strnum.py

#!/usr/bin/env python

"""
More TLA+ inspired coding. Using an explict step() that takes the
current state to the next state makes loop invarients easier to think
about.
"""

def str2int(s):

    def step(pair):
        (s,n) = pair
        digit = ord(s[0]) - ord('0')
        return (s[1:], 10*n + digit)

    p = (s,0)
    while p[0] != '':
        p = step(p)
    return p[1]

if __name__ == '__main__':
    assert str2int('0') == 0
    assert str2int('0000') == 0
    assert str2int('1') == 1
    assert str2int('9') == 9
    assert str2int('09') == 9         # No implied octal conversion
    assert str2int('1000') == 1000
    assert str2int('12345') == 12345
    assert str2int('') == 0           # Odd case, but seems logical because sum([]) == 0
    print 'str2int ok'


def int2str(n):

    def step(p0):
        # Let 
        #     (n',s') = step((n,s))
        # then
        #     10*n' + str2int(s')  ==  n + str2int(s)
        # Remember, I defined str2int('') == 0
        (n,s) = p0
        r = n % 10
        return (n/10, `r`+s)  # inefficient, I know.

    assert n >= 0
    p = step((n,''))
    while p[0] != 0:
        p = step(p)
    return p[1]

if __name__ == '__main__':
    assert int2str(0) == '0'
    assert int2str(1) == '1'
    assert int2str(10) == '10'
    assert int2str(1234567) == '1234567'
    for n in range(100):
        assert str2int(int2str(n)) == n
    print 'int2str ok'