gistfile1.cpp

/*          *
 *  Bython  *
 *          */
#include <stdio.h>
#include <string>

void substr(const char* src, int offset, int length, char* return_buffer) {
	strncpy(return_buffer, src+offset, length);
}

typedef struct{
	int index;
	int error;
} parse;

int expr1(const char* const, parse*);
int expr2(const char* const, parse*);
int expr3(const char* const, parse*);
/*
	{0 to 9}                     ... number
	S {a to z | A to Z | _} S    ... name
	??????                       ... funarg
	name \( funarg \)            ... function
	expr2 {(+ | -) expr2}        ... expr1
	expr3 {(* | /) expr3}        ... expr2
	S ( \( expr1 \) | number ) S ... expr3
*/

void _(char* program, char* result){
	parse option = {0, 0};
	int ret = expr1(program, &option);
	sprintf(result, "%d(%sindex: %d)", ret, option.error?"Error, ":"", option.index);
}
void skips(const char* const program, parse* option){
	while(program[option->index] == ' '){
		option->index++;
	}
}

int number(const char* const program, parse* option){
	int result = 0;
	if(program[option->index] < '0' || '9' < program[option->index]){
		option->error = 1;
		return 0;
	}
	while('0' <= program[option->index] && program[option->index] <= '9'){
		result = result * 10 + program[option->index] - '0';
		option->index++;
	}
	return result;
}

void name(const char* const program, parse* option, char* name){
	int old_index = option->index;
	if(!(  'a' <= program[option->index] && program[option->index] <= 'z'
		|| 'A' <= program[option->index] && program[option->index] <= 'Z'
		|| '_' == program[option->index])){
		option->error = 1;
		return;
	}
	while( 'a' <= program[option->index] && program[option->index] <= 'z'
		|| 'A' <= program[option->index] && program[option->index] <= 'Z'
		|| '0' <= program[option->index] && program[option->index] <= '9'
		|| '_' == program[option->index]){
		option->index++;
	}
	substr(program, old_index, option->index-old_index, name);
}

//int function(const char* const program, parse* option){
/*	name \( funarg \)            ... function*/
/*	int old_index = option->index;
	char name[255];
	int args[255];
	name(program, option, name);
	if(program[option->index] == '('){
		option->index++;
		args = funarg(program, option);
		if(program[option->index] != ')'){
			option->index = old_index;
			option->error = 1;
			return 0;
		}
		option->index++;
	}else{
		option->error = 1;
		return 0;
	}
	name
	return 0;
}*/

int expr1(const char* const program, parse* option){
	int result = expr2(program, option);
	while(1){
		switch(program[option->index]){
		case '+':
			option->index++;
			result += expr2(program, option);
			break;
		case '-':
			option->index++;
			result -= expr2(program, option);
			break;
		default:
			return result;
		}
		if(option->error){
			option->index--;
			option->error=0;
			return result;
		}
	}
}
int expr2(const char* const program, parse* option){
	int result = expr3(program, option);
	while(1){
		switch(program[option->index]){
		case '*':
			option->index++;
			result *= expr3(program, option);
			break;
		case '/':
			option->index++;
			result /= expr3(program, option);
			break;
		default:
			return result;
		}
		if(option->error){
			option->index--;
			option->error=0;
			return result;
		}
	}
}
int expr3(const char* const program, parse* option){
	int result = 0;
	int old_index = option->index;
	skips(program, option);
	if(program[option->index] == '('){
		option->index++;
		result = expr1(program, option);
		if(program[option->index] != ')'){
			option->index = old_index;
			option->error = 1;
			return 0;
		}
		option->index++;
	}else{
		result = number(program, option);
	}
	skips(program, option);
	return result;
}


int main(void){
	string s;
	while(1){
		char result[50];		
		printf("> ");
		fgets(s, 50, stdin);
		if(s == NULL){
			printf("ERROR.\n");
			return -1;
		}else if(!strcmp(s, "exit\n")){
			printf("Exit.\n");
			return 0;
		}
		_(s, result);
		printf("%s\n", result);
	}
}