#!/venv/bin/python3

# prigreypng

# Convert image to grey (L, or LA), but only if that involves no colour change.

import argparse
import array


import png


def as_grey(out, inp):
    """
    Convert image to greyscale, but only when no colour change.
    This works by using the input G channel (green) as
    the output L channel (luminance) and
    checking that every pixel is grey as we go.
    A non-grey pixel will raise an error.
    """

    r = png.Reader(file=inp)
    _, _, rows, info = r.asDirect()
    if info["greyscale"]:
        w = png.Writer(**info)
        return w.write(out, rows)

    planes = info["planes"]
    targetplanes = planes - 2
    alpha = info["alpha"]
    width, height = info["size"]
    typecode = "BH"[info["bitdepth"] > 8]

    # Values per target row
    vpr = width * targetplanes

    def iterasgrey():
        for i, row in enumerate(rows):
            row = array.array(typecode, row)
            targetrow = array.array(typecode, [0] * vpr)
            # Copy G (and possibly A) channel.
            green = row[0::planes]
            if alpha:
                targetrow[0::2] = green
                targetrow[1::2] = row[3::4]
            else:
                targetrow = green
            # Check R and B channel match.
            if green != row[0::planes] or green != row[2::planes]:
                raise ValueError("Row %i contains non-grey pixel." % i)
            yield targetrow

    info["greyscale"] = True
    del info["planes"]
    w = png.Writer(**info)
    return w.write(out, iterasgrey())


def main(argv=None):
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "input", nargs="?", default="-", type=png.cli_open, metavar="PNG"
    )
    args = parser.parse_args()
    return as_grey(png.binary_stdout(), args.input)


if __name__ == "__main__":
    import sys

    sys.exit(main())