Basic implementation

.gitignore

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+*.woff
+*.otf
+*.ttf
+reference.py
+/node_modules

package.json

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+{
+  "name": "woff2otf",
+  "version": "0.0.1",
+  "description": "",
+  "main": "woff2otf.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "python-struct": "^1.1.3"
+  }
+}

woff2otf.js

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+"use strict"
+
+const fs = require('fs');
+const zlib = require("zlib");
+
+const struct = require("python-struct");
+
+const woff2otf = function(infile, outfile) {
+  const woffFile = fs.readFileSync(infile);
+  const WOFFHeader = {
+    "signature": struct.unpack(">I", woffFile.slice(0, 4))[0],
+    "flavor": struct.unpack(">I", woffFile.slice(4, 8))[0],
+    "length": struct.unpack(">I", woffFile.slice(8, 12))[0],
+    "numTables": struct.unpack(">H", woffFile.slice(12, 14))[0],
+    "reserved": struct.unpack(">H", woffFile.slice(14, 16))[0],
+    'totalSfntSize': struct.unpack(">I", woffFile.slice(16, 20))[0],
+    "majorVersion": struct.unpack(">H", woffFile.slice(20, 22))[0],
+    "minorVersion": struct.unpack(">H", woffFile.slice(22, 24))[0],
+    "metaOffset": struct.unpack(">I", woffFile.slice(24, 28))[0],
+    "metaLength": struct.unpack(">I", woffFile.slice(28, 32))[0],
+    "metaOrigLength": struct.unpack(">I", woffFile.slice(32, 36))[0],
+    "privOffset": struct.unpack(">I", woffFile.slice(36, 40))[0],
+    "privLength": struct.unpack(">I", woffFile.slice(40, 44))[0],
+  };
+
+  const flavorB = struct.pack(">I", WOFFHeader['flavor']);
+  const numTablesB = struct.pack(">H", WOFFHeader['numTables']);
+  fs.appendFileSync(outfile, flavorB);
+  fs.appendFileSync(outfile, numTablesB);
+
+  let maximum = null;
+  for (let n=0; n<64; n++) {
+    if (2**n > WOFFHeader['numTables']) {
+      break
+    } else {
+      maximum = [n, 2**n];
+    }
+  }
+
+  const searchRange = maximum[1] * 16;
+  const searchRangeB = struct.pack(">H", searchRange);
+  fs.appendFileSync(outfile, searchRangeB);
+
+  const entrySelector = maximum[0]
+  const entrySelectorB = struct.pack(">H", entrySelector);
+  fs.appendFileSync(outfile, entrySelectorB);
+
+  const rangeShift = WOFFHeader['numTables'] * 16 - searchRange;
+  const rangeShiftB = struct.pack(">H", rangeShift);
+  fs.appendFileSync(outfile, rangeShiftB);
+
+
+  let offset = (
+    flavorB.byteLength + numTablesB.byteLength + searchRangeB.byteLength +
+    entrySelectorB.byteLength + rangeShiftB.byteLength
+  )
+
+  const tableDirectoryEntries = [];
+  for (let i=0; i<WOFFHeader['numTables']; i++) {
+    tableDirectoryEntries.push({
+      "tag": struct.unpack(">I", woffFile.slice(44 + i * 20, 48 + i * 20)),
+      "offset": struct.unpack(">I", woffFile.slice(48 + i * 20, 52 + i * 20))[0],
+      "compLength": struct.unpack(">I", woffFile.slice(52 + i * 20, 56 + i * 20))[0],
+      "origLength": struct.unpack(">I", woffFile.slice(56 + i * 20, 60 + i * 20))[0],
+      "origChecksum": struct.unpack(">I", woffFile.slice(60 + i * 20, 64 + i * 20))[0],
+    })
+    offset += 4 * 4
+  }
+
+  for (let i=0; i<tableDirectoryEntries.length; i++) {
+    let tableDirectoryEntry = tableDirectoryEntries[i];
+    fs.appendFileSync(outfile, struct.pack(">I", tableDirectoryEntry['tag']));
+    fs.appendFileSync(outfile, struct.pack(">I", tableDirectoryEntry['origChecksum']));
+    fs.appendFileSync(outfile, struct.pack(">I", offset));
+    fs.appendFileSync(outfile, struct.pack(">I", tableDirectoryEntry['origLength']));
+    tableDirectoryEntry["outOffset"] = offset;
+    offset += tableDirectoryEntry['origLength'];
+    if ((offset % 4) != 0) {
+      offset += 4 - (offset % 4)
+    }
+  }
+
+  for (let i=0; i<tableDirectoryEntries.length; i++) {
+    let tableDirectoryEntry = tableDirectoryEntries[i];
+    let compressedData = woffFile.slice(
+      tableDirectoryEntry['offset'],
+      tableDirectoryEntry['offset'] + tableDirectoryEntry['compLength'],
+    );
+    let uncompressedData;
+    if (tableDirectoryEntry["compLength"] != tableDirectoryEntry["origLength"]) {
+      uncompressedData = zlib.inflateSync(compressedData);
+    } else {
+      uncompressedData = compressedData;
+    }
+    fs.appendFileSync(outfile, uncompressedData);
+    offset = tableDirectoryEntry['outOffset'] + tableDirectoryEntry['origLength'];
+
+    let padding = 0
+    if ((offset % 4) != 0) {
+      padding = 4 - (offset % 4)
+    }
+    fs.appendFileSync(outfile, Buffer.alloc(padding));
+  }
+}
+
+if (fs.existsSync("SimpleIcons2.otf")) {
+  fs.unlinkSync("SimpleIcons2.otf");
+}
+woff2otf("SimpleIcons.woff", "SimpleIcons2.otf");