132 lines
3.7 KiB
Objective-C
132 lines
3.7 KiB
Objective-C
//
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// BM.m
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// BM-Match
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//
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// Created by Smallfly on 2018/12/9.
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// Copyright © 2018 Smallfly. All rights reserved.
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//
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#import "BM.h"
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#define SIZE 256
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@interface BM()
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@property (nonatomic, strong) NSString *a; // 主串
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@property (nonatomic, strong) NSString *b; // 匹配串
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@property (nonatomic, strong) NSMutableArray *bc; // 匹配串，哈希表，存储字符在匹配串中的下标
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@property (nonatomic, strong) NSMutableArray *suffix;
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@property (nonatomic, strong) NSMutableArray *prifix;
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@end
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@implementation BM
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- (instancetype)initWithA:(NSString *)a andB:(NSString *)b {
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self = [super init];
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if (!self) return nil;
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_a = a;
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_b = b;
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_bc = [NSMutableArray new];
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_suffix = [NSMutableArray new];
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_prifix = [NSMutableArray new];
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[self generateBC];
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[self generateGS];
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return self;
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}
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// 构建坏字符哈希表，记录每个字符在匹配串中最后出现的位置
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- (void)generateBC {
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for (int i = 0; i < SIZE; ++i) {
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[_bc addObject:@-1];
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}
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for (int i = 0; i < _b.length; ++i) {
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int ascii = (int)[_b characterAtIndex:i]; // char to ASCII
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_bc[ascii] = [NSNumber numberWithInteger:i]; // save b's char index
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}
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}
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- (NSInteger)bm {
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NSInteger i = 0; // 主串和匹配串对齐的第一个字符
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NSUInteger n = _a.length;
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NSUInteger m = _b.length;
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while (i <= n - m) {
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NSInteger j;
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// 从后往前匹配
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for (j = m - 1; j >= 0; --j) {
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int aValue = (int)[_a characterAtIndex:(i + j)];
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int bValue = (int)[_b characterAtIndex:j];
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if (aValue != bValue) break; // 找到坏字符下标 j 停止
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}
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if (j < 0) {
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return i; // 匹配成功，返回所在的位置
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}
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// 坏字符在匹配串中最后出现的位置
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id numberInHashTableBC = _bc[(int)[_a characterAtIndex:(i + j)]];
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NSInteger x = j - [numberInHashTableBC integerValue];
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NSInteger y = 0;
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if (j < m - 1) {
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y = [self moveByGSBy:j];
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}
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i = i + MAX(x, y);
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// 这一步比较难理解，不直接滑到过 j，是因为在 j 之前可能存在于坏字符相同的字符
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// 这个于坏字符相同的字符，在匹配串中的最大下标是 numberInHashTableBC
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// i = i + (j - [numberInHashTableBC integerValue]);
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}
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return -1;
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}
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// 好后缀匹配移动
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- (NSInteger)moveByGSBy:(NSInteger)j {
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NSUInteger m = _b.length;
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NSInteger k = m - 1 - j; // 好后缀的长度
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NSInteger t = [_suffix[k] integerValue];
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if (t != -1) return j - t + 1; // 匹配串的前缀，是否匹配好后缀，关键
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for (NSInteger r = j+1; r <= m-1; ++r) {
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if ([_prifix[m-r] boolValue]) { // 关键
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return r;
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}
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}
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return m;
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}
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- (void)generateGS {
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NSUInteger m = _b.length;
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for (NSInteger i = 0; i < m; ++i) {
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_prifix[i] = @(NO);
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_suffix[i] = @(-1);
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}
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for (NSInteger i = 0; i < m - 1; ++i) { // 从 b 中取两个字符对比
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NSInteger j = i;
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NSInteger k = 0; // 公共后缀的长度
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int jValue = (int)[_b characterAtIndex:j];
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int bmValue = (int)[_b characterAtIndex:(m-1-k)];
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while (j >= 0 && jValue == bmValue) { // 与 b[0, m-1] 求公共子串
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++k;
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--j;
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_suffix[k] = [NSNumber numberWithInteger:(j+1)]; //j+1 代表公共子串在 b 中的起始下标
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}
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if (j == -1) _prifix[k] = @(YES);
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}
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}
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#pragma mark -
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// 同步
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- (NSInteger)startMatch {
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return [self bm];
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}
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// 异步
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- (void)startMatchCompeletion:(void (^)(NSInteger))completion {
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dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
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completion([self bm]);
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});
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}
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@end
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