java 实现的Boyer-Moore(BM)算法详解编程语言

BM算法是一种高效的单模查找算法，可以加大查找步长，效率很高, 这是java实现的版本

    import java.util.Arrays;   
    import java.util.HashMap;   
    import java.util.List;   
    import java.util.ArrayList;   
    import java.util.Map;   
 
    public class BoyerMoore {   
        public static List<Integer> match(String pattern, String text) {   
            List<Integer> matches = new ArrayList<Integer>();   
            int m = text.length();   
            int n = pattern.length();   
            Map<Character, Integer> rightMostIndexes = preprocessForBadCharacterShift(pattern);   
            int alignedAt = 0;   
            while (alignedAt + (n - 1) < m) {   
                for (int indexInPattern = n - 1; indexInPattern >= 0; indexInPattern--) {   
                int indexInText = alignedAt + indexInPattern;   
                char x = text.charAt(indexInText);   
                char y = pattern.charAt(indexInPattern);   
                    if (indexInText >= m) break;   
                    if (x != y) {   
                        Integer r = rightMostIndexes.get(x);   
                        if (r == null) {   
                            alignedAt = indexInText + 1;   
                        }   
                        else {   
                            int shift = indexInText - (alignedAt + r);   
                            alignedAt += shift > 0 ? shift : 1;   
                        }   
                        break;   
                    }   
                    else if (indexInPattern == 0) {   
                        matches.add(alignedAt);   
                        alignedAt++;   
                    }   
                }   
            }   
            return matches;   
        }   
        private static Map<Character, Integer> preprocessForBadCharacterShift(   
                String pattern) {   
            Map<Character, Integer> map = new HashMap<Character, Integer>();   
            for (int i = pattern.length() - 1; i >= 0; i--) {   
                char c = pattern.charAt(i);   
                if (!map.containsKey(c)) map.put(c, i);   
            }   
            return map;   
        }   
        public static void main(String[] args) {   
            List<Integer> matches = match("ana", "bananas");   
            for (Integer integer : matches) System.out.println("Match at: " + integer);   
            System.out.println((matches.equals(Arrays.asList(1, 3)) ? "OK" : "Failed"));   
        }   
    }