//! Walk `com.atproto.sync.listRepos` and feed newly discovered repos into the
//! resync queue for the dispatcher to process.
//!
//! Each page of results is processed and the cursor persisted before moving
//! on, so the walk can be safely resumed after a restart. Already-known repos
//! (any state) are skipped — the dispatcher's retry mechanism handles repos
//! that need re-syncing.
//!
//! Accounts listed as non-active (takendown, suspended, deactivated, deleted)
//! have their status recorded without queuing a resync — there's nothing to
//! fetch.

use std::sync::Arc;

use jacquard_api::com_atproto::sync::list_repos::{ListRepos, RepoStatus};
use jacquard_common::{
    error::ClientErrorKind,
    types::{crypto::PublicKey, string::Did},
    url::{Host, Url},
    {IntoStatic, xrpc::XrpcExt},
};
use tokio::time::Duration;
use tokio_util::sync::CancellationToken;
use tracing::{debug, error, info, trace, warn};

use crate::{
    error::Result,
    http::ThrottledClient,
    identity::Resolver,
    storage::{
        self, DbRef,
        backfill_progress::{BackfillProgress, get, set},
        repo::{AccountStatus, RepoInfo, RepoState},
    },
    sync::discovery_queue::{DiscoveryItem, DiscoveryQueue},
    util::TokenExt,
};
use std::sync::atomic::Ordering;
use std::time::SystemTime;

const PAGE_LIMIT: i64 = 500;

// ---------------------------------------------------------------------------
// Mode + retry policy
// ---------------------------------------------------------------------------

/// Which host role this backfill is running against.
///
/// Bundles two decisions that always move together:
/// - whether to validate DIDs against the host (trust model), and
/// - how patiently to retry transient page failures (criticality).
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum BackfillMode {
    /// Primary relay / upstream host. Trusted for DID listings; retried
    /// generously because a transient outage here will take the whole service
    /// down when the task exits.
    Relay,
    /// Untrusted PDS discovered via deep crawl. DIDs are validated against the
    /// host, and a few transient failures are enough to move on.
    DeepCrawl,
}

impl BackfillMode {
    /// True when listed DIDs must be independently verified to actually live
    /// on `host` before we trust their account status.
    fn validates_dids(self) -> bool {
        matches!(self, Self::DeepCrawl)
    }

    fn retry_policy(self) -> RetryPolicy {
        match self {
            Self::Relay => RetryPolicy::RELAY,
            Self::DeepCrawl => RetryPolicy::DEEP_CRAWL,
        }
    }
}

/// How many transient page failures to tolerate and how long to wait between.
struct RetryPolicy {
    max_page_failures: u32,
    retry_delay: Duration,
}

impl RetryPolicy {
    /// Relay budget: 60 attempts × 30s = 30 minutes of retries before giving
    /// up and triggering service shutdown. Enough to absorb typical relay
    /// hiccups without flapping.
    const RELAY: Self = Self {
        max_page_failures: 60,
        retry_delay: Duration::from_secs(30),
    };
    /// Deep-crawl budget: 3 × 10s. Fast give-up so dead PDSes don't monopolise
    /// crawl workers.
    const DEEP_CRAWL: Self = Self {
        max_page_failures: 3,
        retry_delay: Duration::from_secs(10),
    };
}

// ---------------------------------------------------------------------------
// Listed account state
// ---------------------------------------------------------------------------

/// Map the `active`/`status` fields from a listRepos `Repo` entry to a
/// [`ListedAccountState`]. Follows the same mapping as firehose `#account`
/// events in `account_event.rs`.
fn classify_repo(active: Option<bool>, status: &Option<RepoStatus<'_>>) -> RepoState {
    // active-as-fallback is safe:
    if active.unwrap_or(true) {
        return RepoState::Active;
    }
    let Some(jac_status) = status else {
        return RepoState::Active;
    };
    match jac_status {
        RepoStatus::Takendown => RepoState::Takendown,
        RepoStatus::Suspended => RepoState::Suspended,
        RepoStatus::Deleted => RepoState::Deleted,
        RepoStatus::Deactivated => RepoState::Deactivated,
        RepoStatus::Desynchronized => RepoState::Desynchronized,
        RepoStatus::Throttled => RepoState::Throttled,
        RepoStatus::Other(_) => RepoState::Error,
    }
}

// ---------------------------------------------------------------------------
// Main loop
// ---------------------------------------------------------------------------

/// Walk `listRepos` on `host` and enqueue new repos for resync.
///
/// `mode` selects both the trust model (whether DIDs are validated against
/// `host`) and the retry policy (how long we retry transient page failures
/// before giving up). See [`BackfillMode`].
///
/// In `DeepCrawl` mode, DIDs are verified to actually live on `host` and
/// non-matching ones are rejected. Validated DIDs are considered authoritative
/// for account status — if the PDS says an account is deactivated, we trust
/// it even if our local state is Active. In `Relay` mode we only write
/// non-active status for accounts we haven't seen before, to avoid
/// overwriting Active state with stale relay data (e.g. accounts that
/// migrated off and appear deactivated on the old PDS).
pub async fn run(
    host: Host,
    db: DbRef,
    client: ThrottledClient,
    token: CancellationToken,
    resolver: Arc<Resolver>,
    mode: BackfillMode,
    discovery_queue: Arc<DiscoveryQueue>,
) -> Result<bool> {
    let validate = mode.validates_dids();
    let retry_policy = mode.retry_policy();
    let base: jacquard_common::url::Url = format!("https://{host}")
        .parse()
        .map_err(|e: jacquard_common::url::ParseError| crate::error::Error::Other(e.to_string()))?;

    // Resume from the last saved cursor (empty string → start from beginning).
    let mut cursor: Option<String> = {
        let db = db.clone();
        let host = host.clone();
        tokio::task::spawn_blocking(move || get(&db, &host)).await??
    }
    .map(|p| p.cursor);

    info!(
        host = %host,
        resume_cursor = cursor.as_deref().unwrap_or("(start)"),
        "backfill started"
    );

    let mut total_queued: u64 = 0;

    loop {
        if token.is_cancelled() {
            return Ok(false);
        }

        let (dids, next_cursor) = match fetch_page(
            &host,
            &base,
            cursor.as_deref(),
            &client,
            &token,
            &retry_policy,
        )
        .await
        {
            Some(page) => page,
            None => return Ok(false),
        };

        // An absent or empty cursor both signal end-of-list.
        let next_cursor = next_cursor
            .filter(|next| !next.is_empty()) // treat an empty-string cursor as done
            .filter(|next| {
                let Some(current) = cursor else {
                    return true; // none cursor on first request, allow next page
                };
                if *next == current {
                    warn!(host = %host, ?current, ?next,
                        "cursor unchanged, possible crawl-trap, not requesting 'next' listRepos page.");
                    // TODO: mark host as not trustworthy
                    return false;
                }
                true
            });

        let page_len = dids.len();

        // For untrusted hosts (deep crawl), filter DIDs to those whose
        // resolved PDS actually matches this host.
        let dids = if validate {
            validate_dids(dids, &resolver, &host, &token).await
        } else {
            dids
        };

        // Resolve each DID's actual PDS host for discovery queue routing.
        // Cache hits are free; misses fall back to the listed host.
        //
        // TODO: ...this is basically redundant with validate_dids now?
        let dids_with_hosts: Vec<(Did<'static>, Arc<Url>, PublicKey<'static>, RepoState)> = {
            let mut out = Vec::with_capacity(dids.len());
            for (did, account_state) in dids {
                let Some(res) = token.run(resolver.resolve(&did)).await else {
                    return Ok(false); // cancelled
                };
                let resolved = match res {
                    Ok(resolved) => resolved,
                    Err(e) => {
                        error!(did = %did, error = %e, "failed to resolve host for validated did; skipping");
                        continue;
                    }
                };
                out.push((
                    did,
                    resolved.pds.clone(),
                    resolved.pubkey.clone(),
                    account_state,
                ));
            }
            out
        };

        let progress_cursor = next_cursor.clone().unwrap_or_default();
        let (page_queued, page_inactive, new_items) = {
            let db = db.clone();
            let host = host.clone();
            tokio::task::spawn_blocking(move || {
                persist_page(&db, &host, dids_with_hosts, progress_cursor, validate)
            })
            .await??
        };

        // Push newly-discovered repos into the in-memory discovery queue.
        // Each push may block if the queue is full (backpressure).
        for (did, pds, pubkey) in new_items {
            let Some(_) = token
                .run(discovery_queue.push(DiscoveryItem { did, pds, pubkey }))
                .await
            else {
                return Ok(false);
            };
        }

        total_queued += page_queued;

        trace!(
            host = %host,
            page_repos = page_len,
            page_queued,
            page_inactive,
            total_queued,
            next_cursor = next_cursor.as_deref().unwrap_or("(done)"),
            "backfill page"
        );

        // next page: only if we have a next-cursor (omgggggggg sorry everyone)
        if let Some(next) = next_cursor {
            cursor = Some(next.to_string());
            continue;
        }

        // Reached end-of-list (or bailed on a trapping host): persist completion
        // so re-crawls skip this host.
        let db = db.clone();
        let host_owned = host.clone();
        tokio::task::spawn_blocking(move || {
            set(
                &db,
                &host_owned,
                &BackfillProgress {
                    cursor: "".to_string(),
                    completed_at: Some(crate::util::to_millis(SystemTime::now()).to_string()),
                },
            )
        })
        .await??;
        info!(host = %host, total_queued, "backfill complete");
        return Ok(true);
    }
}

// ---------------------------------------------------------------------------
// Page fetch
// ---------------------------------------------------------------------------

/// Fetch one `listRepos` page with retry logic.
///
/// Returns `None` if the host gives a 4xx, exceeds `policy.max_page_failures`
/// transient errors, or the token is cancelled (including mid-request).
async fn fetch_page(
    host: &Host,
    base: &jacquard_common::url::Url,
    cursor: Option<&str>,
    client: &ThrottledClient,
    token: &CancellationToken,
    policy: &RetryPolicy,
) -> Option<(Vec<(Did<'static>, RepoState)>, Option<String>)> {
    let req = ListRepos {
        cursor: cursor.map(Into::into),
        limit: Some(PAGE_LIMIT),
    };
    let mut failures: u32 = 0;
    loop {
        let result = match token.run(client.xrpc(base.clone()).send(&req)).await? {
            Err(e) => {
                let is_client_err = matches!(
                    e.kind(),
                    ClientErrorKind::Http { status } if status.is_client_error()
                );
                if is_client_err {
                    warn!(error = %e, host = %host,
                        "listRepos failed with client error; giving up on this host");
                    return None;
                }
                warn!(error = %e, host = %host, "listRepos request failed");
                None
            }
            Ok(resp) => match resp.parse() {
                Ok(out) => {
                    let next = out.cursor.as_deref().map(str::to_owned);
                    let listed = out
                        .repos
                        .into_iter()
                        .map(|r| {
                            let state = classify_repo(r.active, &r.status);
                            (r.did.into_static(), state)
                        })
                        .collect::<Vec<_>>();
                    Some((listed, next))
                }
                Err(e) => {
                    warn!(error = %e, host = %host, "listRepos response parse failed");
                    None
                }
            },
        };

        match result {
            Some(page) => return Some(page),
            None => {
                failures += 1;
                if failures >= policy.max_page_failures {
                    warn!(host = %host, failures, max = policy.max_page_failures,
                        "listRepos page failed too many times; giving up on this host");
                    return None;
                }
                if !token.sleep(policy.retry_delay).await {
                    return None;
                }
            }
        }
    }
}

// ---------------------------------------------------------------------------
// DID validation
// ---------------------------------------------------------------------------

/// Filter `dids` to those whose resolved PDS endpoint matches `host`.
///
/// Returns early with whatever has been validated so far if the token is cancelled.
async fn validate_dids(
    dids: Vec<(Did<'static>, RepoState)>,
    resolver: &Resolver,
    host: &Host,
    token: &CancellationToken,
) -> Vec<(Did<'static>, RepoState)> {
    let host_str = host.to_string();
    let mut valid = Vec::with_capacity(dids.len());
    for (did, account_state) in dids {
        let Some(r) = token.run(resolver.resolve(&did)).await else {
            break;
        };
        match r {
            Ok(resolved) if resolved.pds.host_str() == Some(host_str.as_str()) => {
                valid.push((did, account_state));
            }
            Ok(resolved) => {
                metrics::counter!("lightrail_backfill_did_rejected_total", "reason" => "pds_mismatch")
                    .increment(1);
                trace!(did = %did, resolved_pds = %resolved.pds, expected = %host,
                    "DID resolves to different PDS; skipping");
            }
            Err(e) => {
                metrics::counter!("lightrail_backfill_did_rejected_total", "reason" => "resolution_failed")
                    .increment(1);
                trace!(did = %did, error = %e,
                    "DID resolution failed during backfill validation; skipping");
            }
        };
    }
    valid
}

// ---------------------------------------------------------------------------
// Storage commit
// ---------------------------------------------------------------------------

type DidWithPds = (Did<'static>, Arc<Url>, PublicKey<'static>);

/// Insert newly-seen DIDs into the repo table and persist the backfill cursor.
///
/// Active accounts are returned for the caller to push into the discovery
/// queue. Non-active accounts have their status written directly (no resync
/// needed). When `authoritative` is false (relay backfill), non-active status
/// is only written for accounts that have no existing Active record — we don't
/// overwrite a locally-Active account with stale relay data.
///
/// Returns `(active_count, inactive_count, new_active_items)`.
fn persist_page(
    db: &DbRef,
    host: &Host,
    items: Vec<(Did<'static>, Arc<Url>, PublicKey<'static>, RepoState)>,
    progress_cursor: String,
    authoritative: bool,
) -> Result<(u64, u64, Vec<DidWithPds>)> {
    let mut new_active: Vec<DidWithPds> = Vec::new();
    let mut inactive_count: u64 = 0;
    for (did, pds, pubkey, repo_state) in items {
        if let Some(deactiveated_account_status) = repo_state.to_account_inactive() {
            if write_inactive_status(
                db,
                &did,
                deactiveated_account_status,
                repo_state,
                authoritative,
            )? {
                inactive_count += 1;
            }
        } else {
            let newly_inserted = storage::repo::ensure_repo(db, &did)?;
            if newly_inserted {
                db.stats.repos_queued_total.fetch_add(1, Ordering::Relaxed);
                new_active.push((did, pds, pubkey));
            }
        }
    }
    // Persist progress before advancing so a crash during the next
    // page restarts from here, not the beginning.
    set(
        db,
        host,
        &BackfillProgress {
            cursor: progress_cursor,
            completed_at: None,
        },
    )?;
    Ok((new_active.len() as u64, inactive_count, new_active))
}

/// Write a non-active account status from a listRepos entry.
///
/// Returns `true` if the status was written, `false` if skipped.
///
/// When `authoritative` is true (deep crawl — PDS confirmed via identity
/// resolution), the status is always written. When false (relay backfill),
/// we only write if the existing local status is not Active, to avoid
/// overwriting with stale data (e.g. an account that migrated off this PDS
/// and shows as deactivated on the old host).
fn write_inactive_status(
    db: &DbRef,
    did: &Did<'_>,
    status: AccountStatus,
    state: RepoState,
    authoritative: bool,
) -> Result<bool> {
    let existing = storage::repo::get_status(db, did)?;
    match existing {
        Some(ref existing_status) if existing_status.is_active() && !authoritative => {
            debug!(
                did = %did,
                listed_status = status.as_str(),
                "skipping non-active status from non-authoritative source; local account is active"
            );
            return Ok(false);
        }
        Some(ref existing_status) if *existing_status == status => {
            return Ok(false); // already matches
        }
        _ => {}
    }
    let info = RepoInfo {
        state,
        status: status.clone(),
        error: None,
    };
    storage::repo::put_info(db, did, &info)?;
    metrics::counter!(
        "lightrail_backfill_inactive_total",
        "status" => status.as_str()
    )
    .increment(1);
    Ok(true)
}