fix_fitcrc16.go

package main

// This script reads a modified fit file (source.fit) and computes the correct
// crc16 value and writes the output to a new file (output.fit)

import (
	"encoding/binary"
	"encoding/hex"
	"fmt"
	"io"
	"log"
	"os"
	"strconv"
)

// https://developer.garmin.com/fit/protocol/#crc
func getFitCRC16(crc uint16, b uint8) uint16 {
	var crc_table = []uint16{
		0x0000, 0xCC01, 0xD801, 0x1400, 0xF001, 0x3C00, 0x2800, 0xE401,
		0xA001, 0x6C00, 0x7800, 0xB401, 0x5000, 0x9C01, 0x8801, 0x4400,
	}

	var tmp uint16

	// compute checksum of lower four bits of byte
	tmp = crc_table[crc&0xF]
	crc = (crc >> 4) & 0x0FFF
	crc = crc ^ tmp ^ crc_table[b&0xF]

	// now compute checksum of upper four bits of byte
	tmp = crc_table[crc&0xF]
	crc = (crc >> 4) & 0x0FFF
	crc = crc ^ tmp ^ crc_table[(b>>4)&0xF]

	return crc
}

func main() {
	// open source src
	// source.fit is the modified fit file with bad crc
	src, err := os.Open("source.fit")
	if err != nil {
		fmt.Println("Error opening source file! " + err.Error())
	}
	defer src.Close()

	srcStat, err := src.Stat()
	if err != nil {
		fmt.Println("Error reading source file! " + err.Error())
	}

	srcSize := srcStat.Size()
	srcCrcB := []byte{}
	outSize := int64(0)

	// open output file
	// output.fit is the modified fit file with computed crc
	out, err := os.Create("output.fit")
	if err != nil {
		fmt.Println("Error opening output file! " + err.Error())
	}
	defer out.Close()

	// create buffer
	buf := make([]byte, 1)

	var crc uint16

	for {
		_, err := src.Read(buf)
		if err != nil {
			if err != io.EOF {
				log.Fatal(err)
			}
			break
		}

		// only process bytes before crc
		if outSize < srcSize-2 {
			_, err = out.Write(buf)
			if err != nil {
				log.Fatal(err)
			}

			crc = getFitCRC16(crc, buf[0])
			outSize++
		} else {
			srcCrcB = append(srcCrcB, buf[0])
		}
	}

	outCrcB := []byte{}
	outCrcB = binary.LittleEndian.AppendUint16(outCrcB, crc)

	// write crc to output
	_, err = out.Write(outCrcB)
	if err != nil {
		log.Fatal(err)
	}

	outSize += 2

	fmt.Println("src size: " + strconv.FormatInt(srcSize, 10))
	fmt.Println("src crc16: " + hex.EncodeToString(srcCrcB))
	fmt.Println("out size: " + strconv.FormatInt(outSize, 10))
	fmt.Println("out crc16: " + hex.EncodeToString(outCrcB))
}