#include <ctype.h>

#include "evalexpr.h"

int parse_number(const char *expr, size_t *offset)
{
    for (*offset = 0;
         expr[*offset] && expr[*offset] >= '0' && expr[*offset] <= '9';
         (*offset)++)
    {
        continue;
    }
    (*offset)--;

    int res = 0;
    int pow = 1;
    for (size_t n = 0; n <= *offset; n++, pow *= 10)
    {
        res += (expr[*offset - n] - '0') * pow;
    }

    return res;
}

int get_operands(struct stack **s, int *op1, int *op2)
{
    if (*s == NULL)
    {
        return 2;
    }
    *op1 = stack_peek(*s);
    *s = stack_pop(*s);
    if (*s == NULL)
    {
        return 2;
    }
    *op2 = stack_peek(*s);
    *s = stack_pop(*s);
    return 0;
}

// Computes a to the bth
int my_pow(int a, int b)
{
    if (b == 0)
    {
        return 1;
    }
    if (a == 0)
    {
        return 0;
    }

    int res = 1;
    int c = b / 2;
    while (c != 0)
    {
        res *= a * a;
        c /= 2;
    }
    if (b % 2 != 0)
        res *= a;
    return res;
}

int dispatch(const char c, struct stack **s, int op1, int op2)
{
    switch (c)
    {
    case '*':
        *s = stack_push(*s, op1 * op2);
        break;
    case '+':
        *s = stack_push(*s, op1 + op2);
        break;
    case '-':
        *s = stack_push(*s, op2 - op1);
        break;
    case '/':
        if (op1 == 0)
        {
            return 3;
        }
        *s = stack_push(*s, op2 / op1);
        break;
    case '%':
        if (op1 == 0)
        {
            return 3;
        }
        *s = stack_push(*s, op2 % op1);
        break;
    case '^':
        if (op1 < 0)
        {
            return 3;
        }
        *s = stack_push(*s, my_pow(op2, op1));
        break;
    default:
        return 1;
    }
    return 0;
}

int evalrpn(const char *expr, int *retval)
{
    struct stack *s = NULL;

    for (size_t i = 0; expr[i]; i++)
    {
        if (expr[i] >= '0' && expr[i] <= '9')
        {
            size_t offset;
            int val = parse_number(expr + i, &offset);
            s = stack_push(s, val);
            i += offset;
        }
        else if (isspace(expr[i]))
            continue;
        else
        {
            int op1;
            int op2;
            if (get_operands(&s, &op1, &op2) == 2)
            {
                return 2;
            }

            int d = dispatch(expr[i], &s, op1, op2);
            if (d != 0)
            {
                return d;
            }
        }
    }

    if (s == NULL)
    {
        return 0;
    }
    *retval = stack_peek(s);
    s = stack_pop(s);
    return (!s) ? 0 : 2;
}