//! Level 9 — special operators. The merge key (`<<`) lets a door
//! inherit a template and override one field.
//!
//! Paired design note: `l09.md`.
//!
//! `serde_yaml` treats `<<` as a literal mapping key (it does NOT
//! perform YAML 1.1 merge-key resolution). The target therefore carries
//! a `<<` key whose value is the defaults dict, exactly as the player's
//! `<<: *defaults` parses.

use rand::seq::SliceRandom;
use rand::{Rng, SeedableRng};
use rand_chacha::ChaCha8Rng;
use serde::Serialize;
use serde_yaml::{Mapping, Value};

use crate::describe::Describer;

use super::{Generated, Level};

pub struct Operators;

const MATERIALS: &[&str] = &["oak", "iron", "stone", "silver", "bone"];

#[derive(Serialize)]
struct DescCtx {
    default_material: String,
    default_locked: bool,
    north_locked: bool,
    south_material: String,
}

impl Level for Operators {
    fn id(&self) -> u8 {
        9
    }

    fn name(&self) -> &'static str {
        "Merge Keys"
    }

    fn generate(&self, seed: u64) -> Generated {
        let mut rng = ChaCha8Rng::seed_from_u64(seed ^ 0x0000_0000_0000_0009);
        let default_material = (*MATERIALS.choose(&mut rng).unwrap()).to_string();
        let default_locked = rng.gen_bool(0.5);
        // The north door overrides locked; choose the opposite so the
        // override is meaningful.
        let north_locked = !default_locked;
        // The south door overrides material; pick anything but default.
        let other_materials: Vec<&str> = MATERIALS
            .iter()
            .filter(|m| **m != default_material)
            .copied()
            .collect();
        let south_material = (*other_materials.choose(&mut rng).unwrap()).to_string();

        // Build the defaults mapping (referenced by all doors).
        let mut defaults = Mapping::new();
        defaults.insert(
            Value::String("material".to_string()),
            Value::String(default_material.clone()),
        );
        defaults.insert(
            Value::String("locked".to_string()),
            Value::Bool(default_locked),
        );

        let mut north_door = Mapping::new();
        north_door.insert(
            Value::String("<<".to_string()),
            Value::Mapping(defaults.clone()),
        );
        north_door.insert(
            Value::String("locked".to_string()),
            Value::Bool(north_locked),
        );

        let mut south_door = Mapping::new();
        south_door.insert(
            Value::String("<<".to_string()),
            Value::Mapping(defaults.clone()),
        );
        south_door.insert(
            Value::String("material".to_string()),
            Value::String(south_material.clone()),
        );

        let mut top = Mapping::new();
        top.insert(
            Value::String("door_defaults".to_string()),
            Value::Mapping(defaults),
        );
        top.insert(
            Value::String("north_door".to_string()),
            Value::Mapping(north_door),
        );
        top.insert(
            Value::String("south_door".to_string()),
            Value::Mapping(south_door),
        );

        let target_yaml =
            serde_yaml::to_string(&Value::Mapping(top)).expect("serialise mapping");

        let mut d = Describer::new();
        d.register(
            "l09",
            "Two doors share a template:\n\
             default material: {{ default_material }}\n\
             default locked:   {{ default_locked }}\n\
             \n\
             north_door overrides locked → {{ north_locked }}\n\
             south_door overrides material → {{ south_material }}\n\
             💡 Anchor the defaults (`door_defaults: &name`) and merge with `<<: *name` in each door.",
        )
        .expect("register template");
        let description = d
            .render(
                "l09",
                &DescCtx {
                    default_material,
                    default_locked,
                    north_locked,
                    south_material,
                },
            )
            .expect("render template");

        Generated {
            target_yaml,
            description,
            flavor: "🚪 Two doors echo a single template.".to_string(),
        }
    }
}