init

node_modules/qrcode/lib/core/galois-field.js

@@ -0,0 +1,69 @@
+const EXP_TABLE = new Uint8Array(512)
+const LOG_TABLE = new Uint8Array(256)
+/**
+ * Precompute the log and anti-log tables for faster computation later
+ *
+ * For each possible value in the galois field 2^8, we will pre-compute
+ * the logarithm and anti-logarithm (exponential) of this value
+ *
+ * ref {@link https://en.wikiversity.org/wiki/Reed%E2%80%93Solomon_codes_for_coders#Introduction_to_mathematical_fields}
+ */
+;(function initTables () {
+  let x = 1
+  for (let i = 0; i < 255; i++) {
+    EXP_TABLE[i] = x
+    LOG_TABLE[x] = i
+
+    x <<= 1 // multiply by 2
+
+    // The QR code specification says to use byte-wise modulo 100011101 arithmetic.
+    // This means that when a number is 256 or larger, it should be XORed with 0x11D.
+    if (x & 0x100) { // similar to x >= 256, but a lot faster (because 0x100 == 256)
+      x ^= 0x11D
+    }
+  }
+
+  // Optimization: double the size of the anti-log table so that we don't need to mod 255 to
+  // stay inside the bounds (because we will mainly use this table for the multiplication of
+  // two GF numbers, no more).
+  // @see {@link mul}
+  for (let i = 255; i < 512; i++) {
+    EXP_TABLE[i] = EXP_TABLE[i - 255]
+  }
+}())
+
+/**
+ * Returns log value of n inside Galois Field
+ *
+ * @param  {Number} n
+ * @return {Number}
+ */
+exports.log = function log (n) {
+  if (n < 1) throw new Error('log(' + n + ')')
+  return LOG_TABLE[n]
+}
+
+/**
+ * Returns anti-log value of n inside Galois Field
+ *
+ * @param  {Number} n
+ * @return {Number}
+ */
+exports.exp = function exp (n) {
+  return EXP_TABLE[n]
+}
+
+/**
+ * Multiplies two number inside Galois Field
+ *
+ * @param  {Number} x
+ * @param  {Number} y
+ * @return {Number}
+ */
+exports.mul = function mul (x, y) {
+  if (x === 0 || y === 0) return 0
+
+  // should be EXP_TABLE[(LOG_TABLE[x] + LOG_TABLE[y]) % 255] if EXP_TABLE wasn't oversized
+  // @see {@link initTables}
+  return EXP_TABLE[LOG_TABLE[x] + LOG_TABLE[y]]
+}