Run Length Encoding

rle.py

def encode(input):
    """
    Returns the Run Length Encoding for an input string.
    """
    if not input:
        return ''

    result = []
    count = 1
    prev = input[0]

    for char in input[1:]:
        if char != prev:
            result.append(str(count) + prev)
            count = 1
            prev = char
        else:
            count += 1
    result.append(str(count) + prev)

    return ''.join(result)

def assertEqual(expected, actual):
    """
    Basic equality assertion. Prints PASS or FAIL.
    """
    result = 'FAIL'

    if expected == actual:
        result = 'PASS'

    print '{0}: {1} -> {2}'.format(result, expected, actual)

if __name__=="__main__":
    tests = [
                ('',''),
                ('a', '1a'),
                ('aa', '2a'),
                ('aaab', '3a1b'),
                ('abc', '1a1b1c'),
                ('aaaaabbcaa', '5a2b1c2a'),
                ('WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW','12W1B12W3B24W1B14W')
            ]
    for test in tests:
        assertEqual(encode(test[0]), test[1])