import struct
from pathlib import Path

from app.fit.crc import fit_crc
from app.fit.models import (
    DeviceFieldValue,
    FieldDefinition,
    FitConversionResult,
    GarminDevice,
    LocalDefinition,
)

FILE_ID_MESG_NUM = 0
DEVICE_INFO_MESG_NUM = 23

# Device-identity fields read back / patched per message type.
FILE_ID_DEVICE_FIELDS = frozenset({1, 2, 3, 8})
DEVICE_INFO_DEVICE_FIELDS = frozenset({2, 3, 4, 27})

# Maximum unsigned value per declared FIT field size, used to reject values that
# cannot be represented in the field the source file actually declares.
_MAX_UNSIGNED_BY_SIZE = {1: 0xFF, 2: 0xFFFF, 4: 0xFFFFFFFF}


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")


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")
        field_num = data[offset]
        field_size = data[offset + 1]
        # A zero-size field is illegal FIT. Rejecting it here (rather than
        # special-casing it downstream) closes a hole where a zero-size
        # device_info field 0 would read back as device_index == 0 via
        # int.from_bytes(b"", ...) and make a sensor record look like the creator.
        if field_size == 0:
            raise FitFormatError(f"FIT field {field_num} declares an invalid size of 0")
        fields.append(FieldDefinition(field_num, field_size, 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]]]]:
    """Walk the FIT data section and return every data record with its field offsets.

    Caller invariant: callers MUST run ``_validate_fit_container(data)`` first. The
    header-derived ``end_offset`` is trusted as-is and never clamped to ``len(data)``,
    so an unvalidated buffer whose declared data size exceeds its real length would be
    parsed out of bounds instead of rejected cleanly.
    """
    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


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


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)
    _verify_patched_metadata(data, resolved)
    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)


def read_device_field_values(path: Path) -> list[DeviceFieldValue]:
    data = bytearray(path.read_bytes())
    _validate_fit_container(data)
    return _read_device_field_values(data)


def _read_device_field_values(data: bytearray) -> list[DeviceFieldValue]:
    values: list[DeviceFieldValue] = []
    for definition, field_offsets in _iter_data_fields(data):
        if definition.global_message_num == FILE_ID_MESG_NUM:
            interesting_fields = FILE_ID_DEVICE_FIELDS
            is_creator = True
        elif definition.global_message_num == DEVICE_INFO_MESG_NUM:
            interesting_fields = DEVICE_INFO_DEVICE_FIELDS
            field_map = {field.num: (field, offset) for field, offset in field_offsets}
            is_creator = _is_creator_device_info(data, definition, field_map)
        else:
            continue

        for field, offset in field_offsets:
            if field.num not in interesting_fields:
                continue
            values.append(
                DeviceFieldValue(
                    definition.global_message_num,
                    field.num,
                    _read_field_value(data, offset, field, definition.endian),
                    is_creator,
                )
            )
    return values


def _is_creator_device_info(
    data: bytearray,
    definition: LocalDefinition,
    field_map: dict[int, tuple[FieldDefinition, int]],
) -> bool:
    """A ``device_info`` record counts as the creator only when it carries an
    explicit device_index (field 0) equal to 0. Records without field 0 are never
    treated as the creator, so paired sensors are never rewritten."""
    device_index_entry = field_map.get(0)
    if device_index_entry is None:
        return False
    index_field, index_offset = device_index_entry
    return _read_field_value(data, index_offset, index_field, definition.endian) == 0


def _patch_device_metadata(data: bytearray, device: GarminDevice) -> int:
    patched_count = 0
    eligible_field_count = 0
    for definition, field_offsets in _iter_data_fields(data):
        field_map = {field.num: (field, offset) for field, offset in field_offsets}
        if definition.global_message_num == DEVICE_INFO_MESG_NUM and not _is_creator_device_info(
            data, definition, field_map
        ):
            continue

        for field, offset in field_offsets:
            target_value: int | str | None = None
            if definition.global_message_num == FILE_ID_MESG_NUM:
                if field.num == 1:
                    target_value = device.manufacturer_id
                elif field.num == 2:
                    target_value = device.product_id
                elif field.num == 3 and device.serial_number is not None:
                    target_value = device.serial_number
                elif field.num == 8:
                    target_value = device.product_name
            elif definition.global_message_num == DEVICE_INFO_MESG_NUM:
                if field.num == 2:
                    target_value = device.manufacturer_id
                elif field.num == 3 and device.serial_number is not None:
                    target_value = device.serial_number
                elif field.num == 4:
                    target_value = device.product_id
                elif field.num == 27:
                    target_value = device.product_name

            if target_value is not None:
                eligible_field_count += 1
                if _write_field_value(data, offset, field, definition.endian, target_value):
                    patched_count += 1

    if eligible_field_count == 0:
        raise FitFormatError("No writable file_id or device_info device fields found")
    return patched_count


def _expected_target_value(
    global_message_num: int, field_num: int, device: GarminDevice
) -> int | str | None:
    """Target value a patched device-identity field must read back as, or None for
    fields that are not verified (e.g. the optional serial number)."""
    if global_message_num == FILE_ID_MESG_NUM:
        return {1: device.manufacturer_id, 2: device.product_id, 8: device.product_name}.get(field_num)
    if global_message_num == DEVICE_INFO_MESG_NUM:
        return {2: device.manufacturer_id, 4: device.product_id, 27: device.product_name}.get(field_num)
    return None


def _verify_patched_metadata(data: bytearray, device: GarminDevice) -> None:
    """Spec 10.4 post-patch step: verify the expected target metadata is readable.

    Reads the patched buffer back and confirms every device-identity field that was
    present in the source (file_id 1/2/8 and creator device_info 2/4/27) now holds
    the target value. A field that could not be written -- e.g. a product_name field
    too small for the target string -- fails the whole conversion with FitFormatError
    instead of silently producing a half-rewritten file.
    """
    for value in _read_device_field_values(data):
        if not value.is_creator:
            continue
        expected = _expected_target_value(value.global_message_num, value.field_num, device)
        if expected is None:
            continue
        if value.value != expected:
            raise FitFormatError(
                f"Patched FIT metadata verification failed for message "
                f"{value.global_message_num} field {value.field_num}: "
                f"expected {expected!r}, read back {value.value!r}"
            )


def _read_field_value(data: bytearray, offset: int, field: FieldDefinition, endian: str) -> int | str:
    base_type = field.base_type & 0x1F
    if base_type in {0x03, 0x04, 0x0B} and field.size >= 2:
        return struct.unpack_from(f"{endian}H", data, offset)[0]
    if base_type in {0x05, 0x06, 0x0C} and field.size >= 4:
        return struct.unpack_from(f"{endian}I", data, offset)[0]
    if base_type == 0x07:
        raw = bytes(data[offset : offset + field.size])
        if 0 in raw:
            raw = raw[: raw.index(0)]
        return raw.decode("utf-8", errors="replace")
    raw = bytes(data[offset : offset + field.size])
    return int.from_bytes(raw, "little" if endian == "<" else "big")


def _write_field_value(
    data: bytearray,
    offset: int,
    field: FieldDefinition,
    endian: str,
    value: int | str,
) -> bool:
    if isinstance(value, str):
        encoded = value.encode("utf-8")
        if not encoded or field.size == 0 or len(encoded) + 1 > field.size:
            return False
        replacement = encoded + b"\x00" + b"\x00" * (field.size - len(encoded) - 1)
        if bytes(data[offset : offset + field.size]) == replacement:
            return False
        data[offset : offset + field.size] = replacement
        return True

    max_value = _MAX_UNSIGNED_BY_SIZE.get(field.size)
    if max_value is None:
        return False
    # Range-check before packing: struct.pack would otherwise raise a raw
    # struct.error, which callers cannot classify alongside FitFormatError.
    if not isinstance(value, int) or value < 0 or value > max_value:
        raise FitFormatError(
            f"Value {value!r} does not fit FIT field {field.num} "
            f"of declared size {field.size} (allowed range 0-{max_value})"
        )

    if field.size == 1:
        replacement = struct.pack("B", value)
    elif field.size == 2:
        replacement = struct.pack(f"{endian}H", value)
    else:
        replacement = struct.pack(f"{endian}I", value)

    if bytes(data[offset : offset + field.size]) == replacement:
        return False
    data[offset : offset + field.size] = replacement
    return True


def _rewrite_header_crc(data: bytearray) -> int | None:
    header_size = data[0]
    if header_size != 14:
        return None
    header_crc = fit_crc(data[:12])
    struct.pack_into("<H", data, 12, header_crc)
    return header_crc


def _rewrite_file_crc(data: bytearray) -> int:
    file_crc = fit_crc(data[:-2])
    struct.pack_into("<H", data, len(data) - 2, file_crc)
    return file_crc