atol.c

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#include <ctype.h>
#include <stdbool.h>
#include <stdlib.h>

long atol(const char* str)
{
    long val = 0;
    bool neg = false;

    while(isspace(*str))
    {
        str++;
    }

    switch(*str)
    {
        case '-':
            neg = true;
        // intentional fallthrough
        case '+':
            str++;
    }

    while(isdigit(*str))
    {
        val = (10 * val) + (*str++ - '0');
    }

    return neg ? -val : val;
}