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Knuth-Morris-Pratt Knuth-Morris-Pratt Algorithm Algorithm Prepared by: Mayank Prepared by: Mayank Agarwal Agarwal Nitesh Maan Nitesh Maan

KMP Pattern Matching algorithm

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Knutth Morris Pratt String Matching Algorithm.

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Page 1: KMP Pattern Matching algorithm

Knuth-Morris-Pratt Knuth-Morris-Pratt AlgorithmAlgorithm

Prepared by: Mayank AgarwalPrepared by: Mayank Agarwal

Nitesh MaanNitesh Maan

Page 2: KMP Pattern Matching algorithm

The problem of String MatchingThe problem of String Matching

Given a string ‘S’, the problem of string Given a string ‘S’, the problem of string matching deals with finding whether a matching deals with finding whether a pattern ‘p’ occurs in ‘S’ and if ‘p’ does pattern ‘p’ occurs in ‘S’ and if ‘p’ does occur then returning position in ‘S’ where occur then returning position in ‘S’ where ‘p’ occurs.‘p’ occurs.

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……. a O(mn) approach. a O(mn) approach

One of the most obvious approach towards the One of the most obvious approach towards the string matching problem would be to compare string matching problem would be to compare the first element of the pattern to be searched the first element of the pattern to be searched ‘p’, with the first element of the string ‘S’ in which ‘p’, with the first element of the string ‘S’ in which to locate ‘p’. If the first element of ‘p’ matches to locate ‘p’. If the first element of ‘p’ matches the first element of ‘S’, compare the second the first element of ‘S’, compare the second element of ‘p’ with second element of ‘S’. If element of ‘p’ with second element of ‘S’. If match found proceed likewise until entire ‘p’ is match found proceed likewise until entire ‘p’ is found. If a mismatch is found at any position, found. If a mismatch is found at any position, shift ‘p’ one position to the right and repeat shift ‘p’ one position to the right and repeat comparison beginning from first element of ‘p’.comparison beginning from first element of ‘p’.

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How does the O(mn) approach How does the O(mn) approach workwork

Below is an illustration of how the previously Below is an illustration of how the previously described O(mn) approach works.described O(mn) approach works.

String S String S aa bb cc aa bb aa aa bb cc aa bb aa cc

Pattern p aa bb aa aa

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Step 1:compare p[1] with S[1]Step 1:compare p[1] with S[1]

S S aa bb cc aa bb aa aa bb cc aa bb aa cc

p aa bb aa aa

Step 2: compare p[2] with S[2]

S aa bb cc aa bb aa aa bb cc aa bb aa cc

p aa bb aa aa

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Step 3: compare p[3] with S[3]Step 3: compare p[3] with S[3]

S S

p aa bb aa aa

Mismatch occurs here..

Since mismatch is detected, shift ‘p’ one position to the left and perform steps analogous to those from step 1 to step 3. At position where mismatch is detected, shift ‘p’ one position to the right and repeat matching procedure.

aa bb cc aa bb aa aa bb cc aa bb aa cc

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S S aa bb cc aa bb aa aa bb cc aa bb aa cc

p aa bb aa aaFinally, a match would be found after shifting ‘p’ three times to the right side.

Drawbacks of this approach: if ‘m’ is the length of pattern ‘p’ and ‘n’ the length of string ‘S’, the matching time is of the order O(mn). This is a certainly a very slow running algorithm. What makes this approach so slow is the fact that elements of ‘S’ with which comparisons had been performed earlier are involved again and again in comparisons in some future iterations. For example: when mismatch is detected for the first time in comparison of p[3] with S[3], pattern ‘p’ would be moved one position to the right and matching procedure would resume from here. Here the first comparison that would take place would be between p[0]=‘a’ and S[1]=‘b’. It should be noted here that S[1]=‘b’ had been previously involved in a comparison in step 2. this is a repetitive use of S[1] in another comparison. It is these repetitive comparisons that lead to the runtime of O(mn).

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The Knuth-Morris-Pratt AlgorithmThe Knuth-Morris-Pratt Algorithm

Knuth, Morris and Pratt proposed a linear Knuth, Morris and Pratt proposed a linear time algorithm for the string matching time algorithm for the string matching problem. problem.

A matching time of O(n) is achieved by A matching time of O(n) is achieved by avoiding comparisons with elements of ‘S’ avoiding comparisons with elements of ‘S’ that have previously been involved in that have previously been involved in comparison with some element of the comparison with some element of the pattern ‘p’ to be matched. i.e., pattern ‘p’ to be matched. i.e., backtracking on the string ‘S’ never occursbacktracking on the string ‘S’ never occurs

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Components of KMP algorithmComponents of KMP algorithm The prefix function, The prefix function, ΠΠThe prefix function,The prefix function,ΠΠ for a pattern encapsulates for a pattern encapsulates

knowledge about how the pattern matches knowledge about how the pattern matches against shifts of itself. This information can be against shifts of itself. This information can be used to avoid useless shifts of the pattern ‘p’. In used to avoid useless shifts of the pattern ‘p’. In other words, this enables avoiding backtracking other words, this enables avoiding backtracking on the string ‘S’.on the string ‘S’.

The KMP MatcherThe KMP MatcherWith string ‘S’, pattern ‘p’ and prefix function ‘With string ‘S’, pattern ‘p’ and prefix function ‘ΠΠ’ as ’ as

inputs, finds the occurrence of ‘p’ in ‘S’ and inputs, finds the occurrence of ‘p’ in ‘S’ and returns the number of shifts of ‘p’ after which returns the number of shifts of ‘p’ after which occurrence is found. occurrence is found.

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The prefix function, The prefix function, ΠΠ

Following pseudocode computes the prefix fucnction, Following pseudocode computes the prefix fucnction, ΠΠ::

Compute-Prefix-Function (p)Compute-Prefix-Function (p)1 m 1 m length[p] //’p’ pattern to be matched length[p] //’p’ pattern to be matched2 2 ΠΠ[1] [1] 0 0 3 k 3 k 0 044 forfor q q 2 to m 2 to m55 do whiledo while k > 0 and p[k+1] != p[q] k > 0 and p[k+1] != p[q]6 6 dodo k k ΠΠ[k][k]77 IfIf p[k+1] = p[q] p[k+1] = p[q]88 thenthen k k k +1 k +199 ΠΠ[q] [q] k k10 10 returnreturn ΠΠ

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Example:Example: compute compute ΠΠ for the pattern ‘p’ below: for the pattern ‘p’ below:

pp aa bb aa bb aa cc aa

Initially: m = length[p] = 7 Π[1] = 0 k = 0

Step 1: q = 2, k=0 Π[2] = 0

Step 2: q = 3, k = 0, Π[3] = 1

Step 3: q = 4, k = 1 Π[4] = 2

qq 11 22 33 44 55 66 77

pp aa bb aa bb aa cc aa

ΠΠ 00 00

qq 11 22 33 44 55 66 77

pp aa bb aa bb aa cc aa

ΠΠ 00 00 11

qq 11 22 33 44 55 66 77

pp aa bb aa bb aa cc AA

ΠΠ 00 00 11 22

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Step 4: Step 4: q = 5, k =2q = 5, k =2 ΠΠ[5] = 3[5] = 3

Step 5:Step 5: q = 6, k = 3 q = 6, k = 3 ΠΠ[6] = 0[6] = 0

Step 6:Step 6: q = 7, k = 0 q = 7, k = 0 ΠΠ[7] = 1[7] = 1

After iterating 6 times, the prefix After iterating 6 times, the prefix function computation is function computation is complete: complete:

qq 11 22 33 44 55 66 77

pp aa bb aa bb aa cc aa

ΠΠ 00 00 11 22 33

qq 11 22 33 44 55 66 77

pp aa bb aa bb aa cc aa

ΠΠ 00 00 11 22 33 00

qq 11 22 33 44 55 66 77

pp aa bb aa bb aa cc aa

ΠΠ 00 00 11 22 33 00 11

qq 11 22 33 44 55 66 77

pp aa bb AA bb aa cc aa

ΠΠ 00 00 11 22 33 00 11

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The KMP MatcherThe KMP MatcherThe KMP Matcher, with pattern ‘p’, string ‘S’ and prefix function ‘The KMP Matcher, with pattern ‘p’, string ‘S’ and prefix function ‘ΠΠ’ as input, finds a match of p in S.’ as input, finds a match of p in S.Following pseudocode computes the matching component of KMP algorithm:Following pseudocode computes the matching component of KMP algorithm:KMP-Matcher(S,p)KMP-Matcher(S,p)1 n 1 n length[S] length[S] 2 m 2 m length[p] length[p]3 3 ΠΠ Compute-Prefix-Function(p) Compute-Prefix-Function(p)4 q 4 q 0 //number of characters matched 0 //number of characters matched 5 5 forfor i i 1 to n //scan S from left to right 1 to n //scan S from left to right6 6 do whiledo while q > 0 and p[q+1] != S[i] q > 0 and p[q+1] != S[i]77 dodo q q ΠΠ[q] //next character does not match[q] //next character does not match88 ifif p[q+1] = S[i] p[q+1] = S[i]99 thenthen q q q + 1 //next character matches q + 1 //next character matches1010 ifif q = m //is all of p matched? q = m //is all of p matched?1111 thenthen print “Pattern occurs with shift” i – m print “Pattern occurs with shift” i – m1212 q q ΠΠ[ q] // look for the next match[ q] // look for the next match

Note: KMP finds every occurrence of a ‘p’ in ‘S’. That is why KMP does not terminate in step 12, Note: KMP finds every occurrence of a ‘p’ in ‘S’. That is why KMP does not terminate in step 12, rather it searches remainder of ‘S’ for any more occurrences of ‘p’.rather it searches remainder of ‘S’ for any more occurrences of ‘p’.

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Illustration:Illustration: given a String ‘S’ and pattern ‘p’ as given a String ‘S’ and pattern ‘p’ as follows: follows:

S S bb aa cc bb aa bb aa bb aa bb aa cc aa cc aa

p aa bb aa bb aa cc aaLet us execute the KMP algorithm to find whether ‘p’ occurs in ‘S’.

For ‘p’ the prefix function, Π was computed previously and is as follows:

qq 11 22 33 44 55 66 77

pp aa bb AA bb aa cc aa

ΠΠ 00 00 11 22 33 11 11

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bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

aa bb aa bb aa cc aa

Initially: n = size of S = 15; m = size of p = 7

Step 1: i = 1, q = 0 comparing p[1] with S[1]

S

pP[1] does not match with S[1]. ‘p’ will be shifted one position to the right.

S

p aa bb aa bb aa cc aa

Step 2: i = 2, q = 0 comparing p[1] with S[2]

P[1] matches S[2]. Since there is a match, p is not shifted.

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Step 3: i = 3, q = 1Step 3: i = 3, q = 1

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

Comparing p[2] with S[3]

S

aa bb aa bb aa cc aa

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

aa bb aa bb aa cc aa

aa bb aa bb aa cc aap

S

p

S

p

p[2] does not match with S[3]

Backtracking on p, comparing p[1] and S[3]

Step 4: i = 4, q = 0 comparing p[1] with S[4] p[1] does not match with S[4]

Step 5: i = 5, q = 0 comparing p[1] with S[5] p[1] matches with S[5]

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bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

aa bb aa bb aa cc aa

aa bb aa bb aa cc aa

aa bb aa bb aa cc aa

Step 6: i = 6, q = 1Step 6: i = 6, q = 1

S

p

Comparing p[2] with S[6] p[2] matches with S[6]

S

p

Step 7: i = 7, q = 2Step 7: i = 7, q = 2Comparing p[3] with S[7] p[3] matches with S[7]

Step 8: i = 8, q = 3Step 8: i = 8, q = 3Comparing p[4] with S[8] p[4] matches with S[8]

S

p

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Step 9: i = 9, q = 4Step 9: i = 9, q = 4

Comparing p[5] with S[9]

Comparing p[6] with S[10]

Comparing p[5] with S[11]

Step 10: i = 10, q = 5Step 10: i = 10, q = 5

Step 11: i = 11, q = 4Step 11: i = 11, q = 4

S

S

S

p

p

p

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

aa bb aa bb aa cc aa

aa bb aa bb aa cc aa

aa bb aa bb aa cc aa

p[6] doesn’t match with S[10]

Backtracking on p, comparing p[4] with S[10] because after mismatch q = Π[5] = 3

p[5] matches with S[9]

p[5] matches with S[11]

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bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

bb aa cc bb aa bb aa bb aa bb aa cc aa aa bb

aa bb aa bb aa cc aa

aa bb aa bb aa cc aa

Step 12: i = 12, q = 5Step 12: i = 12, q = 5

Comparing p[6] with S[12]

Comparing p[7] with S[13]

S

S

p

p

Step 13: i = 13, q = 6Step 13: i = 13, q = 6

p[6] matches with S[12]

p[7] matches with S[13]

Pattern ‘p’ has been found to completely occur in string ‘S’. The total number of shifts that took place for the match to be found are: i – m = 13 – 7 = 6 shifts.

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Running - time analysisRunning - time analysis

Compute-Prefix-Function (Compute-Prefix-Function (ΠΠ))1 m 1 m length[p] //’p’ pattern to be length[p] //’p’ pattern to be

matchedmatched2 2 ΠΠ[1] [1] 0 0 3 k 3 k 0 044 forfor q q 2 to m 2 to m55 do whiledo while k > 0 and p[k+1] != p[q] k > 0 and p[k+1] != p[q]6 6 dodo k k ΠΠ[k][k]77 IfIf p[k+1] = p[q] p[k+1] = p[q]88 thenthen k k k +1 k +199 ΠΠ[q] [q] k k1010 returnreturn ΠΠ

In the above pseudocode for computing the In the above pseudocode for computing the prefix function, the for loop from step 4 prefix function, the for loop from step 4 to step 10 runs ‘m’ times. Step 1 to step to step 10 runs ‘m’ times. Step 1 to step 3 take constant time. Hence the running 3 take constant time. Hence the running time of compute prefix function is time of compute prefix function is ΘΘ(m).(m).

KMP MatcherKMP Matcher1 n 1 n length[S] length[S] 2 m 2 m length[p] length[p]3 3 ΠΠ Compute-Prefix-Function(p) Compute-Prefix-Function(p)4 q 4 q 0 0 5 5 forfor i i 1 to n 1 to n 6 6 do whiledo while q > 0 and p[q+1] != S[i] q > 0 and p[q+1] != S[i]77 dodo q q ΠΠ[q] [q] 88 ifif p[q+1] = S[i] p[q+1] = S[i]99 thenthen q q q + 1 q + 1 1010 ifif q = m q = m 1111 thenthen print “Pattern occurs with shift” i – print “Pattern occurs with shift” i –

mm1212 q q ΠΠ[ q][ q]

The for loop beginning in step 5 runs ‘n’ times, The for loop beginning in step 5 runs ‘n’ times, i.e., as long as the length of the string ‘S’. i.e., as long as the length of the string ‘S’. Since step 1 to step 4 take constant time, Since step 1 to step 4 take constant time, the running time is dominated by this for the running time is dominated by this for loop. Thus running time of matching loop. Thus running time of matching function is function is ΘΘ(n).(n).