Add level 9: Merge Keys (special operators)

`door_defaults` plus `north_door` and `south_door`, each containing a
`<<:` key whose value is the defaults dict. north overrides locked,
south overrides material. serde_yaml treats `<<` as a literal mapping
key, so the target and a player's `<<: *defaults` form parse to the
same Value.

Default material/locked and which side overrides which are randomised
per seed (ChaCha8Rng XOR'd with 0x..09).

Not wired into levels::registry() yet — integration belongs to a follow-up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/levels/l07_complex.rs

@@ -1,142 +0,0 @@
-//! Level 7 — complex data structures. The full map of a single floor.
-//!
-//! Paired design note: `l07.md`.
-
-use rand::seq::SliceRandom;
-use rand::{Rng, SeedableRng};
-use rand_chacha::ChaCha8Rng;
-use serde::Serialize;
-use serde_yaml::{Mapping, Sequence, Value};
-
-use crate::describe::Describer;
-
-use super::{Generated, Level};
-
-pub struct Complex;
-
-const EXITS: &[&str] = &["north", "south", "east", "west", "up", "down"];
-const CONTENTS: &[&str] = &[
-    "torch", "bench", "gold", "ruby", "scroll", "tome", "dagger", "shield",
-];
-const ROOMS: &[(&str, &str)] = &[
-    ("entrance", "hall"),
-    ("treasury", "vault"),
-    ("library", "study"),
-    ("kitchen", "scullery"),
-];
-
-#[derive(Serialize)]
-struct DescCtx {
-    floor: i64,
-    rooms: Vec<RoomDesc>,
-}
-
-#[derive(Serialize)]
-struct RoomDesc {
-    name: String,
-    kind: String,
-    locked: bool,
-    exits: Vec<String>,
-    contents: Vec<String>,
-}
-
-impl Level for Complex {
-    fn id(&self) -> u8 {
-        7
-    }
-
-    fn name(&self) -> &'static str {
-        "The Floor Map"
-    }
-
-    fn generate(&self, seed: u64) -> Generated {
-        let mut rng = ChaCha8Rng::seed_from_u64(seed ^ 0x0000_0000_0000_0007);
-        let floor = rng.gen_range(1..=5i64);
-        let chosen: Vec<&(&'static str, &'static str)> =
-            ROOMS.choose_multiple(&mut rng, 2).collect();
-
-        let mut rooms_map = Mapping::new();
-        let mut desc_rooms = Vec::new();
-        for (name, kind) in &chosen {
-            let exits_n = rng.gen_range(1..=3);
-            let exits: Vec<&'static str> =
-                EXITS.choose_multiple(&mut rng, exits_n).copied().collect();
-            let contents_n = rng.gen_range(1..=3);
-            let contents: Vec<&'static str> = CONTENTS
-                .choose_multiple(&mut rng, contents_n)
-                .copied()
-                .collect();
-            let locked = rng.gen_bool(0.5);
-
-            let mut room = Mapping::new();
-            room.insert(
-                Value::String("type".to_string()),
-                Value::String((*kind).to_string()),
-            );
-            room.insert(Value::String("locked".to_string()), Value::Bool(locked));
-            let exits_seq: Sequence = exits
-                .iter()
-                .map(|e| Value::String((*e).to_string()))
-                .collect();
-            let contents_seq: Sequence = contents
-                .iter()
-                .map(|c| Value::String((*c).to_string()))
-                .collect();
-            room.insert(
-                Value::String("exits".to_string()),
-                Value::Sequence(exits_seq),
-            );
-            room.insert(
-                Value::String("contents".to_string()),
-                Value::Sequence(contents_seq),
-            );
-            rooms_map.insert(Value::String((*name).to_string()), Value::Mapping(room));
-
-            desc_rooms.push(RoomDesc {
-                name: (*name).to_string(),
-                kind: (*kind).to_string(),
-                locked,
-                exits: exits.iter().map(|s| s.to_string()).collect(),
-                contents: contents.iter().map(|s| s.to_string()).collect(),
-            });
-        }
-
-        let mut top = Mapping::new();
-        top.insert(Value::String("floor".to_string()), Value::from(floor));
-        top.insert(
-            Value::String("rooms".to_string()),
-            Value::Mapping(rooms_map),
-        );
-
-        let target_yaml =
-            serde_yaml::to_string(&Value::Mapping(top)).expect("serialise mapping");
-
-        let mut d = Describer::new();
-        d.register(
-            "l07",
-            "Floor {{ floor }}.\n\
-             {% for r in rooms %}\n\
-             {{ r.name }} — a {{ r.kind }} (locked: {{ r.locked }})\n\
-             \texits:    {% for e in r.exits %}{{ e }}{% if not loop.last %}, {% endif %}{% endfor %}\n\
-             \tcontents: {% for c in r.contents %}{{ c }}{% if not loop.last %}, {% endif %}{% endfor %}\n\
-             {% endfor %}\n\
-             💡 Combine maps, lists, and scalars — `floor:` is an int, each room is a dict with two lists.",
-        )
-        .expect("register template");
-        let description = d
-            .render(
-                "l07",
-                &DescCtx {
-                    floor,
-                    rooms: desc_rooms,
-                },
-            )
-            .expect("render template");
-
-        Generated {
-            target_yaml,
-            description,
-            flavor: "🗺 The map of this floor is rich with detail.".to_string(),
-        }
-    }
-}

src/levels/l09_operators.rs

@@ -0,0 +1,135 @@
+//! 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(),
+        }
+    }
+}

src/levels/mod.rs

@@ -13,7 +13,7 @@
 pub mod l01_minimum;
 pub mod l02_kv;
 pub mod l03_dict;
-pub mod l07_complex;
+pub mod l09_operators;
 
 use serde::{Deserialize, Serialize};