atoi.c

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

int atoi(const char* str)
{
    bool neg = false;
    int val = 0;

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

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

    return (neg ? -val : val);
}