Initial commit

Signed-off-by: Thomas Klaehn <thomas.klaehn@perinet.io>
2026-02-08 07:28:32 +00:00
commit 88b74b7dbc
7 changed files with 1567 additions and 0 deletions
--- a/fitparser/decoder.go
+++ b/fitparser/decoder.go
@@ -0,0 +1,332 @@
+package fitparser
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+// Decoder decodes FIT files
+type Decoder struct {
+	reader              *bytes.Reader
+	header              *Header
+	localMessageTypes   [16]*MessageDefinition // Local message type -> definition
+	messages            []*Message
+	crc                 uint16
+	enableCRCCheck      bool
+	offset              int64
+}
+
+// NewDecoder creates a new FIT file decoder
+func NewDecoder(data []byte) (*Decoder, error) {
+	if len(data) < HeaderMinSize {
+		return nil, fmt.Errorf("file too small to be a valid FIT file: %d bytes", len(data))
+	}
+
+	if !IsFIT(data) {
+		return nil, fmt.Errorf("not a valid FIT file: missing .FIT signature")
+	}
+
+	return &Decoder{
+		reader:            bytes.NewReader(data),
+		enableCRCCheck:    true,
+		crc:               0,
+	}, nil
+}
+
+// EnableCRCCheck enables or disables CRC checking
+func (d *Decoder) EnableCRCCheck(enable bool) {
+	d.enableCRCCheck = enable
+}
+
+// Decode decodes the entire FIT file and returns all messages
+func (d *Decoder) Decode() ([]*Message, error) {
+	// Parse header
+	headerData := make([]byte, HeaderWithCRCSize)
+	n, err := d.reader.Read(headerData)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read header: %w", err)
+	}
+
+	header, err := ParseHeader(headerData[:n])
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse header: %w", err)
+	}
+	d.header = header
+
+	// Position reader at start of data records
+	d.reader.Seek(int64(header.Size), io.SeekStart)
+	d.offset = int64(header.Size)
+
+	// Initialize CRC with entire header (all bytes up to data start)
+	// For 14-byte headers, this includes the header CRC (bytes 12-13)
+	// For 12-byte headers, this is just bytes 0-11
+	if d.enableCRCCheck {
+		d.crc = UpdateCRC(d.crc, headerData[:header.Size])
+	}
+
+	// Read data records
+	dataEnd := int64(header.Size) + int64(header.DataSize)
+	for d.offset < dataEnd {
+		msg, err := d.readRecord()
+		if err != nil {
+			return nil, fmt.Errorf("error reading record at offset %d: %w", d.offset, err)
+		}
+
+		if msg != nil {
+			d.messages = append(d.messages, msg)
+		}
+	}
+
+	// Verify file CRC if enabled
+	if d.enableCRCCheck {
+		fileCRCData := make([]byte, 2)
+		_, err := d.reader.Read(fileCRCData)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read file CRC: %w", err)
+		}
+
+		fileCRC := binary.LittleEndian.Uint16(fileCRCData)
+		if fileCRC != d.crc {
+			return nil, fmt.Errorf("file CRC mismatch: calculated 0x%04X, expected 0x%04X", d.crc, fileCRC)
+		}
+	}
+
+	return d.messages, nil
+}
+
+// readRecord reads a single record (definition or data message)
+func (d *Decoder) readRecord() (*Message, error) {
+	recordHeader := make([]byte, 1)
+	_, err := d.reader.Read(recordHeader)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read record header: %w", err)
+	}
+
+	if d.enableCRCCheck {
+		d.crc = UpdateCRC(d.crc, recordHeader)
+	}
+	d.offset++
+
+	header := recordHeader[0]
+
+	// Check if this is a definition message or data message
+	if (header & MesgDefinitionMask) != 0 {
+		// Definition message
+		return d.readDefinitionMessage(header)
+	} else {
+		// Data message (normal or compressed timestamp)
+		if (header & RecordHeaderCompressedMask) != 0 {
+			// Compressed timestamp header
+			return d.readCompressedTimestampMessage(header)
+		} else {
+			// Normal data message
+			return d.readDataMessage(header)
+		}
+	}
+}
+
+// readDefinitionMessage reads a message definition
+func (d *Decoder) readDefinitionMessage(header byte) (*Message, error) {
+	localMesgNum := header & LocalMesgNum
+
+	// Read definition header (5 bytes: reserved, architecture, global_mesg_num, num_fields)
+	defHeader := make([]byte, 5)
+	_, err := d.reader.Read(defHeader)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read definition header: %w", err)
+	}
+
+	if d.enableCRCCheck {
+		d.crc = UpdateCRC(d.crc, defHeader)
+	}
+	d.offset += 5
+
+	msgDef := &MessageDefinition{
+		Reserved:      defHeader[0],
+		Architecture:  defHeader[1],
+		NumFields:     defHeader[4],
+	}
+
+	// Parse global message number based on architecture
+	if msgDef.IsLittleEndian() {
+		msgDef.GlobalMesgNum = binary.LittleEndian.Uint16(defHeader[2:4])
+	} else {
+		msgDef.GlobalMesgNum = binary.BigEndian.Uint16(defHeader[2:4])
+	}
+
+	// Read field definitions (3 bytes each: field_def_num, size, base_type)
+	msgDef.FieldDefinitions = make([]FieldDefinition, msgDef.NumFields)
+	for i := 0; i < int(msgDef.NumFields); i++ {
+		fieldDefData := make([]byte, 3)
+		_, err := d.reader.Read(fieldDefData)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read field definition %d: %w", i, err)
+		}
+
+		if d.enableCRCCheck {
+			d.crc = UpdateCRC(d.crc, fieldDefData)
+		}
+		d.offset += 3
+
+		msgDef.FieldDefinitions[i] = FieldDefinition{
+			Num:      fieldDefData[0],
+			Size:     fieldDefData[1],
+			BaseType: fieldDefData[2],
+		}
+	}
+
+	// Check for developer fields
+	if (header & DevDataMask) != 0 {
+		devFieldsData := make([]byte, 1)
+		_, err := d.reader.Read(devFieldsData)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read num dev fields: %w", err)
+		}
+
+		if d.enableCRCCheck {
+			d.crc = UpdateCRC(d.crc, devFieldsData)
+		}
+		d.offset++
+
+		msgDef.NumDevFields = devFieldsData[0]
+
+		// Read developer field definitions (3 bytes each)
+		msgDef.DevFieldDefinitions = make([]DeveloperFieldDefinition, msgDef.NumDevFields)
+		for i := 0; i < int(msgDef.NumDevFields); i++ {
+			devFieldDefData := make([]byte, 3)
+			_, err := d.reader.Read(devFieldDefData)
+			if err != nil {
+				return nil, fmt.Errorf("failed to read dev field definition %d: %w", i, err)
+			}
+
+			if d.enableCRCCheck {
+				d.crc = UpdateCRC(d.crc, devFieldDefData)
+			}
+			d.offset += 3
+
+			msgDef.DevFieldDefinitions[i] = DeveloperFieldDefinition{
+				Num:                devFieldDefData[0],
+				Size:               devFieldDefData[1],
+				DeveloperDataIndex: devFieldDefData[2],
+			}
+		}
+	}
+
+	// Store the definition for this local message number
+	d.localMessageTypes[localMesgNum] = msgDef
+
+	return nil, nil // Definition messages don't produce output messages
+}
+
+// readDataMessage reads a data message using a previously defined message definition
+func (d *Decoder) readDataMessage(header byte) (*Message, error) {
+	localMesgNum := header & LocalMesgNum
+
+	msgDef := d.localMessageTypes[localMesgNum]
+	if msgDef == nil {
+		return nil, fmt.Errorf("no definition found for local message number %d", localMesgNum)
+	}
+
+	msg := NewMessage(msgDef.GlobalMesgNum)
+
+	// Read and decode each field
+	for _, fieldDef := range msgDef.FieldDefinitions {
+		fieldData := make([]byte, fieldDef.Size)
+		_, err := d.reader.Read(fieldData)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read field %d: %w", fieldDef.Num, err)
+		}
+
+		if d.enableCRCCheck {
+			d.crc = UpdateCRC(d.crc, fieldData)
+		}
+		d.offset += int64(fieldDef.Size)
+
+		// Decode the field value
+		value, err := DecodeField(fieldData, fieldDef, msgDef.IsLittleEndian())
+		if err != nil {
+			return nil, fmt.Errorf("failed to decode field %d: %w", fieldDef.Num, err)
+		}
+
+		msg.Fields[fieldDef.Num] = value
+	}
+
+	// Read developer fields if present
+	for _, devFieldDef := range msgDef.DevFieldDefinitions {
+		devFieldData := make([]byte, devFieldDef.Size)
+		_, err := d.reader.Read(devFieldData)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read dev field %d: %w", devFieldDef.Num, err)
+		}
+
+		if d.enableCRCCheck {
+			d.crc = UpdateCRC(d.crc, devFieldData)
+		}
+		d.offset += int64(devFieldDef.Size)
+
+		msg.DevFields[devFieldDef.Num] = devFieldData
+	}
+
+	return msg, nil
+}
+
+// readCompressedTimestampMessage reads a compressed timestamp data message
+func (d *Decoder) readCompressedTimestampMessage(header byte) (*Message, error) {
+	localMesgNum := header & RecordHeaderLocalMesgNumMask
+	timeOffset := header & RecordHeaderTimeMask
+
+	msgDef := d.localMessageTypes[localMesgNum]
+	if msgDef == nil {
+		return nil, fmt.Errorf("no definition found for local message number %d", localMesgNum)
+	}
+
+	msg := NewMessage(msgDef.GlobalMesgNum)
+
+	// Read and decode each field
+	for _, fieldDef := range msgDef.FieldDefinitions {
+		fieldData := make([]byte, fieldDef.Size)
+		_, err := d.reader.Read(fieldData)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read field %d: %w", fieldDef.Num, err)
+		}
+
+		if d.enableCRCCheck {
+			d.crc = UpdateCRC(d.crc, fieldData)
+		}
+		d.offset += int64(fieldDef.Size)
+
+		// Decode the field value
+		value, err := DecodeField(fieldData, fieldDef, msgDef.IsLittleEndian())
+		if err != nil {
+			return nil, fmt.Errorf("failed to decode field %d: %w", fieldDef.Num, err)
+		}
+
+		msg.Fields[fieldDef.Num] = value
+	}
+
+	// Store the compressed timestamp offset
+	// Note: Full timestamp reconstruction would require tracking the last full timestamp
+	// For now, we just store the offset value
+	msg.Fields[253] = uint32(timeOffset) // Field 253 is typically timestamp
+
+	return msg, nil
+}
+
+// GetHeader returns the parsed header
+func (d *Decoder) GetHeader() *Header {
+	return d.header
+}
+
+// CheckIntegrity checks if the file is a valid FIT file with correct CRC
+func CheckIntegrity(data []byte) error {
+	decoder, err := NewDecoder(data)
+	if err != nil {
+		return err
+	}
+
+	decoder.EnableCRCCheck(true)
+	_, err = decoder.Decode()
+	return err
+}