import numpy as np


def levenshtein(source, target):
    """
    Return the Levenshtein distance between two strings.

    @param source: A C{str}.
    @param target: A C{str}.
    @return: An C{int} distance.
    """
    # Taken from http://en.wikibooks.org/wiki/Algorithm_implementation/\
    # Strings/Levenshtein_distance#Python

    if len(source) < len(target):
        return levenshtein(target, source)

    # So now we have len(source) >= len(target).
    if len(target) == 0:
        return len(source)

    # We call tuple() to force strings to be used as sequences
    # ('c', 'a', 't', 's') - numpy uses them as values by default.
    source = np.array(tuple(source))
    target = np.array(tuple(target))

    # We use a dynamic programming algorithm, but with the
    # added optimization that we only need the last two rows
    # of the matrix.
    previous_row = np.arange(target.size + 1)
    for s in source:
        # Insertion (target grows longer than source):
        current_row = previous_row + 1

        # Substitution or matching:
        # Target and source items are aligned, and either
        # are different (cost of 1), or are the same (cost of 0).
        current_row[1:] = np.minimum(
            current_row[1:],
            np.add(previous_row[:-1], target != s))

        # Deletion (target grows shorter than source):
        current_row[1:] = np.minimum(
            current_row[1:],
            current_row[0:-1] + 1)

        previous_row = current_row

    return previous_row[-1]
