// Save/load ECS extraction (#553)
// Implements D-020 MessagePack format for save files, D-010 determinism.
//
// Two entry points:
//   save_to_file: queries ECS, builds SaveStateV1, writes MessagePack to path.
//   load_from_file: reads path, deserialises SaveStateV1, re-injects ECS state.
//
// IPC: SaveGame / LoadGame PlayerAction variants queue commands here.
// execute_save_load: exclusive system that drains the queue and writes the result
// to SnapshotBuffer.pending_save_result for client feedback.

use std::path::{Path, PathBuf};

use bevy_ecs::prelude::*;
use thiserror::Error;

use crate::bridge::types::SaveLoadResultWire;
use crate::bridge::types::SnapshotBuffer;
use crate::content::template::{TemplateReferenceMap, TriangleState};
use crate::knowledge::graph::KnowledgeGraph;
use crate::knowledge::registry::EntityRegistry;
use crate::npc::Npc;
use crate::npc::relationships::RelationshipGraph;
use crate::simulation::movement::PlayerCharacter;
use crate::simulation::rng::SimRng;
use crate::simulation::save_state::{
    deserialize_npc_from_frozen, serialize_npc_to_frozen, SaveStateV1, SAVE_FORMAT_VERSION,
};
use crate::simulation::tier::BackgroundSim;
use crate::simulation::time::SimulationTime;

/// Errors from save/load operations (#553).
#[derive(Debug, Error)]
pub enum SaveLoadError {
    #[error("I/O error: {0}")]
    Io(String),
    #[error("serialization error: {0}")]
    Serialize(String),
    #[error("deserialization error: {0}")]
    Deserialize(String),
    #[error("format version mismatch: expected {expected}, found {found}")]
    VersionMismatch { expected: u8, found: u8 },
}

/// A queued save or load command (#553).
#[derive(Debug, Clone)]
pub enum SaveLoadCommand {
    Save { path: PathBuf },
    Load { path: PathBuf },
}

/// Pending save/load command resource (#553).
///
/// `process_player_input` queues commands here when it encounters
/// `PlayerAction::SaveGame` or `PlayerAction::LoadGame`. The
/// `execute_save_load` exclusive system drains this queue each tick.
#[derive(Resource, Debug, Default)]
pub struct SaveLoadPending {
    /// Pending command (at most one; new commands overwrite pending ones).
    pub pending: Option<SaveLoadCommand>,
}

/// Extract world state into `SaveStateV1` and write MessagePack bytes to `path` (#553).
///
/// Queries all NPC entities, the player knowledge graph, global relationship graph,
/// simulation time, and RNG seed. Builds `SaveStateV1` and writes to disk.
///
/// NPC states are sorted by `stable_id` ascending for determinism (D-010).
/// NPCs without `StableEntityId` trigger a panic (caller invariant — all live
/// NPCs must be registered before save).
///
/// # Errors
/// `SaveLoadError::Io` on filesystem failure.
/// `SaveLoadError::Serialize` on MessagePack encoding failure.
pub fn save_to_file(path: &Path, world: &mut World) -> Result<(), SaveLoadError> {
    // Simulation clock
    let (tick, tick_rate) = {
        let t = world.resource::<SimulationTime>();
        (t.tick, t.tick_rate)
    };

    // RNG seed for deterministic replay (D-010)
    let seed = world.resource::<SimRng>().seed();

    // Player knowledge graph — the observer's epistemics at save time
    let player_knowledge = {
        let mut q = world.query_filtered::<&KnowledgeGraph, With<PlayerCharacter>>();
        q.single(world).cloned().unwrap_or_else(|_| {
            tracing::warn!("save_to_file: no PlayerCharacter with KnowledgeGraph found — saving empty graph");
            KnowledgeGraph::new()
        })
    };

    // Global NPC social web
    let relationship_graph = world.resource::<RelationshipGraph>().clone();

    // Per-NPC states: collect then sort by stable_id (D-010 determinism)
    let npc_entities: Vec<Entity> = {
        let mut q = world.query_filtered::<Entity, With<Npc>>();
        q.iter(world).collect()
    };
    let mut npc_states: Vec<_> = npc_entities
        .iter()
        .map(|&entity| serialize_npc_to_frozen(entity, world))
        .collect();
    npc_states.sort_by_key(|s| s.stable_id.0);

    let npc_count = npc_states.len();

    // Capture TemplateReferenceMap if present — default to empty if not yet initialised.
    let template_references = world
        .get_resource::<TemplateReferenceMap>()
        .cloned()
        .unwrap_or_default();

    // Capture TriangleState components — sorted by triangle_id for determinism (D-010).
    let mut triangle_states: Vec<TriangleState> = {
        let mut q = world.query::<&TriangleState>();
        q.iter(world).cloned().collect()
    };
    triangle_states.sort_by_key(|t| t.triangle_id.0);

    let state = SaveStateV1 {
        format_version: SAVE_FORMAT_VERSION,
        tick,
        tick_rate,
        seed,
        player_knowledge,
        relationship_graph,
        npc_states,
        template_references,
        triangle_states,
    };

    let bytes = state
        .to_bytes()
        .map_err(|e| SaveLoadError::Serialize(e.to_string()))?;

    std::fs::write(path, &bytes).map_err(|e| SaveLoadError::Io(e.to_string()))?;

    tracing::info!(
        "save_to_file: {:?} (tick={}, npcs={}, {} bytes)",
        path,
        tick,
        npc_count,
        bytes.len()
    );
    Ok(())
}

/// Read `path`, deserialise `SaveStateV1`, and re-inject state into the ECS (#553).
///
/// Steps:
/// 1. Read and deserialise bytes; reject if `format_version != SAVE_FORMAT_VERSION`.
/// 2. Despawn all existing NPC entities and unregister them from `EntityRegistry`.
/// 3. Re-spawn each NPC via `deserialize_npc_from_frozen`; register with
///    `register_existing`; insert `BackgroundSim` tier marker.
/// 4. Advance `EntityRegistry` counter past all restored IDs.
/// 5. Restore `RelationshipGraph`, `SimulationTime`, and `SimRng` resources.
/// 6. Update the player entity's `KnowledgeGraph` if a player entity exists.
///
/// **Gotcha (D-010):** Bevy `Entity` handles are generational. `NpcSaveState` uses
/// `StableId(u64)` throughout — `EntityRegistry` maps restored `StableId`s to the
/// new `Entity` handles after re-spawn.
///
/// # Errors
/// `SaveLoadError::Io` on filesystem failure.
/// `SaveLoadError::Deserialize` on MessagePack decoding failure.
/// `SaveLoadError::VersionMismatch` when the save file predates the current schema.
pub fn load_from_file(path: &Path, world: &mut World) -> Result<(), SaveLoadError> {
    let bytes = std::fs::read(path).map_err(|e| SaveLoadError::Io(e.to_string()))?;
    let state =
        SaveStateV1::from_bytes(&bytes).map_err(|e| SaveLoadError::Deserialize(e.to_string()))?;

    if state.format_version != SAVE_FORMAT_VERSION {
        return Err(SaveLoadError::VersionMismatch {
            expected: SAVE_FORMAT_VERSION,
            found: state.format_version,
        });
    }

    let npc_count = state.npc_states.len();

    // Despawn all existing NPC entities and clear their registry entries.
    let npc_entities: Vec<Entity> = {
        let mut q = world.query_filtered::<Entity, With<Npc>>();
        q.iter(world).collect()
    };
    for entity in npc_entities {
        world.resource_mut::<EntityRegistry>().unregister(entity);
        world.despawn(entity);
    }

    // Track the highest restored StableId so we can advance the counter.
    let mut max_id: u64 = 0;

    // Re-spawn NPCs, assign tier marker, register pre-existing StableIds.
    for npc_state in &state.npc_states {
        let entity = deserialize_npc_from_frozen(npc_state, world);

        // Loaded NPCs start in BackgroundSim; the distance system promotes as needed.
        world.entity_mut(entity).insert(BackgroundSim);

        let stable_id = npc_state.stable_id;
        world
            .resource_mut::<EntityRegistry>()
            .register_existing(entity, stable_id);

        max_id = max_id.max(stable_id.0);
    }

    // Advance the registry counter past all restored IDs so future register()
    // calls produce non-conflicting IDs.
    if npc_count > 0 {
        world.resource_mut::<EntityRegistry>().advance_past(max_id);
    }

    // Restore simulation resources.
    world.insert_resource(state.relationship_graph);
    world.insert_resource(state.template_references);

    // Restore triangle states (#250) — spawn dedicated entities for each.
    for ts in &state.triangle_states {
        world.spawn(ts.clone());
    }
    {
        let mut t = world.resource_mut::<SimulationTime>();
        t.tick = state.tick;
        t.tick_rate = state.tick_rate;
    }
    world.insert_resource(SimRng::new(state.seed));

    // Update the player entity's KnowledgeGraph if a player exists.
    let player_entity = {
        let mut q = world.query_filtered::<Entity, With<PlayerCharacter>>();
        q.single(world).ok()
    };
    if let Some(player_entity) = player_entity {
        world
            .entity_mut(player_entity)
            .insert(state.player_knowledge);
    }

    tracing::info!(
        "load_from_file: {:?} (tick={}, npcs={})",
        path,
        state.tick,
        npc_count,
    );
    Ok(())
}

/// Exclusive system: drain `SaveLoadPending` and execute queued save/load (#553).
///
/// Runs each tick, after `process_player_input`. If a command is pending,
/// executes it and writes `SaveLoadResultWire` to `SnapshotBuffer.pending_save_result`
/// for consumption by `compute_observer_snapshot` the same tick.
pub fn execute_save_load(world: &mut World) {
    // Take the pending command (releases the borrow before we use world again).
    let command = {
        let mut pending = world.resource_mut::<SaveLoadPending>();
        pending.pending.take()
    };

    let Some(command) = command else {
        return;
    };

    let (kind_str, result) = match &command {
        SaveLoadCommand::Save { path } => {
            let r = save_to_file(path, world);
            ("save", r)
        }
        SaveLoadCommand::Load { path } => {
            let r = load_from_file(path, world);
            ("load", r)
        }
    };

    let wire_result = match result {
        Ok(()) => {
            tracing::info!("execute_save_load: {} completed", kind_str);
            SaveLoadResultWire {
                success: true,
                kind: kind_str.to_string(),
                error: None,
            }
        }
        Err(ref e) => {
            tracing::error!("execute_save_load: {} failed: {}", kind_str, e);
            SaveLoadResultWire {
                success: false,
                kind: kind_str.to_string(),
                error: Some(e.to_string()),
            }
        }
    };

    // Write result to SnapshotBuffer for client feedback (one tick only — consumed by
    // compute_observer_snapshot via pending_save_result.take()).
    if let Some(mut buf) = world.get_resource_mut::<SnapshotBuffer>() {
        buf.pending_save_result = Some(wire_result);
    }
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;
    use crate::knowledge::graph::KnowledgeGraph;
    use crate::knowledge::registry::{EntityRegistry, StableEntityId};
    use crate::knowledge::types::StableId;
    use crate::npc::Npc;
    use crate::npc::relationships::RelationshipGraph;
    use crate::simulation::movement::TilePosition;
    use crate::simulation::rng::SimRng;
    use crate::simulation::save_state::{SaveStateV1, SAVE_FORMAT_VERSION};
    use crate::simulation::time::{SimulationTime, TickRate};
    use bevy_ecs::world::World;
    use std::sync::atomic::{AtomicU64, Ordering};

    static COUNTER: AtomicU64 = AtomicU64::new(0);

    fn temp_path() -> PathBuf {
        let id = COUNTER.fetch_add(1, Ordering::Relaxed);
        std::env::temp_dir().join(format!("settled_reach_save_io_test_{}.msgpack", id))
    }

    fn minimal_world() -> World {
        let mut w = World::new();
        w.insert_resource(SimulationTime::default());
        w.insert_resource(SimRng::new(42));
        w.insert_resource(RelationshipGraph::new());
        w.init_resource::<EntityRegistry>();
        w
    }

    fn spawn_test_npc(world: &mut World, stable_id: u64) -> Entity {
        world
            .spawn((
                Npc,
                StableEntityId(StableId(stable_id)),
                TilePosition::new(stable_id as i32, 0, 0),
            ))
            .id()
    }

    // -----------------------------------------------------------------------
    // save_to_file
    // -----------------------------------------------------------------------

    #[test]
    fn save_to_file_creates_valid_msgpack() {
        let mut world = minimal_world();
        spawn_test_npc(&mut world, 1);
        spawn_test_npc(&mut world, 2);

        let path = temp_path();
        save_to_file(&path, &mut world).expect("save should succeed");

        let bytes = std::fs::read(&path).expect("file should exist");
        let state = SaveStateV1::from_bytes(&bytes).expect("bytes must be valid msgpack");
        assert_eq!(state.format_version, SAVE_FORMAT_VERSION);
        assert_eq!(state.npc_states.len(), 2);

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn save_to_file_sorts_npc_states_by_stable_id() {
        let mut world = minimal_world();
        // Spawn in reverse order — save should still sort ascending
        spawn_test_npc(&mut world, 50);
        spawn_test_npc(&mut world, 10);
        spawn_test_npc(&mut world, 30);

        let path = temp_path();
        save_to_file(&path, &mut world).expect("save should succeed");

        let bytes = std::fs::read(&path).expect("read saved file");
        let state = SaveStateV1::from_bytes(&bytes).unwrap();
        let ids: Vec<u64> = state.npc_states.iter().map(|n| n.stable_id.0).collect();
        assert_eq!(ids, vec![10, 30, 50], "npc_states must be sorted by stable_id");

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn save_to_file_preserves_tick_and_seed() {
        let mut world = minimal_world();
        {
            let mut t = world.resource_mut::<SimulationTime>();
            t.tick = 9999;
            t.tick_rate = TickRate::Half;
        }
        world.insert_resource(SimRng::new(0xDEADBEEF));

        let path = temp_path();
        save_to_file(&path, &mut world).expect("save");

        let bytes = std::fs::read(&path).unwrap();
        let state = SaveStateV1::from_bytes(&bytes).unwrap();
        assert_eq!(state.tick, 9999);
        assert_eq!(state.tick_rate, TickRate::Half);
        assert_eq!(state.seed, 0xDEADBEEF);

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn save_to_file_returns_io_error_on_bad_path() {
        let mut world = minimal_world();
        let bad_path = std::path::Path::new("/nonexistent/directory/save.msgpack");
        let result = save_to_file(bad_path, &mut world);
        assert!(
            matches!(result, Err(SaveLoadError::Io(_))),
            "expected Io error for bad path"
        );
    }

    // -----------------------------------------------------------------------
    // load_from_file
    // -----------------------------------------------------------------------

    #[test]
    fn load_from_file_restores_npc_count() {
        let mut world = minimal_world();
        spawn_test_npc(&mut world, 1);
        spawn_test_npc(&mut world, 2);

        let path = temp_path();
        save_to_file(&path, &mut world).expect("save");

        // Spawn an extra NPC — loading should despawn the old NPCs and restore exactly 2
        spawn_test_npc(&mut world, 99);
        let pre_load_count = {
            let mut q = world.query_filtered::<Entity, With<Npc>>();
            q.iter(&world).count()
        };
        assert_eq!(pre_load_count, 3, "three NPCs before load");

        load_from_file(&path, &mut world).expect("load");

        let post_load_count = {
            let mut q = world.query_filtered::<Entity, With<Npc>>();
            q.iter(&world).count()
        };
        assert_eq!(post_load_count, 2, "exactly the two saved NPCs after load");

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn load_from_file_restores_stable_ids_in_registry() {
        let mut world = minimal_world();
        spawn_test_npc(&mut world, 10);
        spawn_test_npc(&mut world, 20);

        let path = temp_path();
        save_to_file(&path, &mut world).expect("save");
        load_from_file(&path, &mut world).expect("load");

        let registry = world.resource::<EntityRegistry>();
        assert!(
            registry.to_entity(&StableId(10)).is_some(),
            "StableId(10) must be in registry after load"
        );
        assert!(
            registry.to_entity(&StableId(20)).is_some(),
            "StableId(20) must be in registry after load"
        );

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn load_from_file_restores_tick_and_seed() {
        let mut world = minimal_world();
        {
            let mut t = world.resource_mut::<SimulationTime>();
            t.tick = 5000;
        }
        world.insert_resource(SimRng::new(12345));

        let path = temp_path();
        save_to_file(&path, &mut world).expect("save");

        // Change time and seed, then load
        {
            let mut t = world.resource_mut::<SimulationTime>();
            t.tick = 1;
        }
        world.insert_resource(SimRng::new(0));

        load_from_file(&path, &mut world).expect("load");

        let t = world.resource::<SimulationTime>();
        assert_eq!(t.tick, 5000, "tick restored from save");
        assert_eq!(
            world.resource::<SimRng>().seed(),
            12345,
            "seed restored from save"
        );

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn load_from_file_rejects_wrong_format_version() {
        use crate::content::template::TemplateReferenceMap;
        // Craft a save with a wrong format_version
        let bad_state = SaveStateV1 {
            format_version: 0xFF, // deliberately wrong
            tick: 0,
            tick_rate: TickRate::Full,
            seed: 0,
            player_knowledge: KnowledgeGraph::new(),
            relationship_graph: RelationshipGraph::new(),
            npc_states: vec![],
            template_references: TemplateReferenceMap::default(),
            triangle_states: vec![],
        };
        let bytes = bad_state.to_bytes().expect("serialize");
        let path = temp_path();
        std::fs::write(&path, &bytes).expect("write");

        let mut world = minimal_world();
        let result = load_from_file(&path, &mut world);
        assert!(
            matches!(result, Err(SaveLoadError::VersionMismatch { .. })),
            "expected VersionMismatch error, got {:?}",
            result
        );

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn load_from_file_returns_io_error_for_missing_file() {
        let mut world = minimal_world();
        let missing = std::path::Path::new("/tmp/settled_reach_nonexistent_42.msgpack");
        let result = load_from_file(missing, &mut world);
        assert!(
            matches!(result, Err(SaveLoadError::Io(_))),
            "expected Io error for missing file"
        );
    }

    #[test]
    fn load_from_file_assigns_background_sim_tier() {
        let mut world = minimal_world();
        spawn_test_npc(&mut world, 1);

        let path = temp_path();
        save_to_file(&path, &mut world).expect("save");
        load_from_file(&path, &mut world).expect("load");

        let has_background: bool = {
            let mut q = world.query_filtered::<Entity, (With<Npc>, With<BackgroundSim>)>();
            q.iter(&world).count() > 0
        };
        assert!(
            has_background,
            "loaded NPC should be in BackgroundSim tier"
        );

        let _ = std::fs::remove_file(&path);
    }

    // -----------------------------------------------------------------------
    // execute_save_load
    // -----------------------------------------------------------------------

    #[test]
    fn execute_save_load_noop_when_no_pending() {
        let mut world = minimal_world();
        world.init_resource::<SaveLoadPending>();
        world.init_resource::<SnapshotBuffer>();

        execute_save_load(&mut world);

        // No result written when no pending command
        let buf = world.resource::<SnapshotBuffer>();
        assert!(
            buf.pending_save_result.is_none(),
            "no pending_save_result when no command was queued"
        );
    }

    #[test]
    fn execute_save_load_writes_success_result() {
        let mut world = minimal_world();
        world.init_resource::<SnapshotBuffer>();

        let path = temp_path();
        world.insert_resource(SaveLoadPending {
            pending: Some(SaveLoadCommand::Save { path: path.clone() }),
        });

        execute_save_load(&mut world);

        let buf = world.resource::<SnapshotBuffer>();
        let result = buf
            .pending_save_result
            .as_ref()
            .expect("result must be written after execute");
        assert!(result.success, "save should succeed");
        assert_eq!(result.kind, "save");
        assert!(result.error.is_none());

        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn execute_save_load_writes_error_result_on_bad_path() {
        let mut world = minimal_world();
        world.init_resource::<SnapshotBuffer>();

        world.insert_resource(SaveLoadPending {
            pending: Some(SaveLoadCommand::Save {
                path: PathBuf::from("/nonexistent/dir/save.msgpack"),
            }),
        });

        execute_save_load(&mut world);

        let buf = world.resource::<SnapshotBuffer>();
        let result = buf
            .pending_save_result
            .as_ref()
            .expect("result must be written even on failure");
        assert!(!result.success, "save should fail with bad path");
        assert_eq!(result.kind, "save");
        assert!(result.error.is_some(), "error message should be present");
    }

    // -----------------------------------------------------------------------
    // Overwrite behaviour
    // -----------------------------------------------------------------------

    /// When two commands arrive in the same tick, the second overwrites the first.
    /// The warn! in process_player_input fires; here we just confirm last-write-wins.
    #[test]
    fn pending_command_overwrite_last_write_wins() {
        let mut pending = SaveLoadPending::default();

        pending.pending = Some(SaveLoadCommand::Save {
            path: PathBuf::from("/tmp/first.msgpack"),
        });
        // Overwrite with a Load command
        pending.pending = Some(SaveLoadCommand::Load {
            path: PathBuf::from("/tmp/second.msgpack"),
        });

        match pending.pending.unwrap() {
            SaveLoadCommand::Load { ref path } => {
                assert_eq!(path.to_str().unwrap(), "/tmp/second.msgpack");
            }
            other => panic!("expected Load, got {:?}", other),
        }
    }

    // -----------------------------------------------------------------------
    // SaveLoadError display
    // -----------------------------------------------------------------------

    #[test]
    fn save_load_error_display() {
        let e = SaveLoadError::Io("disk full".into());
        assert!(e.to_string().contains("disk full"));

        let e2 = SaveLoadError::VersionMismatch {
            expected: 1,
            found: 2,
        };
        assert!(e2.to_string().contains("expected 1"));
        assert!(e2.to_string().contains("found 2"));
    }
}