Decoding lat/long in file name

[deanhystad]

This may be more "pythonic". Added some comments now that I understand what is going on.

def decodejs(sc):
    codes = "-ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_~"
    x = y = z = i = 0
    for ch in sc:
        # Convert asci letter to int using codes.  '-' signals early end of bytes
        if (digit := codes.index(ch)-1) < 0:
            break

        # Each digit contains 3 bits of latitude (y) and 3 bits of longitude (x)
        # interleaved like this y2x2y1x1y0x0.  Extract y2y1y0 and x2x1x0
        # and add to the latitude and longitude.
        x <<= 3
        y <<= 3
        for xbit, ybit, bit in zip((2, 8, 32), (1, 4, 16), (1, 2, 4)):
            x |= bit if digit & xbit else 0
            y |= bit if digit & ybit else 0
        z += 3
        i += 1

    # This scales latitude (x) from 0...0xFFFFFF to -180...180 and
    # the latitude (y) from from 0...0xFFFFFF to -90...90.
    x = x * 2**(2 - 3 * i) * 90 - 180
    y = y * 2**(2 - 3 * i) * 45 - 90

    if i < len(sc) and sc[i] == "-":
        z -= 2
        if i + 1 < len(sc) and sc[i + 1] == "-":
            z += 1

    return z-8, y, x


print(decodejs('0GAjIv8h'))