[Utilities] Conversion of ISO/IEC 19794-2 to nbis-py compatible

[umar-anzar]

Manipulating ISO/IEC 19794-2 Fingerprint Minutiae Record (FMR) data to make it compatible with the NBIS (NIST Biometric Image Software)

def strip_iso_header_metadata(fmr_data: bytes) -> bytes:
    """
    Removes Resolution Y, Finger Count, and Reserved bytes from the ISO header.
    Relocates Fingerprint Quality to the correct offset for NBIS compatibility.
    """
    fmr_mutable = bytearray(fmr_data)
    
    # Move 'Fingerprint Quality' from offset 26 to offset 20
    # In the original ISO spec, offsets 20-23 contain Resolution Y and Finger Count
    fmr_mutable[20:21] = fmr_mutable[26:27]

    # Slice the array to remove the redundant bytes between the new Quality 
    # position and the start of the minutiae data
    return bytes(fmr_mutable[:25] + fmr_mutable[27:])

def update_fmr_payload_length(fmr_data: bytes) -> bytes:
    """
    Updates the 4-byte 'Total Length' field at offset 8 to reflect 
    the current size of the record.
    """
    fmr_mutable = bytearray(fmr_data)
    total_length = len(fmr_mutable)
    
    # Offset 8-12 in ISO/NIST headers is traditionally the Total Record Length
    fmr_mutable[8:12] = total_length.to_bytes(4, "big")
    return bytes(fmr_mutable)

def transform_iso_to_nist_fmr(fmr_data: bytes) -> bytes:
    """
    Converts a standard ISO FMR to a format compatible with NBIS utilities
    by stripping specific header fields and recalculating the record length.
    """
    stripped_fmr = strip_iso_header_metadata(fmr_data)
    final_fmr = update_fmr_payload_length(stripped_fmr)
    
    return final_fmr

[umar-anzar]

def strip_iso_minutiae_padding(fmr: bytes) -> bytes:
    fmr_arr = bytearray(fmr)

    minutiae_num = fmr_arr[27]
    header_size = 28
    minutia_size = 6

    # Find real minutiae count by scanning from end
    real_count = minutiae_num

    for i in reversed(range(minutiae_num)):
        offset = header_size + i * minutia_size
        block = fmr_arr[offset: offset + minutia_size]

        if block != b'\x00\x00\x00\x00\x00\x00':
            break

        real_count -= 1

    # If nothing to trim
    if real_count == minutiae_num:
        return fmr

    # New length
    new_length = header_size + real_count * minutia_size

    # Trim array
    fmr_arr = fmr_arr[:new_length]

    # Update minutiae count (1 byte)
    fmr_arr[27] = real_count

    # Update total_bytes (4 bytes at offset 8–11)
    fmr_arr[8:12] = new_length.to_bytes(4, 'big')

    return bytes(fmr_arr)


def strip_iso_header_metadata(fmr_data: bytes) -> bytes:
    """
    Removes Resolution Y, Finger Count, and Reserved bytes from the ISO header.
    Relocates Fingerprint Quality to the correct offset for NBIS compatibility.
    """
    fmr_mutable = bytearray(fmr_data)

    # print(int.from_bytes(fmr_mutable[14:16], "big"))
    # print(int.from_bytes(fmr_mutable[16:18], "big"))
    
    # Move 'Fingerprint Quality' from offset 26 to offset 20
    # In the original ISO spec, offsets 20-23 contain Resolution Y and Finger Count
    fmr_mutable[20:21] = fmr_mutable[26:27]

    # Slice the array to remove the redundant bytes between the new Quality 
    # position and the start of the minutiae data
    return bytes(fmr_mutable[:25] + fmr_mutable[27:])

def update_fmr_payload_length(fmr_data: bytes) -> bytes:
    """
    Updates the 4-byte 'Total Length' field at offset 8 to reflect 
    the current size of the record.
    """
    fmr_mutable = bytearray(fmr_data)
    total_length = len(fmr_mutable)
    
    # Offset 8-12 in ISO/NIST headers is traditionally the Total Record Length
    fmr_mutable[8:12] = total_length.to_bytes(4, "big")
    return bytes(fmr_mutable)

def transform_iso_to_nist_fmr(fmr_data: bytes) -> bytes:
    """
    Converts a standard ISO FMR to a format compatible with NBIS utilities
    by stripping specific header fields and recalculating the record length.
    """
    removed_padding = strip_iso_minutiae_padding(fmr_data)
    stripped_fmr = strip_iso_header_metadata(removed_padding)
    final_fmr = update_fmr_payload_length(stripped_fmr)
    
    return final_fmr