[c] extension of Week 4, Q5

list.c

#include "list.h"

int
list_head(list *the_list, node_val *ret)
{
	if (!the_list)
		return 1;
	*ret = the_list->val;
	return 0;
}

int
list_rest(list* the_list, list **ret)
{
	if (!the_list || !the_list->next)
		return 1;
	*ret = the_list->next;
	return 0;
}

void
list_free(list** the_list)
{
	list *prev;
	while ((prev = *the_list)) {
		*the_list = (*the_list)->next;
		free(prev);
	}
	*the_list = NULL;
}

static
int list_item_prompt(int *val)
{
	printf("Enter next int: ");
	return scanf("%d", val);
}

int
list_from_stdin(list** ret)
{
	int read_int;
	struct node *curr, *head, *last;

	head = last = NULL;
	while (list_item_prompt(&read_int) == 1) {
		curr = (struct node *) malloc(sizeof(curr));
		curr->val = read_int;
		if (!head) {
			head = curr;
			head->next = NULL;
			last = head;
		} else {
			curr->next = NULL;
			last->next = curr;
			last = curr;
		}
	}

	*ret = head;
	return 0;
}

list.h

#include <stdio.h>
#include <malloc.h>

typedef int node_val;

struct node {
	node_val val;
	struct node *next;
};

typedef struct node list;

int
list_head(list *the_list, node_val *ret);

int
list_rest(list* the_list, list **ret);

void
list_free(list** the_list);

int
list_from_stdin(list** ret);

skeleton.c

#include <stdio.h>
#include "list.h"

int is_non_decreasing(list*);

int main(void) {
	int ret;
	list *the_list;
	list_from_stdin(&the_list);

	ret = is_non_decreasing(the_list);
	if (ret == 0)
		printf("the integers are in non-decreasing order\n");
	else if (ret == 1)
		printf("the integers are not in non-decreasing order\n");
	else
		/* if we reach here, an error occurred; assume the error has
		 * been reported and do nothing */
		; 

	list_free(&the_list);

	return 0;
}

int is_non_decreasing(list* the_list) {
	list *rest_of_list;
	node_val a, b;

	/* base case 1: list is empty -> is non-decreasing */
	if (!the_list)
		return 0;

	/* base case 2: list has one item -> is non-decreasing */
	if (list_rest(the_list, &rest_of_list))
		return 0;

	/* paranoid check */
	if (list_head(the_list, &a)
		|| list_head(rest_of_list, &b)) {
		fprintf(stderr, "bad condition\n");
		return -1;
	}

	/***************************
	 * The condition here is wrong, can you fix it?
	 ***************************/
	if (a == b ||
		is_non_decreasing(rest_of_list) == 0)
		return 0;

	return 1;
}