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fdb merged 3 commits into from
Apr 3, 2026
Merged

Extract font generation helpers into shared module #828

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5dc2d06
Extract shared font generation utilities into test/font-generation-he…
claude Apr 3, 2026
648b0a5
Extract all shared table builders into font-generation-helpers.mjs
claude Apr 3, 2026
6d1f614
Fix cmap length bug and add maxp overrides (PR review feedback)
claude Apr 3, 2026
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276 changes: 276 additions & 0 deletions test/font-generation-helpers.mjs
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Original file line number Diff line number Diff line change
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/**
* Shared utilities for test font generation scripts.
*
* Provides binary encoding helpers, checksum calculation, common OpenType
* table builders, and a font assembly function used by the generate-*.mjs
* scripts in this directory.
*/

// --- binary helpers (big-endian) ---

export function u8(v) { return [v & 0xFF]; }
export function u16(v) { return [(v >> 8) & 0xff, v & 0xff]; }
export function u32(v) { return [(v >> 24) & 0xff, (v >> 16) & 0xff, (v >> 8) & 0xff, v & 0xff]; }
export function i16(v) { return u16(v < 0 ? v + 0x10000 : v); }
export function i64(v) { return [...u32(0), ...u32(v)]; } // simplified LONGDATETIME
export function tag(s) { return [...s].map(c => c.charCodeAt(0)); }
export function pad(arr) { while (arr.length % 4 !== 0) arr.push(0); return arr; }

export function calcChecksum(bytes) {
const padded = [...bytes];
while (padded.length % 4 !== 0) padded.push(0);
let sum = 0;
for (let i = 0; i < padded.length; i += 4) {
sum = (sum + ((padded[i] << 24) | (padded[i+1] << 16) | (padded[i+2] << 8) | padded[i+3])) >>> 0;
}
return sum;
}

// --- common table builders ---

export function makeHead({ indexToLocFormat = 0 } = {}) {
return [
...u16(1), ...u16(0), // majorVersion, minorVersion
...u16(1), ...u16(0), // fontRevision (fixed 1.0)
...u32(0), // checksumAdjustment (filled later)
...u32(0x5F0F3CF5), // magicNumber
...u16(0x000B), // flags
...u16(1000), // unitsPerEm
...i64(0), // created
...i64(0), // modified
...i16(0), ...i16(0), // xMin, yMin
...i16(1000), ...i16(1000), // xMax, yMax
...u16(0), // macStyle
...u16(8), // lowestRecPPEM
...i16(2), // fontDirectionHint
...i16(indexToLocFormat), // indexToLocFormat
...i16(0), // glyphDataFormat
];
}

export function makeHhea(numHMetrics) {
return [
...u16(1), ...u16(0), // version 1.0
...i16(800), // ascender
...i16(-200), // descender
...i16(0), // lineGap
...u16(1000), // advanceWidthMax
...i16(0), // minLeftSideBearing
...i16(0), // minRightSideBearing
...i16(1000), // xMaxExtent
...i16(1), ...i16(0), // caretSlopeRise, caretSlopeRun
...i16(0), // caretOffset
...i16(0), ...i16(0), ...i16(0), ...i16(0), // reserved
...i16(0), // metricDataFormat
...u16(numHMetrics), // numberOfHMetrics
];
}

export function makeMaxp(numGlyphs, { cff = false, ...overrides } = {}) {
if (cff) {
return [
...u16(0), ...u16(0x5000), // version 0.5
...u16(numGlyphs),
];
}
const o = overrides;
return [
...u16(1), ...u16(0), // version 1.0
...u16(numGlyphs),
...u16(o.maxPoints ?? 0),
...u16(o.maxContours ?? 0),
...u16(o.maxCompositePoints ?? 0),
...u16(o.maxCompositeContours ?? 0),
...u16(o.maxZones ?? 1),
...u16(o.maxTwilightPoints ?? 0),
...u16(o.maxStorage ?? 0),
...u16(o.maxFunctionDefs ?? 0),
...u16(o.maxInstructionDefs ?? 0),
...u16(o.maxStackElements ?? 0),
...u16(o.maxSizeOfInstructions ?? 0),
...u16(o.maxComponentElements ?? 0),
...u16(o.maxComponentDepth ?? 0),
];
}

export function makeOS2() {
return [
...u16(1), // version
...i16(500), // xAvgCharWidth
...u16(400), // usWeightClass
...u16(5), // usWidthClass
...u16(0), // fsType
...i16(0), ...i16(0), ...i16(0), ...i16(0), // ySubscript*
...i16(0), ...i16(0), ...i16(0), ...i16(0), // ySuperscript*
...i16(0), ...i16(0), // yStrikeout*
...i16(0), // sFamilyClass
...Array(10).fill(0), // panose
...u32(0), ...u32(0), ...u32(0), ...u32(0), // ulUnicodeRange
...tag(' '), // achVendID
...u16(0), // fsSelection
...u16(0), // usFirstCharIndex
...u16(0), // usLastCharIndex
...i16(800), // sTypoAscender
...i16(-200), // sTypoDescender
...i16(0), // sTypoLineGap
...u16(800), // usWinAscent
...u16(200), // usWinDescent
...u32(0), ...u32(0), // ulCodePageRange 1-2
];
}

export function makeName(familyName) {
const names = [
[0, 'Copyright'],
[1, familyName],
[2, 'Regular'],
[4, familyName],
[5, 'Version 1.0'],
[6, familyName],
];
const stringData = [];
const records = [];
let offset = 0;
for (const [nameID, str] of names) {
const encoded = [...str].flatMap(c => u16(c.charCodeAt(0)));
records.push([
...u16(3), // platformID (Windows)
...u16(1), // encodingID (Unicode BMP)
...u16(0x0409), // languageID (English US)
...u16(nameID),
...u16(encoded.length),
...u16(offset),
]);
stringData.push(...encoded);
offset += encoded.length;
}
const storageOffset = 6 + records.length * 12;
return [
...u16(0), // format
...u16(names.length), // count
...u16(storageOffset), // stringOffset
...records.flat(),
...stringData,
];
}

export function makeHmtx(numGlyphs, advanceWidth = 500) {
const metrics = [];
for (let i = 0; i < numGlyphs; i++) {
metrics.push(...u16(advanceWidth), ...i16(0));
}
return metrics;
}

/**
* Build a format-4 cmap table. If charCode is provided, maps that code point
* to glyph index 1. Otherwise creates a sentinel-only table (no mappings).
*/
export function makeCmap(charCode) {
if (charCode === undefined) {
return [
...u16(0), ...u16(1), // version, numTables
...u16(3), ...u16(1), ...u32(12), // platformID=3, encodingID=1, offset
...u16(4), // format 4
...u16(14 + 2 * 5), // length (14-byte header + 5 per-segment u16 fields × 1 segment)
...u16(0), // language
Comment on lines +172 to +176
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Copilot AI Apr 3, 2026

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makeCmap()'s sentinel-only (no mappings) branch sets the format-4 length field to 14 + 2 * 4 (22 bytes), but the returned subtable is 24 bytes. This produces an internally inconsistent cmap table and may cause strict parsers to reject the font. Consider using a placeholder length and patching it from the computed subtable byte length (as is done in the mapped branch), or update the constant to the correct size.

Copilot uses AI. Check for mistakes.
...u16(2), // segCountX2
...u16(2), ...u16(0), ...u16(0), // searchRange, entrySelector, rangeShift
...u16(0xFFFF), // endCode sentinel
...u16(0), // reservedPad
...u16(0xFFFF), // startCode sentinel
...u16(1), // idDelta sentinel
...u16(0), // idRangeOffset sentinel
];
}
const segCount = 2;
const searchRange = 2 * Math.pow(2, Math.floor(Math.log2(segCount)));
const entrySelector = Math.floor(Math.log2(segCount));
const rangeShift = 2 * segCount - searchRange;

const subtable = [
...u16(4), // format
...u16(0), // length (patched below)
...u16(0), // language
...u16(segCount * 2), // segCountX2
...u16(searchRange), ...u16(entrySelector), ...u16(rangeShift),
...u16(charCode), ...u16(0xFFFF), // endCode
...u16(0), // reservedPad
...u16(charCode), ...u16(0xFFFF), // startCode
...i16(1 - charCode), ...i16(1), // idDelta
...u16(0), ...u16(0), // idRangeOffset
];
subtable[2] = (subtable.length >> 8) & 0xff;
subtable[3] = subtable.length & 0xff;

return [
...u16(0), ...u16(1), // version, numTables
...u16(3), ...u16(1), ...u32(12), // platformID=3, encodingID=1, offset
...subtable,
];
}

export function makePost({ underlinePosition = -100, underlineThickness = 50 } = {}) {
return [
...u16(3), ...u16(0), // version 3.0 (no glyph names)
...u32(0), // italicAngle
...i16(underlinePosition),
...i16(underlineThickness),
...u32(0), // isFixedPitch
...u32(0), // minMemType42
...u32(0), // maxMemType42
...u32(0), // minMemType1
...u32(0), // maxMemType1
];
}

// --- font assembly ---

/**
* Assembles a complete OpenType/TrueType font file from a table map.
* @param {Object} tables - Map of tag string to byte array, e.g. { 'head': [...], 'cmap': [...] }
* @param {Object} [options]
* @param {string|number} [options.sfVersion] - 'OTTO' for CFF fonts, or a 32-bit number (default 0x00010000 for TrueType)
* @returns {Uint8Array}
*/
export function assembleFont(tables, { sfVersion = 0x00010000 } = {}) {
const tags = Object.keys(tables).sort();
const numTables = tags.length;
const searchRange = Math.pow(2, Math.floor(Math.log2(numTables))) * 16;
const entrySelector = Math.floor(Math.log2(numTables));
const rangeShift = numTables * 16 - searchRange;

const headerSize = 12 + numTables * 16;
let dataOffset = headerSize;

const tableRecords = [];
const tableData = [];
for (const t of tags) {
const data = tables[t];
const paddedData = pad([...data]);
tableRecords.push([
...tag(t.padEnd(4, ' ')),
...u32(calcChecksum(data)),
...u32(dataOffset),
...u32(data.length),
]);
tableData.push(...paddedData);
dataOffset += paddedData.length;
}

const sfVersionBytes = typeof sfVersion === 'string'
? tag(sfVersion)
: u32(sfVersion);

const font = [
...sfVersionBytes,
...u16(numTables),
...u16(searchRange),
...u16(entrySelector),
Comment on lines +265 to +269
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assembleFont() computes and writes per-table checksums but never computes/patches head.checksumAdjustment (required so the whole-font checksum equals 0xB1B0AFBA). This means the generated fonts are not fully spec-compliant and may fail validation/loading in stricter environments. Since assembleFont() already knows all table offsets, consider building the font with checksumAdjustment=0, computing the full checksum, and then patching the adjustment into the head table at offset +8 (without changing the head table record checksum, per the OpenType spec).

Copilot uses AI. Check for mistakes.
...u16(rangeShift),
...tableRecords.flat(),
...tableData,
];

return new Uint8Array(font);
}
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