Unit SS, Part B: String Matching, Knuth-Morris-Pratt

• Algorithm in Fig. 10.2
  • k advances in text, j in pattern
  • if pattern mismatch occurs, then use "next" position in pattern
• Explanation of "next" table example
  • mismatch pat[j] => try pat[next[j]]
  • a/0 b/1 r/1 a/0 c/2 a/0 d/2 a/0 b/1 r/1 a/0 X/5 (for X not in text)
• Explanation of "next" table example by tracing pattern pre-processing
• i=1, j=next[1]=0 **
• j==0 => i=2, j=1 (restart w. pat) pat[2]!=pat[1] => next[2]=1 ** "b" failed so backup and try "a"
• j!=0, pat[2]!=pat[1] => j=next[1]=0
• j==0 => i=3, j=1 (restart w. pat) pat[3]!=pat[1] => next[3]=1 ** "r" failed so backup and try "a"
• j!=0, pat[3]!=pat[1] => j=next[1]=0
• j==0 => i=4, j=1 (restart w. pat) pat[4]==pat[1] => next[4]=next[1]=0 ** "a" failed => advance pat
• j!=0, pat[4]==pat[1] => i=5, j=2 pat[5]!=pat[2] => next[5]=2 ** "c" failed but had "a" before so should see if text has "ab" so later would check position 2: "b"