# CG_BoyerMoore

### Preprocessing helper functions

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import string

def z_array(s):
""" Use Z algorithm (Gusfield theorem 1.4.1) to preprocess s """
assert len(s) > 1
z = [len(s)] +  * (len(s)-1)

# Initial comparison of s[1:] with prefix
for i in range(1, len(s)):
if s[i] == s[i-1]:
z += 1
else:
break

r, l = 0, 0
if z > 0:
r, l = z, 1

for k in range(2, len(s)):
assert z[k] == 0
if k > r:
# Case 1
for i in range(k, len(s)):
if s[i] == s[i-k]:
z[k] += 1
else:
break
r, l = k + z[k] - 1, k
else:
# Case 2
# Calculate length of beta
nbeta = r - k + 1
zkp = z[k - l]
if nbeta > zkp:
# Case 2a: zkp wins
z[k] = zkp
else:
# Case 2b: Compare characters just past r
nmatch = 0
for i in range(r+1, len(s)):
if s[i] == s[i - k]:
nmatch += 1
else:
break
l, r = k, r + nmatch
z[k] = r - k + 1
return z

def n_array(s):
""" Compile the N array (Gusfield theorem 2.2.2) from the Z array """
return z_array(s[::-1])[::-1]

def big_l_prime_array(p, n):
""" Compile L' array (Gusfield theorem 2.2.2) using p and N array.
L'[i] = largest index j less than n such that N[j] = |P[i:]| """
lp =  * len(p)
for j in range(len(p)-1):
i = len(p) - n[j]
if i < len(p):
lp[i] = j + 1
return lp

def big_l_array(p, lp):
""" Compile L array (Gusfield theorem 2.2.2) using p and L' array.
L[i] = largest index j less than n such that N[j] >= |P[i:]| """
l =  * len(p)
l = lp
for i in range(2, len(p)):
l[i] = max(l[i-1], lp[i])
return l

def small_l_prime_array(n):
""" Compile lp' array (Gusfield theorem 2.2.4) using N array. """
small_lp =  * len(n)
for i in range(len(n)):
if n[i] == i+1:  # prefix matching a suffix
small_lp[len(n)-i-1] = i+1
for i in range(len(n)-2, -1, -1):  # "smear" them out to the left
if small_lp[i] == 0:
small_lp[i] = small_lp[i+1]
return small_lp

def good_suffix_table(p):
""" Return tables needed to apply good suffix rule. """
n = n_array(p)
lp = big_l_prime_array(p, n)
return lp, big_l_array(p, lp), small_l_prime_array(n)

def good_suffix_mismatch(i, big_l_prime, small_l_prime):
""" Given a mismatch at offset i, and given L/L' and l' arrays,
return amount to shift as determined by good suffix rule. """
length = len(big_l_prime)
assert i < length
if i == length - 1:
return 0
i += 1  # i points to leftmost matching position of P
if big_l_prime[i] > 0:
return length - big_l_prime[i]
return length - small_l_prime[i]

def good_suffix_match(small_l_prime):
""" Given a full match of P to T, return amount to shift as
determined by good suffix rule. """
return len(small_l_prime) - small_l_prime

def dense_bad_char_tab(p, amap):
""" Given pattern string and list with ordered alphabet characters, create
and return a dense bad character table.  Table is indexed by offset
then by character. """
tab = []
nxt =  * len(amap)
for i in range(0, len(p)):
c = p[i]
assert c in amap
tab.append(nxt[:])
nxt[amap[c]] = i+1
return tab

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### Class for preprocessed pattern

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class BoyerMoore(object):
""" Encapsulates pattern and associated Boyer-Moore preprocessing. """

def __init__(self, p, alphabet='ACGT'):
# Create map from alphabet characters to integers
self.amap = {alphabet[i]: i for i in range(len(alphabet))}
# Make bad character rule table
self.bad_char = dense_bad_char_tab(p, self.amap)
# Create good suffix rule table
_, self.big_l, self.small_l_prime = good_suffix_table(p)

def bad_character_rule(self, i, c):
""" Return # skips given by bad character rule at offset i """
assert c in self.amap
assert i < len(self.bad_char)
ci = self.amap[c]
return i - (self.bad_char[i][ci]-1)

def good_suffix_rule(self, i):
""" Given a mismatch at offset i, return amount to shift
as determined by (weak) good suffix rule. """
length = len(self.big_l)
assert i < length
if i == length - 1:
return 0
i += 1  # i points to leftmost matching position of P
if self.big_l[i] > 0:
return length - self.big_l[i]
return length - self.small_l_prime[i]

def match_skip(self):
""" Return amount to shift in case where P matches T """
return len(self.small_l_prime) - self.small_l_prime

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### Matching function

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def boyer_moore(p, p_bm, t):
""" Do Boyer-Moore matching """
i = 0
occurrences = []
while i < len(t) - len(p) + 1:
shift = 1
mismatched = False
for j in range(len(p)-1, -1, -1):
if p[j] != t[i+j]:
skip_bc = p_bm.bad_character_rule(j, t[i+j])
skip_gs = p_bm.good_suffix_rule(j)
shift = max(shift, skip_bc, skip_gs)
mismatched = True
break
if not mismatched:
occurrences.append(i)
skip_gs = p_bm.match_skip()
shift = max(shift, skip_gs)
i += shift
return occurrences

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In :

t = 'haystack needle haystack' # "text" - thing we search in
p = 'needle' # "pattern" - thing we search for

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In :

p_bm = BoyerMoore(p, alphabet='abcdefghijklmnopqrstuvwxyz')

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In :

boyer_moore(p, p_bm, t)

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Out:



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In :

t[9:9+len(p)]  # confirm it really occurs

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Out:

'needle'

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In :

boyer_moore(p, p_bm, 'needleneedleneedle')

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Out:

[0, 6, 12]

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In :

t

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Out:

'haystack needle haystack'

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