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84 lines
2.1 KiB
Python
84 lines
2.1 KiB
Python
from __future__ import annotations
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def kmp(pattern: str, text: str) -> bool:
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"""
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The Knuth-Morris-Pratt Algorithm for finding a pattern within a piece of text
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with complexity O(n + m)
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1) Preprocess pattern to identify any suffixes that are identical to prefixes
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This tells us where to continue from if we get a mismatch between a character
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in our pattern and the text.
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2) Step through the text one character at a time and compare it to a character in
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the pattern updating our location within the pattern if necessary
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"""
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# 1) Construct the failure array
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failure = get_failure_array(pattern)
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# 2) Step through text searching for pattern
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i, j = 0, 0 # index into text, pattern
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while i < len(text):
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if pattern[j] == text[i]:
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if j == (len(pattern) - 1):
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return True
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j += 1
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# if this is a prefix in our pattern
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# just go back far enough to continue
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elif j > 0:
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j = failure[j - 1]
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continue
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i += 1
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return False
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def get_failure_array(pattern: str) -> list[int]:
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"""
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Calculates the new index we should go to if we fail a comparison
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:param pattern:
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:return:
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"""
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failure = [0]
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i = 0
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j = 1
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while j < len(pattern):
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if pattern[i] == pattern[j]:
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i += 1
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elif i > 0:
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i = failure[i - 1]
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continue
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j += 1
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failure.append(i)
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return failure
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if __name__ == "__main__":
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# Test 1)
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pattern = "abc1abc12"
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text1 = "alskfjaldsabc1abc1abc12k23adsfabcabc"
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text2 = "alskfjaldsk23adsfabcabc"
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assert kmp(pattern, text1) and not kmp(pattern, text2)
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# Test 2)
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pattern = "ABABX"
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text = "ABABZABABYABABX"
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assert kmp(pattern, text)
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# Test 3)
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pattern = "AAAB"
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text = "ABAAAAAB"
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assert kmp(pattern, text)
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# Test 4)
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pattern = "abcdabcy"
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text = "abcxabcdabxabcdabcdabcy"
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assert kmp(pattern, text)
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# Test 5)
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pattern = "aabaabaaa"
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assert get_failure_array(pattern) == [0, 1, 0, 1, 2, 3, 4, 5, 2]
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