import struct


def _get_srsi(file_bytes):
    """ Obtains the PCF file Spectral Records Start Index (SRSI).

    Inputs:
        file_bytes: byte array of file contents

    Returns:
        return_value: String indicator for what Deviation pairs type is being used.
        srsi: Spectral Records Start Index.  Index in file where spectra records begin
        after skipping past deviation pairs.
    """
    test_lengths = [30, 20]
    index = 256
    return_value = "other"
    srsi = 2
    for i in test_lengths:
        value = struct.unpack(
            "{}s".format(i),
            file_bytes[index:index + i]
        )[0].decode("utf-8").strip()
        if value in "DeviationPairsInFile" or value in "DeviationPairsInFileCompressed":
            return_value = value
            srsi = 83
            break
    return(return_value, srsi)