str2long.c

#include <assert.h>
#include <stddef.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>

#include "str2long.h"

#if LONG_MAX == 9223372036854775807UL
#define LONG_MAX_DIGITS 19
#else
#define LONG_MAX_DIGITS 10
#endif

static inline int
isdigit(const char x)
{
    if (x >= 0x30 && x <= 0x39) {
        return 1;
    }
    return 0;
}

static int
verify(const char *str)
{
    size_t len, i;
    if (str == NULL) {
        return 1;
    }
    len = strlen(str);
    if (len == 0) {
        return 1;
    }
    if (len > LONG_MAX_DIGITS + 1) {
        return 1;
    }
    i = 0;
    if (str[0] == '-') {
        if (isdigit(str[1]) == 0) {
            return 1;
        }
        i = 1;
    }
    for (; i < len; i++) {
        if (isdigit(str[i]) == 0) {
            return 1;
        }
    }
    return 0;
}

static inline unsigned long int
ord(const char x)
{
    return (unsigned long int) x - 0x30;
}

static inline size_t
min(const size_t x, const size_t y)
{
    if (x < y) {
        return x;
    }
    return y;
}

extern int error;

long int
str2long(const char *str)
{
    int sign;
    unsigned long int result, base, bound, x;
    size_t len, i, j;

    if (verify(str) != 0) {
        error = 1;
        return 0;
    }

    len = strlen(str);
    i = 0;
    result = 0;

    sign = 1;
    bound = LONG_MAX;
    if (str[0] == '-') {
        sign = -1;
        bound += 1;
        i = 1;
    }

    base = 1;
    for (j = 0; j < len - i - 1; j++) {
        base *= 10;
    }

    while (i < min(len, LONG_MAX_DIGITS - 1)) {
        x = base * ord(str[i]);
        result += x;
        base /= 10;
        i++;
    }

    while (i < len) {
        x = base * ord(str[i]);
        if (result > bound - x) {
            error = 1;
            return 0;
        }
        result += x;
        base /= 10;
        i++;
    }

    return (long int) result * sign;
}