eval.c

#include "common.h"
#include "eval.h"

// Used for returning to the main eval() function on syntax errors
static jmp_buf err_jmp_buf;

// The parsing functions below consume as much of the input as can be part of
// the current construction and then delegate to a higher level without looking
// at the rest. This makes it straightforward to catch all syntax errors since
// we detect them at the highest possible level.

static int eval_sum(const char **s);

static int eval_num(const char **s)
{
    int res = 0;

    if (!isdigit(**s))
        longjmp(err_jmp_buf, 1);

    do
        res = 10*res + *(*s)++ - '0';
    while (isdigit(**s));

    return res;
}

static int eval_exp(const char **s)
{
    int base;

    while (isspace(**s))
        ++*s;

    switch (**s) {
    // Unary '+' and '-' operators
    case '+': ++*s; base = eval_exp(s); break;
    case '-': ++*s; base = -eval_exp(s); break;
    case '(':
        ++*s; // Eat "("
        base = eval_sum(s);
        if (**s != ')')
            longjmp(err_jmp_buf, 1);
        ++*s; // Eat ")"
        break;
    default:
        base = eval_num(s);
    }

    // Check if we have an exponent

    while (isspace(**s))
        ++*s;

    if (**s == '*' && *(*s + 1) == '*') {
        *s += 2; // Eat "**"
        return pow(base, eval_exp(s));
    }
    return base;
}

static int eval_product(const char **s)
{
    bool is_times = true;
    int product = 1;

    for (;;) {
        if (is_times)
            product *= eval_exp(s);
        else
            product /= eval_exp(s);

        switch (**s) {
        case '*': ++*s; is_times = true; continue;
        case '/': ++*s; is_times = false; continue;
        default: return product;
        }
    }
}

static int eval_sum(const char **s)
{
    bool is_plus = true;
    int sum = 0;

    for (;;) {
        if (is_plus)
            sum += eval_product(s);
        else
            sum -= eval_product(s);

        switch (**s) {
        case '+': ++*s; is_plus = true; continue;
        case '-': ++*s; is_plus = false; continue;
        default: return sum;
        }
    }
}

bool eval(const char *s, int *res)
{
    int tmp;

    if (setjmp(err_jmp_buf) == 1)
        return false;
    tmp = eval_sum(&s);
    if (*s != '\0')
        // Found extra trailing characters
        return false;
    *res = tmp;

    return true;
}