Created
May 19, 2018 17:48
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Boyer-Moore algorithm
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| import java.util.HashMap; | |
| public class BM { | |
| public static void main(String[] args) { | |
| String source = "abcde"; | |
| String template = "cd"; | |
| System.out.println(getFirstEntry(source, template)); | |
| } | |
| public static HashMap<Character, Integer> makeOffsetTable(String pattern) { | |
| HashMap<Character, Integer> offsetTable = new HashMap<Character, Integer>(); | |
| for (int i = 0; i <= 255; i++) { | |
| offsetTable.put((char) i, pattern.length()); | |
| } | |
| for (int i = 0; i < pattern.length() - 1; i++) { | |
| offsetTable.put(pattern.charAt(i), pattern.length() - i - 1); | |
| } | |
| return offsetTable; | |
| } | |
| public static int getFirstEntry(String s, String p) { | |
| HashMap<Character, Integer> offsetTable = makeOffsetTable(p); | |
| if (s.length() < p.length()) { | |
| return -1; | |
| } | |
| int i = p.length() - 1; | |
| int j = i; | |
| int k = i; | |
| while (j >= 0 && i <= s.length() - 1) { | |
| j = p.length() - 1; | |
| k = i; | |
| while (j >= 0 && s.charAt(k) == p.charAt(j)) { | |
| k--; | |
| j--; | |
| } | |
| i += offsetTable.get(s.charAt(i)); | |
| } | |
| if (k >= s.length() - p.length()) { | |
| return -1; | |
| } else { | |
| return k + 1; | |
| } | |
| } | |
| } |
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