Function readFormatInformation

src/decoder/decoder.ts:190–236

(matrix: BitMatrix)

}

function readFormatInformation(matrix: BitMatrix) {
  let topLeftFormatInfoBits = 0;
  for (let x = 0; x <= 8; x++) {
    if (x !== 6) { // Skip timing pattern bit
      topLeftFormatInfoBits = pushBit(matrix.get(x, 8), topLeftFormatInfoBits);
    }
  }
  for (let y = 7; y >= 0; y--) {
    if (y !== 6) { // Skip timing pattern bit
      topLeftFormatInfoBits = pushBit(matrix.get(8, y), topLeftFormatInfoBits);
    }
  }

  const dimension = matrix.height;
  let topRightBottomRightFormatInfoBits = 0;
  for (let y = dimension - 1; y >= dimension - 7; y--) { // bottom left
    topRightBottomRightFormatInfoBits = pushBit(matrix.get(8, y), topRightBottomRightFormatInfoBits);
  }
  for (let x = dimension - 8; x < dimension; x++) { // top right
    topRightBottomRightFormatInfoBits = pushBit(matrix.get(x, 8), topRightBottomRightFormatInfoBits);
  }

  let bestDifference = Infinity;
  let bestFormatInfo = null;
  for (const {bits, formatInfo} of FORMAT_INFO_TABLE) {
    if (bits === topLeftFormatInfoBits || bits === topRightBottomRightFormatInfoBits) {
      return formatInfo;
    }
    let difference = numBitsDiffering(topLeftFormatInfoBits, bits);
    if (difference < bestDifference) {
      bestFormatInfo = formatInfo;
      bestDifference = difference;
    }
    if (topLeftFormatInfoBits !== topRightBottomRightFormatInfoBits) { // also try the other option
      difference = numBitsDiffering(topRightBottomRightFormatInfoBits, bits);
      if (difference < bestDifference) {
        bestFormatInfo = formatInfo;
        bestDifference = difference;
      }
    }
  }
  // Hamming distance of the 32 masked codes is 7, by construction, so <= 3 bits differing means we found a match
  if (bestDifference <= 3) {
    return bestFormatInfo;
  }
  return null;
}

function getDataBlocks(codewords: number[], version: Version, ecLevel: number) {
  const ecInfo = version.errorCorrectionLevels[ecLevel];