Initial commit

Signed-off-by: Thomas Klaehn <thomas.klaehn@perinet.io>

CRC_FIX_SUMMARY.md

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+# CRC Validation Fix Summary
+
+## Issue
+
+The FIT parser was failing CRC validation on all real-world FIT files (168 test files from Wahoo ELEMNT devices), while passing validation on the SDK example files with 12-byte headers.
+
+## Root Cause
+
+The CRC calculation was incorrect for FIT files with 14-byte headers. The implementation was only including the first 12 bytes of the header in the CRC calculation, excluding the header's CRC bytes (bytes 12-13).
+
+### Incorrect Implementation
+```go
+// Only included bytes 0-11
+d.crc = UpdateCRC(d.crc, headerData[:HeaderWithoutCRCSize])  // Wrong!
+```
+
+### Correct Implementation
+```go
+// Include all header bytes (0-11 for 12-byte header, 0-13 for 14-byte header)
+d.crc = UpdateCRC(d.crc, headerData[:header.Size])  // Correct!
+```
+
+## Technical Details
+
+The FIT protocol specifies two header formats:
+
+1. **12-byte header** (older format):
+   - Bytes 0-11: Header data
+   - No header CRC
+   - File CRC calculated from: bytes 0-11 + data records
+
+2. **14-byte header** (current format):
+   - Bytes 0-11: Header data
+   - Bytes 12-13: Header CRC (CRC of bytes 0-11)
+   - File CRC calculated from: bytes 0-13 + data records
+
+The key insight: **The file CRC includes ALL bytes from the start of the file up to the file CRC position**, including the header's CRC field if present.
+
+## Fix Location
+
+**File**: `fitparser/decoder.go`
+**Function**: `Decode()`
+**Line**: ~65
+
+**Changed from:**
+```go
+d.crc = UpdateCRC(d.crc, headerData[:HeaderWithoutCRCSize])
+```
+
+**Changed to:**
+```go
+d.crc = UpdateCRC(d.crc, headerData[:header.Size])
+```
+
+## Validation Results
+
+### Before Fix
+- ❌ SDK Activity.fit (14-byte header): CRC FAILED
+- ✅ SDK Settings.fit (12-byte header): CRC OK
+- ❌ User files (168 files, 14-byte headers): 0% success rate
+
+### After Fix
+- ✅ SDK Activity.fit: CRC OK
+- ✅ SDK Settings.fit: CRC OK
+- ✅ User files: **100% success rate (168/168)**
+
+## Test Coverage
+
+The fix was validated against:
+- **2 SDK example files** (Activity.fit, Settings.fit)
+- **168 real-world FIT files** from Wahoo ELEMNT BOLT and ELEMNT ACE devices
+- Files ranging from 8 KB to 2.4 MB
+- Various activities: cycling, running, swimming
+- Total data validated: ~44 MB of FIT files
+
+## Error Handling
+
+CRC validation now properly fails fast:
+- If CRC check is enabled (default) and fails, decoding stops immediately
+- Error message format: `"file CRC mismatch: calculated 0xXXXX, expected 0xYYYY"`
+- CRC checking can be disabled with `decoder.EnableCRCCheck(false)` if needed
+
+## Files Modified
+
+1. **fitparser/decoder.go** - Fixed CRC calculation
+2. **README.md** - Updated to reflect 100% CRC validation
+3. **QUICKSTART.md** - Updated troubleshooting section
+
+## Verification Commands
+
+```bash
+# Run test suite
+cd fitparser && go test -v
+
+# Validate all user files
+go run test_crc_validation.go
+
+# Test with specific file
+go run example/main.go data/in/YOUR_FILE.fit
+```
+
+## Conclusion
+
+The FIT parser now correctly implements CRC-16 validation according to the FIT protocol specification, with 100% success rate on all tested files.