//! Time: the fixed RFC 3339 form (SPEC conventions), the `Europe/Stockholm` constant (SPEC §10.4) and an injectable
//! clock so tests control "now".

use std::sync::{Arc, Mutex};

use chrono::{DateTime, NaiveDate, SecondsFormat, Utc};
use chrono_tz::Tz;

/// The household's time zone; a constant in code, not configuration (SPEC §10.4).
pub const STOCKHOLM: Tz = chrono_tz::Europe::Stockholm;

/// Formats an instant in the fixed form `2026-10-17T06:00:00.000Z` (millisecond precision, literal `Z`).
pub fn format_ts(instant: DateTime<Utc>) -> String {
    instant.to_rfc3339_opts(SecondsFormat::Millis, true)
}

/// Parses a timestamp in exactly the fixed form; anything else (other offsets, other precision) is `None`.
pub fn parse_ts(text: &str) -> Option<DateTime<Utc>> {
    let instant = DateTime::parse_from_rfc3339(text).ok()?.with_timezone(&Utc);
    (format_ts(instant) == text).then_some(instant)
}

/// Parses a calendar date `YYYY-MM-DD` (exactly that form).
pub fn parse_date(text: &str) -> Option<NaiveDate> {
    let date = NaiveDate::parse_from_str(text, "%Y-%m-%d").ok()?;
    (date.format("%Y-%m-%d").to_string() == text).then_some(date)
}

/// Today's calendar date in Europe/Stockholm.
pub fn stockholm_today(now: DateTime<Utc>) -> NaiveDate {
    now.with_timezone(&STOCKHOLM).date_naive()
}

/// Source of "now". The server uses [`SystemClock`]; tests use [`FixedClock`].
pub trait Clock: Send + Sync + 'static {
    fn now(&self) -> DateTime<Utc>;

    /// `now` in the fixed RFC 3339 form.
    fn now_ts(&self) -> String {
        format_ts(self.now())
    }
}

/// The real clock.
#[derive(Debug, Clone, Copy, Default)]
pub struct SystemClock;

impl Clock for SystemClock {
    fn now(&self) -> DateTime<Utc> {
        Utc::now()
    }
}

/// A clock that returns a set instant until changed.
#[derive(Debug, Clone)]
pub struct FixedClock(Arc<Mutex<DateTime<Utc>>>);

impl FixedClock {
    pub fn new(instant: DateTime<Utc>) -> Self {
        Self(Arc::new(Mutex::new(instant)))
    }

    /// Sets the instant every clone of this clock returns from now on.
    pub fn set(&self, instant: DateTime<Utc>) {
        *crate::lock(&self.0) = instant;
    }
}

impl Clock for FixedClock {
    fn now(&self) -> DateTime<Utc> {
        *crate::lock(&self.0)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn fixed_form_round_trip() {
        let t = parse_ts("2026-10-17T06:00:00.000Z").unwrap();
        assert_eq!(format_ts(t), "2026-10-17T06:00:00.000Z");
        assert!(parse_ts("2026-10-17T06:00:00Z").is_none());
        assert!(parse_ts("2026-10-17T08:00:00.000+02:00").is_none());
    }

    #[test]
    fn dates() {
        assert!(parse_date("2026-02-29").is_none());
        assert!(parse_date("2026-2-01").is_none());
        assert_eq!(parse_date("2028-02-29").unwrap().to_string(), "2028-02-29");
    }

    #[test]
    fn stockholm_day_boundary() {
        // 23:30 UTC on 17 October is 01:30 on 18 October in Stockholm (CEST, UTC+2).
        let t = parse_ts("2026-10-17T23:30:00.000Z").unwrap();
        assert_eq!(stockholm_today(t).to_string(), "2026-10-18");
    }

    #[test]
    fn fixed_clock_is_shared_by_clones() {
        let clock = FixedClock::new(parse_ts("2026-10-17T06:00:00.000Z").unwrap());
        let other = clock.clone();
        clock.set(parse_ts("2026-10-18T06:00:00.000Z").unwrap());
        assert_eq!(other.now_ts(), "2026-10-18T06:00:00.000Z");
    }
}