Calendar app fail

#!/usr/bin/env rdmd
/** A calendar application to inspired by github:quickfur/dcal.
      * by ARaspiK <araspik@protonmail.com>

  * The final product is envisioned to have every feature that util-linux/cal has, and is
  * meant to be a suitable and hopefully faster replacement of it.

  * This uses component-based programming (I first read of it and was inspired by it from
  * https://wiki.dlang.org/Component_programming_with_ranges) and is easy to read,
  * understand, and modify.

  * Armed with the understanding of the article's "Component Programming" section:
  * Source: List of dates
  * Target: Line-by-line formatted calendar with column-aligned day numbers and month
  * names, with distinct "month blocks".
  * Steps: dates -> by month -> format into set of lines -> add month name (centered) ->
  * join together month blocks -> formatted calendar

  * Configuration (via options):
    * First day as monday/sunday.
    * Show X months (starting with / around / ending with a specified date)
    * Show a full year / current month / X months / this week / X weeks

  * Copyright (c) 2018 ARaspiK <araspik@protonmail.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
  * "Software"), to deal in the Software without restriction, including
  * without limitation the rights to use, copy, modify, merge, publish,
  * distribute, sublicense, and/or sell copies of the Software, and to
  * permit persons to whom the Software is furnished to do so, subject to
  * the following conditions:
  *
  * The above copyright notice and this permission notice shall be
  * included in all copies or substantial portions of the Software.
  *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
  * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
  * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
  * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
  * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  *
  * [Except as contained in this notice, the name of ARaspiK
  * shall not be used in advertising or otherwise to promote the sale, use
  * or other dealings in this Software without prior written authorization
  * from ARaspiK.]
  */

module calendar;

import std.algorithm; // For everything ranges
import std.array; // Arrays and ranges
import std.datetime; // Getting date and getting basic info about it
import std.getopt; // We use standard parsing methods
import std.range; // RANGES EVERYWHERE
import std.stdio; // Input/Output anyone?
import std.string; // String manipulations during formatting

struct CalendarConfig {
  enum StartDay {sun = false, mon = true}
  StartDay startOfWeek = StartDay.sun;
  ubyte monthsPerRow = 3;
}

static CalendarConfig config;

/** Checks if the input type is an input range with element type Date. */
enum isDateRange(R) = isInputRange!R && is(ElementType!R : Date);

// Quick static test
static assert(isDateRange!(typeof(dateList!(a => a.year > 2013)(2013))));

/** Get a list of dates starting from @arg year, and keep going until @targ pred is
  * satisfied.
  */
auto dateList(alias pred)(short year) {
  return Date(year, 1, 1).recurrence!((a,n) => a[n-1] + 1.days).until!(pred);
}

/** Convernience function to be used with chunkBy to split by months. */
auto myMonth(Date a, Date b) {
  return a.month == b.month;
}

/** Convernience function to be used with chunkBy to split by weeks. */
auto myWeek(Date a, Date b) {
  if (config.startOfWeek == CalendarConfig.StartDay.mon)
    return a.isoWeek() == b.isoWeek();
  return (a + 1.days).isoWeek() == (b + 1.days).isoWeek();
}

/** Returns the first day of the week by our configuration. */
auto firstDayOfWeek() {
  if (config.startOfWeek == CalendarConfig.StartDay.mon)
    return DayOfWeek.mon;
  return DayOfWeek.sun;
}

/** Returns the last day of the week by our configuration. */
auto lastDayOfWeek() {
  if (config.startOfWeek == CalendarConfig.StartDay.mon)
    return DayOfWeek.sun;
  return DayOfWeek.sat;
}

/** Formats a single given week into a fixed-length string. */
auto formatWeek(Range)(Range week)
    if (isDateRange!Range) {
  auto firstDayInWeek = week.front.dayOfWeek;
  string result = week.map!(d => format!" %2d"(d.day)).join();
  if (firstDayInWeek != firstDayOfWeek())
    return result.rightJustify(7 * 3);
  if (week.front.dayOfWeek /* Last day */ != lastDayOfWeek())
    return result.leftJustify(7 * 3);
  return result;
}

/** Creates an auto-centered month title given the month. */
string monthTitle(Month month) {
  static immutable string[] monthNames = [
    "January", "Feb", "March", "April", "May", "June",
    "July", "August", "September", "October", "November", "December"
  ];

  return monthNames[month - 1].center(7 * 3);
}

/** Formats a month by joining monthTitle and calls to chunkBy!myWeek. */
auto formatMonth(Range)(Range monthDays)
    if (isDateRange!Range) {
  return chain([monthDays.front.month.monthTitle], monthDays.chunkBy!myWeek.array.formatWeek());
}

/** Formats blocks of pre-formatted months (ranges of lines) into horizontally-placed
  * blocks, without regard to how many go on each block.
  */
auto blockMonths(Range)(Range months, string sep)
    if (isForwardRange!Range && is(ElementType!(ElementType!Range) : string)) {
  struct MonthRow {
    Range months;
    string sep;
    bool isEmpty;

    this(Range months, string sep) {
      this.months = months;
      this.sep = sep;
      isEmpty = months.empty;
    }

    @property bool empty() {return isEmpty;}

    @property auto front() {
      return months.save.map!(a => a.front.leftJustify(7 * 3)).join(sep);
    }

    void popFront() {
      assert (!empty);
      isEmpty = true; // Assume we're out of data
      ror.each!((ElementType!Range r) {
          if (!r.empty) {
            r.popFront();
            if (!r.empty) isEmpty = false;
          }
        });
    }
  }
  return MonthRow(months, sep);
}

/*auto formatYear(short year) {
  return dateList!(a => a.year > year)(year).chunkBy!myMonth.chunks(config.monthsPerRow)
    .map!(r => r.map!formatMonth.array().blockMonths(" ").join("\n")).join("\n\n");
}

int main(string[] args) {

}*/