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def kmp(pattern, text):

"""

The Knuth-Morris-Pratt Algorithm for finding a pattern within a piece of text

with complexity O(n + m)

1) Preprocess pattern to identify any suffixes that are identical to prefixes

This tells us where to continue from if we get a mismatch between a character

in our pattern and the text.

2) Step through the text one character at a time and compare it to a character in

the pattern updating our location within the pattern if necessary

"""

# 1) Construct the failure array

failure = get_failure_array(pattern)

# 2) Step through text searching for pattern

i, j = 0, 0 # index into text, pattern

while i < len(text):

if pattern[j] == text[i]:

if j == (len(pattern) - 1):

return True

j += 1

# if this is a prefix in our pattern

# just go back far enough to continue

elif j > 0:

j = failure[j - 1]

continue

i += 1

return False

def get_failure_array(pattern):

"""

Calculates the new index we should go to if we fail a comparison

:param pattern:

:return:

"""

failure = [0]

i = 0

j = 1

while j < len(pattern):

if pattern[i] == pattern[j]:

i += 1

elif i > 0:

i = failure[i - 1]

continue

j += 1

failure.append(i)

return failure

if __name__ == "__main__":

# Test 1)

pattern = "abc1abc12"

text1 = "alskfjaldsabc1abc1abc12k23adsfabcabc"

text2 = "alskfjaldsk23adsfabcabc"

assert kmp(pattern, text1) and not kmp(pattern, text2)

# Test 2)

pattern = "ABABX"

text = "ABABZABABYABABX"

assert kmp(pattern, text)

# Test 3)

pattern = "AAAB"

text = "ABAAAAAB"

assert kmp(pattern, text)

# Test 4)

pattern = "abcdabcy"

text = "abcxabcdabxabcdabcdabcy"

assert kmp(pattern, text)

# Test 5)

pattern = "aabaabaaa"

assert get_failure_array(pattern) == [0, 1, 0, 1, 2, 3, 4, 5, 2]

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