Chapter 7: help me understand decode_base58

[Seriousaccident]

I'm perplexed by this function on p. 139 and can't work it out for myself:

def decode_base58(s):
    num = 0
    for c in s:  # <1>
        num *= 58
        num += BASE58_ALPHABET.index(c)
    combined = num.to_bytes(25, byteorder='big')  # <2>
    checksum = combined[-4:]
    if hash256(combined[:-4])[:4] != checksum:
        raise ValueError('bad address: {} {}'.format(checksum, 
          hash256(combined[:-4])[:4]))
    return combined[1:-4]  # <3>`

The first thing I would expect us to do here would be to take the base58 string and convert it to the number it's supposed to represent. The opposite of something like this, from the encode_base58 function:

    while num > 0:
        num, mod = divmod(num, 58)
        result = BASE58_ALPHABET[mod] + result

Instead, we do this:

    for c in s: 
        num *= 58
        num += BASE58_ALPHABET.index(c)

Unless I'm reading it wrong, this is not a proper base58-to-int conversion so what is the purpose of this loop? Can someone help me parse this, please?

[Seriousaccident]

Solved. I realized I was calculating from the wrong end.