letters.html

<style>
  html, body {
    font-family: Arial;
  }
  #result {
    font-size: 14px;
    font-family: Arial;
    white-space: pre;
  }
</style>

<select onchange="generateFor(this.value)">
  <option></option>
  <option>Arial</option>
  <option>Calibri</option>
  <option>Times New Roman</option>
</select>

<pre id="result">
  
</pre>

<script>
  function ready(fn){var d=document;(d.readyState=='loading')?d.addEventListener('DOMContentLoaded',fn):fn();}

  let average = (array) => array.reduce((a, b) => a + b) / array.length;

  window.widths = {};
  window.combos = {};
  window.characters = [];

  for (var n = 32; n <= 126; n++) {
    characters.push(String.fromCharCode(n));
  }

  function generateFor(fontFamily) {
    window.widths = {};
    window.combos = {};

    var startTime = (new Date()).getTime();

    var canvas = document.createElement('canvas').getContext('2d');
    canvas.font = `normal 100px ${fontFamily}`;
    canvas.textAlign = 'start';
    canvas.textBaseline = 'alphabetic';

    window.textWidth = function getWidth(value) {
      return canvas.measureText(value).width;
    }

    characters.forEach((char) => {
      widths[char] = textWidth(char);
    });

    characters.forEach((char1) => {
      characters.forEach((char2) => {
        var combined = textWidth(char1 + char2);
        var expected = widths[char1] + widths[char2];

        if (Math.abs(combined - expected) > 0.001) {
          combos[ char1 + char2 ] = [combined, expected];
          //console.log('delta', char1, char2, Math.abs(el.clientWidth - expected));
        }
      });
    });

    printOutput(startTime, fontFamily);
  }

  function printOutput(startTime, fontFamily) {
    //document.querySelector('#result').innerText = JSON.stringify(widths, null, 4) + "\n" + JSON.stringify(combos, null, 4);
    var fontPrefix = fontFamily.replace(/\s+/g, '_').toUpperCase();
    var output = "";

    var averageSize = average(Object.values(widths));

    output += `static float ${fontPrefix}_DEFAULT = ${averageSize};\n`;
    output += `static float ${fontPrefix}_CHAR_WIDTH[127] = {0`;
    for (var n = 1; n <= 126; n++) {
      output += `, ${widths[String.fromCharCode(n)] || '0' }`;
    }
    output += "};\n";

    output += `static float ${fontPrefix}_KERNING[127][127] = {\n`;
    for (var n1 = 0; n1 <= 126; n1++) {
      var char1 = String.fromCharCode(n1);
      var charDisplay = characters.includes(char1) ? char1 : '';

      var prefix = `/* ${n1} : ${charDisplay} */`;
      if (Object.keys(combos).some((pair) => { return pair[0] == char1 })) {
        output += `  ${prefix} { 0`;
        for (var n2 = 1; n2 <= 126; n2++) {
          var char2 = String.fromCharCode(n2);
          output += `, ${combos[char1 + char2] ? combos[char1 + char2][0] : '0' }`;
        }
        output += ` }${n1 == 126 ? '' : ','}\n`;
      } else {
        output += `  ${prefix} { 0 },\n`;
      }
    }
    output += "};\n";

    output += "\nKerning:\n";
    Object.keys(combos).forEach((pair) => {
      output += `${pair} = ${combos[pair][0]} (not ${combos[pair][1]})\n`
    });

    document.querySelector('#result').innerText = output;

    var endTime = (new Date()).getTime();
    //alert(`done ${endTime - startTime} ms`);
    console.log(`done ${endTime - startTime} ms, ${Object.keys(combos).length} combos`);
  }
</script>