Create KMP.py

Blooverh · Blooverh · commit c7375bad4798 · 2023-09-05T23:05:17.000-05:00
diff --git a/TextProcessingChpt/KMP.py b/TextProcessingChpt/KMP.py
@@ -0,0 +1,89 @@
+
+
+def find_KMP(T, P):
+    """Return the lowest index of T at which substring P begins else -1"""
+
+    n = len(T)
+    m = len(P)
+
+    if m == 0 :
+        return 0
+    
+    fail = compute_kmp_fail(P) # utility method to precompute
+
+    j=0
+    k=0
+
+    while j < n:
+        if T[j] == P[k]:
+            if k == m-1: # P[0: 1+k] matched thus far
+                return j - m+1 # match is complete return first index where P starts in T
+            
+            j+=1
+            k+=1
+        elif k > 0: 
+            k =  fail[k-1] # reuse suffix of P[0:k]
+        else:
+            j+=1
+    
+    return -1 #reached end without match
+
+"""
+a t c a m a l g a m a m a l g a m a t i o n
+  a |m a l g a m a t i o n
+    a |m a l g a m a t i o n
+      a m a l g a m a|t i o n
+                a m a|l g a m a t i o n
+                    a m a l g a m a t i o n
+"""
+
+def compute_kmp_fail(P):
+    """Utility that computes and returns KMP fail list"""
+
+    m=  len(P)
+
+    fail = [0] * m # by default presum overlap of 0 everywhere
+    j=1
+    k=0
+
+    while j < m: # compute f(j) during this pass is nonzero
+        if P[j] == P[k]: # k+1 character match thus far
+            fail[j] = k+1
+            j+=1
+            k+=1
+        elif k >0:
+            k = fail[k-1] # k follows a matching prefix
+        else: # no match foung starting at j
+            j+=1
+
+    return fail
+
+
+"""
+a m a l g a m a t i o n
+0 1 2 3 4 5 6 7 8 9 10 11
+
+    m    ==  a
+    p[1] == p[0] - false j+=1 = j= 2 - fail -> [0,0,0,0,0,0,0,0,0,0,0,0]
+    j=2 
+    k=0
+
+    a   ==  a
+    p[2] == P[0] - true fail[2] = 1 - fail -> [0,0,1,0,0,0,0,0,0,0,0,0]
+    j+=1 - j=3
+    k+=1 = k=1
+
+    l    ==   m
+    p[3] == k[1] - false k = fail[k-1] - k = fail[1-1] - [0,0,1,0,0,0,0,0,0,0,0,0]
+    j=3
+    k=0
+
+    .
+    .
+    .
+    .
+
+    fail = [0,0,1,0,0,1,2,3,0,0,0,0]
+
+
+"""
