/*

Prints a calendar for the given year with every 2 months adjacent

Michael Sanders 2024
https://busybox.neocities.org/notes/adjacent-calendars.txt

Example...

Jan                            Feb
Sun Mon Tue Wed Thu Fri Sat    Sun Mon Tue Wed Thu Fri Sat
                          1              1   2   3   4   5
  2   3   4   5   6   7   8      6   7   8   9  10  11  12
  9  10  11  12  13  14  15     13  14  15  16  17  18  19
 16  17  18  19  20  21  22     20  21  22  23  24  25  26
 23  24  25  26  27  28  29     27  28
 30  31

*/

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

#define HEIGHT   6 // matrix height
#define WIDTH    7 // matrix width
#define OFFSET1  1 // offset 1st calendar
#define OFFSET2 25 // offset 2nd calendar

const int daysInMonth[12] = { 31, 28, 31, 30, 31, 30,
                              31, 31, 30, 31, 30, 31 };

const char *WeekDays[12] = { "Sun", "Mon", "Tue", "Wed",
                             "Thu", "Fri", "Sat" };

const char *Months[12] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                           "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };

int isLeapYear(int year) {
    return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
}

int dayOfMonth(int d, int m, int y) {

    // function stolen from Kenny McCormack - mucho thanks kind sir

    if (m < 3) {
        m += 12;
        y--;
    }

    return (d + (13*m-27)/5 + y + y/4 - y/100 + y/400) % 7;
}

void popluateMatrix(int year, int month, int calendar[HEIGHT][WIDTH]) {

    // initialize matrix with zeros
    for (int i = 0; i < HEIGHT; i++) {
        for (int j = 0; j < WIDTH; j++) calendar[i][j] = 0;
    }

    int totalDays = daysInMonth[month - 1];
    if (month == 2 && isLeapYear(year)) totalDays++;
    int day = 1;
    int firstDay = dayOfMonth(day, month, year);

    for (int week = 0; week < HEIGHT; week++) {
        for (int weekday = 0; weekday < WIDTH; weekday++) {
            if ((week == 0 && weekday < firstDay) || day > totalDays)
                 calendar[week][weekday] = 0;
            else calendar[week][weekday] = day++;
        }
    }
}

void printTwoCalendars(int year, int month1, int month2) {

    int cal1[HEIGHT][WIDTH], cal2[HEIGHT][WIDTH];

    popluateMatrix(year, month1, cal1);
    popluateMatrix(year, month2, cal2);

    printf("%*s%s%*s%s\n", OFFSET1, "", Months[month1 - 1],
        OFFSET2 + 3, "", Months[month2 - 1]);

    for (int i = 0; i < OFFSET1; i++) printf(" ");
    for (int i = 0; i < WIDTH; i++) printf("%s ", WeekDays[i]);
    for (int i = 0; i < OFFSET2 - OFFSET1 - 3 * WIDTH; i++) printf(" ");
    for (int i = 0; i < WIDTH; i++) printf("%s ", WeekDays[i]);
    printf("\n");

    for (int i = 0; i < HEIGHT; i++) {
        int isEmptyRow1 = 1, isEmptyRow2 = 1;
        for (int j = 0; j < WIDTH; j++) {
            if (cal1[i][j] != 0) isEmptyRow1 = 0;
            if (cal2[i][j] != 0) isEmptyRow2 = 0;
        }

        if (!isEmptyRow1 || !isEmptyRow2) {
            for (int j = 0; j < OFFSET1; j++) printf(" ");
            for (int j = 0; j < WIDTH; j++) {
                if (cal1[i][j] == 0) printf("    ");
                else printf("%3d ", cal1[i][j]);
            }
            for (int j = 0; j < OFFSET2 - OFFSET1 - 3 * WIDTH; j++) printf(" ");
            for (int j = 0; j < WIDTH; j++) {
                if (cal2[i][j] == 0) printf("    ");
                else printf("%3d ", cal2[i][j]);
            }
            printf("\n");
        }
    }
}

int main() {

    int year = 2022;

    printf("%*sCalendar for %d\n\n", OFFSET1, "", year);
    for (int month = 1; month <= 11; month += 2) {
        printTwoCalendars(year, month, month + 1);
        printf("\n"); // separate each pair of months
    }

    return 0;
}

// eof