The Levenshtein distance is a metric for measuring the amount
of difference between two sequences (i.e., the so called edit
distance). The Levenshtein distance between two sequences is
given by the minimum number of operations needed to transform
one sequence into the other, where an operation is an
insertion, deletion, or substitution of a single element.

The two sequences can be two strings, two arrays, or two other
objects responding to :each. All sequences are by generic
(fast) C code.

All objects in the sequences should respond to :hash and :eql?.

More information about the Levenshtein distance algorithm:
http://en.wikipedia.org/wiki/Levenshtein_distance .