lib-font/src/opentype/woff.js

A JS based OpenType font inspector

import { SimpleTable } from "./tables/simple-table.js";
import lazy from "../lazy.js";

const gzipDecode = globalThis.pako ? globalThis.pako.inflate : undefined;
let nativeGzipDecode = undefined;

if (!gzipDecode) {
  import("zlib").then((zlib) => {
    nativeGzipDecode = (buffer) => zlib.unzipSync(buffer);
  });
}

/**
 * The WOFF header
 *
 * See https://www.w3.org/TR/WOFF for WOFF information
 * See https://docs.microsoft.com/en-us/typography/opentype/spec/overview for font information
 */
class WOFF extends SimpleTable {
  constructor(font, dataview, createTable) {
    const { p } = super({ offset: 0, length: 44 }, dataview, `woff`);

    this.signature = p.tag;
    this.flavor = p.uint32;
    this.length = p.uint32;
    this.numTables = p.uint16;
    p.uint16;
    this.totalSfntSize = p.uint32;
    this.majorVersion = p.uint16;
    this.minorVersion = p.uint16;
    this.metaOffset = p.uint32;
    this.metaLength = p.uint32;
    this.metaOrigLength = p.uint32;
    this.privOffset = p.uint32;
    this.privLength = p.uint32;

    p.verifyLength();

    this.directory = [...new Array(this.numTables)].map(
      (_) => new WoffTableDirectoryEntry(p)
    );
    buildWoffLazyLookups(this, dataview, createTable);
  }
}

/**
 * ...
 */
class WoffTableDirectoryEntry {
  constructor(p) {
    this.tag = p.tag;
    this.offset = p.uint32;
    this.compLength = p.uint32;
    this.origLength = p.uint32;
    this.origChecksum = p.uint32;
  }
}

/**
 * Build late-evaluating properties for each table in a
 * woff/woff2 font, so that accessing a table via the
 * woff.tables.tableName or woff2.tables.tableName
 * property kicks off a table parse on first access.
 *
 * @param {*} woff the woff or woff2 font object
 * @param {DataView} dataview passed when dealing with woff
 * @param {buffer} decoded passed when dealing with woff2
 */
function buildWoffLazyLookups(woff, dataview, createTable) {
  woff.tables = {};
  woff.directory.forEach((entry) => {
    lazy(woff.tables, entry.tag.trim(), () => {
      let offset = 0;
      let view = dataview;
      // compressed data?
      if (entry.compLength !== entry.origLength) {
        const data = dataview.buffer.slice(
          entry.offset,
          entry.offset + entry.compLength
        );

        let unpacked;
        if (gzipDecode) {
          unpacked = gzipDecode(new Uint8Array(data));
        } else if (nativeGzipDecode) {
          unpacked = nativeGzipDecode(new Uint8Array(data));
        } else {
          const msg = `no brotli decoder available to decode WOFF2 font`;
          if (font.onerror) font.onerror(msg);
          throw new Error(msg);
        }

        view = new DataView(unpacked.buffer);
      }
      // uncompressed data.
      else {
        offset = entry.offset;
      }
      return createTable(
        woff.tables,
        { tag: entry.tag, offset, length: entry.origLength },
        view
      );
    });
  });
}

export { WOFF };