摘要：在很多的遊戲程式設計中，我們使用了有限狀態自動機作為模型。有限狀態自動機作為變成模型，具有通用性好，方便理解的特點。本文主要結合前一個系列的兩篇文章（1）C語言宣告解析器的實現和（2）用C語言實現有限狀態自動機來說明如何用有限狀態自動機模型實現一個C語言的宣告解析器。

一、狀態機的設計

typedef struct
{
   state current;
   trasition trasitions[STATENUM][CONDITIONS];
} StateMachine, * pStateMachine;

我們在解析一個C語言宣告的時候，大致過程是這樣的：

1）讀取宣告到第一個變數名稱識別符號name，此為狀態S0

2）讀取name後面的下一個識別符號，確定型別並採取相應的動作：

         “［”：這是一個陣列，我們要處理這個陣列，然後進入狀態S1

         “（”：這是一個函式，我們要處理這個函式，然後進入狀態S2

         “）”：它的前面是一個指標，我們要處理這個指標，然後進入狀態S3

         “／0”：宣告讀取完畢，可以結束了，進入狀態S4

         ”other“：出錯

這個狀態機如下：

注意：S1,S2，S3，S4的狀態圖沒有從圖中體現出來，實際上，S1～S4的出邊和S0是幾乎一樣的，所不同的就是S1沒有經過function到達S2的出邊，因為陣列的元素不能是函式（可以是函式指標）。這一部分可以參考前面的C語言宣告規則。

S0：讀完宣告到第一個變數識別符號

S1：讀完一個陣列

S2：讀完後面的一個函式

S3：讀完前面的一個指標

S4：解析完畢

二、資料結構的設計

我們僅僅談與自動機相關的部分，其他的部分可以在 另外一篇文章 一個C語言宣告解析器的設計與實現裡找到。

typedef int (* actiontype)( );//函式指標，是一個動作
typedef struct{
  state next;
  actiontype action;
}trasition, *ptrasition;//轉移動作：下一個狀態+動作

typedef struct
{
   state current;
   int statenum;
   int conditionnum;
   trasition trasitions[STATENUM][CONDITIONS];
} StateMachine, * pStateMachine;//自動機：關鍵是 當前狀態+轉移函式集合
int initMachine(pStateMachine mymachine)//初始化狀態機，設定初始狀態和轉移函式
state step(pStateMachine machine, condition mycondition)//狀態機的運轉函式

三、詳細程式碼

#include<stdio.h>
#include<ctype.h>
#include<unistd.h>
#include<stdlib.h>
#include<string.h>
#define MAXTOKENLEN 64
#define MAXTOKENS 100
#define MAXDECLLEN 200

typedef int state;
typedef char condition;

#define STATENUM 5
#define STATE0 0
#define STATE1 1
#define STATE2 2
#define STATE3 3
#define STATE4 4
#define STATETRAP 6

#define CONDITIONS 5 

struct token{
  char type;
  char string[MAXTOKENLEN];
};
struct decl{
  char string[MAXDECLLEN];
  int current_location;
};

struct decl mydecl;
int top=-1;
struct token this;//the current token
struct token stack[MAXTOKENS];//the tokens before the first identifer
#define IDENTIFIER 'I'
#define QUALIFIER 'Q'
#define TYPE 'T'
#define pop stack[top--]
#define push(s) stack[++top]=s
//#define debug 
char classify_string(const char *string);


/*
 * init the value of mydecl*/
int init_parse_decl(){
  if( fgets(mydecl.string,MAXDECLLEN,stdin) == NULL){
    perror("can not read from the stdin");
  }
  if(strlen(mydecl.string)==MAXDECLLEN-1)
    printf("you input is too long, you may get an error!");
  mydecl.current_location=0;

  #ifdef debug
  printf("init:we get last char of mydecl:%c,%d\n",mydecl.string[strlen(mydecl.string)],mydecl.string[strlen(mydecl.string)]);
  #endif
}


/*
  get a token from the current string,value the "this" and move the pointer
notice: we may get a '\0' at the end of the string
  */
int gettoken(){
  char *ch_pointer=this.string;
  //leave out the blank
  while(mydecl.string[mydecl.current_location]==' '){
    mydecl.current_location++;
  }

  *ch_pointer=mydecl.string[mydecl.current_location];
  if(isalnum(*ch_pointer)){
  	while(isalnum(*ch_pointer)){
	  mydecl.current_location++;
	  ch_pointer++;
	  *ch_pointer=mydecl.string[mydecl.current_location];
	}

	*ch_pointer='\0';
	this.type=classify_string(this.string );//indentifier,qualifier,type
	#ifdef debug 
	printf("we get token:%s ",this.string );
	#endif
	return 1;
  }

  //else:(, ), [, ] 
  
//  printf("current location is:%d\n",mydecl.current_location);
 // printf("(%d)",mydecl.string[mydecl.current_location]);
  switch(*ch_pointer){
    case '*':
    case '(':
    case ')':
    case '[':
    case ']':
	      this.type=*ch_pointer;
	      this.string[1]='\0';
	      mydecl.current_location++;
	      #ifdef debug 
	      printf("we get token:%s ",this.string );
	      #endif
	      break;
    case '\n':
    case '\0':this.type='\0'; 
	     // printf("we come to the end\n");
	      strcpy(this.string,"then end");
	      return 0;

   default :
	      printf("we can not read this indentifier %d\n",*ch_pointer);
	      parse_error();
  }
  return 1;
}

char classify_string(const char *string){
  if(isspace(*string))
    return '\0';
  if(!strcmp(string,"const")) 
   return QUALIFIER;
  if(!strcmp(string,"volatile")) 
   return QUALIFIER;
  

  if(!strcmp(string,"void")) 
    return TYPE;
  if(!strcmp(string,"char")) 
    return TYPE;
  if(!strcmp(string,"signed")) 
    return TYPE;
  if(!strcmp(string,"unsigned")) 
    return TYPE;
  if(!strcmp(string,"short")) 
    return TYPE;
  if(!strcmp(string,"int")) 
    return TYPE;
  if(!strcmp(string,"long")) 
    return TYPE;
  if(!strcmp(string,"float")) 
    return TYPE;
  if(!strcmp(string,"struct")) 
    return TYPE;
  if(!strcmp(string,"union")) 
    return TYPE;
  if(!strcmp(string,"enum")) 
    return TYPE;

  return IDENTIFIER ;
}

/*follow the dec string until the first identifier,push token to a stack*/
int read_to_first_identifier(){
  if(gettoken()==0){
    printf("readtofirst:missing the identifier,please put in your string...\n");
    return 0;
  }
  //the token is a struct with member type and string
  while(this.type!=IDENTIFIER)
  {
    push(this);//a stack with element of token
    if(gettoken()==0){
      printf("readtofirst:missing the identifier\n");
      return 0;
    }
  }
  printf("%s is ", this.string);
  
  return 1;
}



/*
deal with the next token in diff ways according to the token type
we just go to right
1) "(":deal in a function way,deal_with_declaration,
2) "[":deal in a array way,deal_with_declaration,
3) ")":deal with a pointer way,deal_with_declaration
4) NULL:move_left,
*/
int  deal_with_declaration(){
  if(gettoken()==0){
    move_left();
    return 1;
   }  
  //
  switch(this.type){
    case '(': deal_with_function();break;
    case ')': deal_with_pointer();break;
    case '[': deal_with_array();break;
    default : printf("there should not be a %c after the identifier\n",this.type );parse_error();
  }
  deal_with_declaration();
}


/*the current token is [,the content in the '[]' may be digtal,al,and other*/
int deal_with_array(){
  printf("array of ");
  while(this.type!=']'){
    gettoken();//we may get an "]" or digital
    if(isdigit(this.string[0])){
      printf("%d  ", atoi(this.string));
      //the next token must be ]
      gettoken();
      if(this.type!=']'){
	printf("the array miss the ']'before %s",this.string);
	parse_error();
      }
    }
  }
  return 0;
}

int parse_error(){
	printf("press any key to exit\n");
	getchar();
	exit(0);
}

/*the current token is ')' */
int deal_with_pointer(){
  while(stack[top].type=='*'){
    printf("pointer pointing to \n");
    pop;
  }
  if(stack[top].type!='('){
    printf(" missing an '(' after %s\n",stack[top].string );
    parse_error();
  }
  else{
    pop;
    return 1;

  }
}

/*the current token is '('*/
int deal_with_function(){
  while(this.type!=')'){
    gettoken();
    if(this.type=='\0'){
      printf(" missing an ')' before %s",this.string);
      parse_error();
    }
  }
  printf(" function returning ");
  return 1;
}

int move_left(){
  while(top>=0){
    switch (stack[top].type){
      case '*': printf(" pointer pointing to ");break;
      case  QUALIFIER: printf(" %s ",stack[top].string );break;
      case  TYPE: printf(" %s ",stack[top].string );break;
      default :printf("there is someting wrong about %s",stack[top].string);parse_error();break;
    }
    top--;
  }
}

typedef int (* actiontype)( );
typedef struct{
  state next;
  actiontype action;
}trasition, *ptrasition;

typedef struct
{
   state current;
   int statenum;
   int conditionnum;
   trasition trasitions[STATENUM][CONDITIONS];
} StateMachine, * pStateMachine;


int initMachine(pStateMachine mymachine){
  trasition   array={
  STATE1,deal_with_array 
  },
  fun={
    STATE2,deal_with_function 
  },
  pointer={
    STATE3,deal_with_pointer 
  },
  left={
    STATE4,move_left
  },
  acerr={
    STATE4,parse_error 
  },
  end={
    STATE4,parse_error/*it will no be execed*/
  };

  trasition mytrasitions[STATENUM][CONDITIONS]={
         /*[, (, ), \0,other*/
  /*s0*/ array,fun,pointer,left,acerr,
  /*s1*/ array,fun,pointer,left,acerr,
  /*s2*/ array,fun,pointer,left,acerr,
  /*s3*/ array,fun,pointer,left,acerr,
  /*s4*/ end,  end,  end  ,end ,  end

  };
  
  mymachine->statenum=STATENUM;
  mymachine->conditionnum=CONDITIONS;
  mymachine->current=STATE0;

  int i,j;

  for (i = 0; i < STATENUM ; ++i)
  {
    for (j = 0; j < CONDITIONS; ++j)
    {
      mymachine->trasitions[i][j]=mytrasitions[i][j];
    }
  }

}
 
state step(pStateMachine machine, condition mycondition)
{
    int index;
    switch (mycondition){
      case '[': index=0;break;
      case '(': index=1;break;
      case ')': index=2;break;
      case '\0': index=3;break;
      default: index=4;
    
    }
    trasition t = machine->trasitions[machine->current][index];
    (*(t.action))( );
    if(machine->current==1&&index==1){
      printf("\n\n\nerror:the element of array should not be function\n");
      parse_error();
    }
    if(machine->current==2&&index==1){
      printf("\n\n\nerror:the returning type of a funciton should not be function\n");
      parse_error();
    }
    if(machine->current==2&&index==0){
      printf("\n\n\nerror:the returning type of a funciton should not be array\n");
      parse_error();
    }
    machine->current = t.next;
    //printf("the current state is %d\n",t->next );
    return machine->current;
   
}
int main(int argc, char *argv[])
{
  StateMachine mymachine;
  init_parse_decl();
  read_to_first_identifier();

  initMachine(&mymachine);
  char mycon;
  while(mymachine.current!=STATE4){
    gettoken();
    mycon=this.type;
    step(&mymachine,mycon);
  }
  printf("\n");
  return 0;
}