#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <math.h>
#include <string.h>
#include <ctype.h>

char arg[256], item;
int badArgument;
double x; // an actual argument

int arrLength(char* string)
{
    int length = 0;
    while (*string++) length++;
    return length;
}

/*
void getArg() // v. 1
{
    scanf("%s", arg);

    while((getchar()) != '\n'); // an alternative way to flush stdin

    x = strtod(arg, NULL);

    if (errno == ERANGE || x == 0.0) // exception handling in strtod function
    {
        badArgument = 1; // so that the user's argument is bad
    }

    badArgument = 0; // so that the user's argument is good
}
*/

int checkArg()
{
    int dots = 0, weird_symbols = 0;

    for (int i = 0; i < arrLength(arg); i++)
    {
        if (arg[i] == '.') dots++;

        if (!isdigit(arg[i]) && dots > 1 && arg[i] != '-' && arg[i] != 'e' && arg[i] != 'E')
        {
            badArgument = 1;
            return 0;
        }
    }

    badArgument = 0;
    return 1;
}

void displayMenu()
{
    printf("\n1. Compute a function.\n");
    printf("2. Exit.\n");
    printf("Choose a menu item: ");
}

void getArg() // v. 2
{
    scanf("%s", arg);
    if (checkArg())
    {
        x = strtod(arg, NULL);
    }
}

void compute(double x)
{
    printf("Z1 = %f\n", cos(x) + sin(x) + cos(3*x) + sin(3*x));
    printf("Z2 = %f\n", 2 * sqrt(2) * cos(x) * sin(M_PI/4 + 2*x));
}

void getItem()
{
    scanf(" %c", &item);
    while((getchar()) != '\n'); // an alternative way to flush stdin
}


int main()
{
    displayMenu();
    getItem();

    while (item != '1' && item != '2')
    {
        printf("\nPlease, choose a correct menu item: ");
        getItem();
    }

    if (item == '2')
    {
        exit(0);
    }

    while (1)
    {
        printf("\nZ1 = cosA + sinA + cos(3A) + sin(3A)\n");
        printf("Z2 = 2 * sqrt(2) * cosA * sin(pi/4 + 2A)\n");
        printf("Input argument A: ");

        getArg();

        if (badArgument)
        {
            printf("Wrong input argument A. Press 'y' to continue, another key to return back: ");
            getItem();
            if (item == 'y')
            {
                continue;
            } else break;
        }

        compute(x);

        break;
    }

    return 0;
}