/*
 * Program to find difference in days between two dates.
 * 
 * Approach is to convert both dates into number of days since 
 * a semi-arbitrary but conventional start day of 1/1/1970,
 * and then subtract.
 */
#include <stdio.h>
#include <string.h>
#include <stdbool.h>

/* global variables -- usually frowned on but here seems simpler */

/*
 * table of month names and data
 */
typedef struct {
	char *abbrev;
	int days;
	int extra_leap_year_days;
} month_table_entry_t;
const month_table_entry_t month_table[] = {
	{ "jan", 31, 0 },
	{ "feb", 28, 1 },
	{ "mar", 31, 0 },
	{ "apr", 30, 0 },
	{ "may", 31, 0 },
	{ "jun", 30, 0 },
	{ "jul", 31, 0 },
	{ "aug", 31, 0 },
	{ "sep", 30, 0 },
	{ "oct", 31, 0 },
	{ "nov", 30, 0 },
	{ "dec", 31, 0 }
};
const int months = sizeof(month_table) / sizeof(month_table_entry_t);

const int start_year = 1970;

bool is_leap(int year);

int date_to_day(char *month, int day, int year);

/* main program */

int main(void) {

	char month[4];
	int day;
	int year;

	printf("enter start date (three-char month (lower case), day, year)\n");
	if (scanf("%3s %d %d", month, &day, &year) != 3) {
		printf("error in input\n");
		return 1;
	}
	int day1 = date_to_day(month, day, year);
	if (day1 == -1) {
		return 1;
	}

	printf("enter end date (three-char month (lower case), day, year)\n");
	if (scanf("%3s %d %d", month, &day, &year) != 3) {
		printf("error in input\n");
		return 1;
	}
	int day2 = date_to_day(month, day, year);
	if (day2 == -1) {
		return 1;
	}
	printf("difference of %d days\n", day2-day1);
	return 0;
}

/* functions */

/* 
 * function to turn m/d/y into day since 1/1/1970
 * print error message and return -1 on error
 */
int date_to_day(char *month, int day, int year) {
	int month_index = -1;
	for (int i = 0; i < months; ++i) {
		if (strcmp(month, month_table[i].abbrev) == 0) {
			month_index = i;
		}
	}
	if (month_index == -1) {
		printf("bad month\n");
		return -1;
	}

	if (year < start_year) {
		printf("bad year\n");
		return -1;
	}

	int days_in_month = month_table[month_index].days;
	if (is_leap(year)) {
		days_in_month += month_table[month_index].extra_leap_year_days;
	}
	if ((day < 1) || (day > days_in_month)) {
		printf("bad day\n");
		return -1;
	}

	int days = 0;
	for (int y = start_year; y < year; ++y) {
		if (is_leap(y))
			days += 366;
		else 
			days += 365;
	}

	for (int m = 0; m < month_index; ++m) {
		days += month_table[m].days;
		if (is_leap(year)) 
			days += month_table[m].extra_leap_year_days;
	}

	days += day;

	return days;
}

bool is_leap(int year) {
	if ((year % 400) == 0)
		return true;
	else if (((year % 4) == 0) && ((year % 100) != 0))
		return true;
	else
		return false;
}