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## Two Way Algorithm

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**Two Way Algorithm**Two-way string-matching Journal of the ACM 38(3):651-675, 1991 Crochemore M., Perrin D. Advisor: Prof. R. C. T. Lee Speaker: C. C. Yen**In 2003 ,Rytter proposed a constant space and linear time**string matching algorithm • To achieving the good constant space , this algorithm avoids the preprocessing function table of the KMP algorithm • Before introducing this algorithm , we shall define some characteristic of the strings**The Property of Maximal Suffix**• Consider a string P. Let P = uv where v = MaxSuf(P). The property of the maximal suffix of a string is: If u is non-empty, no suffix of u will be equal to a prefix of v. Example ： Consider a pattern = ababadada. Let P = uv =ababa.dada No suffix of u is equal to a prefix of v.**Short Maximal Suffix**• If a maximal suffix of a string x satisfies , we say that this maximal suffix of x is a short maximal suffix of x. Example： Consider a string x = abcdda ,dda is a maximal suffix of x and . Hence we say that dda is a short maximal suffix of x**Short Prefixes Lemma**• Let the decomposition of P = uv, where v is the maximal suffix of P and v is also a short maximal suffix. Suppose that we start to match v with T at position i, a part of v is matched and a mismatch occurs at the j +1-th position on v. Then we can shift P safely by j + 1 positions without missing any occurrence of P in T. i i+j+1 T: mismatch j j P: u v j P: v u**j**i v’ T: Why do we have to use short maximal suffix? Suppose V’ is very long, then we move pattern which is incorrect. j i v’ P: u v j j+1 T: j i P: u v**In the following , we will introduce the basic rule of the**Two Way Matching algorithm with short maximal pattern strings The basic rules are given in the next slides.**Basic rule of the Two-Way algorithm with short maximal**1. Let the decomposition of P=uv, where v is the maximal suffix of P and v is also a short maximal suffix. • We then find where v appears in T from left to right. Assume the comparison starts at position i. When a mismatch occurs at v[j + 1], we shift v with j + 1 characters and start next comparison at P[1] with T[i + j + 1]. • When the part of v has be found in T, we scan the part of u from right to left. If a mismatch occurs when scanning u, we shift P with Period(P) 4. If we find both the parts of v and u in T, we report an occurrence of P in T. We then shift v with Period(P)**Full Example**T=adadadaddadababadada P=u.v = ababa .dada**T=adadadaddadababadada**P=u.v = ababa .dada Shift 4 steps**T=adadadaddadababadada**P=u.v = ababa .dada Shift 1 steps**T=adadadaddadababadada**P=u.v = ababa .dada Shift Preiod(P) = 8 steps Rule 1 again!**T=adadadaddadababadada**P=u.v = ababa .dada Match!! Shift Preiod(P) = 8 steps**References**BRESLAUER, D., 1996, Saving comparisons in the Crochemore-Perrin string matching algorithm, Theoretical Computer Science 158(1-2):177-192. CROCHEMORE, M., 1997. Off-line serial exact string searching, in Pattern Matching Algorithms, ed. A. Apostolico and Z. Galil, Chapter 1, pp 1-53, Oxford University Press. CROCHEMORE M., PERRIN D., 1991, Two-way string-matching, Journal of the ACM 38(3):651-675. CROCHEMORE, M., RYTTER, W., 1994, Text Algorithms, Oxford University Press.