pack 8 row-bytes (bit x set, x=0 leftmost) into the 8x8 bitmap.
fn rows(r: [8]u8) u64 {
    var b: u64 = 0;
    for (r, 0..) |row, y| b |= @as(u64, row) << @intCast(y * 8);
    return b;
}

// the horizontal half blocks and the ten 2x2 quadrants — sextants are 2x3, so they
// can't express an exact 4-row half or a quarter; these fill that gap.
const block_glyphs = [_]Glyph{
    .{ .cp = 0x2580, .bits = rows(.{ 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0 }) }, // ▀ top half
    .{ .cp = 0x2584, .bits = rows(.{ 0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff }) }, // ▄ bottom half
    .{ .cp = 0x2598, .bits = rows(.{ 0x0f, 0x0f, 0x0f, 0x0f, 0, 0, 0, 0 }) }, // ▘ TL
    .{ .cp = 0x259d, .bits = rows(.{ 0xf0, 0xf0, 0xf0, 0xf0, 0, 0, 0, 0 }) }, // ▝ TR
    .{ .cp = 0x2596, .bits = rows(.{ 0, 0, 0, 0, 0x0f, 0x0f, 0x0f, 0x0f }) }, // ▖ BL
    .{ .cp = 0x2597, .bits = rows(.{ 0, 0, 0, 0, 0xf0, 0xf0, 0xf0, 0xf0 }) }, // ▗ BR
    .{ .cp = 0x259a, .bits = rows(.{ 0x0f, 0x0f, 0x0f, 0x0f, 0xf0, 0xf0, 0xf0, 0xf0 }) }, // ▚ TL+BR
    .{ .cp = 0x259e, .bits = rows(.{ 0xf0, 0xf0, 0xf0, 0xf0, 0x0f, 0x0f, 0x0f, 0x0f }) }, // ▞ TR+BL
    .{ .cp = 0x259b, .bits = rows(.{ 0xff, 0xff, 0xff, 0xff, 0x0f, 0x0f, 0x0f, 0x0f }) }, // ▛ ¬BR
    .{ .cp = 0x259c, .bits = rows(.{ 0xff, 0xff, 0xff, 0xff, 0xf0, 0xf0, 0xf0, 0xf0 }) }, // ▜ ¬BL
    .{ .cp = 0x2599, .bits = rows(.{ 0x0f, 0x0f, 0x0f, 0x0f, 0xff, 0xff, 0xff, 0xff }) }, // ▙ ¬TR
    .{ .cp = 0x259f, .bits = rows(.{ 0xf0, 0xf0, 0xf0, 0xf0, 0xff, 0xff, 0xff, 0xff }) }, // ▟ ¬TL
};

// line/diagonal glyphs add the characteristic PETSCII "drawn" look on edges.
const line_glyphs = [_]Glyph{
    .{ .cp = 0x2500, .bits = rows(.{ 0, 0, 0, 0xff, 0xff, 0, 0, 0 }) }, // ─
    .{ .cp = 0x2502, .bits = rows(.{ 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18 }) }, // │
    .{ .cp = 0x253c, .bits = rows(.{ 0x18, 0x18, 0x18, 0xff, 0xff, 0x18, 0x18, 0x18 }) }, // ┼
    .{ .cp = 0x2572, .bits = rows(.{ 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80 }) }, // ╲
    .{ .cp = 0x2571, .bits = rows(.{ 0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01 }) }, // ╱
    .{ .cp = 0x2573, .bits = rows(.{ 0x81, 0x42, 0x24, 0x18, 0x18, 0x24, 0x42, 0x81 }) }, // ╳
};

// Real 8x8 font bitmaps for the printable ASCII range (extracted from the cp850-8x8
// console font). This is how petsciinator's charset works: the FULL character set is
// matched, each glyph scored against the cell by its actual bitmap (ink = fg), then
// rendered by its codepoint — the terminal draws the letter in its own font. (bit
// y*8+x set = ink; x=0 leftmost. Generated, not hand-drawn.)
const ascii_glyphs = [_]Glyph{
    .{ .cp = 0x21, .bits = 0x00180018183c3c18 }, // !
    .{ .cp = 0x22, .bits = 0x0000000000246666 }, // "
    .{ .cp = 0x23, .bits = 0x0036367f367f3636 }, // #
    .{ .cp = 0x24, .bits = 0x00183e603c067c18 }, // $
    .{ .cp = 0x25, .bits = 0x0063660c18336300 }, // %
    .{ .cp = 0x26, .bits = 0x006e333b6e1c361c }, // &
    .{ .cp = 0x27, .bits = 0x00000000000c1818 }, // '
    .{ .cp = 0x28, .bits = 0x0030180c0c0c1830 }, // (
    .{ .cp = 0x29, .bits = 0x000c18303030180c }, // )
    .{ .cp = 0x2a, .bits = 0x0000663cff3c6600 }, // *
    .{ .cp = 0x2b, .bits = 0x000018187e181800 }, // +
    .{ .cp = 0x2c, .bits = 0x0c18180000000000 }, // ,
    .{ .cp = 0x2d, .bits = 0x000000007e000000 }, // -
    .{ .cp = 0x2e, .bits = 0x0018180000000000 }, // .
    .{ .cp = 0x2f, .bits = 0x000103060c183060 }, // /
    .{ .cp = 0x30, .bits = 0x001c36636b63361c }, // 0
    .{ .cp = 0x31, .bits = 0x007e181818181c18 }, // 1
    .{ .cp = 0x32, .bits = 0x007f660c3860633e }, // 2
    .{ .cp = 0x33, .bits = 0x003e63603c60633e }, // 3
    .{ .cp = 0x34, .bits = 0x0078307f33363c38 }, // 4
    .{ .cp = 0x35, .bits = 0x003e63603f03037f }, // 5
    .{ .cp = 0x36, .bits = 0x003e63633f03061c }, // 6
    .{ .cp = 0x37, .bits = 0x000c0c0c1830637f }, // 7
    .{ .cp = 0x38, .bits = 0x003e63633e63633e }, // 8
    .{ .cp = 0x39, .bits = 0x001e30607e63633e }, // 9
    .{ .cp = 0x3a, .bits = 0x0018180000181800 }, // :
    .{ .cp = 0x3b, .bits = 0x0c18180000181800 }, // ;
    .{ .cp = 0x3c, .bits = 0x006030180c183060 }, // <
    .{ .cp = 0x3d, .bits = 0x00007e00007e0000 }, // =
    .{ .cp = 0x3e, .bits = 0x00060c1830180c06 }, // >
    .{ .cp = 0x3f, .bits = 0x001800181830633e }, // ?
    .{ .cp = 0x40, .bits = 0x001e037b7b7b633e }, // @
    .{ .cp = 0x41, .bits = 0x006363637f63361c }, // A
    .{ .cp = 0x42, .bits = 0x003f66663e66663f }, // B
    .{ .cp = 0x43, .bits = 0x003c66030303663c }, // C
    .{ .cp = 0x44, .bits = 0x001f36666666361f }, // D
    .{ .cp = 0x45, .bits = 0x007f46161e16467f }, // E
    .{ .cp = 0x46, .bits = 0x000f06161e16467f }, // F
    .{ .cp = 0x47, .bits = 0x005c66730303663c }, // G
    .{ .cp = 0x48, .bits = 0x006363637f636363 }, // H
    .{ .cp = 0x49, .bits = 0x003c18181818183c }, // I
    .{ .cp = 0x4a, .bits = 0x001e333330303078 }, // J
    .{ .cp = 0x4b, .bits = 0x006766361e366667 }, // K
    .{ .cp = 0x4c, .bits = 0x007f66460606060f }, // L
    .{ .cp = 0x4d, .bits = 0x0063636b7f7f7763 }, // M
    .{ .cp = 0x4e, .bits = 0x006363737b6f6763 }, // N
    .{ .cp = 0x4f, .bits = 0x003e63636363633e }, // O
    .{ .cp = 0x50, .bits = 0x000f06063e66663f }, // P
    .{ .cp = 0x51, .bits = 0x703e73636363633e }, // Q
    .{ .cp = 0x52, .bits = 0x006766363e66663f }, // R
    .{ .cp = 0x53, .bits = 0x003c6630180c663c }, // S
    .{ .cp = 0x54, .bits = 0x003c1818185a7e7e }, // T
    .{ .cp = 0x55, .bits = 0x003e636363636363 }, // U
    .{ .cp = 0x56, .bits = 0x001c366363636363 }, // V
    .{ .cp = 0x57, .bits = 0x00367f6b6b636363 }, // W
    .{ .cp = 0x58, .bits = 0x006363361c366363 }, // X
    .{ .cp = 0x59, .bits = 0x003c18183c666666 }, // Y
    .{ .cp = 0x5a, .bits = 0x007f664c1831637f }, // Z
    .{ .cp = 0x5b, .bits = 0x003c0c0c0c0c0c3c }, // [
    .{ .cp = 0x5c, .bits = 0x00406030180c0603 }, // \\
    .{ .cp = 0x5d, .bits = 0x003c30303030303c }, // ]
    .{ .cp = 0x5e, .bits = 0x0000000063361c08 }, // ^
    .{ .cp = 0x5f, .bits = 0xff00000000000000 }, // _
    .{ .cp = 0x60, .bits = 0x000000000030180c }, // `
    .{ .cp = 0x61, .bits = 0x006e333e301e0000 }, // a
    .{ .cp = 0x62, .bits = 0x003b6666663e0607 }, // b
    .{ .cp = 0x63, .bits = 0x003e6303633e0000 }, // c
    .{ .cp = 0x64, .bits = 0x006e3333333e3038 }, // d
    .{ .cp = 0x65, .bits = 0x003e037f633e0000 }, // e
    .{ .cp = 0x66, .bits = 0x000f06061f06663c }, // f
    .{ .cp = 0x67, .bits = 0x1f303e33336e0000 }, // g
    .{ .cp = 0x68, .bits = 0x006766666e360607 }, // h
    .{ .cp = 0x69, .bits = 0x003c1818181c0018 }, // i
    .{ .cp = 0x6a, .bits = 0x3c66666060600060 }, // j
    .{ .cp = 0x6b, .bits = 0x0067361e36660607 }, // k
    .{ .cp = 0x6c, .bits = 0x003c18181818181c }, // l
    .{ .cp = 0x6d, .bits = 0x006b6b6b7f370000 }, // m
    .{ .cp = 0x6e, .bits = 0x00666666663b0000 }, // n
    .{ .cp = 0x6f, .bits = 0x003e6363633e0000 }, // o
    .{ .cp = 0x70, .bits = 0x0f063e66663b0000 }, // p
    .{ .cp = 0x71, .bits = 0x78303e33336e0000 }, // q
    .{ .cp = 0x72, .bits = 0x000f06066e3b0000 }, // r
    .{ .cp = 0x73, .bits = 0x003f603e037e0000 }, // s
    .{ .cp = 0x74, .bits = 0x00386c0c0c3f0c0c }, // t
    .{ .cp = 0x75, .bits = 0x006e333333330000 }, // u
    .{ .cp = 0x76, .bits = 0x001c366363630000 }, // v
    .{ .cp = 0x77, .bits = 0x00367f6b6b630000 }, // w
    .{ .cp = 0x78, .bits = 0x0063361c36630000 }, // x
    .{ .cp = 0x79, .bits = 0x3f607e6363630000 }, // y
    .{ .cp = 0x7a, .bits = 0x007e4c18327e0000 }, // z
    .{ .cp = 0x7b, .bits = 0x007018180e181870 }, // {
    .{ .cp = 0x7c, .bits = 0x0018181818181818 }, // |
    .{ .cp = 0x7d, .bits = 0x000e18187018180e }, // }
    .{ .cp = 0x7e, .bits = 0x0000000000003b6e }, // ~
};

// the block glyph table (sextants + half/quadrant blocks + line glyphs), built at
// comptime, and the same set extended with the ASCII glyphs. `render(ascii=…)` picks.
const glyphs = blk: {
    @setEvalBranchQuota(100000);
    var list: [64 + block_glyphs.len + line_glyphs.len]Glyph = undefined;
    var p: usize = 0;
    while (p < 64) : (p += 1) list[p] = .{ .cp = sextantCp(@intCast(p)), .bits = sextantBits(@intCast(p)) };
    for (block_glyphs, 0..) |bg, i| list[64 + i] = bg;
    for (line_glyphs, 0..) |lg, i| list[64 + block_glyphs.len + i] = lg;
    break :blk list;
};
const glyphs_ascii = glyphs ++ ascii_glyphs;

// match one 8x8 RGB cell to the best (glyph, fg, bg). Candidate colors are the
// most-common palette colors among the 64 pixels; for each ordered pair the glyph
// cost is base(all-bg) + sum over the glyph's ink bits of (dist_fg - dist_bg).
fn matchCell(cell: *const [64][3]u8, pal: Palette, gset: []const Glyph) Cell {
    var counts = [_]u16{0} ** 16;
    for (cell) |px| counts[nearest(px, pal)] += 1;
    var cand: [4]u4 = undefined;
    var ncand: usize = 0;
    var used = [_]bool{false} ** 16;
    while (ncand < 4) : (ncand += 1) {
        var best: ?usize = null;
        for (counts, 0..) |c, i| {
            if (used[i] or c == 0) continue;
            if (best == null or c > counts[best.?]) best = i;
        }
        if (best) |bi| {
            cand[ncand] = @intCast(bi);
            used[bi] = true;
        } else break;
    }
    if (ncand == 0) return .{}; // can't happen (64 pixels), but keep it total
    if (ncand == 1) return encode(' ', cand[0], cand[0]); // solid color

    var best_cost: i64 = std.math.maxInt(i64);
    var best = encode(' ', cand[0], cand[0]);
    var fi: usize = 0;
    while (fi < ncand) : (fi += 1) {
        var bi: usize = 0;
        while (bi < ncand) : (bi += 1) {
            if (fi == bi) continue;
            const fg = cand[fi];
            const bg = cand[bi];
            var dfg: [64]u32 = undefined;
            var dbg: [64]u32 = undefined;
            var base: i64 = 0;
            for (cell, 0..) |px, p| {
                dfg[p] = dist2(px, pal[fg]);
                dbg[p] = dist2(px, pal[bg]);
                base += dbg[p];
            }
            for (gset) |g| {
                var delta: i64 = 0;
                var bits = g.bits;
                while (bits != 0) : (bits &= bits - 1) {
                    const p: usize = @ctz(bits);
                    delta += @as(i64, dfg[p]) - @as(i64, dbg[p]);
                }
                const cost = base + delta;
                if (cost < best_cost) {
                    best_cost = cost;
                    best = encode(g.cp, fg, bg);
                }
            }
        }
    }
    return best;
}

fn encode(cp: u21, fg: u4, bg: u4) Cell {
    var c = Cell{ .fg = fg, .bg = bg };
    const n = std.unicode.utf8Encode(cp, &c.glyph) catch 1;
    c.glen = @intCast(n);
    return c;
}

// Render `rgba` (iw x ih, 4 bytes/px) into a grid of at most cols x rows cells,
// preserving the image aspect with the terminal cell aspect (~1:2) corrected.
// Caller owns Grid.cells (gpa).
pub fn render(gpa: std.mem.Allocator, rgba: []const u8, iw: usize, ih: usize, cols: usize, rows_: usize, pal: Palette, ascii: bool) !Grid {
    if (iw == 0 or ih == 0 or cols == 0 or rows_ == 0) return .{ .cells = try gpa.alloc(Cell, 0), .gw = 0, .gh = 0 };
    const gset: []const Glyph = if (ascii) &glyphs_ascii else &glyphs;
    // contain-fit; cells are ~twice as tall as wide, so a row spans 2 width-units.
    var gw = cols;
    var gh = (cols * ih) / (2 * iw);
    if (gh > rows_) {
        gh = rows_;
        gw = (rows_ * 2 * iw) / ih;
    }
    gw = std.math.clamp(gw, 1, cols);
    gh = std.math.clamp(gh, 1, rows_);

    const cells = try gpa.alloc(Cell, gw * gh);
    var cy: usize = 0;
    while (cy < gh) : (cy += 1) {
        const ry0 = cy * ih / gh;
        const ry1 = @max(ry0 + 1, (cy + 1) * ih / gh);
        var cx: usize = 0;
        while (cx < gw) : (cx += 1) {
            const rx0 = cx * iw / gw;
            const rx1 = @max(rx0 + 1, (cx + 1) * iw / gw);
            var cell: [64][3]u8 = undefined;
            var sy: usize = 0;
            while (sy < 8) : (sy += 1) {
                const py0 = ry0 + sy * (ry1 - ry0) / 8;
                const py1 = @max(py0 + 1, ry0 + (sy + 1) * (ry1 - ry0) / 8);
                var sx: usize = 0;
                while (sx < 8) : (sx += 1) {
                    const px0 = rx0 + sx * (rx1 - rx0) / 8;
                    const px1 = @max(px0 + 1, rx0 + (sx + 1) * (rx1 - rx0) / 8);
                    var rs: usize = 0;
                    var gs: usize = 0;
                    var bs: usize = 0;
                    var n: usize = 0;
                    var yy = py0;
                    while (yy < py1 and yy < ih) : (yy += 1) {
                        var xx = px0;
                        while (xx < px1 and xx < iw) : (xx += 1) {
                            const i = (yy * iw + xx) * 4;
                            rs += rgba[i];
                            gs += rgba[i + 1];
                            bs += rgba[i + 2];
                            n += 1;
                        }
                    }
                    if (n == 0) n = 1;
                    cell[sy * 8 + sx] = .{ @intCast(rs / n), @intCast(gs / n), @intCast(bs / n) };
                }
            }
            cells[cy * gw + cx] = matchCell(&cell, pal, gset);
        }
    }
    return .{ .cells = cells, .gw = gw, .gh = gh };
}

test "sextant codepoints: blocks + endpoints" {
    try std.testing.expectEqual(@as(u21, ' '), sextantCp(0));
    try std.testing.expectEqual(@as(u21, 0x2588), sextantCp(63));
    try std.testing.expectEqual(@as(u21, 0x258C), sextantCp(21));
    try std.testing.expectEqual(@as(u21, 0x2590), sextantCp(42));
    try std.testing.expectEqual(@as(u21, 0x1FB00), sextantCp(1)); // first sextant
    try std.testing.expectEqual(@as(u21, 0x1FB3B), sextantCp(62)); // last sextant
}

test "matchCell: solid color -> space on that bg" {
    var cell: [64][3]u8 = undefined;
    for (&cell) |*p| p.* = commodore[5]; // all green
    const m = matchCell(&cell, commodore, &glyphs);
    try std.testing.expectEqual(@as(u4, 5), m.bg);
    try std.testing.expectEqual(@as(u8, ' '), m.glyph[0]);
}

test "matchCell: clean top/bottom split picks the two colors" {
    var cell: [64][3]u8 = undefined;
    for (0..64) |p| cell[p] = if (p < 32) commodore[1] else commodore[6]; // white over blue
    const m = matchCell(&cell, commodore, &glyphs);
    // both palette colors must be chosen (in some fg/bg order)
    const a = @as(u4, @min(m.fg, m.bg));
    const b = @as(u4, @max(m.fg, m.bg));
    try std.testing.expectEqual(@as(u4, 1), a);
    try std.testing.expectEqual(@as(u4, 6), b);
    // a clean top/bottom split must resolve to a real block glyph (the top-4-rows
    // half block ▀), never a blank cell.
    const cp = std.unicode.utf8Decode(m.glyph[0..m.glen]) catch 0;
    try std.testing.expectEqual(@as(u21, 0x2580), cp);
}

test "ascii option only enables the ascii glyphs" {
    // the ascii glyph codepoints must be reachable exactly when ascii is on.
    var seen_block = false;
    var seen_ascii = false;
    for (glyphs) |g| if (g.cp == '#') {
        seen_block = true;
    };
    for (glyphs_ascii) |g| if (g.cp == '#') {
        seen_ascii = true;
    };
    try std.testing.expect(!seen_block); // '#' is an ascii-only glyph
    try std.testing.expect(seen_ascii);
    try std.testing.expectEqual(glyphs.len + ascii_glyphs.len, glyphs_ascii.len);
}

test "ascii glyph is chosen when a cell has its exact shape" {
    // a cell shaped exactly like the font 'S' (ink=white on black) must match 'S'
    // with ascii on (cost 0), and fall back to some block glyph with ascii off.
    const s_bits: u64 = 0x003c6630180c663c;
    var cell: [64][3]u8 = undefined;
    for (0..64) |p| cell[p] = if ((s_bits >> @intCast(p)) & 1 != 0) commodore[1] else commodore[0];
    const on = matchCell(&cell, commodore, &glyphs_ascii);
    try std.testing.expectEqual(@as(u21, 'S'), std.unicode.utf8Decode(on.glyph[0..on.glen]) catch 0);
    const off = matchCell(&cell, commodore, &glyphs);
    try std.testing.expect((std.unicode.utf8Decode(off.glyph[0..off.glen]) catch 0) != 'S');
}

test "render: tiny image produces a grid within bounds" {
    const a = std.testing.allocator;
    // 2x2 checker, RGBA
    var img = [_]u8{0} ** (2 * 2 * 4);
    img[0] = 255; img[1] = 255; img[2] = 255; img[3] = 255; // (0,0) white
    img[(3) * 4 + 0] = 255; img[(3) * 4 + 1] = 255; img[(3) * 4 + 2] = 255; img[(3) * 4 + 3] = 255; // (1,1) white
    const g = try render(a, &img, 2, 2, 10, 10, commodore, true);
    defer a.free(g.cells);
    try std.testing.expect(g.gw >= 1 and g.gw <= 10);
    try std.testing.expect(g.gh >= 1 and g.gh <= 10);
    try std.testing.expectEqual(g.gw * g.gh, g.cells.len);
}