//! Moving an aggregate to a different target endpoint.
//!
//! This is the sharpest failure mode in the design, which is why it is a command
//! of its own rather than something a config edit triggers. A plain `sync` after
//! the target changed would read the new, empty calendar as an aggregate whose
//! every event had been deleted — and with delete propagation on, remove them
//! from every source. `sync` therefore refuses on target drift and sends the user
//! here.
//!
//! Two properties of the design make the move mechanical rather than a rebuild:
//! the aggregate is a derived replica, so the sources stay authoritative
//! throughout, and UID derivation excludes the target, so every derived UID is
//! unchanged by the move. Only the rendered content and its location change.

use std::path::Path;

use thiserror::Error;

use crate::config::{Config, Endpoint};
use crate::mirror;
use crate::provenance;
use crate::state::{State, Target};
use crate::sync::{self, SyncError, vdir_path};
use crate::vdir::{self, VdirError};

#[derive(Debug, Error)]
pub enum RetargetError {
    #[error(transparent)]
    Sync(#[from] SyncError),
    #[error(transparent)]
    Vdir(#[from] VdirError),
    #[error(transparent)]
    State(#[from] crate::state::StateError),
    #[error("no aggregate named `{0}` in the configuration")]
    UnknownAggregate(String),
    #[error("no endpoint named `{0}` in the configuration")]
    UnknownEndpoint(String),
    #[error("endpoint `{0}` is read-only, so an aggregate cannot be published to it")]
    NotWritable(String),
    #[error(
        "endpoint `{endpoint}` is a source of aggregate `{aggregate}`, so targeting it would sync the aggregate into itself"
    )]
    TargetIsSource { endpoint: String, aggregate: String },
    #[error(
        "aggregate `{0}` has never been synced, so there is nothing to move; edit the config and run `calcalist sync`"
    )]
    NeverSynced(String),
}

#[derive(Debug)]
pub struct Outcome {
    pub aggregate: String,
    pub from: String,
    pub to: String,
    /// Events flushed to a sink before the move, which existed nowhere else.
    pub routed_first: usize,
    pub materialised: usize,
    pub purged: usize,
}

/// Moves `aggregate_id` to `new_target_id`.
pub fn retarget(
    config: &Config,
    state_dir: &Path,
    aggregate_id: &str,
    new_target_id: &str,
    purge_old: bool,
) -> Result<Outcome, RetargetError> {
    let aggregate = config
        .aggregates
        .iter()
        .find(|candidate| candidate.id == aggregate_id)
        .ok_or_else(|| RetargetError::UnknownAggregate(aggregate_id.to_string()))?;
    let new_target = endpoint(config, new_target_id)?;
    if !new_target.kind.is_writable() {
        return Err(RetargetError::NotWritable(new_target_id.to_string()));
    }
    if aggregate
        .sources
        .iter()
        .any(|source| source == new_target_id)
    {
        return Err(RetargetError::TargetIsSource {
            endpoint: new_target_id.to_string(),
            aggregate: aggregate_id.to_string(),
        });
    }

    let state_path = state_dir.join(crate::state::FILE_NAME);
    let mut state = State::load(&state_path)?;
    let recorded = state
        .aggregate(aggregate_id)
        .map(|entry| entry.target.clone())
        .ok_or_else(|| RetargetError::NeverSynced(aggregate_id.to_string()))?;
    let old_target = endpoint(config, &recorded.endpoint)?.clone();

    sync::prepare_vdirs(config, state_dir)?;
    let pimsync_config = if sync::needs_pimsync(config) {
        Some(crate::pimsync::write_config(config, state_dir).map_err(SyncError::from)?)
    } else {
        None
    };
    if let Some(path) = &pimsync_config {
        crate::pimsync::sync(path).map_err(SyncError::from)?;
    }

    // Settle against the old target first. Events a user created in the
    // aggregate and that have not yet reached a sink exist only there, and would
    // be lost the moment we stop looking at it.
    let settled = sync::sync_aggregate(
        config,
        aggregate,
        &old_target,
        state_dir,
        &mut state,
        false,
        false,
    )?;

    let materialised = materialise(config, aggregate, new_target, state_dir, &mut state)?;
    let purged = if purge_old {
        purge(aggregate_id, &old_target, state_dir, &state)?
    } else {
        0
    };

    let entry = state.aggregate_mut(aggregate_id, target_of(new_target));
    entry.target = target_of(new_target);
    state.save(&state_path)?;

    if let Some(path) = &pimsync_config {
        crate::pimsync::sync(path).map_err(SyncError::from)?;
    }

    Ok(Outcome {
        aggregate: aggregate_id.to_string(),
        from: old_target.id.clone(),
        to: new_target.id.clone(),
        routed_first: settled.written_back,
        materialised,
        purged,
    })
}

/// Rebuilds every mapped event on the new target, using that backend's transform.
///
/// The rendered content differs between backends — a target that can suppress
/// scheduling keeps real attendees, one that cannot has them demoted — so this is
/// a rewrite of every event rather than a copy.
fn materialise(
    config: &Config,
    aggregate: &crate::config::Aggregate,
    new_target: &Endpoint,
    state_dir: &Path,
    state: &mut State,
) -> Result<usize, RetargetError> {
    let new_dir = vdir_path(state_dir, &new_target.id);
    let existing = vdir::read(&new_dir)?;
    let suppression = new_target.kind.scheduling_suppression();

    let links = state
        .aggregate(&aggregate.id)
        .map(|entry| entry.links.clone())
        .unwrap_or_default();

    let mut written = 0;
    let mut updated = links.clone();
    for (aggregate_uid, link) in &links {
        let source = config.endpoint(&link.source_id);
        let Some(source) = source else { continue };
        let source_items = vdir::read(&vdir_path(state_dir, &source.id))?;
        let Some(item) = source_items.get(&link.source_uid) else {
            // The source event is gone; the next ordinary sync will retire it.
            continue;
        };
        let rebuilt = mirror::to_aggregate(
            &item.calendar,
            aggregate_uid,
            &source.id,
            &link.source_uid,
            source.kind.owner(),
            suppression,
        );
        let replaces = existing.get(aggregate_uid).map(|held| held.path.clone());
        vdir::write(&new_dir, aggregate_uid, &rebuilt, replaces.as_deref())?;
        if let Some(entry) = updated.get_mut(aggregate_uid) {
            entry.aggregate_hash = rebuilt.content_hash();
        }
        written += 1;
    }
    state
        .aggregate_mut(&aggregate.id, target_of(new_target))
        .links = updated;
    Ok(written)
}

/// Removes this aggregate's mirrors from the calendar it has left.
///
/// Bounded by the derivation: only events whose UID this aggregate would have
/// produced are touched, so anything the user keeps in that calendar survives.
fn purge(
    aggregate_id: &str,
    old_target: &Endpoint,
    state_dir: &Path,
    state: &State,
) -> Result<usize, RetargetError> {
    let old_dir = vdir_path(state_dir, &old_target.id);
    let items = vdir::read(&old_dir)?;
    let links = state
        .aggregate(aggregate_id)
        .map(|entry| &entry.links)
        .ok_or_else(|| RetargetError::NeverSynced(aggregate_id.to_string()))?;

    let mut removed = 0;
    for (uid, item) in &items {
        if provenance::is_derived(uid) && links.contains_key(uid) {
            vdir::remove(&item.path)?;
            removed += 1;
        }
    }
    Ok(removed)
}

/// The state record describing where an aggregate is published.
fn target_of(endpoint: &Endpoint) -> Target {
    Target {
        endpoint: endpoint.id.clone(),
        kind: endpoint.kind.kind_name().to_string(),
    }
}

fn endpoint<'a>(config: &'a Config, id: &str) -> Result<&'a Endpoint, RetargetError> {
    config
        .endpoint(id)
        .ok_or_else(|| RetargetError::UnknownEndpoint(id.to_string()))
}

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

    /// Google endpoints are used as stand-ins: they need no pimsync, so a whole
    /// cycle runs against local directories with nothing to reach over a network.
    const CONFIG: &str = r#"
version = 1

[[endpoint]]
id = "src"
type = "google"
calendar_id = "src@example.com"
client_id = "x.apps.googleusercontent.com"

[[endpoint]]
id = "agg1"
type = "google"
calendar_id = "agg1@example.com"
client_id = "x.apps.googleusercontent.com"

[[endpoint]]
id = "agg2"
type = "google"
calendar_id = "agg2@example.com"
client_id = "x.apps.googleusercontent.com"

[[aggregate]]
id = "unified"
target = "agg1"
sources = ["src"]
default_sink = "src"
"#;

    fn config() -> Config {
        toml::from_str(CONFIG).expect("test config should parse")
    }

    fn event(uid: &str, summary: &str) -> String {
        format!(
            "BEGIN:VCALENDAR\r\nVERSION:2.0\r\nBEGIN:VEVENT\r\nUID:{uid}\r\nDTSTART:20260910T090000Z\r\nDTSTAMP:20260101T000000Z\r\nSUMMARY:{summary}\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n"
        )
    }

    /// Reconciles one aggregate and records the result, without running a full
    /// cycle. A full cycle would contact Google for real, which these tests have
    /// no business doing — they are about what happens to local files.
    fn settle(config: &Config, dir: &Path) {
        let aggregate = &config.aggregates[0];
        let target = config
            .endpoint(&aggregate.target)
            .expect("target endpoint")
            .clone();
        let state_path = dir.join(crate::state::FILE_NAME);
        let mut state = State::load(&state_path).expect("state");
        sync::sync_aggregate(config, aggregate, &target, dir, &mut state, false, false)
            .expect("reconcile");
        state.save(&state_path).expect("save state");
    }

    fn seed(config: &Config) -> tempfile::TempDir {
        let dir = tempfile::tempdir().expect("temp dir");
        let src = vdir_path(dir.path(), "src");
        fs::create_dir_all(&src).expect("create src vdir");
        fs::write(src.join("a.ics"), event("a@example.com", "A")).expect("write");
        fs::write(src.join("b.ics"), event("b@example.com", "B")).expect("write");
        settle(config, dir.path());
        dir
    }

    /// A state directory with two source events already mirrored into `agg1`.
    fn synced() -> tempfile::TempDir {
        seed(&config())
    }

    fn count(dir: &Path, endpoint: &str) -> usize {
        vdir::read(&vdir_path(dir, endpoint))
            .expect("read vdir")
            .len()
    }

    #[test]
    fn rebuilds_every_event_on_the_new_target() {
        let dir = synced();
        assert_eq!(count(dir.path(), "agg1"), 2);

        let outcome = retarget(&config(), dir.path(), "unified", "agg2", false).expect("retarget");
        assert_eq!(outcome.materialised, 2);
        assert_eq!(count(dir.path(), "agg2"), 2);
        // Kept by default: the old calendar may be wanted as a snapshot.
        assert_eq!(count(dir.path(), "agg1"), 2);
        assert_eq!(outcome.purged, 0);
    }

    #[test]
    fn records_the_new_target_so_sync_stops_refusing() {
        let dir = synced();
        retarget(&config(), dir.path(), "unified", "agg2", false).expect("retarget");

        let state = State::load(&dir.path().join(crate::state::FILE_NAME)).expect("state");
        assert_eq!(
            state.aggregate("unified").expect("entry").target.endpoint,
            "agg2"
        );
    }

    /// A purge is bounded by the derivation, so a calendar the user also keeps
    /// their own events in does not lose them.
    ///
    /// The aggregate has no sink here deliberately: with one configured, a
    /// foreign event in the aggregate is a user-created event and gets adopted
    /// into the sink rather than staying foreign, so it would never reach the
    /// purge as an outsider.
    #[test]
    fn purging_removes_only_this_aggregates_mirrors() {
        let sinkless: Config = toml::from_str(&CONFIG.replace(r#"default_sink = "src""#, ""))
            .expect("config should parse");
        let dir = seed(&sinkless);

        let old = vdir_path(dir.path(), "agg1");
        fs::write(old.join("mine.ics"), event("my-own@example.com", "Mine")).expect("write");
        assert_eq!(count(dir.path(), "agg1"), 3);

        let outcome = retarget(&sinkless, dir.path(), "unified", "agg2", true).expect("retarget");
        assert_eq!(outcome.purged, 2, "only the aggregate's own mirrors go");

        let remaining = vdir::read(&old).expect("read");
        assert_eq!(remaining.len(), 1);
        assert!(
            remaining.contains_key("my-own@example.com"),
            "the user's own event was destroyed"
        );
    }

    /// The converse, and the reason the test above needs a sinkless aggregate:
    /// an event created in an aggregate that has a sink is adopted into it.
    #[test]
    fn a_foreign_event_is_adopted_when_a_sink_exists() {
        let dir = synced();
        let old = vdir_path(dir.path(), "agg1");
        fs::write(old.join("mine.ics"), event("my-own@example.com", "Mine")).expect("write");

        retarget(&config(), dir.path(), "unified", "agg2", true).expect("retarget");
        let sources = vdir::read(&vdir_path(dir.path(), "src")).expect("read");
        assert!(sources.contains_key("my-own@example.com"));
    }

    /// An event created in the aggregate and not yet routed exists nowhere else,
    /// so the move must flush it to a sink before it stops looking at that
    /// calendar.
    #[test]
    fn an_unrouted_creation_reaches_the_sink_before_the_move() {
        let dir = synced();
        let old = vdir_path(dir.path(), "agg1");
        fs::write(old.join("hand.ics"), event("hand@phone", "Handwritten")).expect("write");

        let outcome = retarget(&config(), dir.path(), "unified", "agg2", true).expect("retarget");
        assert_eq!(outcome.routed_first, 1);

        let sources = vdir::read(&vdir_path(dir.path(), "src")).expect("read");
        assert!(
            sources.contains_key("hand@phone"),
            "the only copy of the event was lost"
        );
    }

    #[test]
    fn refuses_a_target_that_is_one_of_the_sources() {
        let dir = tempfile::tempdir().expect("temp dir");
        let error =
            retarget(&config(), dir.path(), "unified", "src", false).expect_err("should refuse");
        assert!(
            matches!(error, RetargetError::TargetIsSource { .. }),
            "{error}"
        );
    }

    /// Writability is checked before any state is touched, so this needs no
    /// prior sync — which keeps a webcal endpoint out of the shared fixture.
    #[test]
    fn refuses_a_read_only_target() {
        let with_feed: Config = toml::from_str(&format!(
            "{CONFIG}\n[[endpoint]]\nid = \"feed\"\ntype = \"webcal\"\nurl = \"https://example.org/f.ics\"\n"
        ))
        .expect("config should parse");
        let dir = tempfile::tempdir().expect("temp dir");
        let error =
            retarget(&with_feed, dir.path(), "unified", "feed", false).expect_err("should refuse");
        assert!(matches!(error, RetargetError::NotWritable(_)), "{error}");
    }

    #[test]
    fn refuses_unknown_names() {
        let dir = tempfile::tempdir().expect("temp dir");
        assert!(matches!(
            retarget(&config(), dir.path(), "nope", "agg2", false).expect_err("refuse"),
            RetargetError::UnknownAggregate(_)
        ));
        assert!(matches!(
            retarget(&config(), dir.path(), "unified", "nowhere", false).expect_err("refuse"),
            RetargetError::UnknownEndpoint(_)
        ));
    }

    /// Nothing has been published yet, so there is nothing to move.
    #[test]
    fn refuses_an_aggregate_that_has_never_synced() {
        let dir = tempfile::tempdir().expect("temp dir");
        let error =
            retarget(&config(), dir.path(), "unified", "agg2", false).expect_err("should refuse");
        assert!(matches!(error, RetargetError::NeverSynced(_)), "{error}");
    }
}