fitreader/crc.ts at master · jsimpson/fitreader · GitHub

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import { BinaryReader } from "./deps.ts";

export function calculateCrc(
  io: BinaryReader,
  start: number,
  end: number,
): number {
  const crcTable = [
    0x0000,
    0xcc01,
    0xd801,
    0x1400,
    0xf001,
    0x3c00,
    0x2800,
    0xe401,
    0xa001,
    0x6c00,
    0x7800,
    0xb401,
    0x5000,
    0x9c01,
    0x8801,
    0x4400,
  ];

  let crc = 0;

  // Copied from the FIT SDK
  for (let i = start; i < end; i++) {
    const byte = io.readUint8();

    let tmp = crcTable[crc & 0xf];

    crc = (crc >> 4) & 0x0fff;
    crc = crc ^ tmp ^ crcTable[byte & 0xf];

    tmp = crcTable[crc & 0xf];

    crc = (crc >> 4) & 0x0fff;
    crc = crc ^ tmp ^ crcTable[(byte >> 4) & 0xf];
  }

  return crc;
}