Create a routine that will generate a text calendar for any year.
Test the calendar by generating a calendar for the year 1969, on a device of the time.
Choose one of the following devices:

 A line printer with a width of 132 characters.
 An IBM 3278 model 4 terminal (80×43 display with accented characters). Target formatting the months of the year to fit nicely across the 80 character width screen. Restrict number of lines in test output to 43.


(Ideally, the program will generate well-formatted calendars for any page width from 20 characters up.)

Kudos (κῦδος) for routines that also transition from Julian to Gregorian calendar.

This task is inspired by Real Programmers Don't Use PASCAL by Ed Post, Datamation, volume 29 number 7, July 1983.

THE REAL PROGRAMMER'S NATURAL HABITAT
"Taped to the wall is a line-printer Snoopy calender for the year 1969."

For further Kudos see task CALENDAR, where all code is to be in UPPERCASE.

For economy of size, do not actually include Snoopy generation in either the code or the output, instead just output a place-holder.



Related task

   Five weekends


// Assume your binary name is 'calendar'.
// Command line:
// >>$ calendar 2019 150
// First argument: year number.
// Second argument (optional): text area width (in characters).
 
extern crate chrono;
 
use std::{env, cmp};
use chrono::{NaiveDate, Datelike};
 
const MONTH_WIDTH: usize = 22;
 
fn print_header(months: &[&str]) {
    const DAYS_OF_WEEK: &str = "SU MO TU WE TH FR SA  ";
    println!();
    for m in months {
        print!("{:^20}  ", m);
    }
    println!("\n{}", DAYS_OF_WEEK.repeat(months.len()));
}
 
fn get_week_str(days: i32, week_num: i32, start_day_of_week: i32) -> Option<String> {
    let start = week_num * 7 - start_day_of_week + 1;
    let end = (week_num + 1) * 7 - start_day_of_week;
    let mut ret = String::with_capacity(MONTH_WIDTH);
    if start > days {
        None
    } else {
        for i in start..(end + 1) {
            if i <= 0 || i > days {
                ret.push_str("  ");
            } else {
                if i < 10 {
                    ret.push_str(" ");
                }
                ret.push_str(&i.to_string());
            }
            ret.push_str(" ");
        }
        ret.push_str(" ");
        Some(ret)
    }
}
 
fn main() {
    const MONTH_NAMES: [&str; 12] = ["JANUARY", "FEBRUARY", "MARCH", "APRIL", "MAY", "JUNE", "JULY",
                                     "AUGUST", "SEPTEMBER", "OCTOBER", "NOVEMBER", "DECEMBER"];
    const DEFAULT_TEXT_WIDTH: usize = 100;
 
    let args: Vec<String> = env::args().collect();
    let year: i32 = args[1].parse().expect("The first argument must be a year");
    let width: usize = if args.len() > 2 {
        cmp::max(MONTH_WIDTH, args[2].parse().expect("The second argument should be text width"))
    } else {
        DEFAULT_TEXT_WIDTH
    };
    let months_in_row = width / MONTH_WIDTH;
    let month_rows = if MONTH_NAMES.len() % months_in_row == 0 {
        MONTH_NAMES.len() / months_in_row
    } else {
        MONTH_NAMES.len() / months_in_row + 1
    };
 
    let start_days_of_week: Vec<i32> =
        (1..13).map(|x| NaiveDate::from_ymd(year, x, 1).weekday().num_days_from_sunday() as i32).collect();
 
    let month_days: [i32; 12] = if NaiveDate::from_ymd_opt(year, 2, 29).is_some() {
        [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    } else {
        [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    };
 
    println!("{year:^w$}", w=width, year=year.to_string());
    for i in 0..month_rows {
        let start = i * months_in_row;
        let end = cmp::min((i + 1) * months_in_row, MONTH_NAMES.len());
        print_header(&MONTH_NAMES[start..end]);
        let mut count = 0;
        let mut row_num = 0;
        while count < months_in_row {
            let mut row_str = String::with_capacity(width);
            for j in start..end {
                match get_week_str(month_days[j], row_num, start_days_of_week[j]) {
                    None => {
                        count += 1;
                        row_str.push_str(&" ".repeat(MONTH_WIDTH));
                    },
                    Some(week_str) => row_str.push_str(&week_str)
                }
            }
            if count < months_in_row {
                println!("{}", row_str);
            }
            row_num += 1;
        }
    }
}
 