/*******************************************************************************
 * K&R 4-2
 *
 * atof extended: convert string s to double, allow for possible scientific
 * notation accepting e or E and an optionally signed exponent.
 *
 * program: atof_extended
 * by.....: yetimach
 * date...: 06 November 2024
 *
 ******************************************************************************/
#include <stdio.h>
#include <ctype.h>

#define MAX 21

char gs[MAX];


void read_line(char s[], int max){
    int c, i = 0;
    while (i < max-1 && (c = getchar()) != '\n' && c != EOF)
        s[i++] = c;
}

double atof(char s[]){
    double val, power;
    int i, sign, e, e_sign, is_e = 0;

    for (i = 0; isspace(s[i]); i++)
        ;
    sign = (s[i] == '-') ? -1 : 1;
    if (s[i] == '+' || s[i] == '-')
        i++;
    for (val = 0.0; isdigit(s[i]); i++)
        val = 10.0 * val + (s[i] - '0');
    if (s[i] == '.')
        i++;
    for (power = 1.0; isdigit(s[i]); i++){
        val = 10.0 * val + (s[i] - '0');
        power *= 10.0;
    }
    val = sign * val / power;
    if (s[i] == 'e' || s[i] == 'E'){
        i++;
        is_e = 1;
    }
    e_sign = (s[i] == '-') ? -1 : 1;
    if (s[i] == '+' || s[i] == '-')
        i++;
    for (e = 0; isdigit(s[i]); i++)
        e = 10 * e + (s[i] - '0');
    e *= e_sign;
    if (is_e && e == 0)
        return 1.0 * sign; 
    while (e < 0){
        val *= 0.1;
        e++;
    }
    while (e-- > 0)
        val *= 10;
    return  val;
}

int main(void)
{
    printf("(Accepts decimal values and scientific notation, "
            "20 character limit)\nEnter number value: ");
    read_line(gs, MAX);
    double converted_value = atof(gs);
    printf("\nThe converted number is %f\n", converted_value);
    return 0;
}