// Copyright 2023 Greptime Team
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use std::collections::{HashMap, HashSet};
use std::time::Instant;

use common_catalog::consts::MITO_ENGINE;
use common_meta::datanode::{RegionManifestInfo, RegionStat};
use common_meta::peer::Peer;
use common_meta::rpc::ddl::EventContext;
use common_telemetry::tracing::Instrument as _;
use common_telemetry::{debug, error, info, warn};
use futures::StreamExt;
use itertools::Itertools;
use ordered_float::OrderedFloat;
use store_api::region_engine::RegionRole;
use store_api::storage::{GcReport, RegionId};
use table::metadata::TableId;

use crate::error::Result;
use crate::gc::Region2Peers;
use crate::gc::candidate::GcCandidate;
use crate::gc::dropped::DroppedRegionCollector;
use crate::gc::scheduler::{GcJobReport, GcScheduler};
use crate::gc::tracker::RegionGcInfo;
use crate::metrics::METRIC_META_GC_CANDIDATE_REGIONS;

impl GcScheduler {
    pub(crate) async fn trigger_gc(&self, event_context: EventContext) -> Result<GcJobReport> {
        let start_time = Instant::now();
        info!("Starting GC cycle");

        // limit gc region scope to regions whose datanode have reported stats(by heartbeat)
        let table_to_region_stats = self
            .ctx
            .get_table_to_region_stats()
            .instrument(common_telemetry::tracing::info_span!(
                "meta_gc_get_region_stats"
            ))
            .await?;
        info!(
            "Fetched region stats for {} tables",
            table_to_region_stats.len()
        );

        let per_table_candidates = self
            .select_gc_candidates(&table_to_region_stats)
            .instrument(common_telemetry::tracing::info_span!(
                "meta_gc_select_candidates"
            ))
            .await?;

        let table_reparts = self
            .ctx
            .get_table_reparts()
            .instrument(common_telemetry::tracing::info_span!(
                "meta_gc_get_table_reparts"
            ))
            .await?;
        let dropped_collector =
            DroppedRegionCollector::new(self.ctx.as_ref(), &self.config, &self.region_gc_tracker);
        let dropped_assignment = dropped_collector
            .collect_and_assign(&table_reparts)
            .instrument(common_telemetry::tracing::info_span!(
                "meta_gc_collect_dropped_regions"
            ))
            .await?;
        let candidate_count: usize = per_table_candidates.values().map(|c| c.len()).sum();
        let dropped_count: usize = dropped_assignment
            .regions_by_peer
            .values()
            .map(|regions| regions.len())
            .sum();
        METRIC_META_GC_CANDIDATE_REGIONS.set((candidate_count + dropped_count) as i64);

        if per_table_candidates.is_empty() && dropped_assignment.regions_by_peer.is_empty() {
            info!("No GC candidates found, skipping GC cycle");
            return Ok(Default::default());
        }

        let mut datanode_to_candidates = self
            .aggregate_candidates_by_datanode(per_table_candidates)
            .instrument(common_telemetry::tracing::info_span!(
                "meta_gc_aggregate_by_datanode"
            ))
            .await?;

        self.merge_dropped_regions(&mut datanode_to_candidates, &dropped_assignment);

        if datanode_to_candidates.is_empty() {
            info!("No valid datanode candidates found, skipping GC cycle");
            return Ok(Default::default());
        }

        let report = self
            .parallel_process_datanodes(
                datanode_to_candidates,
                dropped_assignment.force_full_listing,
                dropped_assignment.region_routes_override,
                event_context,
            )
            .instrument(common_telemetry::tracing::info_span!(
                "meta_gc_dispatch_to_datanodes"
            ))
            .await;

        let duration = start_time.elapsed();
        match &report {
            GcJobReport::PerDatanode {
                per_datanode_reports,
                failed_datanodes,
            } => {
                info!(
                    "Finished GC cycle. Processed {} datanodes ({} failed). Duration: {:?}",
                    per_datanode_reports.len(),
                    failed_datanodes.len(),
                    duration
                );
            }
            GcJobReport::Combined { report } => {
                info!(
                    "Finished GC cycle with combined report. Deleted files: {}, deleted indexes: {}. Duration: {:?}",
                    report.deleted_files.len(),
                    report.deleted_indexes.len(),
                    duration
                );
            }
        }
        debug!("Detailed GC Job Report: {report:#?}");

        Ok(report)
    }

    fn merge_dropped_regions(
        &self,
        datanode_to_candidates: &mut HashMap<Peer, Vec<(TableId, GcCandidate)>>,
        assignment: &crate::gc::dropped::DroppedRegionAssignment,
    ) {
        for (peer, dropped_infos) in &assignment.regions_by_peer {
            let entry = datanode_to_candidates.entry(peer.clone()).or_default();
            for info in dropped_infos {
                entry.push((info.table_id, dropped_candidate(info.region_id)));
            }
        }
    }

    pub(crate) async fn aggregate_candidates_by_datanode(
        &self,
        per_table_candidates: HashMap<TableId, Vec<GcCandidate>>,
    ) -> Result<HashMap<Peer, Vec<(TableId, GcCandidate)>>> {
        let mut datanode_to_candidates: HashMap<Peer, Vec<(TableId, GcCandidate)>> = HashMap::new();

        for (table_id, candidates) in per_table_candidates {
            if candidates.is_empty() {
                continue;
            }

            // Get table route information to map regions to peers
            let (phy_table_id, table_peer) = self.ctx.get_table_route(table_id).await?;

            if phy_table_id != table_id {
                // Skip logical tables
                continue;
            }

            let region_to_peer = table_peer
                .region_routes
                .iter()
                .filter_map(|r| {
                    r.leader_peer
                        .as_ref()
                        .map(|peer| (r.region.id, peer.clone()))
                })
                .collect::<HashMap<RegionId, Peer>>();

            for candidate in candidates {
                if let Some(peer) = region_to_peer.get(&candidate.region_id) {
                    datanode_to_candidates
                        .entry(peer.clone())
                        .or_default()
                        .push((table_id, candidate));
                } else {
                    warn!(
                        "Skipping region {} for table {}: no leader peer found",
                        candidate.region_id, table_id
                    );
                }
            }
        }

        info!(
            "Aggregated GC candidates for {} datanodes",
            datanode_to_candidates.len()
        );
        Ok(datanode_to_candidates)
    }

    /// Process multiple datanodes concurrently with limited parallelism.
    pub(crate) async fn parallel_process_datanodes(
        &self,
        datanode_to_candidates: HashMap<Peer, Vec<(TableId, GcCandidate)>>,
        force_full_listing_by_peer: HashMap<Peer, HashSet<RegionId>>,
        region_routes_override_by_peer: HashMap<Peer, Region2Peers>,
        event_context: EventContext,
    ) -> GcJobReport {
        let mut per_datanode_reports = HashMap::new();
        let mut failed_datanodes: HashMap<_, Vec<_>> = HashMap::new();

        // Create a stream of datanode GC tasks with limited concurrency
        let results: Vec<_> = futures::stream::iter(
            datanode_to_candidates
                .into_iter()
                .filter(|(_, candidates)| !candidates.is_empty()),
        )
        .map(|(peer, candidates)| {
            let scheduler = self;
            let peer_clone = peer.clone();
            let force_full_listing = force_full_listing_by_peer
                .get(&peer)
                .cloned()
                .unwrap_or_default();
            let region_routes_override = region_routes_override_by_peer
                .get(&peer)
                .cloned()
                .unwrap_or_default();
            let event_context = event_context.clone();
            async move {
                (
                    peer,
                    scheduler
                        .process_datanode_gc(
                            peer_clone,
                            candidates,
                            force_full_listing,
                            region_routes_override,
                            event_context,
                        )
                        .await,
                )
            }
        })
        .buffer_unordered(self.config.max_concurrent_tables) // Reuse table concurrency limit for datanodes
        .collect()
        .await;

        // Process all datanode GC results and collect regions that need retry from table reports
        for (peer, result) in results {
            match result {
                Ok(dn_report) => {
                    per_datanode_reports.insert(peer.id, dn_report);
                }
                Err(e) => {
                    error!(e; "Failed to process datanode GC for peer {}", peer);
                    // Note: We don't have a direct way to map peer to table_id here,
                    // so we just log the error. The table_reports will contain individual region failures.
                    failed_datanodes.entry(peer.id).or_default().push(e);
                }
            }
        }

        GcJobReport::PerDatanode {
            per_datanode_reports,
            failed_datanodes,
        }
    }

    /// Process GC for a single datanode with all its candidate regions.
    /// Returns the table reports for this datanode.
    pub(crate) async fn process_datanode_gc(
        &self,
        peer: Peer,
        candidates: Vec<(TableId, GcCandidate)>,
        force_full_listing: HashSet<RegionId>,
        region_routes_override: Region2Peers,
        event_context: EventContext,
    ) -> Result<GcReport> {
        info!(
            "Starting GC for datanode {} with {} candidate regions",
            peer,
            candidates.len()
        );

        if candidates.is_empty() {
            return Ok(Default::default());
        }

        let all_region_ids: Vec<RegionId> = candidates.iter().map(|(_, c)| c.region_id).collect();

        // Step 2: Run GC for all regions on this datanode in a single batch
        let (gc_report, fully_listed_regions) = {
            // Partition regions into full listing and fast listing in a single pass

            let batch_full_listing_decisions = self
                .batch_should_use_full_listing(&all_region_ids, &force_full_listing)
                .await;

            let need_full_list_regions = batch_full_listing_decisions
                .iter()
                .filter_map(
                    |(&region_id, &need_full)| {
                        if need_full { Some(region_id) } else { None }
                    },
                )
                .collect_vec();
            let fast_list_regions = batch_full_listing_decisions
                .iter()
                .filter_map(
                    |(&region_id, &need_full)| {
                        if !need_full { Some(region_id) } else { None }
                    },
                )
                .collect_vec();

            let mut combined_report = GcReport::default();

            // First process regions that can fast list
            if !fast_list_regions.is_empty() {
                match self
                    .ctx
                    .gc_regions(
                        &fast_list_regions,
                        false,
                        self.config.mailbox_timeout,
                        region_routes_override.clone(),
                        event_context.clone(),
                    )
                    .instrument(common_telemetry::tracing::info_span!(
                        "meta_gc_call_datanode",
                        peer = %peer,
                        mode = "fast",
                        region_count = fast_list_regions.len()
                    ))
                    .await
                {
                    Ok(report) => combined_report.merge(report),
                    Err(e) => {
                        error!(
                            e; "Failed to GC regions {:?} on datanode {}",
                            fast_list_regions, peer,
                        );

                        // Add to need_retry_regions since it failed
                        combined_report
                            .need_retry_regions
                            .extend(fast_list_regions.clone());
                    }
                }
            }

            if !need_full_list_regions.is_empty() {
                match self
                    .ctx
                    .gc_regions(
                        &need_full_list_regions,
                        true,
                        self.config.mailbox_timeout,
                        region_routes_override,
                        event_context,
                    )
                    .instrument(common_telemetry::tracing::info_span!(
                        "meta_gc_call_datanode",
                        peer = %peer,
                        mode = "full",
                        region_count = need_full_list_regions.len()
                    ))
                    .await
                {
                    Ok(report) => combined_report.merge(report),
                    Err(e) => {
                        error!(
                            e; "Failed to GC regions {:?} on datanode {}",
                            need_full_list_regions, peer,
                        );

                        // Add to need_retry_regions since it failed
                        combined_report
                            .need_retry_regions
                            .extend(need_full_list_regions.clone());
                    }
                }
            }
            let fully_listed_regions = need_full_list_regions
                .into_iter()
                .filter(|r| !combined_report.need_retry_regions.contains(r))
                .collect::<HashSet<_>>();

            (combined_report, fully_listed_regions)
        };

        // Step 3: Process the combined GC report and update table reports
        for region_id in &all_region_ids {
            self.update_full_listing_time(*region_id, fully_listed_regions.contains(region_id))
                .await;
        }

        info!(
            "Completed GC for datanode {}: {} regions processed",
            peer,
            all_region_ids.len()
        );

        Ok(gc_report)
    }

    async fn batch_should_use_full_listing(
        &self,
        region_ids: &[RegionId],
        force_full_listing: &HashSet<RegionId>,
    ) -> HashMap<RegionId, bool> {
        let mut result = HashMap::new();
        let mut gc_tracker = self.region_gc_tracker.lock().await;
        let now = Instant::now();
        for &region_id in region_ids {
            if force_full_listing.contains(&region_id) {
                gc_tracker
                    .entry(region_id)
                    .and_modify(|info| {
                        info.last_full_listing_time = Some(now);
                        info.last_gc_time = now;
                    })
                    .or_insert_with(|| RegionGcInfo {
                        last_gc_time: now,
                        last_full_listing_time: Some(now),
                    });
                result.insert(region_id, true);
                continue;
            }
            let use_full_listing = {
                if let Some(gc_info) = gc_tracker.get(&region_id) {
                    if let Some(last_full_listing) = gc_info.last_full_listing_time {
                        // check if pass cooling down interval after last full listing
                        let elapsed = now.saturating_duration_since(last_full_listing);
                        elapsed >= self.config.full_file_listing_interval
                    } else {
                        // Never did full listing for this region, do it now
                        true
                    }
                } else {
                    // First time GC for this region, skip doing full listing, for this time
                    gc_tracker.insert(
                        region_id,
                        RegionGcInfo {
                            last_gc_time: now,
                            last_full_listing_time: Some(now),
                        },
                    );
                    false
                }
            };
            result.insert(region_id, use_full_listing);
        }
        result
    }
}

fn dropped_candidate(region_id: RegionId) -> GcCandidate {
    GcCandidate {
        region_id,
        score: OrderedFloat(0.0),
        region_stat: dropped_region_stat(region_id),
    }
}

fn dropped_region_stat(region_id: RegionId) -> RegionStat {
    RegionStat {
        id: region_id,
        rcus: 0,
        wcus: 0,
        approximate_bytes: 0,
        engine: MITO_ENGINE.to_string(),
        role: RegionRole::Leader,
        num_rows: 0,
        memtable_size: 0,
        manifest_size: 0,
        sst_size: 0,
        sst_num: 0,
        index_size: 0,
        region_manifest: RegionManifestInfo::Mito {
            manifest_version: 0,
            flushed_entry_id: 0,
            file_removed_cnt: 0,
        },
        written_bytes: 0,
        query_cpu_time: 0,
        query_scanned_bytes: 0,
        data_topic_latest_entry_id: 0,
        metadata_topic_latest_entry_id: 0,
    }
}
