In computer science, the Apostolico–Giancarlo algorithm is a variant of the Boyer–Moore string search algorithm, the basic application of which is searching for occurrences of a pattern P in a text T . As with other comparison-based string searches, this is done by aligning P to a certain index of T and checking whether a match occurs at that index. P is then shifted relative to T according to the rules of the Boyer-Moore algorithm, and the process repeats until the end of T has been reached. Application of the Boyer-Moore shift rules often results in large chunks of the text being skipped entirely.
With regard to the shift operation, Apostolico-Giancarlo is exactly equivalent in functionality to Boyer-Moore. The utility of Apostolico-Giancarlo is to speed up the match-checking operation at any index. With Boyer-Moore, finding an occurrence of P in T requires that all n characters of P be explicitly matched. For certain patterns and texts, this is very inefficient - a simple example is when both pattern and text consist of the same repeated character, in which case Boyer-Moore runs in O ( n m ) where m is the length in characters of T . Apostolico-Giancarlo speeds this up by recording the number of characters matched at the alignments of T in a table, which is combined with data gathered during the pre-processing of P to avoid redundant equality checking for sequences of characters that are known to match.
