calculate crc codes

index.js

@@ -323,6 +323,102 @@ function updatePacketStats() {
   document.getElementById('packet-error-bits-per-block').innerText = (HAMMING_ERROR_CORRECTION ? ERROR_CORRECTION_BLOCK_SIZE : 0).toLocaleString();
   document.getElementById('packet-error-bit-count').innerText = getPacketErrorBitCount();
 }
+function calcCrc(
+  bytes,
+  size,
+  polynomial,
+  {
+    initialization = 0,
+    reflectIn = false,
+    reflectOut = false,
+    xorOut = 0
+  } = {}
+) {
+  if(bytes.length === 0) return 0;
+  const validBits = (1 << size) - 1;
+  const mostSignificantBit = 1 << size - 1;
+  const bitsBeforeLastByte = size - 8;
+
+  // setup our initial value
+  let crc = initialization;
+
+  function reverseBits(value, size) {
+    let reversed = 0;
+    for(let i = 0; i < size; i++) {
+      // if bit position is on
+      if(value & (1<<i)) {
+        // turn on bit in reverse order
+        reversed |= 1 << (size - 1 - i);
+      }
+    }
+    return reversed;
+  }
+
+  for(let byte of bytes) {
+    // reflect incoming bits?
+    if(reflectIn){
+      byte = reverseBits(byte, 8);
+    }
+    // xor current byte against first byte of crc
+    crc ^= byte << bitsBeforeLastByte;
+    // loop through the first 8 bits of the crc
+    for(let i = 0; i < 8; i++) {
+      // is first bit 1?
+      const isFlagged = crc & mostSignificantBit;
+      // if flagged, xor the first bit to prevent overflow
+      if(isFlagged) crc ^= mostSignificantBit;
+      // shift bits left
+      crc <<= 1;
+      // remove invalid bits
+      crc &= validBits;
+      // xor the polynomial
+      if(isFlagged) crc ^= polynomial;
+    }
+  }
+
+  // We only want the last [size] bits
+  crc &= validBits;
+
+  // reflect final bits?
+  if(reflectOut) crc = reverseBits(crc, size);
+
+  // xor the final value going out
+  crc ^= xorOut;
+
+  // remove sign
+  if(size >= 32 && crc & mostSignificantBit) {
+    crc >>>= 0;
+  }
+  return crc;
+}
+function crc8(bytes) { return calcCrc(bytes, 8, 0x07); }
+function crc16(bytes) { 
+  return calcCrc(
+    bytes,
+    16,
+    0x8005,
+    {
+      initialization: 0,
+      reflectIn: true,
+      reflectOut: true,
+      xorOut: 0
+    }
+  );
+}
+function crc32(bytes) {
+  return calcCrc(
+    bytes,
+    32,
+    0x04C11DB7,
+    {
+      initialization: 0xFFFFFFFF,
+      reflectIn: true,
+      reflectOut: true,
+      xorOut: 0x0
+    }
+  );
+}
+
 function drawChannels() {
   const sampleRate = getAudioContext().sampleRate;
   const fftSize = 2 ** FFT_SIZE_POWER;