str_to_int_iterative

def _str_to_int_inner(s):
    DIGLIM = 2048

    w5pow = {}
    w = len(s)
    while w >= DIGLIM:
        w2 = w >> 1
        if w & 1:
            w5pow[w2 + 1] = None
        w5pow[w2] = None
        w = w2
    if w5pow:
        it = reversed(w5pow)
        w = next(it)
        w5pow[w] = 5 ** w
        for w in it:
            if w - 1 in w5pow:
                val = w5pow[w - 1] * 5
            else:
                w2 = w >> 1
                val = w5pow[w2] * w5pow[w - w2]
            w5pow[w] = val

    def inner(a, b):
        if b - a <= DIGLIM:
            return int(s[a:b])
        split = (b - a) >> 1
        x = (inner(a, b - split) * w5pow[split]) << split
        y = inner(b - split, b)
        return x + y
    return inner(0, len(s))