<?php

// CALENDAR_PATH :: string
// The absolute path to the `calendar` program.
const CALENDAR_PATH = '/usr/bin/calendar';

// CALEVENTS_PATH :: string
// The absolute path to the `calendar-events` program.
const CALEVENTS_PATH = '/path/to/calendar-events';

// EMAIL_RECEIVER :: string
// Where to send the email?
const EMAIL_RECEIVER = 'you@where.ever';

// EMAIL_SENDER :: string
// Who should send the email?
const EMAIL_SENDER = 'who@this.server';

// EMAIL_SUBJECT :: string
const EMAIL_SUBJECT  = "Upcoming events on your calendar";

// getMessage :: void -> [string]
function getMessage() {
    $blocks = [
        "Today is ".date('l, F jS, Y')
    ];

    array_push(
        $blocks,
        makeMessageBlock(
            "Next 10 days",
            makeCalendarRangeCommand(10, ((date('n') == 12) ? '-M' : null))
        )
    );

    if (date('j') === '1') {
        array_push(
            $blocks,
            makeMessageBlock(
                "This Month",
                makeCalendarMonthCommand('this', ((date('n') == 12) ? '-M' : null))
            ),
            makeMessageBlock(
                "Next Month",
                makeCalendarMonthCommand('next', ((date('n') == 12) ? '-M' : null))
            )
        );
    }

    return implode("\n\n", $blocks);
}

// makeCalendarRangeCommand :: (numeric, string?) -> string
function makeCalendarRangeCommand($days, $sortOpts = null) {
    return (
        "(".CALENDAR_PATH." -A {$days} ; ".
        CALEVENTS_PATH." -A {$days} -co) ".
        "| sort ".($sortOpts ?? '')
    );
}

// makeCalendarMonthCommand :: (which, string?) -> string
// which = (string) must be 'this' or 'next'
function makeCalendarMonthCommand($which, $sortOpts) {
    $start = [
        date("Ymd", strtotime("first day of {$which} month")),
        date("Y-m-d", strtotime("first day of {$which} month"))
    ];
    $end = date("Y-m-d", (strtotime("last day of {$which} month")));
    $days = date("d", (strtotime("last day of {$which} month") - 86400));
    return (
        "(".CALENDAR_PATH." -t {$start[0]} -A {$days} ; ".
        CALEVENTS_PATH." -ge {$start[1]} -lt {$end} -co) ".
        "| sort ".($sortOpts ?? '')
    );
}

// makeMessageBlock :: (string, string) -> string
function makeMessageBlock($header, $command) {
    $output = `{$command}`;
    if ($output) {
        return "[{$header}]\n{$output}";
    }
    return '';
}

// getHeaders :: void -> [string => string]
function getHeaders() {
    return [
        'to' => EMAIL_RECEIVER,
        'extra' => "From: ".EMAIL_SENDER."\n".
        "X-Mailer: PHP/".phpversion(),
    ];
}

// sendMail :: (string, string, [string => string]) -> void
function sendMail($subject, $message, $headers) {
    mail(
        $headers['to'],
        $subject,
        $message,
        $headers['extra']
    );
}

sendMail(
    EMAIL_SUBJECT,
    getMessage(),
    getHeaders()
);