Arithmatic parser

math.c

// process arbitrary 1+2-(3+(5-1))-2 style equations
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int parse(char[]);
int process(int, char, int);
struct State {
    int total;
    char operator;
    struct State* prevstate;
};

int main(int argc, char* argv[]){
    if( argc != 2 ){
      printf("Fail.\n");
      exit(1);
    }
    char* equation = argv[1];
    int result = parse(equation);
    printf("%s = %d\n", equation, result);
    exit( 0 );
}

// assume all numbers, operators, or parens
// spaces or anything else will break
int parse(char equation[]){
    // largest 32-bit integer is 10 chars
    int maxintlen = 10;
    char rhs[maxintlen + 1];
    char operator;
    char c2s[2]; // for single-char string conversion
    char c = '\0';
    int  i = 0;
    int total = 0;
    struct State* currentstate = NULL;
    struct State* newstate = NULL;
    
    c2s[1] = '\0';

    //strcpy(rhs, "");
    rhs[0] = '\0';
    operator = '+';
    total = 0;
    for ( i=0; i < strlen(equation); i++ ){
        c = equation[i];
        switch(c){
            case '+':
            case '-':
                // hit an operator; perform last operation then proceed
                // atoi is dangerous, but we're ignoring error conditions here
                total = process(total, operator, atoi(rhs));
                operator = c;
                rhs[0] = '\0';
                break;
            case '(':
                // only comes after operator, so just save state and
                // essentially start over
                newstate = malloc(sizeof(struct State));
                newstate->total = total;
                newstate->operator = operator;
                newstate->prevstate = currentstate;
                currentstate = newstate;
                total = 0;
                operator = '+';
                rhs[0] = '\0';
                break;
            case ')':
                // end of block; total is new RHS, so prepare for next operator
                // finish the math in the block
                total = process(total, operator, atoi(rhs));
                // prepare current total to be processed
                sprintf( rhs, "%d", total );
                total = currentstate->total;
                operator =  currentstate->operator,
                currentstate = currentstate->prevstate;
                break;
            default:
                // still working on a number
                c2s[0] = c;
                strcat(rhs, c2s);
        }
    }
    // process final value
    total = process(total, operator, atoi(rhs));

    return total;
}

int process(int lhs, char operator, int rhs){
    printf( "processing '%d%c%d'\n", lhs, operator, rhs );
    switch(operator){
        case '+':
            return( lhs + rhs );
        case '-':
            return( lhs - rhs );
    }
}