Unit SS, Part B: String Matching, Shift-Or

Preprocessing:

Let pat= pattern, A=pattern alphabet
Each entry "a" in A is represented by a |pat| bit string, indicating where "a" occurs in the pattern

Searching:

Initial state = |pat| 1's
Match=0 state: (state<<1)OR T[char]

For pattern "ababc" get T[a] = 11010 because "a" is in position 1 and 3 in the pattern.

Example of searching for "ababc" in text "abdabadabc"

text:    a     b     d     a     b     a     b     a     b     c
T[x]:  11010 10101 11111 11010 10101 11010 10101 11010 10101 01111
state: 11110 11101 11111 11110 11101 11010 10101 11010 10101 01111

Ex. state "10101 means there are 2 partial matches, of lengths 2,4 (starting at the right)

Advantages:

Easy to implement in hardware
Only need to store 1 char. of text (no buffering or text storage)
Can handle regular expressions: sets of chars, don't cares
Extends to don't cares or errors in text