feat: add FIT CRC calculation

app/fit/crc.py

@@ -0,0 +1,18 @@
+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

tests/fit/test_crc.py

@@ -0,0 +1,11 @@
+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