//! Scenario file format and runner (MetricsAndScenarios §2, GameKernel
//! K17). Games opt in by implementing [`ScenarioGame`]; [`run`] executes a
//! scenario twice with the same seed and fails on hash divergence (K8).

use cb_events::{state_hash_hex, Envelope, EventLog};
use cb_kernel::{Actor, Aggregate, EventSeq, GameId, PlayerId};
use serde::{Deserialize, Serialize};
use std::collections::{BTreeMap, BTreeSet};

/// One scenario file (`scenarios/<game>/<slug>.yaml`).
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct ScenarioFile {
    pub scenario: String,
    #[serde(default)]
    pub description: String,
    /// Numbered rule IDs this scenario covers (feeds M-D1-COV / AM-1).
    pub covers: Vec<String>,
    /// Marks scenarios encoding a PROVISIONAL U-item default
    /// (GroundRules §Underdetermined): a ground-game ruling flips the
    /// scenario, not the kernel.
    #[serde(default)]
    pub provisional: bool,
    pub seed: u64,
    pub setup: Setup,
    pub commands: Vec<CommandStep>,
    pub expect: Expect,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct Setup {
    pub players: u8,
    /// Named setup preset from the game spec (e.g. "standard-3p").
    pub preset: String,
    /// Explicit state overrides applied after the preset (dot-paths).
    #[serde(default)]
    pub patch: BTreeMap<String, serde_yaml::Value>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct CommandStep {
    pub actor: String,
    pub cmd: String,
    #[serde(default)]
    pub args: BTreeMap<String, serde_yaml::Value>,
}

#[derive(Debug, Clone, Serialize, Deserialize, Default)]
#[serde(deny_unknown_fields)]
pub struct Expect {
    /// Ordered subsequence of event kinds that must occur.
    #[serde(default)]
    pub events: Vec<BTreeMap<String, serde_yaml::Value>>,
    /// Partial end-state assertions: dot-path → expected value.
    #[serde(default)]
    pub state: BTreeMap<String, serde_yaml::Value>,
    /// Indices into `commands` that must be rejected.
    #[serde(default)]
    pub rejects: Vec<usize>,
    /// Optional full-state golden hash.
    #[serde(default)]
    pub state_hash: Option<String>,
}

impl ScenarioFile {
    pub fn from_yaml(yaml: &str) -> Result<Self, serde_yaml::Error> {
        serde_yaml::from_str(yaml)
    }
}

/// Result of executing one scenario (twice, per the K8 double-run rule).
#[derive(Debug)]
pub enum RunOutcome {
    Passed { covers: Vec<String> },
    Failed { reason: String },
}

/// A game aggregate the scenario runner can drive (GameKernel K17). The
/// game owns setup presets and the command vocabulary; the runner owns
/// execution, assertions, and the determinism check.
pub trait ScenarioGame: Aggregate + Serialize + serde::de::DeserializeOwned + Sized {
    /// Build the initial state for a named preset (GR-S rules for GROUND).
    fn setup(setup: &Setup, seed: u64) -> Result<Self, String>;

    /// Resolve one scenario step into an actor and a typed command.
    fn parse_command(step: &CommandStep) -> Result<(Actor, Self::Command), String>;

    /// Current round, for the event envelope (GameKernel K4).
    fn round(&self) -> u8;
}

/// `P1` → `PlayerId(0)`, `SYSTEM` → [`Actor::System`]. Seat numbering is
/// 1-based in scenario files and 0-based in state, matching the
/// `players.0.*` assertion paths.
pub fn parse_actor(actor: &str) -> Result<Actor, String> {
    if actor == "SYSTEM" {
        return Ok(Actor::System);
    }
    actor
        .strip_prefix('P')
        .and_then(|n| n.parse::<u8>().ok())
        .filter(|n| *n >= 1)
        .map(|n| Actor::Player(PlayerId(n - 1)))
        .ok_or_else(|| format!("unparseable actor {actor:?} (expected P<n> or SYSTEM)"))
}

/// One execution pass: returns the end state as JSON, its hash, the
/// emitted events, and the indices of rejected commands.
struct Pass {
    state: serde_json::Value,
    hash: String,
    events: Vec<serde_json::Value>,
    rejected: BTreeSet<usize>,
}

fn execute<G>(scenario: &ScenarioFile) -> Result<Pass, String>
where
    G: ScenarioGame,
    G::Event: Serialize,
{
    let mut state = G::setup(&scenario.setup, scenario.seed)?;
    // The runner owns `setup.patch` so no game reimplements it: overrides
    // are applied to the canonical form and read back.
    if !scenario.setup.patch.is_empty() {
        state = apply_patch(&state, &scenario.setup.patch)?;
    }
    let mut log: EventLog<G::Event> = EventLog::new();
    let mut events = Vec::new();
    let mut rejected = BTreeSet::new();
    let mut seq = 0u64;

    for (index, step) in scenario.commands.iter().enumerate() {
        let (actor, command) = G::parse_command(step)?;
        match state.validate(actor, &command) {
            Err(_) => {
                rejected.insert(index);
            }
            Ok(produced) => {
                for event in produced {
                    events.push(
                        serde_json::to_value(&event).map_err(|e| format!("event encode: {e}"))?,
                    );
                    state.fold(&event);
                    // K4/K11: everything applied is logged in envelope order.
                    log.append(Envelope {
                        seq: EventSeq(seq),
                        game_id: GameId(0),
                        round: state.round(),
                        schema_ver: 1,
                        payload: event,
                    })
                    .map_err(|e| format!("event log: {e}"))?;
                    seq += 1;
                }
            }
        }
    }

    Ok(Pass {
        state: serde_json::to_value(&state).map_err(|e| format!("state encode: {e}"))?,
        hash: state_hash_hex(&state),
        events,
        rejected,
    })
}

/// Execute a scenario against game `G`: run twice with the same seed,
/// compare hashes (K8), then check the `expect` block.
pub fn run<G>(scenario: &ScenarioFile) -> RunOutcome
where
    G: ScenarioGame,
    G::Event: Serialize,
{
    let first = match execute::<G>(scenario) {
        Ok(pass) => pass,
        Err(reason) => return RunOutcome::Failed { reason },
    };
    let second = match execute::<G>(scenario) {
        Ok(pass) => pass,
        Err(reason) => return RunOutcome::Failed { reason },
    };

    if first.hash != second.hash {
        return RunOutcome::Failed {
            reason: format!(
                "K8 divergence: run 1 hash {} != run 2 hash {}",
                first.hash, second.hash
            ),
        };
    }

    if let Err(reason) = check(scenario, &first) {
        return RunOutcome::Failed { reason };
    }

    RunOutcome::Passed {
        covers: scenario.covers.clone(),
    }
}

fn check(scenario: &ScenarioFile, pass: &Pass) -> Result<(), String> {
    let expected_rejects: BTreeSet<usize> = scenario.expect.rejects.iter().copied().collect();
    if pass.rejected != expected_rejects {
        return Err(format!(
            "rejects mismatch: expected {expected_rejects:?}, got {:?}",
            pass.rejected
        ));
    }

    for (path, want) in &scenario.expect.state {
        let want = to_json(want)?;
        let got = lookup(&pass.state, path)
            .ok_or_else(|| format!("state path {path:?} not found in end state"))?;
        if got != &want {
            return Err(format!("state {path:?}: expected {want}, got {got}"));
        }
    }

    // `expect.events` is an ordered subsequence: each entry must match a
    // later event than the previous one, by field subset.
    let mut cursor = 0usize;
    for want in &scenario.expect.events {
        let want = to_json(want)?;
        let found = pass.events[cursor..]
            .iter()
            .position(|event| is_subset(&want, event));
        match found {
            Some(offset) => cursor += offset + 1,
            None => return Err(format!("expected event {want} not found in order")),
        }
    }

    if let Some(want) = &scenario.expect.state_hash {
        if want != &pass.hash {
            return Err(format!(
                "state_hash mismatch: expected {want}, got {}",
                pass.hash
            ));
        }
    }

    Ok(())
}

/// Apply `setup.patch` dot-path overrides to a freshly built state. Every
/// path must already exist — a typo is an error, never a silent no-op.
fn apply_patch<G: ScenarioGame>(
    state: &G,
    patch: &BTreeMap<String, serde_yaml::Value>,
) -> Result<G, String> {
    let mut json = serde_json::to_value(state).map_err(|e| format!("state encode: {e}"))?;
    for (path, value) in patch {
        let value = to_json(value)?;
        let (parent_path, key) = path.rsplit_once('.').unwrap_or(("", path.as_str()));
        // The parent must exist — a typo mid-path is an error, never a
        // silent no-op. The final key may be new, so scenarios can seed
        // open-ended maps (relations, focus) that start empty.
        let parent = if parent_path.is_empty() {
            &mut json
        } else {
            lookup_mut(&mut json, parent_path).ok_or_else(|| {
                format!("patch path {path:?}: {parent_path:?} does not exist in the initial state")
            })?
        };
        match parent {
            serde_json::Value::Object(map) => {
                map.insert(key.to_string(), value);
            }
            serde_json::Value::Array(items) => {
                let index = key
                    .parse::<usize>()
                    .map_err(|_| format!("patch path {path:?}: {key:?} is not an array index"))?;
                let slot = items
                    .get_mut(index)
                    .ok_or_else(|| format!("patch path {path:?}: index {index} out of range"))?;
                *slot = value;
            }
            _ => return Err(format!("patch path {path:?}: parent is not a container")),
        }
    }
    serde_json::from_value(json).map_err(|e| format!("patch produced invalid state: {e}"))
}

fn lookup_mut<'v>(
    root: &'v mut serde_json::Value,
    path: &str,
) -> Option<&'v mut serde_json::Value> {
    path.split('.').try_fold(root, |node, segment| match node {
        serde_json::Value::Object(map) => map.get_mut(segment),
        serde_json::Value::Array(items) => items.get_mut(segment.parse::<usize>().ok()?),
        _ => None,
    })
}

fn to_json<T: Serialize>(value: &T) -> Result<serde_json::Value, String> {
    serde_json::to_value(value).map_err(|e| format!("expectation encode: {e}"))
}

/// Dot-path lookup: object keys and array indices, so `players.0.stress`
/// reads the same whether the container is a map or a sequence.
fn lookup<'v>(root: &'v serde_json::Value, path: &str) -> Option<&'v serde_json::Value> {
    path.split('.').try_fold(root, |node, segment| match node {
        serde_json::Value::Object(map) => map.get(segment),
        serde_json::Value::Array(items) => items.get(segment.parse::<usize>().ok()?),
        _ => None,
    })
}

/// Field-subset match: every key in `want` must be present and equal in
/// `got`, so scenarios can assert on the fields they care about.
fn is_subset(want: &serde_json::Value, got: &serde_json::Value) -> bool {
    match (want, got) {
        (serde_json::Value::Object(w), serde_json::Value::Object(g)) => w
            .iter()
            .all(|(key, value)| g.get(key).is_some_and(|found| is_subset(value, found))),
        _ => want == got,
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    const SAMPLE: &str = r#"
scenario: ground/smoke-setup
description: Parses the documented scenario shape end to end.
covers: [GR-S02, GR-S03]
provisional: false
seed: 42
setup:
  players: 3
  preset: standard-3p
  patch:
    "players.0.stress": 4
commands:
  - actor: P1
    cmd: select_action
    args: { action: ATTACK, target: P2 }
expect:
  state:
    "round": 1
  rejects: []
"#;

    #[test]
    fn scenario_format_roundtrips() {
        let parsed = ScenarioFile::from_yaml(SAMPLE).unwrap();
        assert_eq!(parsed.scenario, "ground/smoke-setup");
        assert_eq!(parsed.covers, vec!["GR-S02", "GR-S03"]);
        assert_eq!(parsed.setup.players, 3);
        assert_eq!(parsed.commands.len(), 1);
    }

    #[test]
    fn actors_parse_1_based_into_0_based_seats() {
        assert_eq!(parse_actor("P1").unwrap(), Actor::Player(PlayerId(0)));
        assert_eq!(parse_actor("P3").unwrap(), Actor::Player(PlayerId(2)));
        assert_eq!(parse_actor("SYSTEM").unwrap(), Actor::System);
        assert!(parse_actor("P0").is_err());
        assert!(parse_actor("bogus").is_err());
    }

    #[test]
    fn dot_paths_index_objects_and_arrays() {
        let state = serde_json::json!({ "players": { "0": { "stress": 2 } }, "deck": [7, 9] });
        assert_eq!(lookup(&state, "players.0.stress").unwrap(), &2);
        assert_eq!(lookup(&state, "deck.1").unwrap(), &9);
        assert!(lookup(&state, "players.9.stress").is_none());
    }

    #[test]
    fn unknown_fields_are_rejected() {
        let bad = SAMPLE.replace("seed: 42", "seed: 42\ntypo_field: 1");
        assert!(ScenarioFile::from_yaml(&bad).is_err());
    }
}