# -*- coding: utf-8 -*-

"""

    sphinx.util.png

    ~~~~~~~~~~~~~~~

    PNG image manipulation helpers.

    :copyright: 2008 by Georg Brandl.

    :license: BSD.

"""

import struct

import binascii

LEN_IEND = 12

LEN_DEPTH = 22

DEPTH_CHUNK_LEN = struct.pack('!i', 10)

DEPTH_CHUNK_START = 'tEXtDepth\x00'

IEND_CHUNK = '\x00\x00\x00\x00IEND\xAE\x42\x60\x82'

def read_png_depth(filename):

    """

    Read the special tEXt chunk indicating the depth from a PNG file.

    """

    result = None

    f = open(filename, 'rb')

    try:

        f.seek(- (LEN_IEND + LEN_DEPTH), 2)

        depthchunk = f.read(LEN_DEPTH)

        if not depthchunk.startswith(DEPTH_CHUNK_LEN + DEPTH_CHUNK_START):

            # either not a PNG file or not containing the depth chunk

            return None

        result = struct.unpack('!i', depthchunk[14:18])[0]

    finally:

        f.close()

    return result

def write_png_depth(filename, depth):

    """

    Write the special tEXt chunk indicating the depth to a PNG file.

    The chunk is placed immediately before the special IEND chunk.

    """

    data = struct.pack('!i', depth)

    f = open(filename, 'r+b')

    try:

        # seek to the beginning of the IEND chunk

        f.seek(-LEN_IEND, 2)

        # overwrite it with the depth chunk

        f.write(DEPTH_CHUNK_LEN + DEPTH_CHUNK_START + data)

        # calculate the checksum over chunk name and data

        f.write(struct.pack('!i', binascii.crc32(DEPTH_CHUNK_START + data)))

        # replace the IEND chunk

        f.write(IEND_CHUNK)

    finally:

        f.close()