# FIT Edge 1030 Plus Rewriter Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Implement a binary-preserving FIT metadata rewriter that changes only Garmin creator-device metadata plus required CRC bytes, targeting Garmin Edge 1030 Plus product ID `3570`.

**Architecture:** Parse FIT definition/data records only far enough to locate `file_id` and creator `device_info` fields, patch bytes in place, recalculate header/file CRC, and validate the output. Do not fully decode/re-encode the activity, so unknown/developer fields and all non-target bytes remain untouched.

**Tech Stack:** Python 3.12 standard library (`struct`, `dataclasses`, `pathlib`), pytest.

## Global Constraints

- Garmin manufacturer ID is `1`.
- Garmin Edge 1030 Plus product ID is `3570`.
- Product name is `Edge 1030 Plus`.
- Support FIT header sizes 12 and 14.
- Validate declared data size and `.FIT` signature.
- Validate header CRC when a 14-byte header is present.
- Validate file CRC before and after modification.
- Support little- and big-endian definition architectures, compressed timestamp records, developer fields, and changing local message definitions.
- Patch `file_id` device fields when present.
- Patch `device_info` only when it is safely identified as the creator (`device_index == 0`).
- If creator-specific `device_info` cannot be identified safely, leave it unchanged rather than rewriting all device records.
- Preserve every non-target byte except CRC fields.

---

## File Structure

```text
app/fit/
  __init__.py
  crc.py
  models.py
  rewriter.py
tests/fit/
  builders.py
  test_crc.py
  test_rewriter_validation.py
  test_rewriter_patching.py
  test_rewriter_preservation.py
```

## Task 1: Implement Garmin FIT CRC

**Files:**
- Create: `app/fit/crc.py`
- Create: `tests/fit/test_crc.py`

**Interfaces:**
- Produces: `fit_crc(data: bytes | bytearray | memoryview) -> int`.

- [ ] **Step 1: Write failing CRC vector tests**

```python
# tests/fit/test_crc.py
from app.fit.crc import fit_crc


def test_empty_crc_is_zero() -> None:
    assert fit_crc(b"") == 0


def test_crc_is_incremental_equivalent() -> None:
    payload = b".FIT-device-metadata"
    assert fit_crc(payload) == fit_crc(memoryview(payload))
    assert 0 <= fit_crc(payload) <= 0xFFFF
```

- [ ] **Step 2: Run and verify failure**

Run: `pytest tests/fit/test_crc.py -v`

Expected: import failure.

- [ ] **Step 3: Implement the FIT nibble-table CRC algorithm**

```python
# app/fit/crc.py
CRC_TABLE = (
    0x0000, 0xCC01, 0xD801, 0x1400,
    0xF001, 0x3C00, 0x2800, 0xE401,
    0xA001, 0x6C00, 0x7800, 0xB401,
    0x5000, 0x9C01, 0x8801, 0x4400,
)


def fit_crc(data: bytes | bytearray | memoryview) -> int:
    crc = 0
    for byte in data:
        tmp = CRC_TABLE[crc & 0xF]
        crc = (crc >> 4) & 0x0FFF
        crc ^= tmp ^ CRC_TABLE[byte & 0xF]
        tmp = CRC_TABLE[crc & 0xF]
        crc = (crc >> 4) & 0x0FFF
        crc ^= tmp ^ CRC_TABLE[(byte >> 4) & 0xF]
    return crc & 0xFFFF
```

- [ ] **Step 4: Run tests**

Run: `pytest tests/fit/test_crc.py -v`

Expected: PASS.

- [ ] **Step 5: Commit**

```bash
git add app/fit/crc.py tests/fit/test_crc.py
git commit -m "feat: add FIT CRC calculation"
```

## Task 2: Parse and validate the FIT container safely

**Files:**
- Create: `app/fit/models.py`
- Create: `app/fit/rewriter.py`
- Create: `tests/fit/builders.py`
- Create: `tests/fit/test_rewriter_validation.py`

**Interfaces:**
- Produces: `FitFormatError`, `FieldDefinition`, `LocalDefinition`, `_validate_fit_container()`, `_iter_data_fields()`.
- `_iter_data_fields(data)` returns data records with exact field offsets without decoding unrelated values.

- [ ] **Step 1: Create a deterministic minimal FIT fixture builder**

```python
# tests/fit/builders.py
import struct

from app.fit.crc import fit_crc


def make_fit(data_records: bytes, *, header_size: int = 14) -> bytes:
    if header_size not in {12, 14}:
        raise ValueError(header_size)
    header = bytearray(header_size)
    header[0] = header_size
    header[1] = 0x20
    struct.pack_into("<H", header, 2, 0x0100)
    struct.pack_into("<I", header, 4, len(data_records))
    header[8:12] = b".FIT"
    if header_size == 14:
        struct.pack_into("<H", header, 12, fit_crc(header[:12]))
    body = header + data_records
    return bytes(body + struct.pack("<H", fit_crc(body)))


def definition(local: int, global_num: int, fields: list[tuple[int, int, int]], *, endian: str = "<") -> bytes:
    architecture = 1 if endian == ">" else 0
    payload = bytearray([0, architecture])
    payload.extend(struct.pack(f"{endian}H", global_num))
    payload.append(len(fields))
    for num, size, base_type in fields:
        payload.extend(bytes([num, size, base_type]))
    return bytes([0x40 | local]) + bytes(payload)


def data(local: int, payload: bytes) -> bytes:
    return bytes([local]) + payload
```

- [ ] **Step 2: Write validation tests**

```python
# tests/fit/test_rewriter_validation.py
from pathlib import Path

import pytest

from app.fit.rewriter import FitFormatError, is_fit_file
from tests.fit.builders import make_fit


def test_valid_12_and_14_byte_headers(tmp_path: Path) -> None:
    for size in (12, 14):
        path = tmp_path / f"valid-{size}.fit"
        path.write_bytes(make_fit(b"", header_size=size))
        assert is_fit_file(path) is True


def test_bad_file_crc_is_rejected(tmp_path: Path) -> None:
    payload = bytearray(make_fit(b""))
    payload[-1] ^= 0xFF
    path = tmp_path / "bad.fit"
    path.write_bytes(payload)
    assert is_fit_file(path) is False
```

- [ ] **Step 3: Run and verify failure**

Run: `pytest tests/fit/test_rewriter_validation.py -v`

Expected: import failure.

- [ ] **Step 4: Implement the parser structures and validation**

```python
# app/fit/models.py
from dataclasses import dataclass


@dataclass(frozen=True)
class FieldDefinition:
    num: int
    size: int
    base_type: int


@dataclass(frozen=True)
class LocalDefinition:
    global_message_num: int
    endian: str
    fields: tuple[FieldDefinition, ...]
    developer_field_size: int
```

Implement in `app/fit/rewriter.py` the validated logic from the known working implementation:

```python
class FitFormatError(ValueError):
    pass


def _validate_fit_container(data: bytearray) -> None:
    if len(data) < 14:
        raise FitFormatError("FIT file is too small")
    header_size = data[0]
    if header_size not in {12, 14}:
        raise FitFormatError(f"Unsupported FIT header size: {header_size}")
    if len(data) < header_size + 2:
        raise FitFormatError("FIT file is shorter than its header")
    if bytes(data[8:12]) != b".FIT":
        raise FitFormatError("Missing .FIT signature")
    data_size = struct.unpack_from("<I", data, 4)[0]
    expected_size = header_size + data_size + 2
    if len(data) != expected_size:
        raise FitFormatError(f"FIT size mismatch: header says {expected_size} bytes, file has {len(data)}")
    if header_size == 14:
        expected_header_crc = struct.unpack_from("<H", data, 12)[0]
        if expected_header_crc != fit_crc(data[:12]):
            raise FitFormatError("FIT header CRC check failed")
    expected_file_crc = struct.unpack_from("<H", data, len(data) - 2)[0]
    if expected_file_crc != fit_crc(data[:-2]):
        raise FitFormatError("FIT file CRC check failed")
```

Implement the parser helpers explicitly:

```python
def _read_definition(
    data: bytearray,
    offset: int,
    has_developer_fields: bool,
    end_offset: int,
) -> tuple[LocalDefinition, int]:
    if offset + 5 > end_offset:
        raise FitFormatError("Truncated FIT definition message")
    offset += 1  # reserved byte
    architecture = data[offset]
    offset += 1
    if architecture not in {0, 1}:
        raise FitFormatError(f"Unsupported FIT architecture: {architecture}")
    endian = ">" if architecture == 1 else "<"
    global_message_num = struct.unpack_from(f"{endian}H", data, offset)[0]
    offset += 2
    field_count = data[offset]
    offset += 1

    fields: list[FieldDefinition] = []
    for _ in range(field_count):
        if offset + 3 > end_offset:
            raise FitFormatError("Truncated FIT field definition")
        fields.append(FieldDefinition(data[offset], data[offset + 1], data[offset + 2]))
        offset += 3

    developer_field_size = 0
    if has_developer_fields:
        if offset >= end_offset:
            raise FitFormatError("Truncated FIT developer field count")
        developer_count = data[offset]
        offset += 1
        for _ in range(developer_count):
            if offset + 3 > end_offset:
                raise FitFormatError("Truncated FIT developer field definition")
            developer_field_size += data[offset + 1]
            offset += 3

    return LocalDefinition(global_message_num, endian, tuple(fields), developer_field_size), offset


def _collect_field_offsets(
    definition: LocalDefinition,
    offset: int,
    end_offset: int,
) -> tuple[list[tuple[FieldDefinition, int]], int]:
    result: list[tuple[FieldDefinition, int]] = []
    current = offset
    for field in definition.fields:
        if current + field.size > end_offset:
            raise FitFormatError("Truncated FIT data record")
        result.append((field, current))
        current += field.size
    if current + definition.developer_field_size > end_offset:
        raise FitFormatError("Truncated FIT developer field payload")
    current += definition.developer_field_size
    return result, current


def _iter_data_fields(
    data: bytearray,
) -> list[tuple[LocalDefinition, list[tuple[FieldDefinition, int]]]]:
    header_size = data[0]
    data_size = struct.unpack_from("<I", data, 4)[0]
    offset = header_size
    end_offset = header_size + data_size
    definitions: dict[int, LocalDefinition] = {}
    records: list[tuple[LocalDefinition, list[tuple[FieldDefinition, int]]]] = []

    while offset < end_offset:
        record_header = data[offset]
        offset += 1

        if record_header & 0x80:
            local = (record_header >> 5) & 0x03
            definition = definitions.get(local)
            if definition is None:
                raise FitFormatError(f"Compressed timestamp record used unknown local definition {local}")
            field_offsets, offset = _collect_field_offsets(definition, offset, end_offset)
            records.append((definition, field_offsets))
            continue

        local = record_header & 0x0F
        is_definition = bool(record_header & 0x40)
        has_developer_fields = bool(record_header & 0x20)
        if is_definition:
            definition, offset = _read_definition(data, offset, has_developer_fields, end_offset)
            definitions[local] = definition
            continue

        definition = definitions.get(local)
        if definition is None:
            raise FitFormatError(f"Data record used unknown local definition {local}")
        field_offsets, offset = _collect_field_offsets(definition, offset, end_offset)
        records.append((definition, field_offsets))

    if offset != end_offset:
        raise FitFormatError("FIT parser did not end on data boundary")
    return records
```

- [ ] **Step 5: Expose `is_fit_file` and verify parser boundary failures**

```python
def is_fit_file(path: Path) -> bool:
    try:
        data = bytearray(path.read_bytes())
        _validate_fit_container(data)
        _iter_data_fields(data)
    except (OSError, FitFormatError):
        return False
    return True
```

- [ ] **Step 6: Run validation tests**

Run: `pytest tests/fit/test_rewriter_validation.py -v`

Expected: PASS.

- [ ] **Step 7: Commit**

```bash
git add app/fit/models.py app/fit/rewriter.py tests/fit/builders.py tests/fit/test_rewriter_validation.py
git commit -m "feat: parse and validate FIT containers"
```

## Task 3: Patch `file_id` and only creator `device_info`

**Files:**
- Modify: `app/fit/models.py`
- Modify: `app/fit/rewriter.py`
- Create: `tests/fit/test_rewriter_patching.py`

**Interfaces:**
- Produces: `GarminDevice`, `FitConversionResult`, `DeviceFieldValue`, `convert_fit_device()`, `read_device_field_values()`.
- Default device is Garmin manufacturer `1`, Edge 1030 Plus product `3570`, name `Edge 1030 Plus`.

- [ ] **Step 1: Add model types**

```python
# app/fit/models.py additions
from pathlib import Path

GARMIN_MANUFACTURER_ID = 1


@dataclass(frozen=True)
class GarminDevice:
    manufacturer_id: int = GARMIN_MANUFACTURER_ID
    product_id: int = 3570
    product_name: str = "Edge 1030 Plus"
    serial_number: int | None = None


@dataclass(frozen=True)
class FitConversionResult:
    source_path: Path
    output_path: Path
    patched_field_count: int
    header_crc: int | None
    file_crc: int


@dataclass(frozen=True)
class DeviceFieldValue:
    global_message_num: int
    field_num: int
    value: int | str
```

- [ ] **Step 2: Write a fixture containing one creator and one sensor `device_info`**

Use these FIT field numbers in `tests/fit/test_rewriter_patching.py`:

```python
FILE_ID_MESG_NUM = 0
DEVICE_INFO_MESG_NUM = 23

# file_id: manufacturer(1/u16), product(2/u16)
file_def = definition(0, FILE_ID_MESG_NUM, [(1, 2, 0x84), (2, 2, 0x84)])
file_data = data(0, struct.pack("<HH", 255, 999))

# device_info: device_index(0/u8), manufacturer(2/u16), product(4/u16)
device_def = definition(1, DEVICE_INFO_MESG_NUM, [(0, 1, 0x02), (2, 2, 0x84), (4, 2, 0x84)])
creator = data(1, struct.pack("<BHH", 0, 255, 999))
sensor = data(1, struct.pack("<BHH", 1, 32, 1234))
```

Write assertions that `file_id` and creator become `(1, 3570)` while sensor remains `(32, 1234)`.

- [ ] **Step 3: Run the test and verify failure**

Run: `pytest tests/fit/test_rewriter_patching.py -v`

Expected: missing conversion functions.

- [ ] **Step 4: Implement conservative creator detection**

In `_patch_device_metadata`, process each `device_info` record as a unit:

```python
field_map = {field.num: (field, offset) for field, offset in field_offsets}
if definition.global_message_num == DEVICE_INFO_MESG_NUM:
    device_index_entry = field_map.get(0)
    if device_index_entry is None:
        continue
    index_field, index_offset = device_index_entry
    device_index = _read_field_value(data, index_offset, index_field, definition.endian)
    if device_index != 0:
        continue
```

Only after this check may fields `2`, `3`, `4`, and `27` be patched. Do not patch a `device_info` message that lacks field `0`.

Always patch eligible `file_id` fields `1`, `2`, optional `3`, and `8`.

- [ ] **Step 5: Implement numeric/string read-write helpers and conversion entry point**

Use the supplied working `_read_field_value()` and `_write_field_value()` behavior, then expose:

```python
def convert_fit_device(source_path: Path, output_path: Path, device: GarminDevice | None = None) -> FitConversionResult:
    resolved = device or GarminDevice()
    data = bytearray(source_path.read_bytes())
    _validate_fit_container(data)
    patched_count = _patch_device_metadata(data, resolved)
    header_crc = _rewrite_header_crc(data)
    file_crc = _rewrite_file_crc(data)
    _validate_fit_container(data)
    output_path.parent.mkdir(parents=True, exist_ok=True)
    output_path.write_bytes(data)
    return FitConversionResult(source_path, output_path, patched_count, header_crc, file_crc)
```

- [ ] **Step 6: Run patching tests**

Run: `pytest tests/fit/test_rewriter_patching.py -v`

Expected: PASS.

- [ ] **Step 7: Commit**

```bash
git add app/fit tests/fit/test_rewriter_patching.py
git commit -m "feat: patch FIT creator as Edge 1030 Plus"
```

## Task 4: Prove binary preservation and advanced record support

**Files:**
- Create: `tests/fit/test_rewriter_preservation.py`
- Modify: `tests/fit/builders.py`
- Modify: `app/fit/rewriter.py` only if a test exposes an actual parser defect

**Interfaces:**
- No new public interface; this task hardens `convert_fit_device()`.

- [ ] **Step 1: Write a preservation test that records changed byte positions**

```python
# tests/fit/test_rewriter_preservation.py
from pathlib import Path

from app.fit.rewriter import convert_fit_device


def test_only_target_fields_and_crcs_change(tmp_path: Path, complex_fit_bytes: bytes) -> None:
    source = tmp_path / "source.fit"
    output = tmp_path / "output.fit"
    source.write_bytes(complex_fit_bytes)

    convert_fit_device(source, output)

    before = source.read_bytes()
    after = output.read_bytes()
    assert len(before) == len(after)

    changed = {index for index, (a, b) in enumerate(zip(before, after)) if a != b}
    expected_metadata_offsets = set(find_expected_device_metadata_offsets(before))
    crc_offsets = {12, 13, len(before) - 2, len(before) - 1}
    assert changed <= expected_metadata_offsets | crc_offsets
```

The fixture helper `find_expected_device_metadata_offsets()` must use the test fixture's known construction offsets, not production parser code, so the test is independent.

- [ ] **Step 2: Add fixtures for compressed timestamps, developer fields, and local-definition replacement**

Construct one synthetic FIT data section containing:

1. a normal `file_id` definition/data pair;
2. a `device_info` definition with creator record;
3. a record definition with one developer field and one data record;
4. a compressed-timestamp data header referring to a known local definition;
5. a later replacement definition for the same local message number.

The test passes if conversion completes, output CRC validates, and non-target payload bytes remain identical.

- [ ] **Step 3: Run the focused advanced tests**

Run: `pytest tests/fit/test_rewriter_preservation.py -v`

Expected: PASS. If a parser guard fails, fix only the smallest parser defect required by the test.

- [ ] **Step 4: Add rejection tests for malformed/truncated definitions**

```python
def test_truncated_definition_is_non_recoverable(tmp_path: Path) -> None:
    path = tmp_path / "truncated.fit"
    path.write_bytes(make_fit(bytes([0x40, 0x00, 0x00])))
    assert is_fit_file(path) is False
```

Also assert that `convert_fit_device()` raises `FitFormatError` for the same input.

- [ ] **Step 5: Run the entire FIT suite**

Run: `pytest tests/fit -v`

Expected: PASS.

- [ ] **Step 6: Commit**

```bash
git add tests/fit app/fit/rewriter.py
git commit -m "test: verify FIT binary preservation"
```