//! Transforming events between a source calendar and its aggregate copy.
//!
//! Mirroring is not a byte copy. The aggregate copy must be *scheduling inert* —
//! writing it must never cause a server to mail invitations or cancellations for
//! a meeting that was already invited from its source. How that is achieved
//! depends on the target backend, which is what [`SchedulingSuppression`] selects.
//!
//! Writing an edit back to the source is the inverse, with one asymmetry: the
//! demoted form has thrown away the live guest list, so the source event itself
//! is used as the donor to put it back. Otherwise editing a meeting's time in the
//! aggregate would silently drop its guests.

use crate::config::SchedulingSuppression;
use crate::ical::Calendar;

/// Records which source an aggregate copy came from.
pub const SOURCE_PROPERTY: &str = "X-CALCALIST-SOURCE";
/// Records the UID the event has in its source calendar.
pub const ORIGIN_UID_PROPERTY: &str = "X-CALCALIST-ORIGIN-UID";
/// Carries the guest list inertly when it cannot be kept as real attendees.
pub const ATTENDEES_PROPERTY: &str = "X-CALCALIST-ATTENDEES";

/// Separates the original description from the appended guest list, so the
/// addition can be found and removed again when writing an edit back.
pub const GUEST_MARKER: &str = "-- guests (calcalist) --";

/// Properties that only ever exist on an aggregate copy.
const OWN_PROPERTIES: &[&str] = &[SOURCE_PROPERTY, ORIGIN_UID_PROPERTY, ATTENDEES_PROPERTY];

/// Live scheduling properties, whose presence is what makes a server send mail.
const SCHEDULING_PROPERTIES: &[&str] = &["ATTENDEE", "ORGANIZER"];

/// The marker that keeps an event in the aggregate rather than filing it under
/// a source. Reserved: an endpoint may not be given this id.
pub const LOCAL_SINK: &str = "local";

/// Whether a routing hint asks for the event to stay where it is.
pub fn is_local_sink(hint: &str) -> bool {
    hint.eq_ignore_ascii_case(LOCAL_SINK)
}

/// Builds the aggregate copy of a source event.
pub fn to_aggregate(
    source: &Calendar,
    aggregate_uid: &str,
    source_id: &str,
    source_uid: &str,
    owner: Option<&str>,
    suppression: SchedulingSuppression,
) -> Calendar {
    let mut mirrored = source.clone();
    mirrored.set_uid(aggregate_uid);
    mirrored.remove_properties("VEVENT", OWN_PROPERTIES);
    mirrored.add_property("VEVENT", SOURCE_PROPERTY, source_id);
    mirrored.add_property("VEVENT", ORIGIN_UID_PROPERTY, source_uid);

    if suppression == SchedulingSuppression::Native {
        return mirrored;
    }
    demote_attendees(&mut mirrored, owner);
    mirrored
}

/// Applies a target's scheduling rule to an event that is not a mirror.
///
/// An event living only in the aggregate still has to obey the rule that writes
/// to an aggregate never emit scheduling mail — moving one between calendars is
/// as capable of mailing a guest list as mirroring is. It carries no provenance,
/// having none, so this is the demotion alone.
pub fn make_inert(
    calendar: &Calendar,
    owner: Option<&str>,
    suppression: SchedulingSuppression,
) -> Calendar {
    let mut inert = calendar.clone();
    if suppression == SchedulingSuppression::None {
        demote_attendees(&mut inert, owner);
    }
    inert
}

/// Removes the live guest list, keeping its information in inert form.
fn demote_attendees(calendar: &mut Calendar, owner: Option<&str>) {
    let guests: Vec<String> = calendar
        .properties("VEVENT", "ATTENDEE")
        .map(|attendee| {
            let name = attendee
                .param("CN")
                .unwrap_or_else(|| address(attendee.value));
            match attendee.param("PARTSTAT") {
                Some(status) => format!("{name} <{}> ({status})", address(attendee.value)),
                None => format!("{name} <{}>", address(attendee.value)),
            }
        })
        .collect();

    // A meeting the owner declined must not read as busy in the aggregate.
    if owner.is_some_and(|owner| declined(calendar, owner)) {
        calendar.set_property("VEVENT", "TRANSP", "TRANSPARENT");
    }

    if !guests.is_empty() {
        calendar.add_property(
            "VEVENT",
            ATTENDEES_PROPERTY,
            &escape_text(&guests.join(", ")),
        );
        append_guests_to_description(calendar, &guests);
    }
    calendar.remove_properties("VEVENT", SCHEDULING_PROPERTIES);
}

/// Whether `owner` is an attendee who has declined.
fn declined(calendar: &Calendar, owner: &str) -> bool {
    calendar.properties("VEVENT", "ATTENDEE").any(|attendee| {
        address(attendee.value).eq_ignore_ascii_case(owner)
            && attendee.param("PARTSTAT") == Some("DECLINED")
    })
}

fn append_guests_to_description(calendar: &mut Calendar, guests: &[String]) {
    let existing = calendar
        .properties("VEVENT", "DESCRIPTION")
        .next()
        .map(|property| property.value.to_string())
        .unwrap_or_default();
    let block = format!("{GUEST_MARKER}\\n{}", escape_text(&guests.join("\n")));
    let combined = if existing.is_empty() {
        block
    } else {
        format!("{existing}\\n\\n{block}")
    };
    calendar.set_property("VEVENT", "DESCRIPTION", &combined);
}

/// Marks which source an event created in the aggregate should be filed under.
///
/// A line of its own in the description, `@endpoint-id`. The description is used
/// rather than the title because every calendar client exposes a notes field and
/// editing it does not disfigure the event's name, and rather than CATEGORIES
/// because many mobile clients do not surface categories at all — though a
/// matching category is honoured too, that being the field iCalendar intends.
pub const ROUTE_MARKER: char = '@';

/// The source an event asks to be filed under, if it names one.
pub fn routing_hint(calendar: &Calendar) -> Option<String> {
    if let Some(description) = calendar
        .properties("VEVENT", "DESCRIPTION")
        .next()
        .map(|property| property.value)
        && let Some(hint) = description_hint(description)
    {
        return Some(hint);
    }
    calendar
        .properties("VEVENT", "CATEGORIES")
        .flat_map(|property| {
            property
                .value
                .split(',')
                .map(|category| category.trim().to_string())
                .collect::<Vec<_>>()
        })
        .find(|category| !category.is_empty())
}

/// Finds a line consisting only of `@name`. Requiring the whole line keeps an
/// address written in prose from being mistaken for a routing instruction.
fn description_hint(description: &str) -> Option<String> {
    description
        .split("\\n")
        .map(str::trim)
        .find_map(|line| line.strip_prefix(ROUTE_MARKER))
        .filter(|name| !name.is_empty() && !name.contains(char::is_whitespace))
        .map(str::to_string)
}

/// Removes the routing instruction, which is calcalist's own bookkeeping and has
/// no business appearing in the calendar the event lands in.
pub fn strip_routing_hint(calendar: &mut Calendar, hint: &str) {
    let categories: Vec<String> = calendar
        .properties("VEVENT", "CATEGORIES")
        .map(|property| property.value.to_string())
        .collect();
    if !categories.is_empty() {
        calendar.remove_properties("VEVENT", &["CATEGORIES"]);
        let kept: Vec<String> = categories
            .iter()
            .flat_map(|value| value.split(','))
            .map(str::trim)
            .filter(|category| !category.eq_ignore_ascii_case(hint) && !category.is_empty())
            .map(str::to_string)
            .collect();
        if !kept.is_empty() {
            calendar.add_property("VEVENT", "CATEGORIES", &kept.join(","));
        }
    }

    let Some(description) = calendar
        .properties("VEVENT", "DESCRIPTION")
        .next()
        .map(|property| property.value.to_string())
    else {
        return;
    };
    let marker = format!("{ROUTE_MARKER}{hint}");
    let kept: Vec<&str> = description
        .split("\\n")
        .filter(|line| line.trim() != marker)
        .collect();
    if kept.iter().all(|line| line.trim().is_empty()) {
        calendar.remove_properties("VEVENT", &["DESCRIPTION"]);
    } else {
        calendar.set_property("VEVENT", "DESCRIPTION", &kept.join("\\n"));
    }
}

/// Rebuilds a source-side event from an edited aggregate copy.
///
/// `donor` is the event as it currently stands in the source calendar, when
/// there is one. It supplies the properties the mirror transform removed, so an
/// edit made in the aggregate cannot silently strip a meeting's guests. Routing
/// a newly created event has no donor: nothing was stripped from it.
pub fn to_source(edited: &Calendar, donor: Option<&Calendar>, source_uid: &str) -> Calendar {
    let mut restored = edited.clone();
    restored.set_uid(source_uid);
    restored.remove_properties("VEVENT", OWN_PROPERTIES);
    strip_guest_block(&mut restored);

    // If the mirror kept live attendees, the edit owns them. If it demoted them,
    // they are missing here and must come back from the source.
    if let Some(donor) = donor
        && restored.properties("VEVENT", "ATTENDEE").next().is_none()
    {
        for line in donor.property_lines("VEVENT", "ORGANIZER") {
            restored.add_raw_property("VEVENT", line);
        }
        for line in donor.property_lines("VEVENT", "ATTENDEE") {
            restored.add_raw_property("VEVENT", line);
        }
    }
    restored
}

/// Removes the appended guest block from DESCRIPTION, leaving any real text.
fn strip_guest_block(calendar: &mut Calendar) {
    let Some(description) = calendar
        .properties("VEVENT", "DESCRIPTION")
        .next()
        .map(|property| property.value.to_string())
    else {
        return;
    };
    let Some(index) = description.find(GUEST_MARKER) else {
        return;
    };
    // Strip exactly the separator this transform inserted. Trimming a character
    // set here would eat a trailing "n" from real text ("Plan" -> "Pla").
    let before = &description[..index];
    let kept = before.strip_suffix("\\n\\n").unwrap_or(before).to_string();
    if kept.is_empty() {
        calendar.remove_properties("VEVENT", &["DESCRIPTION"]);
    } else {
        calendar.set_property("VEVENT", "DESCRIPTION", &kept);
    }
}

/// Strips a `mailto:` (or other) scheme from a calendar user address.
fn address(value: &str) -> &str {
    value.split_once(':').map_or(value, |(_, rest)| rest)
}

/// Escapes a value for an iCalendar TEXT property, per RFC 5545 section 3.3.11.
pub(crate) fn escape_text(value: &str) -> String {
    let mut escaped = String::with_capacity(value.len());
    for ch in value.chars() {
        match ch {
            '\\' => escaped.push_str(r"\\"),
            ';' => escaped.push_str(r"\;"),
            ',' => escaped.push_str(r"\,"),
            '\n' => escaped.push_str(r"\n"),
            _ => escaped.push(ch),
        }
    }
    escaped
}

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

    const OWNER: &str = "me@example.com";

    fn source_event(extra: &str) -> Calendar {
        let text = format!(
            "BEGIN:VCALENDAR\r\nVERSION:2.0\r\nBEGIN:VEVENT\r\nUID:event-1@example.com\r\nDTSTART:20260910T090000Z\r\nSUMMARY:Weekly sync\r\n{extra}BEGIN:VALARM\r\nACTION:DISPLAY\r\nTRIGGER:-PT15M\r\nEND:VALARM\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n"
        );
        Calendar::parse(&text).expect("fixture should parse")
    }

    const GUESTS: &str = concat!(
        "ORGANIZER;CN=Boss:mailto:boss@example.com\r\n",
        "ATTENDEE;CN=Me;PARTSTAT=ACCEPTED:mailto:me@example.com\r\n",
        "ATTENDEE;CN=Them;PARTSTAT=NEEDS-ACTION:mailto:them@example.com\r\n",
    );

    fn mirror(source: &Calendar, suppression: SchedulingSuppression) -> Calendar {
        to_aggregate(
            source,
            "deadbeef@calcalist",
            "work",
            "event-1@example.com",
            Some(OWNER),
            suppression,
        )
    }

    #[test]
    fn a_mirror_records_where_it_came_from() {
        let mirrored = mirror(&source_event(""), SchedulingSuppression::Native);
        assert_eq!(mirrored.uid(), Some("deadbeef@calcalist"));
        assert_eq!(
            mirrored
                .properties("VEVENT", SOURCE_PROPERTY)
                .next()
                .map(|p| p.value),
            Some("work")
        );
        assert_eq!(
            mirrored
                .properties("VEVENT", ORIGIN_UID_PROPERTY)
                .next()
                .map(|p| p.value),
            Some("event-1@example.com")
        );
    }

    /// Google can suppress notification, so the guest list survives intact.
    #[test]
    fn a_suppressible_backend_keeps_real_attendees() {
        let mirrored = mirror(&source_event(GUESTS), SchedulingSuppression::Native);
        assert_eq!(mirrored.properties("VEVENT", "ATTENDEE").count(), 2);
        assert_eq!(mirrored.properties("VEVENT", "ORGANIZER").count(), 1);
        assert_eq!(mirrored.properties("VEVENT", ATTENDEES_PROPERTY).count(), 0);
    }

    /// CalDAV cannot, so the live properties must go.
    #[test]
    fn an_unsuppressible_backend_emits_no_live_scheduling_properties() {
        let mirrored = mirror(&source_event(GUESTS), SchedulingSuppression::None);
        assert_eq!(mirrored.properties("VEVENT", "ATTENDEE").count(), 0);
        assert_eq!(mirrored.properties("VEVENT", "ORGANIZER").count(), 0);
        let output = mirrored.to_ics();
        assert!(!output.contains("\r\nATTENDEE"));
        assert!(!output.contains("\r\nORGANIZER"));
    }

    #[test]
    fn the_demoted_guest_list_is_still_readable() {
        let mirrored = mirror(&source_event(GUESTS), SchedulingSuppression::None);
        let inert = mirrored
            .properties("VEVENT", ATTENDEES_PROPERTY)
            .next()
            .expect("guest list carried")
            .value;
        assert!(inert.contains("Me <me@example.com> (ACCEPTED)"));
        assert!(inert.contains("Them <them@example.com> (NEEDS-ACTION)"));

        let description = mirrored
            .properties("VEVENT", "DESCRIPTION")
            .next()
            .expect("description added")
            .value;
        assert!(description.contains(GUEST_MARKER));
    }

    /// A meeting the owner declined should read as free, not busy.
    #[test]
    fn declining_marks_the_mirror_transparent() {
        let declined_by_owner = concat!(
            "ATTENDEE;CN=Me;PARTSTAT=DECLINED:mailto:me@example.com\r\n",
            "ATTENDEE;CN=Them;PARTSTAT=ACCEPTED:mailto:them@example.com\r\n",
        );
        let mirrored = mirror(
            &source_event(declined_by_owner),
            SchedulingSuppression::None,
        );
        assert_eq!(
            mirrored
                .properties("VEVENT", "TRANSP")
                .next()
                .map(|p| p.value),
            Some("TRANSPARENT")
        );
    }

    /// Someone else declining says nothing about the owner's availability.
    #[test]
    fn another_guest_declining_does_not_free_the_owner() {
        let other_declined = concat!(
            "ATTENDEE;CN=Me;PARTSTAT=ACCEPTED:mailto:me@example.com\r\n",
            "ATTENDEE;CN=Them;PARTSTAT=DECLINED:mailto:them@example.com\r\n",
        );
        let mirrored = mirror(&source_event(other_declined), SchedulingSuppression::None);
        assert_eq!(mirrored.properties("VEVENT", "TRANSP").count(), 0);
    }

    #[test]
    fn alarms_are_never_touched() {
        for suppression in [SchedulingSuppression::Native, SchedulingSuppression::None] {
            let mirrored = mirror(&source_event(GUESTS), suppression);
            assert!(mirrored.to_ics().contains("BEGIN:VALARM"));
            assert!(mirrored.to_ics().contains("TRIGGER:-PT15M"));
        }
    }

    #[test]
    fn mirroring_twice_is_stable() {
        let source = source_event(GUESTS);
        let once = mirror(&source, SchedulingSuppression::None);
        let twice = mirror(&source, SchedulingSuppression::None);
        assert_eq!(once.to_ics(), twice.to_ics());
    }

    #[test]
    fn writing_back_restores_the_source_uid_and_drops_our_properties() {
        let source = source_event(GUESTS);
        let mirrored = mirror(&source, SchedulingSuppression::None);
        let back = to_source(&mirrored, Some(&source), "event-1@example.com");

        assert_eq!(back.uid(), Some("event-1@example.com"));
        for property in OWN_PROPERTIES {
            assert_eq!(back.properties("VEVENT", property).count(), 0, "{property}");
        }
    }

    /// The demoted mirror has no guests to give back, so editing it must not
    /// silently strip the meeting's attendees.
    #[test]
    fn writing_back_a_demoted_mirror_recovers_the_guest_list() {
        let source = source_event(GUESTS);
        let mut mirrored = mirror(&source, SchedulingSuppression::None);
        mirrored.set_property("VEVENT", "SUMMARY", "Weekly sync (moved)");

        let back = to_source(&mirrored, Some(&source), "event-1@example.com");
        assert_eq!(back.properties("VEVENT", "ATTENDEE").count(), 2);
        assert_eq!(back.properties("VEVENT", "ORGANIZER").count(), 1);
        // The edit itself survives.
        assert_eq!(
            back.properties("VEVENT", "SUMMARY").next().map(|p| p.value),
            Some("Weekly sync (moved)")
        );
        // And the parameters came back with them.
        let attendee = back
            .properties("VEVENT", "ATTENDEE")
            .find(|a| a.value.contains("them@"))
            .expect("guest restored");
        assert_eq!(attendee.param("PARTSTAT"), Some("NEEDS-ACTION"));
    }

    #[test]
    fn writing_back_removes_the_appended_guest_block() {
        let source = source_event(GUESTS);
        let mirrored = mirror(&source, SchedulingSuppression::None);
        let back = to_source(&mirrored, Some(&source), "event-1@example.com");
        assert_eq!(back.properties("VEVENT", "DESCRIPTION").count(), 0);
    }

    #[test]
    fn a_real_description_survives_the_round_trip() {
        let with_description = format!("DESCRIPTION:Bring the plan\r\n{GUESTS}");
        let source = source_event(&with_description);
        let mirrored = mirror(&source, SchedulingSuppression::None);
        let back = to_source(&mirrored, Some(&source), "event-1@example.com");
        assert_eq!(
            back.properties("VEVENT", "DESCRIPTION")
                .next()
                .map(|p| p.value),
            Some("Bring the plan")
        );
    }

    /// Regression: trimming a character set would truncate text ending in "n".
    #[test]
    fn a_description_ending_in_n_is_not_truncated() {
        let with_description = format!("DESCRIPTION:Bring the plan\r\n{GUESTS}");
        let source = source_event(&with_description);
        let mirrored = mirror(&source, SchedulingSuppression::None);
        let back = to_source(&mirrored, Some(&source), "event-1@example.com");
        let description = back
            .properties("VEVENT", "DESCRIPTION")
            .next()
            .expect("description kept")
            .value;
        assert!(
            description.ends_with("plan"),
            "truncated to {description:?}"
        );
    }

    #[test]
    fn text_escaping_follows_the_specification() {
        assert_eq!(escape_text("a,b;c\\d\ne"), r"a\,b\;c\\d\ne");
    }
}