// Textbook fragment 12.03 ---- Goodrich /** Simplified version of the Boyer-Moore algorithm, which uses * only the looking-glass and bad character-jump heuristics. * @return Index of the beginning of the leftmost substring of the text T * matching the pattern P, or -1 if there is no match. */ /* -------------------------------- Code simplified by S.Y.Cheung -------------------------------- */ import java.util.*; public class BMG { /* =================================================================== buildLastFunction(P): find the last occurrence of each char in pattern P 01234567 Example: P = abcddacx Result: lastOcc['a'] = 5 lastOcc['b'] = 1 lastOcc['c'] = 1 lastOcc['d'] = 4 lastOcc['x'] = 7 All other array elements has -1 (= no last char) =================================================================== */ public static int[] computeLastOcc(String P) { int[] lastOcc = new int[128]; // assume ASCII character set for (int i = 0; i < 128; i++) { lastOcc[i] = -1; // initialize all elements to -1 } for (int i = 0; i < P.length(); i++) { lastOcc[P.charAt(i)] = i; // The LAST value will be store } return lastOcc; } public static int BMH (String T, String P) { int[] lastOcc; int i0, j, m, n; n = T.length(); m = P.length(); lastOcc = computeLastOcc(P); // Find last occurence of all characters in P printLastOcc(lastOcc); i0 = 0; // Line P up at T[0] while ( i0 < (n-m) ) { j = m-1; // Start at the last char in P System.out.println("+++++++++++++++++++++++++++++++++++++"); printState( T, P, i0+j, j); while ( P.charAt(j) == T.charAt(i0+j) ) { j--; // Check "next" (= previous) character printState( T, P, i0+j, j); if ( j < 0 ) return (i0); // P found ! } if ( j < lastOcc[T.charAt(i0+j)] ) { System.out.println("** Bad character caveat detected - slide 1 pos...."); i0++; } else { System.out.println("****** lastOcc['" + T.charAt(i0+j) + "'] = " + lastOcc[T.charAt(i0+j)] ); i0 = i0 + j - lastOcc[T.charAt(i0+j)]; } } return -1; // no match } public static void printLastOcc(int[] lastOcc) { int i, j=0; System.out.println(); for (i = (int)'a'; i <= (int)'z'; i++) { System.out.print((char)i + " " + lastOcc[i] + "; "); if (++j % 13 == 0) System.out.println(); } System.out.println(); System.out.println(); } public static void main(String[] args) { Scanner in = new Scanner(System.in); String T, P; int r; System.out.println("Try"); System.out.println("T = abacaxbaccabacbbaabb"); System.out.println("P = abacbb"); System.out.println(); System.out.print("T = "); T = in.next(); System.out.print("P = "); P = in.next(); System.out.println(); System.out.println("012345678901234567890"); System.out.println(T); System.out.println("012345678901234567890"); System.out.println(P); System.out.println(); r = BMH (T, P); System.out.println("Found " + P + " at pos: " + r); } /* ===================================================== Variables and Methods to make the algorithm visual ===================================================== */ public static String T_ruler, P_ruler; public static String ruler(int n) { String out = ""; char x = '0'; for ( int i = 0; i < n; i++ ) { out = out + x; x++; if ( x > '9' ) x = '0'; } return out; } public static void printState(String T, String P, int i, int j) { if ( T_ruler == null ) T_ruler = ruler( T.length() ); if ( P_ruler == null ) P_ruler = ruler( P.length() ); System.out.println("====================================="); System.out.println("Matching: i = " + i + ", j = " + j); System.out.println(" " + T_ruler ); System.out.println(" " + T); System.out.print(" "); for ( int k = 0; k < i-j; k++) System.out.print(" "); System.out.println(P); System.out.print(" "); for ( int k = 0; k < i-j; k++) System.out.print(" "); System.out.println( P_ruler ); System.out.print(" "); for ( int k = 0; k < i; k++) System.out.print(" "); System.out.println("^"); System.out.print(" "); for ( int k = 0; k < i; k++) System.out.print(" "); System.out.println("|"); System.out.println(); } }