//! Query execution helpers: fast path, filtering, decay, ranking.
//!
//! This module handles the post-candidate-fetch logic:
//! - Fast path: materialized view lookup and staleness checks
//! - Filtering: lifecycle, epoch, as_of, since
//! - Decay: time-based confidence decay (uniform or source-class-aware)
//! - Limit/pagination

use std::time::{SystemTime, UNIX_EPOCH};

use stemedb_core::types::{Assertion, MaterializedView};
use stemedb_storage::{key_codec, KVStore};
use tracing::debug;

use crate::decay::{apply_decay, apply_source_class_decay};
use crate::error::Result;
use crate::query::{Query, QueryResult};

use super::QueryEngine;

impl<S: KVStore + 'static> QueryEngine<S> {
    /// Try the fast path: read a pre-computed MaterializedView for this subject+predicate.
    ///
    /// Returns `Some(QueryResult)` if a materialized view exists, is fresh enough,
    /// and the winner passes the query's filters. Returns `None` to fall through
    /// to the slow path.
    ///
    /// # Staleness Check
    ///
    /// If `query.max_stale` is set, the MV's age is checked against this threshold.
    /// If the MV is older than `max_stale` seconds, we fall through to the slow path.
    pub(super) async fn try_fast_path(
        &self,
        subject: &str,
        predicate: &str,
        query: &Query,
    ) -> Result<Option<QueryResult>> {
        let mv_key = key_codec::mv_key(subject, predicate);

        let data = match self.store.get(&mv_key).await? {
            Some(data) => data,
            None => return Ok(None),
        };

        let view: MaterializedView = match stemedb_core::serde::deserialize(&data) {
            Ok(v) => v,
            Err(e) => {
                debug!(
                    subject,
                    predicate,
                    error = %e,
                    "Malformed materialized view, falling back to slow path"
                );
                return Ok(None);
            }
        };

        // Check staleness if max_stale is specified
        if let Some(max_stale) = query.max_stale {
            let now =
                SystemTime::now().duration_since(UNIX_EPOCH).map(|d| d.as_secs()).unwrap_or(0);
            let age = now.saturating_sub(view.materialized_at);

            if age > max_stale {
                debug!(
                    subject,
                    predicate,
                    age_seconds = age,
                    max_stale_seconds = max_stale,
                    materialized_at = view.materialized_at,
                    "Materialized view is stale, falling back to slow path"
                );
                return Ok(None);
            }
        }

        // Apply query filters to the winner
        if !query.matches(&view.winner) {
            // Winner doesn't match filters (e.g., wrong lifecycle stage).
            // Fall through to slow path which considers all candidates.
            return Ok(None);
        }

        // Check conflict score filters if specified
        if let Some(min_score) = query.min_conflict_score {
            if view.conflict_score < min_score {
                debug!(
                    conflict_score = view.conflict_score,
                    min_conflict_score = min_score,
                    "Materialized view conflict score below threshold"
                );
                return Ok(Some(QueryResult {
                    assertions: vec![],
                    total_count: 0,
                    has_more: false,
                }));
            }
        }

        if let Some(max_score) = query.max_conflict_score {
            if view.conflict_score > max_score {
                debug!(
                    conflict_score = view.conflict_score,
                    max_conflict_score = max_score,
                    "Materialized view conflict score above threshold"
                );
                return Ok(Some(QueryResult {
                    assertions: vec![],
                    total_count: 0,
                    has_more: false,
                }));
            }
        }

        Ok(Some(QueryResult { assertions: vec![view.winner], total_count: 1, has_more: false }))
    }

    /// Apply filters to candidates and construct the final QueryResult.
    ///
    /// This is used by similarity search paths to apply post-filtering
    /// (subject, predicate, lifecycle, epoch) and handle limit/truncation.
    pub(super) async fn apply_filters_and_return(
        &self,
        candidates: Vec<Assertion>,
        query: &Query,
    ) -> Result<QueryResult> {
        debug!(candidate_count = candidates.len(), "Applying filters to candidates");

        // Apply filters
        let mut matching: Vec<Assertion> =
            candidates.into_iter().filter(|a| query.matches(a)).collect();

        // Apply decay if decay_halflife is set
        if let Some(halflife) = query.decay_halflife {
            // Use as_of timestamp if set (time-travel), otherwise current time
            let now = query.as_of.unwrap_or_else(|| {
                SystemTime::now().duration_since(UNIX_EPOCH).map(|d| d.as_secs()).unwrap_or(0)
            });

            if query.source_class_decay {
                debug!(
                    halflife_seconds = halflife,
                    now,
                    source_class_aware = true,
                    "Applying source-class-aware decay"
                );
                matching = apply_source_class_decay(&matching, halflife, now);
            } else {
                debug!(halflife_seconds = halflife, now, "Applying uniform decay");
                matching = apply_decay(&matching, halflife, now);
            }
        }

        let total_count = matching.len();

        // Apply limit if specified
        let has_more = if let Some(limit) = query.limit {
            if matching.len() > limit {
                matching.truncate(limit);
                true
            } else {
                false
            }
        } else {
            false
        };

        debug!(matched_count = matching.len(), total_count, has_more, "Query complete");

        Ok(QueryResult { assertions: matching, total_count, has_more })
    }
}