feat: parse and validate FIT containers

tests/fit/builders.py

@@ -0,0 +1,32 @@
+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

tests/fit/test_rewriter_validation.py

@@ -0,0 +1,21 @@
+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