Add dialogue tree data model with tests

Implements a TOML-based dialogue tree system for NPCs with:
- DialogueChoice: player response options with optional conditions
- DialogueNode: NPC text with choices and optional actions
- DialogueTree: complete tree with root node and node graph
- Validation for root_node and next_node references
- load_dialogue() for single files, load_all_dialogues() for directories

Includes librarian dialogue example with nested conversation flow.

content/dialogue/librarian.toml

@@ -0,0 +1,49 @@
+npc_name = "librarian"
+root_node = "greeting"
+
+[nodes.greeting]
+text = "Welcome to the library. How can I help you today?"
+
+[[nodes.greeting.choices]]
+text = "I'm looking for a book."
+next_node = "book_search"
+
+[[nodes.greeting.choices]]
+text = "Tell me about this place."
+next_node = "about_library"
+
+[[nodes.greeting.choices]]
+text = "Goodbye."
+next_node = "farewell"
+
+[nodes.book_search]
+text = "We have quite the collection. Fairy tales, mostly. Browse the shelves — you might find something that catches your eye."
+
+[[nodes.book_search.choices]]
+text = "Any recommendations?"
+next_node = "recommendations"
+
+[[nodes.book_search.choices]]
+text = "Thanks, I'll look around."
+next_node = "farewell"
+
+[nodes.recommendations]
+text = "The Brothers Grimm collected some wonderful tales. 'The Golden Bird' is a personal favorite — a story about patience and trust."
+
+[[nodes.recommendations.choices]]
+text = "I'll keep an eye out for it."
+next_node = "farewell"
+
+[[nodes.recommendations.choices]]
+text = "Tell me about this place instead."
+next_node = "about_library"
+
+[nodes.about_library]
+text = "This library was built to preserve the old stories. Every book here was carefully transcribed. Take your time — the stories aren't going anywhere."
+
+[[nodes.about_library.choices]]
+text = "Thanks for the information."
+next_node = "farewell"
+
+[nodes.farewell]
+text = "Happy reading. Come back anytime."

src/mudlib/dialogue.py

@@ -0,0 +1,102 @@
+"""Dialogue tree data model and TOML loading."""
+
+import tomllib
+from dataclasses import dataclass, field
+from pathlib import Path
+
+
+@dataclass
+class DialogueChoice:
+    """A player response option in a dialogue."""
+
+    text: str
+    next_node: str
+    condition: str | None = None
+
+
+@dataclass
+class DialogueNode:
+    """A single node in a dialogue tree."""
+
+    id: str
+    text: str
+    choices: list[DialogueChoice] = field(default_factory=list)
+    action: str | None = None
+
+
+@dataclass
+class DialogueTree:
+    """A complete dialogue tree for an NPC."""
+
+    npc_name: str
+    root_node: str
+    nodes: dict[str, DialogueNode] = field(default_factory=dict)
+
+
+def load_dialogue(path: Path) -> DialogueTree:
+    """Load a dialogue tree from a TOML file.
+
+    Args:
+        path: Path to the TOML file
+
+    Returns:
+        DialogueTree instance
+
+    Raises:
+        ValueError: If root_node or any next_node reference doesn't exist
+    """
+    with open(path, "rb") as f:
+        data = tomllib.load(f)
+
+    npc_name = data["npc_name"]
+    root_node = data["root_node"]
+    nodes: dict[str, DialogueNode] = {}
+
+    # Parse all nodes
+    for node_id, node_data in data.get("nodes", {}).items():
+        choices = []
+        for choice_data in node_data.get("choices", []):
+            choices.append(
+                DialogueChoice(
+                    text=choice_data["text"],
+                    next_node=choice_data["next_node"],
+                    condition=choice_data.get("condition"),
+                )
+            )
+
+        nodes[node_id] = DialogueNode(
+            id=node_id,
+            text=node_data["text"],
+            choices=choices,
+            action=node_data.get("action"),
+        )
+
+    # Validate root_node exists
+    if root_node not in nodes:
+        msg = f"root_node '{root_node}' not found in nodes"
+        raise ValueError(msg)
+
+    # Validate all next_node references exist
+    for node in nodes.values():
+        for choice in node.choices:
+            if choice.next_node not in nodes:
+                msg = f"next_node '{choice.next_node}' not found in nodes"
+                raise ValueError(msg)
+
+    return DialogueTree(npc_name=npc_name, root_node=root_node, nodes=nodes)
+
+
+def load_all_dialogues(directory: Path) -> dict[str, DialogueTree]:
+    """Load all dialogue trees from TOML files in a directory.
+
+    Args:
+        directory: Path to directory containing .toml files
+
+    Returns:
+        Dict mapping npc_name to DialogueTree instances
+    """
+    trees: dict[str, DialogueTree] = {}
+    for path in sorted(directory.glob("*.toml")):
+        tree = load_dialogue(path)
+        trees[tree.npc_name] = tree
+    return trees

tests/test_dialogue.py

@@ -0,0 +1,269 @@
+"""Tests for dialogue tree data model and TOML loading."""
+
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from mudlib.dialogue import (
+    load_all_dialogues,
+    load_dialogue,
+)
+
+
+def test_load_dialogue_from_toml():
+    """Load a dialogue tree from TOML with correct structure."""
+    toml_content = """
+npc_name = "guard"
+root_node = "greeting"
+
+[nodes.greeting]
+text = "Halt! State your business."
+
+[[nodes.greeting.choices]]
+text = "I'm just passing through."
+next_node = "passing"
+
+[[nodes.greeting.choices]]
+text = "I have a delivery."
+next_node = "delivery"
+
+[nodes.passing]
+text = "Move along then, quickly."
+
+[nodes.delivery]
+text = "Leave it by the gate."
+"""
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".toml", delete=False) as f:
+        f.write(toml_content)
+        f.flush()
+        path = Path(f.name)
+
+    try:
+        tree = load_dialogue(path)
+        assert tree.npc_name == "guard"
+        assert tree.root_node == "greeting"
+        assert len(tree.nodes) == 3
+
+        # Check greeting node
+        greeting = tree.nodes["greeting"]
+        assert greeting.id == "greeting"
+        assert greeting.text == "Halt! State your business."
+        assert len(greeting.choices) == 2
+        assert greeting.choices[0].text == "I'm just passing through."
+        assert greeting.choices[0].next_node == "passing"
+        assert greeting.choices[1].text == "I have a delivery."
+        assert greeting.choices[1].next_node == "delivery"
+
+        # Check terminal nodes
+        passing = tree.nodes["passing"]
+        assert passing.id == "passing"
+        assert passing.text == "Move along then, quickly."
+        assert len(passing.choices) == 0
+
+        delivery = tree.nodes["delivery"]
+        assert delivery.id == "delivery"
+        assert delivery.text == "Leave it by the gate."
+        assert len(delivery.choices) == 0
+    finally:
+        path.unlink()
+
+
+def test_node_traversal():
+    """Follow choices from root to next nodes."""
+    toml_content = """
+npc_name = "merchant"
+root_node = "greeting"
+
+[nodes.greeting]
+text = "Welcome to my shop!"
+
+[[nodes.greeting.choices]]
+text = "What do you sell?"
+next_node = "wares"
+
+[nodes.wares]
+text = "Potions and scrolls, finest in town."
+"""
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".toml", delete=False) as f:
+        f.write(toml_content)
+        f.flush()
+        path = Path(f.name)
+
+    try:
+        tree = load_dialogue(path)
+        current = tree.nodes[tree.root_node]
+        assert current.text == "Welcome to my shop!"
+
+        # Follow the first choice
+        next_id = current.choices[0].next_node
+        current = tree.nodes[next_id]
+        assert current.text == "Potions and scrolls, finest in town."
+    finally:
+        path.unlink()
+
+
+def test_terminal_node_ends_conversation():
+    """Nodes with no choices are terminal."""
+    toml_content = """
+npc_name = "beggar"
+root_node = "plea"
+
+[nodes.plea]
+text = "Spare a coin?"
+
+[[nodes.plea.choices]]
+text = "Here you go."
+next_node = "thanks"
+
+[nodes.thanks]
+text = "Bless you, kind soul."
+"""
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".toml", delete=False) as f:
+        f.write(toml_content)
+        f.flush()
+        path = Path(f.name)
+
+    try:
+        tree = load_dialogue(path)
+        thanks = tree.nodes["thanks"]
+        assert len(thanks.choices) == 0  # Terminal node
+    finally:
+        path.unlink()
+
+
+def test_choice_with_condition():
+    """Choices can have optional conditions."""
+    toml_content = """
+npc_name = "gatekeeper"
+root_node = "greeting"
+
+[nodes.greeting]
+text = "The gate is locked."
+
+[[nodes.greeting.choices]]
+text = "I have the key."
+next_node = "unlock"
+condition = "has_item:gate_key"
+
+[nodes.unlock]
+text = "Very well, pass through."
+"""
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".toml", delete=False) as f:
+        f.write(toml_content)
+        f.flush()
+        path = Path(f.name)
+
+    try:
+        tree = load_dialogue(path)
+        greeting = tree.nodes["greeting"]
+        choice = greeting.choices[0]
+        assert choice.condition == "has_item:gate_key"
+        assert choice.next_node == "unlock"
+    finally:
+        path.unlink()
+
+
+def test_node_with_action():
+    """Nodes can have optional actions."""
+    toml_content = """
+npc_name = "quest_giver"
+root_node = "offer"
+
+[nodes.offer]
+text = "Will you help me find my lost cat?"
+
+[[nodes.offer.choices]]
+text = "I'll do it."
+next_node = "accept"
+
+[nodes.accept]
+text = "Thank you! Here's a map."
+action = "give_item:cat_map"
+"""
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".toml", delete=False) as f:
+        f.write(toml_content)
+        f.flush()
+        path = Path(f.name)
+
+    try:
+        tree = load_dialogue(path)
+        accept = tree.nodes["accept"]
+        assert accept.action == "give_item:cat_map"
+    finally:
+        path.unlink()
+
+
+def test_load_all_dialogues():
+    """Load multiple dialogue files from a directory."""
+    guard_content = """
+npc_name = "guard"
+root_node = "greeting"
+
+[nodes.greeting]
+text = "Halt!"
+"""
+    merchant_content = """
+npc_name = "merchant"
+root_node = "greeting"
+
+[nodes.greeting]
+text = "Welcome!"
+"""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        tmppath = Path(tmpdir)
+        (tmppath / "guard.toml").write_text(guard_content)
+        (tmppath / "merchant.toml").write_text(merchant_content)
+
+        trees = load_all_dialogues(tmppath)
+        assert len(trees) == 2
+        assert "guard" in trees
+        assert "merchant" in trees
+        assert trees["guard"].npc_name == "guard"
+        assert trees["merchant"].npc_name == "merchant"
+
+
+def test_missing_root_node():
+    """Raise error if root_node doesn't exist in nodes."""
+    toml_content = """
+npc_name = "broken"
+root_node = "nonexistent"
+
+[nodes.greeting]
+text = "Hello."
+"""
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".toml", delete=False) as f:
+        f.write(toml_content)
+        f.flush()
+        path = Path(f.name)
+
+    try:
+        with pytest.raises(ValueError, match="root_node.*not found"):
+            load_dialogue(path)
+    finally:
+        path.unlink()
+
+
+def test_missing_next_node():
+    """Raise error if a choice references a non-existent node."""
+    toml_content = """
+npc_name = "broken"
+root_node = "greeting"
+
+[nodes.greeting]
+text = "Hello."
+
+[[nodes.greeting.choices]]
+text = "Goodbye."
+next_node = "nonexistent"
+"""
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".toml", delete=False) as f:
+        f.write(toml_content)
+        f.flush()
+        path = Path(f.name)
+
+    try:
+        with pytest.raises(ValueError, match="next_node.*not found"):
+            load_dialogue(path)
+    finally:
+        path.unlink()