Created
March 20, 2023 14:20
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| 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)) | |
| } |
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