Ratcliff

The Ratcliff/Obershelp Pattern Recognition algorithm (often referred to as Ratcliff/Obershelp Similarity or Gestalt Pattern Matching) is a string-matching algorithm used to find the longest common subsequence between two strings. It was described by John W. Ratcliff and David Metzener in their 1988 Dr. Dobb's Journal article.

The core concept revolves around finding the longest matching subsequence. Once found, this subsequence is considered "locked," and the algorithm recursively searches for the longest matching subsequences on the remaining parts of the original strings both before and after the locked subsequence. This process continues until no further matches can be found.

The algorithm then calculates a similarity factor based on the total length of the matched subsequences compared to the total length of the input strings. The similarity is typically expressed as twice the number of matching characters divided by the total number of characters in both strings. This results in a value between 0 and 1, where 1 represents an exact match and 0 represents no match.

The Ratcliff/Obershelp algorithm is particularly useful for fuzzy string matching, spell correction, and plagiarism detection, where minor variations between strings are expected. Its strengths lie in its simplicity and relatively good performance for many common use cases. However, its time complexity can be O(n*m) in the worst case, where n and m are the lengths of the two strings. Therefore, for very large strings, more sophisticated algorithms may be preferred. Alternatives include Levenshtein distance-based methods and more advanced sequence alignment techniques.