// date4.cpp

#include "date4.h"

inline int isleap(int y)
  {return y%4 == 0 && y%100 != 0 || y%400 == 0;}

static int dtab[2][13] =
{
  {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
  {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};

Date * Date::interval(const Date& d2) const
{
    static Date result;
    int months, days, years, prev_month;

    // Compute the interval - assume d1 precedes d2
    years = d2.year - year;
    months = d2.month - month;
    days = d2.day - day;

    // Do obvious corrections (days before months!)
    //
    // This is a loop in case the previous month is
    // February, and days < -28.
    prev_month = d2.month - 1;
    while (days < 0)
    {
        // Borrow from the previous month
        if (prev_month == 0)
            prev_month = 12;
        --months;
        days += dtab[isleap(d2.year)][prev_month--];
    }

    if (months < 0)
    {
        // Borrow from the previous year
        --years;
        months += 12;
    }

    // Prepare output
    result.month = months;
    result.day = days;
    result.year = years;
    return &result;
}

int Date::compare(const Date& d2) const
{
    int months, days, years, order;
    
    years = year - d2.year;
    months = month - d2.month;
    days = day - d2.day;

    // return <0, 0, or >0, like strcmp()
    if (years == 0 && months == 0 && days == 0)
        return 0;
    else if (years == 0 && months == 0)
        return days;
    else if (years == 0)
        return months;
    else
        return years;
}

