sync

internal/fit/encoder.go

@@ -0,0 +1,121 @@
+package fit
+
+import (
+	"bytes"
+	"encoding/binary"
+	"time"
+)
+
+// FitBaseType represents FIT base type definitions
+type FitBaseType struct {
+	ID      int
+	Name    string
+	Size    int
+	Invalid uint64
+	Field   byte
+}
+
+// Base types definitions
+var (
+	FitEnum = FitBaseType{0, "enum", 1, 0xFF, 0x00}
+	FitUint8 = FitBaseType{2, "uint8", 1, 0xFF, 0x02}
+	FitUint16 = FitBaseType{4, "uint16", 2, 0xFFFF, 0x84}
+	FitUint32 = FitBaseType{6, "uint32", 4, 0xFFFFFFFF, 0x86}
+	FitString = FitBaseType{7, "string", 1, 0x00, 0x07}
+	FitFloat32 = FitBaseType{8, "float32", 4, 0xFFFFFFFF, 0x88}
+	FitByte = FitBaseType{13, "byte", 1, 0xFF, 0x0D}
+)
+
+// FitEncoder encodes FIT activity files
+type FitEncoder struct {
+	buf bytes.Buffer
+	headerSize int
+	activityDefined bool
+}
+
+const (
+	FitHeaderSize = 12
+	FileTypeActivity = 4
+	GarminEpochOffset = 631065600 // UTC 00:00 Dec 31 1989
+)
+
+// NewFitEncoder creates a new FIT encoder
+func NewFitEncoder() *FitEncoder {
+	e := &FitEncoder{headerSize: FitHeaderSize}
+	e.writeHeader(0) // Initial header with 0 data size
+	return e
+}
+
+// writeHeader writes the FIT file header
+func (e *FitEncoder) writeHeader(dataSize int) {
+	e.buf.Reset()
+	header := []byte{
+		byte(e.headerSize), // Header size
+		16,                 // Protocol version
+		0, 0, 0, 108,       // Profile version (108.0)
+	}
+	e.buf.Write(header)
+
+	// Write data size (4 bytes, little-endian)
+	sizeBytes := make([]byte, 4)
+	binary.LittleEndian.PutUint32(sizeBytes, uint32(dataSize))
+	e.buf.Write(sizeBytes)
+
+	// Write file type signature
+	e.buf.Write([]byte(".FIT"))
+}
+
+// AddActivity adds activity data to the FIT file
+func (e *FitEncoder) AddActivity(activity FitActivity) error {
+	// TODO: Implement activity message encoding
+	return nil
+}
+
+// Encode returns the encoded FIT file bytes
+func (e *FitEncoder) Encode() ([]byte, error) {
+	dataSize := e.buf.Len() - e.headerSize
+	e.writeHeader(dataSize)
+	e.writeCRC()
+	return e.buf.Bytes(), nil
+}
+
+// writeCRC calculates and appends the FIT CRC
+func (e *FitEncoder) writeCRC() {
+	data := e.buf.Bytes()
+	crc := uint16(0)
+	for _, b := range data {
+		crc = calcCRC(crc, b)
+	}
+	crcBytes := make([]byte, 2)
+	binary.LittleEndian.PutUint16(crcBytes, crc)
+	e.buf.Write(crcBytes)
+}
+
+// calcCRC calculates FIT CRC
+func calcCRC(crc uint16, byteVal byte) uint16 {
+	table := [...]uint16{
+		0x0000, 0xCC01, 0xD801, 0x1400, 0xF001, 0x3C00, 0x2800, 0xE401,
+		0xA001, 0x6C00, 0x7800, 0xB401, 0x5000, 0x9C01, 0x8801, 0x4400,
+	}
+	tmp := table[crc & 0xF]
+	crc = (crc >> 4) & 0x0FFF
+	crc = crc ^ tmp ^ table[byteVal & 0xF]
+	
+	tmp = table[crc & 0xF]
+	crc = (crc >> 4) & 0x0FFF
+	return crc ^ tmp ^ table[(byteVal >> 4) & 0xF]
+}
+
+// timestamp converts Go time to FIT timestamp
+func timestamp(t time.Time) uint32 {
+	return uint32(t.Unix() - GarminEpochOffset)
+}
+
+// FitActivity represents basic activity data for FIT encoding
+type FitActivity struct {
+	Name      string
+	Type      string
+	StartTime time.Time
+	Duration  time.Duration
+	Distance  float32 // in meters
+}