总结：
AC算法的核心是三张查找表：goto、failure和output，共包含四种具体的算法，分别是计算三张查找表的算法以及AC算法本身。
可以先阅读博客（http://tech.meituan.com/ac.html）了解goto表、failure表。
下述文章（http://blog.csdn.net/myjoying/article/details/7960534）博客转发。
早在1975年贝尔实验室的两位研究人员Alfred V. Aho 和Margaret J. Corasick就提出了以他们的名字命名的高效的匹配算法—AC算法。该算法几乎与KMP算法同时问世。与KMP算法相同，AC算法时至今日仍然在模式匹配领域被广泛应用。
最近本人由于兴趣所致，在学习完KMP和BM单模式匹配算法之后，开始学习多模式匹配算法。看了一些博文和中文文章之后发现学习AC算法的最有效方式仍然是Aho和Corasick的论文《Efficient String Matching: An Aid to Bibliographic Search》。本文算是对这篇文章的一些总结。
AC算法思想
多模式匹配AC算法的核心仍然是寻找模式串内部规律，达到在每次失配时的高效跳转。这一点与单模式匹配KMP算法。不同的是，AC算法寻找的是模式串之间的相同前缀关系。
构造goto表
goto表本质上是一个有限状态机，这里称作模式匹配机（Pattern Matching Machine，PMM）。下面以论文中的例子来说明goto表的构造过程。对于模式串集合K{he, she, his, hers}
第一步：PMM初始状态为0，然后向PMM中加入第一个模式串K[0] = “he”。
第二步：继续向PMM中添加第二个模式串K[1] = “she”，每次添加都是从状态0开始扫描。
第三步：从状态0开始继续添加第三个模式串K[2] = “his”，这里值得注意的是遇到相同字符跳转时要重复利用以前已经生成的跳转。如这里的’h’在第一步中已经存在。
第四步：添加模式串K[3] = “hers”。至此，goto表已经构造完成。
构造failure表
failure表作用是在goto表中匹配失败后状态跳转的依据，这点与KMP中next表的作用相似。首先引入状态深度的概念，状态s的深度depth(s)定义为在goto表中从起始状态0到状态s的最短路径长度。如goto表中状态1和3的深度为1。
构造failure表利用到了递归的思想。
1、若depth(s) = 1，则f(s) = 0；
2、假设对于depth(r) < d的所有状态r，已近计算出了f(r)；
3、对于深度为d的状态s：
(1) 若g(r,a) = fail，对于所有的a，则不动作；（注：a为字符，g为状态转移函数）；
(2) 否则，对于a使得g(r,a) = s，则如下步骤：
a、使state = f(r)
b、重复步骤state = f(state)，直到g(state, a) != fail。（注意对于任意的a，状态0的g(0,a) != fail）
c、使f(s) = g(state, a)。
根据以上算法，得到该例子的failure表为：
i 1 2 3 4 5 6 7 8 9
f(i) 0 0 0 1 2 0 3 0 3 构造output表
output表示输出，即代表到达某个状态后某个模式串匹配成功。该表的构造过程融合在goto表和failure表的构造过程中。
i output(i)
2 {he}
5 {she}
7 {his}
9 {hers}
2、在构造failure表时，若f(s) = s’，则将s和s‘对应的output集合求并集。如f(5) = 2，则得到最终的output表为：
i output(i)
2 {he}
5 {she，he}
7 {his}
9 {hers}
AC算法实现
根据上面已经构造好的goto、failure和output表就可以方便地应用AC算法了。
u s h e r s
0 0 3 4 5 8 9
2
说明：在状态5发生失配，查找failure表，转到状态2继续比较。在状态5和状态9有输出。
算法改进
值得注意的是在AC算法的以上实现中，对于输入字符a的跳转次数是不确定的。因为有可能在输入a后发生失配，需要查找failure表重新跳转。能不能在PMM的基础上实现确定型的有限状态机，即对于任意字符a，都只用进行一次确定的状态跳转？答案是肯定的。在Aho和Corasick论文中给出了处理的方法：构造一个与KMP算法中相似的next表，实现确定性跳转。
next表的构造需要在goto表和failure表的基础上得到。next表如下所示：
输入字符 下一状态
state 0: h 1
s 3
* 0
state 1: e 2
i 6
h 1
s 3
* 0
state9,7,3: h 4
s 3
* 0
state5,2: r 8
h 1
s 3
* 0
state 6: s 7
h 1
* 0
state 4: e 5
i 6
h 1
s 3
* 0
state 8: s 9
h 1
* 0注：*表示除了以上字符以外的其他字符。存储next数组所需要的内存空间比存储goto表更大。这样就可以用next表替代goto表和failure表，实现每次字符输入的唯一跳转。
acsmx.h
/* ** Copyright (C) 2002 Martin Roesch <[email protected]> ** ** This program is free software; you can redistribute it and/or modify ** it under the terms of the GNU General Public License as published by ** the Free Software Foundation; either version 2 of the License, or ** (at your option) any later version. ** ** This program is distributed in the hope that it will be useful, ** but WITHOUT ANY WARRANTY; without even the implied warranty of ** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ** GNU General Public License for more details. ** ** You should have received a copy of the GNU General Public License ** along with this program; if not, write to the Free Software ** Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */
/* ** ACSMX.H ** ** */
#ifndef ACSMX_H
#define ACSMX_H
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* * Prototypes */
#define ALPHABET_SIZE 256
#define MAXLEN 256
#define ACSM_FAIL_STATE -1
typedef struct _acsm_pattern {
struct _acsm_pattern *next;
unsigned char *patrn;
unsigned char *casepatrn;
int n;
int nocase;
void * id;
int nmatch;
} ACSM_PATTERN;
typedef struct {
/* Next state - based on input character */
int NextState[ ALPHABET_SIZE ];
/* Failure state - used while building NFA & DFA */
int FailState;
/* List of patterns that end here, if any */
ACSM_PATTERN *MatchList;
}ACSM_STATETABLE;
/* * State machine Struct */
typedef struct {
int acsmMaxStates;
int acsmNumStates;
ACSM_PATTERN * acsmPatterns;
ACSM_STATETABLE * acsmStateTable;
}ACSM_STRUCT;
/* * Prototypes */
ACSM_STRUCT * acsmNew ();
int acsmAddPattern( ACSM_STRUCT * p, unsigned char * pat, int n,int nocase);
int acsmCompile ( ACSM_STRUCT * acsm );
//int acsmSearch ( ACSM_STRUCT * acsm,unsigned char * T, int n, int (*Match) (ACSM_PATTERN * pattern,ACSM_PATTERN * mlist, int nline,int index));
int acsmSearch (ACSM_STRUCT * acsm, unsigned char *Tx, int n,void (*PrintMatch) (ACSM_PATTERN * pattern,ACSM_PATTERN * mlist, int nline,int index));
void acsmFree ( ACSM_STRUCT * acsm );
void PrintMatch (ACSM_PATTERN * pattern,ACSM_PATTERN * mlist, int nline,int index) ;
void PrintSummary (ACSM_PATTERN * pattern) ;
#endifacsmx.c
/*
**
** Multi-Pattern Search Engine
**
** Aho-Corasick State Machine - uses a Deterministic Finite Automata - DFA
**
** Copyright (C) 2002 Sourcefire,Inc.
** Marc Norton
**
**
** This program is free software; you can redistribute it and/or modify
** it under the terms of the GNU General Public License as published by
** the Free Software Foundation; either version 2 of the License, or
** (at your option) any later version.
**
** This program is distributed in the hope that it will be useful,
** but WITHOUT ANY WARRANTY; without even the implied warranty of
** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
** GNU General Public License for more details.
**
** You should have received a copy of the GNU General Public License
** along with this program; if not, write to the Free Software
** Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
**
**
** Reference - Efficient String matching: An Aid to Bibliographic Search
** Alfred V Aho and Margaret J Corasick
** Bell Labratories
** Copyright(C) 1975 Association for Computing Machinery,Inc
**
** Implemented from the 4 algorithms in the paper by Aho & Corasick
** and some implementation ideas from 'Practical Algorithms in C'
**
** Notes:
** 1) This version uses about 1024 bytes per pattern character - heavy on the memory.
** 2) This algorithm finds all occurrences of all patterns within a
** body of text.
** 3) Support is included to handle upper and lower case matching.
** 4) Some comopilers optimize the search routine well, others don't, this makes all the difference.
** 5) Aho inspects all bytes of the search text, but only once so it's very efficient,
** if the patterns are all large than the Modified Wu-Manbar method is often faster.
** 6) I don't subscribe to any one method is best for all searching needs,
** the data decides which method is best,
** and we don't know until after the search method has been tested on the specific data sets.
**
**
** May 2002 : Marc Norton 1st Version
** June 2002 : Modified interface for SNORT, added case support
** Aug 2002 : Cleaned up comments, and removed dead code.
** Nov 2,2002: Fixed queue_init() , added count=0
**
** Wangyao : [email protected]
**
** Apr 24,2007: WangYao Combined Build_NFA() and Convert_NFA_To_DFA() into Build_DFA();
** And Delete Some redundancy Code
**
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include "acsmx.h"
#define MEMASSERT(p,s) if(!p){fprintf(stderr,"ACSM-No Memory: %s!\n",s);exit(0);}
/*Define the number of the line,when match a keyword*/
extern int nline=1;
/*
* Malloc the AC Memory
*/
static void *AC_MALLOC (int n)
{
void *p;
p = malloc (n);
return p;
}
/*
*Free the AC Memory
*/
static void AC_FREE (void *p)
{
if (p)
free (p);
}
/*
* Simple QUEUE NODE
*/
typedef struct _qnode
{
int state;
struct _qnode *next;
}QNODE;
/*
* Simple QUEUE Structure
*/
typedef struct _queue
{
QNODE * head, *tail;
int count;
}QUEUE;
/*
*Init the Queue
*/
static void queue_init (QUEUE * s)
{
s->head = s->tail = 0;
s->count = 0;
}
/*
* Add Tail Item to queue
*/
static void queue_add (QUEUE * s, int state)
{
QNODE * q;
/*Queue is empty*/
if (!s->head)
{
q = s->tail = s->head = (QNODE *) AC_MALLOC (sizeof (QNODE));
/*if malloc failed,exit the problom*/
MEMASSERT (q, "queue_add");
q->state = state;
q->next = 0; /*Set the New Node's Next Null*/
}
else
{
q = (QNODE *) AC_MALLOC (sizeof (QNODE));
MEMASSERT (q, "queue_add");
q->state = state;
q->next = 0;
/*Add the new Node into the queue*/
s->tail->next = q;
/*set the new node is the Queue's Tail*/
s->tail = q;
}
s->count++;
}
/*
* Remove Head Item from queue
*/
static int queue_remove (QUEUE * s)
{
int state = 0;
QNODE * q;
/*Remove A QueueNode From the head of the Queue*/
if (s->head)
{
q = s->head;
state = q->state;
s->head = s->head->next;
s->count--;
/*If Queue is Empty,After Remove A QueueNode*/
if (!s->head)
{
s->tail = 0;
s->count = 0;
}
/*Free the QueNode Memory*/
AC_FREE (q);
}
return state;
}
/*
*Return The count of the Node in the Queue
*/
static int queue_count (QUEUE * s)
{
return s->count;
}
/*
*Free the Queue Memory
*/
static void queue_free (QUEUE * s)
{
while (queue_count (s))
{
queue_remove (s);
}
}
/*
** Case Translation Table
*/
static unsigned char xlatcase[256];
/*
* Init the xlatcase Table,Trans alpha to UpperMode
* Just for the NoCase State
*/
static void init_xlatcase ()
{
int i;
for (i = 0; i < 256; i++)
{
xlatcase[i] = toupper (i);
}
}
/*
*Convert the pattern string into upper
*/
static void ConvertCaseEx (unsigned char *d, unsigned char *s, int m)
{
int i;
for (i = 0; i < m; i++)
{
d[i] = xlatcase[s[i]];
}
}
/*
* Add a pattern to the list of patterns terminated at this state.
* Insert at front of list.
*/
static void AddMatchListEntry (ACSM_STRUCT * acsm, int state, ACSM_PATTERN * px)
{
ACSM_PATTERN * p;
p = (ACSM_PATTERN *) AC_MALLOC (sizeof (ACSM_PATTERN));
MEMASSERT (p, "AddMatchListEntry");
memcpy (p, px, sizeof (ACSM_PATTERN));
/*Add the new pattern to the pattern list*/
p->next = acsm->acsmStateTable[state].MatchList;
acsm->acsmStateTable[state].MatchList = p;
}
/*
* Add Pattern States
*/
static void AddPatternStates (ACSM_STRUCT * acsm, ACSM_PATTERN * p)
{
unsigned char *pattern;
int state=0, next, n;
n = p->n; /*The number of alpha in the pattern string*/
pattern = p->patrn;
/*
* Match up pattern with existing states
*/
for (; n > 0; pattern++, n--)
{
next = acsm->acsmStateTable[state].NextState[*pattern];
if (next == ACSM_FAIL_STATE)
break;
state = next;
}
/*
* Add new states for the rest of the pattern bytes, 1 state per byte
*/
for (; n > 0; pattern++, n--)
{
acsm->acsmNumStates++;
acsm->acsmStateTable[state].NextState[*pattern] = acsm->acsmNumStates;
state = acsm->acsmNumStates;
}
/*Here,An accept state,just add into the MatchListof the state*/
AddMatchListEntry (acsm, state, p);
}
/*
* Build Non-Deterministic Finite Automata
*/
static void Build_DFA (ACSM_STRUCT * acsm)
{
int r, s;
int i;
QUEUE q, *queue = &q;
ACSM_PATTERN * mlist=0;
ACSM_PATTERN * px=0;
/* Init a Queue */
queue_init (queue);
/* Add the state 0 transitions 1st */
/*1st depth Node's FailState is 0, fail(x)=0 */
for (i = 0; i < ALPHABET_SIZE; i++)
{
s = acsm->acsmStateTable[0].NextState[i];
if (s)
{
queue_add (queue, s);
acsm->acsmStateTable[s].FailState = 0;
}
}
/* Build the fail state transitions for each valid state */
while (queue_count (queue) > 0)
{
r = queue_remove (queue);
/* Find Final States for any Failure */
for (i = 0; i < ALPHABET_SIZE; i++)
{
int fs, next;
/*** Note NextState[i] is a const variable in this block ***/
if ((s = acsm->acsmStateTable[r].NextState[i]) != ACSM_FAIL_STATE)
{
queue_add (queue, s);
fs = acsm->acsmStateTable[r].FailState;
/*
* Locate the next valid state for 'i' starting at s
*/
/**** Note the variable "next" ****/
/*** Note "NextState[i]" is a const variable in this block ***/
while ((next=acsm->acsmStateTable[fs].NextState[i]) ==
ACSM_FAIL_STATE)
{
fs = acsm->acsmStateTable[fs].FailState;
}
/*
* Update 's' state failure state to point to the next valid state
*/
acsm->acsmStateTable[s].FailState = next;
}
else
{
acsm->acsmStateTable[r].NextState[i] =
acsm->acsmStateTable[acsm->acsmStateTable[r].FailState].NextState[i];
}
}
}
/* Clean up the queue */
queue_free (queue);
}
/*
* Init the acsm DataStruct
*/
ACSM_STRUCT * acsmNew ()
{
ACSM_STRUCT * p;
init_xlatcase ();
p = (ACSM_STRUCT *) AC_MALLOC (sizeof (ACSM_STRUCT));
MEMASSERT (p, "acsmNew");
if (p)
memset (p, 0, sizeof (ACSM_STRUCT));
return p;
}
/*
* Add a pattern to the list of patterns for this state machine
*/
int acsmAddPattern (ACSM_STRUCT * p, unsigned char *pat, int n, int nocase)
{
ACSM_PATTERN * plist;
plist = (ACSM_PATTERN *) AC_MALLOC (sizeof (ACSM_PATTERN));
MEMASSERT (plist, "acsmAddPattern");
plist->patrn = (unsigned char *) AC_MALLOC (n+1);
memset(plist->patrn+n,0,1);
ConvertCaseEx (plist->patrn, pat, n);
plist->casepatrn = (unsigned char *) AC_MALLOC (n+1);
memset(plist->casepatrn+n,0,1);
memcpy (plist->casepatrn, pat, n);
plist->n = n;
plist->nocase = nocase;
plist->nmatch=0;
/*Add the pattern into the pattern list*/
plist->next = p->acsmPatterns;
p->acsmPatterns = plist;
return 0;
}
/*
* Compile State Machine
*/
int acsmCompile (ACSM_STRUCT * acsm)
{
int i, k;
ACSM_PATTERN * plist;
/* Count number of states */
acsm->acsmMaxStates = 1; /*State 0*/
for (plist = acsm->acsmPatterns; plist != NULL; plist = plist->next)
{
acsm->acsmMaxStates += plist->n;
}
acsm->acsmStateTable = (ACSM_STATETABLE *) AC_MALLOC (sizeof (ACSM_STATETABLE) * acsm->acsmMaxStates);
MEMASSERT (acsm->acsmStateTable, "acsmCompile");
memset (acsm->acsmStateTable, 0,sizeof (ACSM_STATETABLE) * acsm->acsmMaxStates);
/* Initialize state zero as a branch */
acsm->acsmNumStates = 0;
/* Initialize all States NextStates to FAILED */
for (k = 0; k < acsm->acsmMaxStates; k++)
{
for (i = 0; i < ALPHABET_SIZE; i++)
{
acsm->acsmStateTable[k].NextState[i] = ACSM_FAIL_STATE;
}
}
/* This is very import */
/* Add each Pattern to the State Table */
for (plist = acsm->acsmPatterns; plist != NULL; plist = plist->next)
{
AddPatternStates (acsm, plist);
}
/* Set all failed state transitions which from state 0 to return to the 0'th state */
for (i = 0; i < ALPHABET_SIZE; i++)
{
if (acsm->acsmStateTable[0].NextState[i] == ACSM_FAIL_STATE)
{
acsm->acsmStateTable[0].NextState[i] = 0;
}
}
/* Build the NFA */
Build_DFA (acsm);
return 0;
}
/*64KB Memory*/
static unsigned char Tc[64*1024];
/*
* Search Text or Binary Data for Pattern matches
*/
int acsmSearch (ACSM_STRUCT * acsm, unsigned char *Tx, int n,void (*PrintMatch)(ACSM_PATTERN * pattern,ACSM_PATTERN * mlist, int nline,int index)) { int state; ACSM_PATTERN * mlist; unsigned char *Tend; ACSM_STATETABLE * StateTable = acsm->acsmStateTable; int nfound = 0; /*Number of the found(matched) patten string*/ unsigned char *T; int index; /* Case conversion */ ConvertCaseEx (Tc, Tx, n); T = Tc; Tend = T + n; for (state = 0; T < Tend; T++) { state = StateTable[state].NextState[*T]; /* State is a accept state? */ if( StateTable[state].MatchList != NULL ) { for( mlist=StateTable[state].MatchList; mlist!=NULL; mlist=mlist->next ) { /*Get the index of the Match Pattern String in the Text*/ index = T - mlist->n + 1 - Tc; //mlist->nmatch++; nfound++; PrintMatch (acsm->acsmPatterns,mlist, nline,index); } } } return nfound; } /* * Free all memory */ void acsmFree (ACSM_STRUCT * acsm) { int i; ACSM_PATTERN * mlist, *ilist; for (i = 0; i < acsm->acsmMaxStates; i++) { if (acsm->acsmStateTable[i].MatchList != NULL) { mlist = acsm->acsmStateTable[i].MatchList; while (mlist) { ilist = mlist; mlist = mlist->next; AC_FREE (ilist); } } } AC_FREE (acsm->acsmStateTable); } /* * Print A Match String's Information */ void PrintMatch (ACSM_PATTERN * pattern,ACSM_PATTERN * mlist, int nline,int index) { /* Count the Each Match Pattern */ ACSM_PATTERN *temp = pattern; for (;temp!=NULL;temp=temp->next) { if (!strcmp(temp->patrn,mlist->patrn)) //strcmp succeed return 0,So here use "!" operation { temp->nmatch++; } } if(mlist->nocase) fprintf (stdout, "Match KeyWord %s at %d line %d char\n", mlist->patrn,nline,index); else fprintf (stdout, "Match KeyWord %s at %d line %d char\n", mlist->casepatrn,nline,index); } /* * Print Summary Information of the AC Match */ void PrintSummary (ACSM_PATTERN * pattern) { ACSM_PATTERN * mlist = pattern; printf("\n### Summary ###\n"); for (;mlist!=NULL;mlist=mlist->next) { if(mlist->nocase) printf("%12s : %5d\n",mlist->patrn,mlist->nmatch); else printf("%12s : %5d\n",mlist->casepatrn,mlist->nmatch); } } main.c
/* Author: wangyao Email: [email protected] */
#include "acsmx.h"
/* * Text Data Buffer */
unsigned char text[MAXLEN];
extern int nline;
int main (int argc, char **argv)
{
int i, nocase = 0;
FILE *fd;
char filename[20];
ACSM_STRUCT * acsm;
if (argc < 3)
{
fprintf (stderr,"Usage: acsmx filename pattern1 pattern2 ... -nocase\n");
exit (0);
}
acsm = acsmNew ();
strcpy (filename, argv[1]);
fd = fopen(filename,"r");
if(fd == NULL)
{
fprintf(stderr,"Open file error!\n");
exit(1);
}
for (i = 1; i < argc; i++)
if (strcmp (argv[i], "-nocase") == 0)
nocase = 1;
for (i = 2; i < argc; i++)
{
if (argv[i][0] == '-')
continue;
acsmAddPattern (acsm, argv[i], strlen (argv[i]), nocase);
}
/* Generate GtoTo Table and Fail Table */
acsmCompile (acsm);
/*Search Pattern*/
while ( fgets(text,MAXLEN,fd) )
{
acsmSearch (acsm, text, strlen (text), PrintMatch);
nline++;
}
PrintSummary(acsm->acsmPatterns);
acsmFree (acsm);
printf ("\n### AC Match Finished ###\n");
// system("pause");
return (0);
} 