06d3fd0c75830a06257727fa580ef90f51e15ab55c3f48061e2c1cc7eb4238ec.json raw

   1  {"ast":null,"code":"const EXP_TABLE = new Uint8Array(512);\nconst LOG_TABLE = new Uint8Array(256)\n/**\n * Precompute the log and anti-log tables for faster computation later\n *\n * For each possible value in the galois field 2^8, we will pre-compute\n * the logarithm and anti-logarithm (exponential) of this value\n *\n * ref {@link https://en.wikiversity.org/wiki/Reed%E2%80%93Solomon_codes_for_coders#Introduction_to_mathematical_fields}\n */;\n(function initTables() {\n  let x = 1;\n  for (let i = 0; i < 255; i++) {\n    EXP_TABLE[i] = x;\n    LOG_TABLE[x] = i;\n    x <<= 1; // multiply by 2\n\n    // The QR code specification says to use byte-wise modulo 100011101 arithmetic.\n    // This means that when a number is 256 or larger, it should be XORed with 0x11D.\n    if (x & 0x100) {\n      // similar to x >= 256, but a lot faster (because 0x100 == 256)\n      x ^= 0x11D;\n    }\n  }\n\n  // Optimization: double the size of the anti-log table so that we don't need to mod 255 to\n  // stay inside the bounds (because we will mainly use this table for the multiplication of\n  // two GF numbers, no more).\n  // @see {@link mul}\n  for (let i = 255; i < 512; i++) {\n    EXP_TABLE[i] = EXP_TABLE[i - 255];\n  }\n})();\n\n/**\n * Returns log value of n inside Galois Field\n *\n * @param  {Number} n\n * @return {Number}\n */\nexports.log = function log(n) {\n  if (n < 1) throw new Error('log(' + n + ')');\n  return LOG_TABLE[n];\n};\n\n/**\n * Returns anti-log value of n inside Galois Field\n *\n * @param  {Number} n\n * @return {Number}\n */\nexports.exp = function exp(n) {\n  return EXP_TABLE[n];\n};\n\n/**\n * Multiplies two number inside Galois Field\n *\n * @param  {Number} x\n * @param  {Number} y\n * @return {Number}\n */\nexports.mul = function mul(x, y) {\n  if (x === 0 || y === 0) return 0;\n\n  // should be EXP_TABLE[(LOG_TABLE[x] + LOG_TABLE[y]) % 255] if EXP_TABLE wasn't oversized\n  // @see {@link initTables}\n  return EXP_TABLE[LOG_TABLE[x] + LOG_TABLE[y]];\n};","map":null,"metadata":{},"sourceType":"script","externalDependencies":[]}