test: verify FIT binary preservation

tests/fit/builders.py

@@ -18,15 +18,43 @@ def make_fit(data_records: bytes, *, header_size: int = 14) -> bytes:
     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:
+def definition(
+    local: int,
+    global_num: int,
+    fields: list[tuple[int, int, int]],
+    *,
+    endian: str = "<",
+    developer_fields: list[tuple[int, int, int]] | None = None,
+) -> bytes:
+    """Build a FIT definition message.
+
+    ``developer_fields`` is an optional list of (field_num, size, developer_data_index)
+    triples. When provided (and non-empty), the record header's "has developer fields"
+    bit (0x20) is set and a developer field definition block is appended after the
+    native field definitions, per the FIT format.
+    """
     architecture = 1 if endian == ">" else 0
+    header_byte = 0x40 | local
+    if developer_fields:
+        header_byte |= 0x20
     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)
+    if developer_fields:
+        payload.append(len(developer_fields))
+        for num, size, dev_index in developer_fields:
+            payload.extend(bytes([num, size, dev_index]))
+    return bytes([header_byte]) + bytes(payload)
 
 
 def data(local: int, payload: bytes) -> bytes:
     return bytes([local]) + payload
+
+
+def compressed_timestamp_data(local: int, time_offset: int, payload: bytes) -> bytes:
+    """Build a compressed-timestamp data record header (bit 0x80 set, local message
+    type in bits 5-6, time offset in bits 0-4) followed by the record payload."""
+    header = 0x80 | ((local & 0x03) << 5) | (time_offset & 0x1F)
+    return bytes([header]) + payload