//! Establish ground truth about a repository's collections.
//!
//! "Resync" covers three scenarios:
//!
//! - **Backfill**: first-time indexing of a repo found via `listRepos`; diff
//!   against an empty set.
//! - **`#sync` event**: the PDS declared a repo discontinuity; diff against
//!   the current index entries.
//! - **Detected discontinuity**: a firehose commit's `prev` CID doesn't match
//!   our record; diff against the current index entries.
//!
//! Each submodule implements one approach to fetching the ground-truth
//! collection list as `fetch_collections(client, base, did: Did<'_>)
//! -> std::result::Result<RepoSnapshot, GetCollectionsError>`.
//! This uniform signature makes it straightforward to test approaches in
//! isolation and to add new ones later.

pub mod describe_repo;
pub mod dispatcher;
pub mod get_repo;

use std::collections::BTreeSet;
use std::sync::Arc;
use std::sync::atomic::Ordering;
use std::time::Duration;

use cid::Cid as RawCid;
use jacquard_common::{
    IntoStatic,
    http_client::{HttpClient, HttpClientExt},
    types::string::{Did, Nsid, Tid},
};
use tracing::{error, info};

use crate::storage::DbRef;
use crate::storage::repo::{AccountStatus, RepoInfo, RepoPrev, RepoState};
use crate::util::TokenExt;

pub use dispatcher::DispatcherConfig;

type Result<T> = std::result::Result<T, ResyncError>;

/// Errors that can occur during a resync operation.
#[derive(Debug, thiserror::Error)]
pub enum ResyncError {
    #[error("identity resolution failed: {0}")]
    Identity(#[from] crate::identity::IdentityError),
    #[error("collection fetch failed: {0}")]
    Fetch(GetCollectionsError),
    #[error(transparent)]
    Storage(#[from] crate::storage::StorageError),
    /// The PDS returned HTTP 429. Carries the PDS URL for host-level cooldown.
    #[error("rate limited by {0}")]
    RateLimited(Arc<jacquard_common::url::Url>),
    #[error("invalid DID in queue: {0}")]
    InvalidDid(String),
    #[error("blocking storage task panicked: {0}")]
    TaskPanic(String),
    /// The PDS returned a definitive "repo not found" for this DID.
    /// The repo state has been updated; the caller may schedule a slow retry.
    #[error("repo not found on PDS")]
    RepoNotFound,
    #[error("externally cancelled")]
    Cancelled,
    #[error("wtf, {0}")]
    Wtf(String),
}

/// A snapshot of a repository's state as observed during a resync.
#[derive(Debug, PartialEq)]
pub struct RepoSnapshot {
    /// Sorted list of collection NSIDs present in the repository.
    pub collections: BTreeSet<Nsid<'static>>, // TODO btreeset
    /// Revision TID of the latest commit.
    pub rev: Tid,
    /// MST root CID
    pub data: RawCid,
}

/// Why fetching a repository's collection list failed.
#[derive(Debug, thiserror::Error)]
pub enum GetCollectionsError {
    /// No repository exists for this DID on the PDS.
    #[error("repo not found")]
    RepoNotFound,
    /// The repo exists but its current state makes it inaccessible.
    #[error("repo {0}")]
    RepoGone(RepoGoneReason),
    /// The PDS rate-limited this request (HTTP 429).
    #[error("rate limited by {0} (HTTP 429)")]
    RateLimited(String),
    /// Network or HTTP failure; may be transient.
    #[error("request failed: {0}")]
    Request(String),
    /// The server returned data that could not be parsed or is structurally invalid.
    #[error("invalid data: {0}")]
    InvalidData(String),
    /// The server returned an unrecognised XRPC error code.
    #[error("unexpected XRPC error: {0}")]
    UnexpectedXrpc(String),
    /// The repo is likely tiny, intentionally fall through to sync.getRepo
    #[error("should getRepo because it's likely tiny")]
    GetSmallRepo,
    /// The request was externally cancelled
    #[error("externally cancelled")]
    Cancelled,
    #[error("wtf!?, {0}")]
    Wtf(String),
}

/// The specific reason a repository is inaccessible.
#[derive(Debug, thiserror::Error)]
pub enum RepoGoneReason {
    #[error("taken down")]
    Takendown,
    #[error("suspended")]
    Suspended,
    #[error("deactivated")]
    Deactivated,
}

/// Bundle of useful stuff from the app
///
/// should probably become a proper AppState
#[derive(Clone)]
pub struct AppStuff {
    resolver: Arc<crate::identity::Resolver>,
    client: crate::http::ThrottledClient,
    db: DbRef,
    token: tokio_util::sync::CancellationToken,
}

/// Establish the current collection set for `did` and write it to the index.
///
/// Tries `describeRepo` first (cheap). Falls back to a full `getRepo` CAR walk
/// if the PDS doesn't support it or returns an empty collection list.
///
/// Returns `Ok(())` silently if the repo is not found or inaccessible;
/// the caller decides whether to retry on transient errors.
pub async fn index_repo(
    AppStuff {
        resolver,
        client,
        db,
        token,
    }: AppStuff,
    did: Did<'_>,
    describe_timeout: Duration,
    get_repo_timeout: Duration,
    force_get_repo: bool,
) -> Result<()> {
    // Own the DID for the duration of the function so we can move it into
    // spawn_blocking closures (which require 'static captures).
    let did: Did<'static> = did.into_static();

    let Some(resolved) = token.run(resolver.resolve(&did)).await else {
        return Err(ResyncError::Cancelled);
    };
    let resolved = resolved?;
    let base = &*resolved.pds;
    let Some(_) = base.host_str() else {
        error!(did = %did, host = %resolved.pds, "index_repo received a URL without a host???");
        return Err(ResyncError::Wtf(format!(
            "URL without host?? {:?}",
            resolved.pds
        )));
    };

    let repo_snapshot = match fetch_collections(
        &client,
        base,
        did.clone(),
        token,
        describe_timeout,
        get_repo_timeout,
        force_get_repo,
    )
    .await
    {
        Ok(s) => s,
        Err(GetCollectionsError::RepoNotFound) => {
            info!(did = %did.as_str(), "repo not found on PDS; marking state");
            let db = db.clone();
            tokio::task::spawn_blocking(move || mark_not_found(&db, &did))
                .await
                .map_err(|e| ResyncError::TaskPanic(e.to_string()))??;
            return Err(ResyncError::RepoNotFound);
        }
        Err(GetCollectionsError::RepoGone(reason)) => {
            let (state, status) = match reason {
                RepoGoneReason::Takendown => (RepoState::Takendown, AccountStatus::Takendown),
                RepoGoneReason::Suspended => (RepoState::Suspended, AccountStatus::Suspended),
                RepoGoneReason::Deactivated => (RepoState::Deactivated, AccountStatus::Deactivated),
            };
            info!(did = %did.as_str(), status = status.as_str(), "repo gone; updating account status");
            let db = db.clone();
            tokio::task::spawn_blocking(move || update_account_status(&db, &did, state, status))
                .await
                .map_err(|e| ResyncError::TaskPanic(e.to_string()))??;
            return Ok(());
        }
        Err(GetCollectionsError::RateLimited(_)) => {
            return Err(ResyncError::RateLimited(resolved.pds.clone()));
        }
        Err(e) => return Err(ResyncError::Fetch(e)),
    };

    let db_w = db.clone();
    let collections = repo_snapshot.collections;
    let rev = repo_snapshot.rev;
    let prev_data = repo_snapshot.data.to_bytes();
    let (n_inserted, n_removed) = tokio::task::spawn_blocking(move || {
        crate::storage::repo::put_prev(&db_w, &did, &RepoPrev { rev, prev_data })?;
        crate::storage::collection_index::sync_collections(&db_w, &did, &collections)
    })
    .await
    .map_err(|e| ResyncError::TaskPanic(e.to_string()))??;
    if n_inserted > 0 {
        db.stats
            .collection_births_resync
            .fetch_add(n_inserted as u64, Ordering::Relaxed);
        metrics::counter!("lightrail_collection_births_total", "source" => "resync")
            .increment(n_inserted as u64);
    }
    if n_removed > 0 {
        db.stats
            .collection_deaths_resync
            .fetch_add(n_removed as u64, Ordering::Relaxed);
        metrics::counter!("lightrail_collection_deaths_total", "source" => "resync")
            .increment(n_removed as u64);
    }
    Ok(())
}

/// Update the stored `RepoState` and `AccountStatus` for a repo that the PDS
/// reports as gone (takendown, suspended, or deactivated).
///
/// If no record exists yet (e.g. a backfill race), one is created. The `error`
/// field is cleared since the gone state is fully described by `state`/`status`.
fn update_account_status(
    db: &DbRef,
    did: &Did<'_>,
    state: RepoState,
    status: AccountStatus,
) -> Result<()> {
    let new_info = match crate::storage::repo::get(db, did)? {
        Some((mut info, _)) => {
            info.state = state;
            info.status = status;
            info.error = None;
            info
        }
        None => RepoInfo {
            state,
            status,
            error: None,
        },
    };
    crate::storage::repo::put_info(db, did, &new_info)?;
    Ok(())
}

/// Set `RepoState::NotFound` for a DID whose PDS reports no repo.
///
/// Preserves the existing `AccountStatus` if a record exists (we don't know
/// the true account status from a not-found response). The `error` field is
/// cleared since the state is fully described by `RepoState::NotFound`.
fn mark_not_found(db: &DbRef, did: &Did<'_>) -> Result<()> {
    let new_info = match crate::storage::repo::get(db, did)? {
        Some((mut info, _)) => {
            info.state = RepoState::NotFound;
            info.error = None;
            info
        }
        None => RepoInfo {
            state: RepoState::NotFound,
            status: AccountStatus::Active,
            error: None,
        },
    };
    crate::storage::repo::put_info(db, did, &new_info)?;
    Ok(())
}

/// Try each approach in preference order, falling back to `get_repo` selectively.
///
/// Falls back to the `get_repo` CAR walk when `describe_repo`:
///   - returns an empty collection list (possible PDS bug or unpaginated large repo),
///   - fails with any error other than rate-limiting or a definitive gone/not-found.
///
/// A 429 rate-limit response is propagated immediately rather than escalated to a
/// heavier `getRepo` request. Definitive `RepoNotFound`/`RepoGone` results are also
/// propagated, since `getRepo` would return the same answer.
async fn fetch_collections<C>(
    client: &C,
    base: &jacquard_common::url::Url,
    did: Did<'_>,
    token: tokio_util::sync::CancellationToken,
    describe_timeout: Duration,
    get_repo_timeout: Duration,
    force_get_repo: bool,
) -> std::result::Result<RepoSnapshot, GetCollectionsError>
where
    C: HttpClient + HttpClientExt + Sync,
{
    if !force_get_repo {
        let Some(describe_result) = token
            .timeout(
                describe_timeout,
                describe_repo::fetch_collections(client, base, did.clone()),
            )
            .await
        else {
            return Err(GetCollectionsError::Cancelled);
        };

        match describe_result {
            Ok(Ok(snapshot)) => {
                metrics::counter!("lightrail_resync_fetch_total", "source" => "describe_repo")
                    .increment(1);
                return Ok(snapshot);
            }
            // Rate-limited: don't escalate to a heavier getRepo request.
            Ok(Err(e @ GetCollectionsError::RateLimited(_))) => return Err(e),
            // Definitively gone: getRepo would return the same answer.
            Ok(Err(e @ (GetCollectionsError::RepoNotFound | GetCollectionsError::RepoGone(_)))) => {
                return Err(e);
            }
            // Any other failure or timeout: fall through. The PDS may not implement
            // describeRepo, or may have a bug this endpoint doesn't hit.
            Ok(Err(GetCollectionsError::GetSmallRepo)) | Ok(Err(_)) | Err(_) => {}
        }
    }

    let res = token
        .timeout(
            get_repo_timeout,
            get_repo::fetch_collections(client, base, did),
        )
        .await
        .ok_or(GetCollectionsError::Cancelled)?
        .unwrap_or_else(|_| {
            Err(GetCollectionsError::Request(
                "getRepo timed out".to_string(),
            ))
        })?;
    metrics::counter!("lightrail_resync_fetch_total", "source" => "get_repo").increment(1);
    Ok(res)
}