/* Program to print a picture of a calendar month.

   Input must be the number of days in a month, and the day in the
   week of the first day of the month.  There must be at least 8
   days in a month, and the day in the week must be from 0 to 6
   (0=Sunday, 6=Saturday).

   Output is a 'calendar' style picture of the month.
*/

#include <stdio.h>

/***** Global variables to specify calendar required *****/

int last_day;           /* Number of days in the month */
int start_weekday;      /* Week day of the first of the month */

/********************* Functions **********************/

void obtain_user_input (void);
void check_input_legality (void);
void print_calendar_month (void);

main () {
    /* Input the number of days and the starting day of the week. */
    obtain_user_input();

    /* Make sure input is legal, ask user to re-enter until it is. */
    check_input_legality();

    /* Print the picture of the month. */
    print_calendar_month();
}

/****************** obtain_user_input ******************

    Gets user's values for last_day and start_weekday.
*/

void obtain_user_input (void) {
    printf("How many days are in the month? ");
    scanf("%d", &last_day);
    printf("Which week day is the first of the month?\n");
    printf("(0=sun, 1=mon, 2=tue. 3=wed, 4=thu, 5=fri, 6=sat)\n");
    scanf("%d", &start_weekday);
}

/**************** check_input_legality ******************

    Makes sure user has entered last_day >= 8 and start_weekday
    between 0 and 6.

    Note: last_day is not restricted to 28..31 because some
    calendars used in the world have months of other lengths.
*/

void check_input_legality (void) {
    void obtain_user_input (void);

    while (
        last_day < 8       ||
        start_weekday < 0  ||
        start_weekday > 6
    ) {
        if (last_day < 8) {
            printf("The month must have eight or more days.\n");
        }
        if (start_weekday < 0  ||  start_weekday > 6) {
            printf("The starting week day must be between 0 & 6\n");
        }
        printf("Please re-enter both values.\n");
        obtain_user_input();
    }
}


/************ print_calendar_month ******************************
    Prints the calendar picture.

    On entry:
        last_day is the number of days in the month, and must be
        greater than seven.
        start_weekday is the day-of-the-week of the first of the
        month, and must indicate a valid week day (0 to 6).
*/

void print_calendar_month (void) {
    void print_top_asterisks (int start_weekday);
    void print_calendar_row (int sun_date, int last_day);
    void print_full_line_asterisks (void);
    void print_bottom_asterisks (int sun_date, int last_day);
    int sun_date;       /* date of Sunday in the week to be printed */

    /* prepare to print the first week.
       (The date of Sunday of week 1 is on or before the first of the
       month.  Since start_weekday is the 1st of the month, Sunday (0)
       must be the (1-start_weekday)st.  If this is less than 1, Sunday
       is not in the month, so some blank days will be printed.) */
    sun_date = 1 - start_weekday;

    /* print the top line of asterisks. */
    print_top_asterisks(start_weekday);

    /* while we have not printed the second-last row, print another.
       (We have reached the last week if sun_date is within the final
       seven days of the month.) */
    while (sun_date < last_day - 6) {
        /* Here sun_date is Sunday's date in the week to be printed. */

        print_calendar_row(sun_date, last_day);

        sun_date += 7;        /* prepare for the next week */

        print_full_line_asterisks();
    }

    /* print the last calendar row */
    /* Here sun_date is Sunday's date in the final week. */
    print_calendar_row(sun_date, last_day);

    print_bottom_asterisks(sun_date, last_day);
}


/******************************************************************/
/*            Functions for outputting the calendar               */

/* The following #defines allow us to change our minds about the
   appearance of the calendar with minimal effort. */

/* Full width of a square (cannot be less than 4):   */
#define SQUARE_WIDTH 9

/* Height of area BETWEEN rows of asterisks (must be at least 1): */
#define SQUARE_HEIGHT 3

void putout (int number, char whichchar);

/************************ print_calendar_row: **********************
  On entry: sun_date is the date of Sunday in the week to be printed.
            If this is the first week, sun_date can be less than one.
            If so, sun_date should be 1 - start_weekday so that the
            first falls on start_weekday.  last_date must be correct.

  Prints a calendar row (i.e. minus top & bottom asterisks) for the
  indicated week.  If a day has a date less than 1 or greater than
  last_day, its square in the calendar is blanked (as the day is not
  within the month).
*/

void print_calendar_row (int sun_date, int last_day) {
    int line, day;

    for (line=1; line<=SQUARE_HEIGHT; line++) {
        /*Print one text line */

        for (day=sun_date; day<sun_date+7; day++) {
            /* Print the segment belonging to a single day */
            if (day<1) {
                /* day is before start of month - print blanks */
                putout(SQUARE_WIDTH, ' ');
            } else if (day>last_day) {
                /* day is beyond end of month - print nothing */
                /* NOTHING TO DO */
            } else {
                /* day is within month - print square pieces */

                putchar('*');              /* print left asterisk */

                if (line==1) {
                    /* First line needs a number */
                    printf("%3d", day);
                    putout(SQUARE_WIDTH - 4, ' '); /* rest of square */
                } else {
                    /* other lines are an empty box */
                    putout(SQUARE_WIDTH - 1, ' ');
                }
            }
        }
        printf("*\n");   /* Terminate final box and add a newline */
    }
}

/********************* putout *************************
A short utility function to output repeated characters.
   On entry: number is how many characters to print, whichchar is
             the character to be printed.
   Prints whichchar, number times.
*/

void putout (int number, char whichchar) {
    int i;
    for (i=0; i<number;i++) {
        putchar(whichchar);
    }
}


/****************** print_top_asterisks: *************************

  On entry: start_weekday is the weekday of the first of the month.

  Prints a row of asterisks to cover the top of the 'used' squares
  in the first week.
*/

void print_top_asterisks (int start_weekday) {
    /* There are start_weekday blank squares at the start,
       then (7 - start_weekday) used squares. */

    /* SQUARE_WIDTH blank characters per blank square: */
    putout(start_weekday * SQUARE_WIDTH, ' ');

    /* Now ditto for asterisks, plus one to cover the right hand
       vertical column of asterisks */
    putout((7 - start_weekday) * SQUARE_WIDTH + 1, '*');

    putchar('\n');     /* Complete the line */
}

/******************* print_full_line_asterisks: *******************
  Prints a full line of asterisks to separate calendar rows.
*/

void print_full_line_asterisks (void) {
    putout(7 * SQUARE_WIDTH + 1, '*');
    putchar('\n');     /* Complete the line */
}

/********************* print_bottom_asterisks: ********************
  On entry: sun_date is the date of Sunday in the final week of the
            month, and last_day is the last day in the month.

  Prints asterisks to underline days sun_date to last_day.
*/

void print_bottom_asterisks (int sun_date, int last_day) {
    putout((last_day - sun_date + 1) * SQUARE_WIDTH + 1, '*');
    putchar('\n');     /* Complete the line */
}