use std::env::args;
use std::str::FromStr;

use chrono::{DateTime, Local, NaiveTime, TimeDelta, TimeZone, Utc};
use chrono_tz::Tz;
use icalendar::{Calendar, CalendarComponent, CalendarDateTime, Component, DatePerhapsTime, EventLike};
use walkdir::WalkDir;

fn to_utc(dpt: DatePerhapsTime) -> Option<DateTime<Utc>> {
    match dpt {
        DatePerhapsTime::DateTime(cdt) => match cdt {
            CalendarDateTime::Utc(dt) => Some(dt),
            CalendarDateTime::Floating(naive) => Local
                .from_local_datetime(&naive)
                .earliest()
                .map(|dt| dt.with_timezone(&Utc)),
            CalendarDateTime::WithTimezone { date_time, tzid } => {
                let tz = tzid.parse::<Tz>().ok()?;
                tz.from_local_datetime(&date_time)
                    .earliest()
                    .map(|dt| dt.with_timezone(&Utc))
            }
        },
        DatePerhapsTime::Date(date) => {
            let naive = date.and_time(NaiveTime::from_hms_opt(0, 0, 0).unwrap());
            Local
                .from_local_datetime(&naive)
                .earliest()
                .map(|dt| dt.with_timezone(&Utc))
        }
    }
}

fn is_imminent(time_delta: TimeDelta) -> bool {
    time_delta <= TimeDelta::hours(2)
}
fn is_soon(time_delta: TimeDelta) -> bool {
    time_delta <= TimeDelta::hours(6)
}

fn show_distant_event(time: DateTime<Local>, summary: &str) -> String {
    format!("{} at {}", summary, time.format("%H:%M"))
}

fn format_time_delta(td: TimeDelta) -> String {
    if td.num_hours() == 0 {
        format!("{}m", td.num_minutes())
    } else {
        format!("{}h{}m", td.num_hours(), td.num_minutes() % 60)
    }
}

fn show_imminent(time_delta: TimeDelta, summary: &str, location: Option<String>) -> String {
    if let Some(loc) = location {
        format!("{} in {} in {}", summary, format_time_delta(time_delta), loc)
    } else {
        format!("{} in {}", summary, format_time_delta(time_delta))
    }
}

fn parenthesize_location(location: Option<String>) -> String {
    location.map(|u| format!("({}) ", u)).unwrap_or("".to_string())
}

fn is_ending_imminent(time_delta: TimeDelta) -> bool {
    time_delta <= TimeDelta::minutes(30)
}
fn show_imminent_ending(time_delta: TimeDelta, summary: &str, location: Option<String>) -> String {
    format!("{} {}ending in {}", summary, parenthesize_location(location), format_time_delta(time_delta))
}
fn show_distant_ending(end: DateTime<Local>, summary: &str, location: Option<String>) -> String {
    format!("in {} {}until {}", summary, parenthesize_location(location), end.format("%H:%M"))
}

fn main() {
    let vdir = args().nth(1).expect("usage: calstr <vdir> [withloc]");

    let with_loc = args().nth(2) == Some("withloc".to_string());

    let now = Utc::now();
    let mut upcoming: Vec<(DateTime<Utc>, DateTime<Utc>, String, Option<String>)> = Vec::new();

    for entry in WalkDir::new(&vdir)
        .follow_links(true)
        .into_iter()
        .filter_map(|e| e.ok())
    {
        if !entry.file_name().to_string_lossy().ends_with(".ics") {
            continue;
        }
        let content = match std::fs::read_to_string(entry.path()) {
            Ok(c) => c,
            Err(_) => continue,
        };
        let cal = match Calendar::from_str(&content) {
            Ok(c) => c,
            Err(_) => continue,
        };
        for component in cal.components {
            if let CalendarComponent::Event(event) = component {
                let Some(start) = event.get_start() else {
                    continue;
                };
                let Some(end) = event.get_end() else {
                    continue;
                };
                let Some(dt) = to_utc(start) else { continue };
                let Some(dte) = to_utc(end) else { continue };
                if dte > now {
                    let summary = event.get_summary().unwrap_or("(no title)").to_string();
                    let location = event.get_location().map(|u| u.to_string());
                    upcoming.push((dt, dte, summary, if with_loc { location } else { None }));
                }
            }
        }
    }

    upcoming.sort_by_key(|(dt, _, _, _)| *dt);

    // write the message
    // "nothing left today" if next event is past midnight or there are no events
    if upcoming.is_empty() {
        println!("nothing left today");
        return;
    }
    let next = &upcoming[0];
    let next_local = next.0.with_timezone(&Local);

    let now = Utc::now();

    let mut midnight = Local::now().with_time(NaiveTime::from_hms_milli_opt(0, 0, 0, 0).unwrap()).unwrap();
    midnight += TimeDelta::days(1);

    let time_until_next = next.0 - now;

    if next.0 > midnight && !is_imminent(time_until_next) {
        print!("nothing left today");
        // exception: if event is soon (<6h), include it
        if is_soon(time_until_next) {
            print!(", {} tomorrow", show_distant_event(next_local, &next.2))
        }
        println!();
        return;
    }

    // has event started?
    if time_until_next < TimeDelta::zero() {
        let time_until_end = next.1 - now;
        let end_local = next.1.with_timezone(&Local);
        // is end imminent
        if is_ending_imminent(time_until_end) {
            let followup = upcoming.get(1);
            if let Some((following_start, _, following, following_loc)) = followup {
                print!("{}", show_imminent_ending(time_until_end, &next.2, next.3.clone()));
                let turnaround_time = *following_start - next.1;

                if !is_soon(turnaround_time) {
                    println!();
                    return;
                }

                if is_imminent(turnaround_time) {
                    println!(", then {}", show_imminent(turnaround_time, following, following_loc.clone()));
                    return;
                } else {
                    println!(", then {}", show_distant_event(following_start.with_timezone(&Local), following));
                    return;
                }
            } else {
                println!("{}", show_imminent_ending(time_until_end, &next.2, next.3.clone()));
                return;
            }
        } else {
            println!("{}", show_distant_ending(end_local, &next.2, next.3.clone()));
            return;
        }
    }

    // next event is today or imminent (or both)
    // is it imminent?
    if is_imminent(time_until_next) {
        println!("{}", show_imminent(time_until_next, &next.2, next.3.clone()));
        return;
    }

    // today, show distant
    println!("{}", show_distant_event(next_local, &next.2));
}