//! Camera-offset cascade and display-zone helpers for the true-instant date
//! contract (see `docs/adr/0001-true-utc-dates.md`).
//!
//! The legacy `image_exif.date_taken` column mixes conventions: EXIF-sourced
//! rows hold camera wall-clock *as UTC*, exiftool / filesystem rows hold true
//! UTC instants. The new columns — `date_utc`, `camera_offset_minutes`,
//! `date_taken_tz_source` — are populated at ingest by the offset cascade:
//!
//! 1. **offset tag** — the EXIF `OffsetTime*` tag, when the camera stamped
//!    it (explicit, most accurate).
//! 2. **gps** — the capture location resolved to an IANA zone (bundled
//!    offline TZF geometry) and that zone's offset for the capture *date*
//!    (DST-correct via chrono-tz).
//! 3. **assumed_server** — the server's own IANA zone, offset at the
//!    capture date. A camera without offset or GPS was probably carried
//!    around by its owner, i.e. in the operator's home zone.
//!
//! The cascade always returns `(offset_minutes, source)` so the caller can
//! compute `date_utc = wall − offset` and audit why the row has the offset
//! it does. Branches that already produce true UTC instants (exiftool,
//! filesystem time) skip the cascade entirely and are recorded as
//! [`TzSource::UtcDirect`].

use std::str::FromStr;
use std::sync::OnceLock;

use chrono::{Local, LocalResult, NaiveDateTime, Offset, TimeZone};
use chrono_tz::Tz;

/// Why a `date_utc` row has the offset it does — or that it is a direct
/// true instant. Written verbatim to `image_exif.date_taken_tz_source`.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum TzSource {
    /// An EXIF `OffsetTime*` tag supplied the camera offset directly.
    OffsetTag,
    /// GPS → IANA-zone lookup resolved the offset for the capture date.
    Gps,
    /// No offset tag and no usable GPS: assumed the server's zone.
    AssumedServer,
    /// The resolver branch already produced a true UTC instant (exiftool,
    /// filesystem time, unix-timestamp filename). No offset is involved.
    UtcDirect,
    /// An operator set the date manually (POST /image/exif/date, ticket
    /// #115): the wall the user picked, interpreted in their display zone.
    Manual,
}

impl TzSource {
    pub fn as_str(self) -> &'static str {
        match self {
            TzSource::OffsetTag => "offset_tag",
            TzSource::Gps => "gps",
            TzSource::AssumedServer => "assumed_server",
            TzSource::UtcDirect => "utc_direct",
            TzSource::Manual => "manual",
        }
    }
}

/// Evidence available to the offset cascade for one file.
#[derive(Copy, Clone, Debug, Default)]
pub struct OffsetEvidence {
    /// Signed minutes from the EXIF `OffsetTime*` tag (e.g. EDT = `-240`),
    /// when the camera stamped one.
    pub offset_tag_minutes: Option<i32>,
    /// `(latitude, longitude)` from the EXIF GPS tags, when present.
    pub gps: Option<(f64, f64)>,
}

/// A resolved camera offset for one capture.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub struct OffsetResolution {
    /// Camera local time minus UTC, in minutes (negative = west of UTC).
    pub minutes: i32,
    /// Which rung of the cascade produced it.
    pub source: TzSource,
}

/// GPS → IANA-zone seam. The production adapter wraps the bundled offline
/// TZF geometry; tests inject a stub.
pub trait GpsZoneLookup {
    fn zone_at(&self, lat: f64, lon: f64) -> Option<Tz>;
}

/// Production [`GpsZoneLookup`]: bundled lite TZF geometry. No network,
/// ~4 MB, loaded once per process on first use.
pub struct TzfGpsZoneLookup {
    finder: OnceLock<tzf_rs::DefaultFinder>,
}

impl TzfGpsZoneLookup {
    pub fn new() -> Self {
        Self {
            finder: OnceLock::new(),
        }
    }

    fn finder(&self) -> &tzf_rs::DefaultFinder {
        self.finder.get_or_init(tzf_rs::DefaultFinder::new)
    }
}

impl Default for TzfGpsZoneLookup {
    fn default() -> Self {
        Self::new()
    }
}

impl GpsZoneLookup for TzfGpsZoneLookup {
    fn zone_at(&self, lat: f64, lon: f64) -> Option<Tz> {
        // tzf-rs takes (lng, lat) — the reverse of our (lat, lon) convention.
        let name = self.finder().get_tz_name(lon, lat);
        if name.is_empty() {
            return None;
        }
        Tz::from_str(name).ok()
    }
}

/// The zone offset, in minutes, for a naive wall-clock reading.
///
/// DST-correct: resolves the zone's offset for the wall-clock *date itself*
/// (the same zone rule the camera's own clock followed). Ambiguous
/// fall-back wall times resolve to the earlier (pre-transition) offset;
/// nonexistent spring-forward wall times to the later one — each within one
/// hour of the true capture instant, never off by a full day.
pub fn offset_minutes_for_zone(zone: Tz, wall: NaiveDateTime) -> i32 {
    let dt = match zone.from_local_datetime(&wall) {
        LocalResult::Single(dt) => dt,
        LocalResult::Ambiguous(early, _) => early,
        // Nonexistent spring-forward wall time: use the zone's offset at
        // that wall reading interpreted as UTC — within an hour of truth.
        LocalResult::None => zone.from_utc_datetime(&wall),
    };
    dt.offset().fix().local_minus_utc() / 60
}

/// True UTC instant (unix seconds) for a camera wall-clock reading and its
/// resolved offset: `instant = wall − offset`.
///
/// e.g. wall `2024-07-04 12:00:00` at EDT (`-240`) → `16:00:00Z`.
pub fn wall_to_instant(wall: NaiveDateTime, offset_minutes: i32) -> i64 {
    wall.and_utc().timestamp() - (offset_minutes as i64) * 60
}

/// The offset cascade: offset tag → GPS zone → server zone.
///
/// `server_zone` is the operator's home zone (see [`server_zone`]); when it
/// is `None` (unresolvable on this host) the cascade falls back to the
/// process-local offset, which is only exact for fixed-offset zones.
pub fn resolve_camera_offset(
    wall: NaiveDateTime,
    evidence: &OffsetEvidence,
    server_zone: Option<Tz>,
    zone_lookup: &dyn GpsZoneLookup,
) -> OffsetResolution {
    if let Some(minutes) = evidence.offset_tag_minutes {
        return OffsetResolution {
            minutes,
            source: TzSource::OffsetTag,
        };
    }
    if let Some((lat, lon)) = evidence.gps
        && let Some(zone) = zone_lookup.zone_at(lat, lon)
    {
        return OffsetResolution {
            minutes: offset_minutes_for_zone(zone, wall),
            source: TzSource::Gps,
        };
    }
    let minutes = match server_zone {
        Some(zone) => offset_minutes_for_zone(zone, wall),
        None => Local::now().offset().fix().local_minus_utc() / 60,
    };
    OffsetResolution {
        minutes,
        source: TzSource::AssumedServer,
    }
}

/// Resolve the server's IANA zone from the host environment, in order:
/// `TZ` env var (zone-name form), `/etc/timezone`, then the
/// `/etc/localtime` symlink target. Returns `None` when nothing parses —
/// callers then fall back to the process-local offset.
pub fn server_zone() -> Option<Tz> {
    if let Ok(tz) = std::env::var("TZ")
        && let Some(zone) = Tz::from_str(tz.trim()).ok()
    {
        return Some(zone);
    }
    if let Ok(contents) = std::fs::read_to_string("/etc/timezone")
        && let Some(zone) = Tz::from_str(contents.trim()).ok()
    {
        return Some(zone);
    }
    if let Ok(target) = std::fs::read_link("/etc/localtime") {
        let target = target.to_string_lossy().into_owned();
        let zone_name = target
            .trim_start_matches("/usr/share/zoneinfo/")
            .trim_start_matches("/etc/zoneinfo/")
            .trim_start_matches("/var/db/timezone/zoneinfo/");
        if !zone_name.is_empty()
            && zone_name != "localtime"
            && let Ok(zone) = Tz::from_str(zone_name)
        {
            return Some(zone);
        }
    }
    None
}

/// App default display zone when the user hasn't stored one
/// (`user_ai_prefs.display_tz` NULL).
pub const DEFAULT_DISPLAY_TZ: &str = "America/New_York";

/// Resolve the user's stored display zone to a usable IANA name.
/// NULL / empty / unparseable prefs fall back to the app default, so
/// read paths (tickets #119/#120) never have to special-case the
/// unset state themselves.
pub fn display_zone_for(stored: Option<&str>) -> String {
    match stored.map(str::trim).filter(|s| !s.is_empty()) {
        Some(name) if Tz::from_str(name).is_ok() => name.to_string(),
        _ => DEFAULT_DISPLAY_TZ.to_string(),
    }
}

/// Pure seam for the manual date-edit endpoints (ADR-0001, ticket #115):
/// reinterpret the naive-as-UTC epoch the UI sent as a wall clock in the
/// user's display zone, and produce the new-contract
/// `(date_utc, camera_offset_minutes)` pair. The caller writes
/// `date_taken_tz_source = 'manual'` itself and stores the epoch verbatim
/// in the legacy `date_taken` column (byte-identical to the pre-rework
/// behavior).
///
/// `stored_zone` is `user_ai_prefs.display_tz` (NULL / unparseable → app
/// default). The offset follows the wall's own date through the zone's DST
/// rules — ambiguous fall-back walls take the earlier (pre-transition)
/// offset; nonexistent spring-forward walls take the zone's offset at the
/// wall-as-UTC reading, within an hour of truth (see
/// [`offset_minutes_for_zone`]). `None` only when the zone name is
/// unparseable, which [`display_zone_for`] already rules out — the caller
/// then leaves the new-contract triple untouched.
pub fn manual_date_resolution(
    stored_zone: Option<&str>,
    wall_naive_epoch: i64,
) -> Option<(i64, i32)> {
    let zone = Tz::from_str(&display_zone_for(stored_zone)).ok()?;
    let wall = chrono::DateTime::from_timestamp(wall_naive_epoch, 0).map(|dt| dt.naive_utc())?;
    let minutes = offset_minutes_for_zone(zone, wall);
    Some((wall_to_instant(wall, minutes), minutes))
}

/// Display-zone offset, in minutes, for a true UTC instant — DST-aware.
/// `None` when the zone name doesn't parse.
#[allow(dead_code)] // consumed by the read-side cutover (#119/#120)
pub fn display_zone_offset_minutes(zone_name: &str, instant: i64) -> Option<i32> {
    let zone = Tz::from_str(zone_name).ok()?;
    let dt = zone.timestamp_opt(instant, 0).single()?;
    Some(dt.offset().fix().local_minus_utc() / 60)
}

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

    #[test]
    fn display_zone_for_falls_back_to_default() {
        assert_eq!(display_zone_for(None).as_str(), DEFAULT_DISPLAY_TZ);
        assert_eq!(display_zone_for(Some("")).as_str(), DEFAULT_DISPLAY_TZ);
        assert_eq!(display_zone_for(Some("  ")).as_str(), DEFAULT_DISPLAY_TZ);
        assert_eq!(
            display_zone_for(Some("Not/A_Zone")).as_str(),
            DEFAULT_DISPLAY_TZ
        );
    }

    #[test]
    fn display_zone_for_passes_through_valid_names() {
        assert_eq!(display_zone_for(Some("Europe/Berlin")), "Europe/Berlin");
        assert_eq!(display_zone_for(Some("  Asia/Tokyo ")), "Asia/Tokyo");
    }

    /// Stub lookup for cascade tests: one fixed zone or none.
    struct StubLookup(Option<Tz>);

    impl GpsZoneLookup for StubLookup {
        fn zone_at(&self, _lat: f64, _lon: f64) -> Option<Tz> {
            self.0
        }
    }

    fn ny() -> Tz {
        chrono_tz::America::New_York
    }

    fn wall_july() -> NaiveDateTime {
        NaiveDateTime::parse_from_str("2024-07-04 12:00:00", "%Y-%m-%d %H:%M:%S").unwrap()
    }

    fn wall_january() -> NaiveDateTime {
        NaiveDateTime::parse_from_str("2024-01-15 12:00:00", "%Y-%m-%d %H:%M:%S").unwrap()
    }

    #[test]
    fn offset_tag_wins_over_everything() {
        let evidence = OffsetEvidence {
            offset_tag_minutes: Some(330), // +05:30
            gps: Some((40.71, -74.00)),
        };
        let res =
            resolve_camera_offset(wall_july(), &evidence, Some(ny()), &StubLookup(Some(ny())));
        assert_eq!(res.minutes, 330);
        assert_eq!(res.source, TzSource::OffsetTag);
    }

    #[test]
    fn gps_zone_wins_when_no_tag() {
        let evidence = OffsetEvidence {
            offset_tag_minutes: None,
            gps: Some((40.71, -74.00)),
        };
        let res =
            resolve_camera_offset(wall_july(), &evidence, Some(ny()), &StubLookup(Some(ny())));
        assert_eq!(res.minutes, -240, "July wall time in NY is EDT (UTC-4)");
        assert_eq!(res.source, TzSource::Gps);
    }

    #[test]
    fn server_zone_is_last_resort() {
        let evidence = OffsetEvidence::default();
        let res = resolve_camera_offset(
            wall_january(),
            &evidence,
            Some(ny()),
            &StubLookup(Some(ny())), // stub is ignored: no GPS evidence
        );
        assert_eq!(res.minutes, -300, "January wall time in NY is EST (UTC-5)");
        assert_eq!(res.source, TzSource::AssumedServer);
    }

    #[test]
    fn gps_miss_falls_through_to_server_zone() {
        let evidence = OffsetEvidence {
            offset_tag_minutes: None,
            gps: Some((30.0, -40.0)), // open ocean: lookup returns None
        };
        let res = resolve_camera_offset(wall_january(), &evidence, Some(ny()), &StubLookup(None));
        assert_eq!(res.source, TzSource::AssumedServer);
    }

    #[test]
    fn zone_offset_is_dst_correct_for_capture_date() {
        assert_eq!(offset_minutes_for_zone(ny(), wall_july()), -240);
        assert_eq!(offset_minutes_for_zone(ny(), wall_january()), -300);
        // Berlin: CEST +120 in summer, CET +60 in winter.
        let berlin = chrono_tz::Europe::Berlin;
        assert_eq!(offset_minutes_for_zone(berlin, wall_july()), 120);
        assert_eq!(offset_minutes_for_zone(berlin, wall_january()), 60);
    }

    #[test]
    fn wall_to_instant_edt_example() {
        // Wall 2024-07-04 12:00:00 at EDT (-240) is 16:00:00 UTC
        // = 1720108800. This is the exact pair the mixed-convention audit
        // used to prove the legacy EXIF branch was off by 4 hours.
        assert_eq!(wall_to_instant(wall_july(), -240), 1_720_108_800);
    }

    #[test]
    fn wall_to_instant_positive_offset() {
        // Wall 2024-07-04 12:00:00 at +05:30 (330) is 06:30:00 UTC.
        let expected = wall_july().and_utc().timestamp() - 330 * 60;
        assert_eq!(wall_to_instant(wall_july(), 330), expected);
    }

    #[test]
    fn display_zone_offset_minutes_is_dst_aware() {
        // 2024-07-04 16:00:00Z — New York is on EDT.
        assert_eq!(
            display_zone_offset_minutes("America/New_York", 1_720_108_800),
            Some(-240)
        );
        // 2024-01-15 17:00:00Z — New York is on EST.
        assert_eq!(
            display_zone_offset_minutes("America/New_York", 1_705_335_600),
            Some(-300)
        );
    }

    #[test]
    fn manual_date_resolution_display_zone_edt() {
        // The UI sends the wall rendered in the display zone, as a
        // naive-as-UTC epoch: 1720080000 = "2024-07-04 04:00:00". July in
        // New York is EDT (-240): the instant is 08:00:00Z.
        let (utc, off) = manual_date_resolution(Some("America/New_York"), 1_720_080_000).unwrap();
        assert_eq!(off, -240);
        assert_eq!(utc, 1_720_080_000 + 4 * 3600);
    }

    #[test]
    fn manual_date_resolution_display_zone_est() {
        // Same function in January: EST (-300). Wall 2024-01-15 04:00:00
        // (naive epoch 1705291200) → 09:00:00Z.
        let (utc, off) = manual_date_resolution(Some("America/New_York"), 1_705_291_200).unwrap();
        assert_eq!(off, -300);
        assert_eq!(utc, 1_705_291_200 + 5 * 3600);
    }

    #[test]
    fn manual_date_resolution_positive_zone_and_default() {
        // Tokyo is +09:00 year-round: wall 04:00 → previous day 19:00Z.
        let (utc, off) = manual_date_resolution(Some("Asia/Tokyo"), 1_720_080_000).unwrap();
        assert_eq!(off, 540);
        assert_eq!(utc, 1_720_080_000 - 540 * 60);
        // NULL stored zone → app default (America/New_York), July → EDT.
        let (_, off) = manual_date_resolution(None, 1_720_080_000).unwrap();
        assert_eq!(off, -240);
        // Unparseable stored zone also falls back to the default.
        let (_, off) = manual_date_resolution(Some("Not/A_Zone"), 1_720_080_000).unwrap();
        assert_eq!(off, -240);
    }

    #[test]
    fn display_zone_offset_minutes_unknown_zone_is_none() {
        assert_eq!(
            display_zone_offset_minutes("Not/A_Zone", 1_700_000_000),
            None
        );
    }

    #[test]
    fn tz_source_as_str_round_trip() {
        let pairs = [
            (TzSource::OffsetTag, "offset_tag"),
            (TzSource::Gps, "gps"),
            (TzSource::AssumedServer, "assumed_server"),
            (TzSource::UtcDirect, "utc_direct"),
            (TzSource::Manual, "manual"),
        ];
        for (src, expected) in pairs {
            assert_eq!(src.as_str(), expected);
        }
    }

    #[test]
    fn tzf_lookup_resolves_new_york() {
        let lookup = TzfGpsZoneLookup::new();
        assert_eq!(
            lookup.zone_at(40.7128, -74.0060),
            Some(chrono_tz::America::New_York)
        );
    }

    #[test]
    fn tzf_lookup_resolves_berlin() {
        let lookup = TzfGpsZoneLookup::new();
        assert_eq!(
            lookup.zone_at(52.5200, 13.4050),
            Some(chrono_tz::Europe::Berlin)
        );
    }

    #[test]
    fn tzf_lookup_open_ocean_resolves_etc_zone() {
        // The bundled lite data covers open ocean with fixed-offset
        // Etc/GMT* zones — a mid-Atlantic photo gets a sane local offset
        // rather than falling through to the server zone.
        let lookup = TzfGpsZoneLookup::new();
        let zone = lookup
            .zone_at(30.0, -40.0)
            .expect("ocean resolves to Etc/GMT*");
        assert!(zone.name().starts_with("Etc/GMT"), "got {}", zone.name());
    }

    #[test]
    fn server_zone_resolves_on_this_host_or_none() {
        // The host's zone is whatever it is — the contract is that a
        // parseable result is a real Tz and a missing config is None, not a
        // panic. Both outcomes are valid; assert only the no-panic shape.
        let _ = server_zone();
    }
}