core: generate mermaid syntax and render visual graph (langchain-ai#1…

…9599)

- **Description:** Add functionality to generate Mermaid syntax and
render flowcharts from graph data. This includes support for custom node
colors and edge curve styles, as well as the ability to export the
generated graphs to PNG images using either the Mermaid.INK API or
Pyppeteer for local rendering.
- **Dependencies:** Optional dependencies are `pyppeteer` if rendering
wants to be done using Pypeteer and Javascript code.

---------

Co-authored-by: Angel Igareta <angel.igareta@klarna.com>
Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/core/langchain_core/runnables/graph.py

@@ -2,9 +2,11 @@
 
 import inspect
 from dataclasses import dataclass, field
+from enum import Enum
 from typing import (
     TYPE_CHECKING,
     Any,
+    Callable,
     Dict,
     List,
     NamedTuple,
@@ -51,6 +53,46 @@ class Node(NamedTuple):
     data: Union[Type[BaseModel], RunnableType]
 
 
+class Branch(NamedTuple):
+    """Branch in a graph."""
+
+    condition: Callable[..., str]
+    ends: Optional[dict[str, str]]
+
+
+class CurveStyle(Enum):
+    """Enum for different curve styles supported by Mermaid"""
+
+    BASIS = "basis"
+    BUMP_X = "bumpX"
+    BUMP_Y = "bumpY"
+    CARDINAL = "cardinal"
+    CATMULL_ROM = "catmullRom"
+    LINEAR = "linear"
+    MONOTONE_X = "monotoneX"
+    MONOTONE_Y = "monotoneY"
+    NATURAL = "natural"
+    STEP = "step"
+    STEP_AFTER = "stepAfter"
+    STEP_BEFORE = "stepBefore"
+
+
+@dataclass
+class NodeColors:
+    """Schema for Hexadecimal color codes for different node types"""
+
+    start: str = "#ffdfba"
+    end: str = "#baffc9"
+    other: str = "#fad7de"
+
+
+class MermaidDrawMethod(Enum):
+    """Enum for different draw methods supported by Mermaid"""
+
+    PYPPETEER = "pyppeteer"  # Uses Pyppeteer to render the graph
+    API = "api"  # Uses Mermaid.INK API to render the graph
+
+
 def node_data_str(node: Node) -> str:
     from langchain_core.runnables.base import Runnable
 
@@ -112,6 +154,7 @@ class Graph:
 
     nodes: Dict[str, Node] = field(default_factory=dict)
     edges: List[Edge] = field(default_factory=list)
+    branches: Optional[Dict[str, List[Branch]]] = field(default_factory=dict)
 
     def to_json(self) -> Dict[str, List[Dict[str, Any]]]:
         """Convert the graph to a JSON-serializable format."""
@@ -277,3 +320,59 @@ def draw_png(
                 edges=labels["edges"] if labels is not None else {},
             ),
         ).draw(self, output_file_path)
+
+    def draw_mermaid(
+        self,
+        curve_style: CurveStyle = CurveStyle.LINEAR,
+        node_colors: NodeColors = NodeColors(
+            start="#ffdfba", end="#baffc9", other="#fad7de"
+        ),
+        wrap_label_n_words: int = 9,
+    ) -> str:
+        from langchain_core.runnables.graph_mermaid import draw_mermaid
+
+        nodes = {node.id: node_data_str(node) for node in self.nodes.values()}
+
+        first_node = self.first_node()
+        first_label = node_data_str(first_node) if first_node is not None else None
+
+        last_node = self.last_node()
+        last_label = node_data_str(last_node) if last_node is not None else None
+
+        return draw_mermaid(
+            nodes=nodes,
+            edges=self.edges,
+            branches=self.branches,
+            first_node_label=first_label,
+            last_node_label=last_label,
+            curve_style=curve_style,
+            node_colors=node_colors,
+            wrap_label_n_words=wrap_label_n_words,
+        )
+
+    def draw_mermaid_png(
+        self,
+        curve_style: CurveStyle = CurveStyle.LINEAR,
+        node_colors: NodeColors = NodeColors(
+            start="#ffdfba", end="#baffc9", other="#fad7de"
+        ),
+        wrap_label_n_words: int = 9,
+        output_file_path: str = "graph.png",
+        draw_method: MermaidDrawMethod = MermaidDrawMethod.API,
+        background_color: str = "white",
+        padding: int = 10,
+    ) -> None:
+        from langchain_core.runnables.graph_mermaid import draw_mermaid_png
+
+        mermaid_syntax = self.draw_mermaid(
+            curve_style=curve_style,
+            node_colors=node_colors,
+            wrap_label_n_words=wrap_label_n_words,
+        )
+        draw_mermaid_png(
+            mermaid_syntax=mermaid_syntax,
+            output_file_path=output_file_path,
+            draw_method=draw_method,
+            background_color=background_color,
+            padding=padding,
+        )