#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define PROMPT_COMMAND "Enter command:\n"
#define COMMAND_FAIL "Command failed.\n"

typedef char * string;
typedef enum { SUCCESS, FAIL } result;
typedef enum { INTEGER, DOUBLE, STRING } item_tag;

typedef struct item_t *item_ptr;

typedef struct item_t {
    item_tag tag;
    union {
        int i;
        double d;
        string s;
    } value;
    item_ptr next_item;
} item;

item *head;

/*
track number of nodes on stack
*/
int STACK_SIZE = 0;

/*
push item i onto stack
if successful returns SUCCESS
else returns FAIL
*/
result Push(item i) {
    item *temp;
    temp = malloc(sizeof(item));

    temp->value = i.value;
    temp->next_item = head;

    head = temp;

    free(temp);
    STACK_SIZE++;
    return SUCCESS;
}

/*
retrieves topmost item from stack and stores in location *ip
if successful returns SUCCESS
else returns FAIL
*/
result Pop(item *ip) {
    if ( head->next_item == NULL ) {
        printf(COMMAND_FAIL);
        return FAIL;
    } else {

        ip->value = head->value;
        ip->next_item = NULL;

        head = head->next_item;

        STACK_SIZE--;
        return SUCCESS;
    }
}

/*
prints contents of top item on stack
*/
result Print() {
    if ( STACK_SIZE == 0 ) {
        printf(COMMAND_FAIL);
        return FAIL;
    } else {
        if ( head->tag == STRING ) {
            printf("Head value is %s \n", head->value.s);
        } else if ( head->tag == INTEGER ) {
            printf("Head value is %i \n", head->value.i);
        } else if ( head->tag == DOUBLE ) {
            printf("Head value is %f \n", head->value.d);
        }
    }
    return SUCCESS;
}

int main() {

    item *temp1;
    temp1 = malloc(sizeof(item));

    item testI;
    testI.value.i = 5;

    Push(testI);
    Print();

    testI.value.i = 7;
    Push(testI);
    Print();

    testI.value.i = 3;
    Push(testI);
    Print();

    Pop(temp1);
    printf("Popped value is: %i\n", temp1->value.i);
    Print();

    return 0;
}