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Parameterized Matching Amir, Farach, Muthukrishnan Orgad Keller Modified by Ariel Rosenfeld

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Orgad Keller - Algorithms 2 - Recitation 9 2 Definition: Two strings over the alphabet, parametrized match (p-match) if the following 3 conditions apply : Parametrized Match Relation

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Orgad Keller - Algorithms 2 - Recitation 9 3 Conditions

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Orgad Keller - Algorithms 2 - Recitation 9 4 We can see it as a bijection : Example

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Orgad Keller - Algorithms 2 - Recitation 9 5 Parametrized Matching Input: Output: All locations where p-matches.

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Orgad Keller - Algorithms 2 - Recitation 9 6 Given we’ll define : In linear time… Observation

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Orgad Keller - Algorithms 2 - Recitation 9 7 Now is over and is over and. We get the algorithm for p-match: Create Find all the places appears in (using KMP) (cond. 1+2) Find all the places m-matches in (We’ll show later how) (cond. 3) Return Observation

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Orgad Keller - Algorithms 2 - Recitation 9 8 Why is that enough? In other words: Prove there is a p-match at location iff. (HW) We are left with the question: How do we solve step 3 efficiently? Exercise

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Ariel Rosenfeld- Algorithms 2 - Recitation 9 9 M-match

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Orgad Keller - Algorithms 2 - Recitation 9 10 When is the last occurrence? We’ll build an array : So, if, we know hasn’t appeared before. Otherwise, we’ll know exactly where it had appeared last. Can we do this efficiently?

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Orgad Keller - Algorithms 2 - Recitation 9 11 Building the Array We’ll hold a Balanced Binary Search Tree for the symbols of the alphabet. Initially it will be empty. We’ll go over the pattern. For each symbol, if it isn’t in the tree, we’ll add it with it’s index and update. Otherwise, we know exactly where it had last appeared, so we’ll update and then update the symbol in the tree with the new index. Time: where.

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Orgad Keller - Algorithms 2 - Recitation 9 12 The Matching Itself We move forward if either and. We’ll hold and update a balanced BST as we go over the text as well. Time: So overall algorithm time is Can we improve this further?

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Orgad Keller - Algorithms 2 - Recitation 9 13 The Trick We’ll split the text into overlapping segments of size like this: So every match in the text must appear in whole in one of the segments. We’ll run the algorithm for each such segment. Time: where. Overall for all segments:

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© 2004 Goodrich, Tamassia Pattern Matching1. © 2004 Goodrich, Tamassia Pattern Matching2 Strings A string is a sequence of characters Examples of strings:

© 2004 Goodrich, Tamassia Pattern Matching1. © 2004 Goodrich, Tamassia Pattern Matching2 Strings A string is a sequence of characters Examples of strings:

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