feuerwerk/src/core/graph/emitGraph.ts

import { edgesInto, findNode, type Graph, type GraphNode } from './Graph';
import { nodeDef, type NodeDef, type PortDef } from './NodeRegistry';

export interface EmitRules<D extends NodeDef, T> {
  isDef: (def: NodeDef) => def is D;
  /** Names the node kind in "Not a … node". */
  noun: string;
  /** A node's outputs from its inputs; called once per node, in dependency order. */
  emit: (def: D, inputs: Record<string, T>, node: GraphNode) => Record<string, T>;
  /** An input without an edge: the typed-in value or what the node says it means unfed. */
  unfed: (node: GraphNode, port: PortDef, def: D | undefined) => T | null;
  /** An edge's value on its way into `port`; null (after reporting a problem) when it cannot go in. */
  conform?: (node: GraphNode, port: PortDef, value: T) => T | null;
}

export interface GraphEmitter<T> {
  /** Per node id, the first thing that keeps the graph from compiling. */
  readonly problems: Record<string, string>;
  problem(node: GraphNode, text: string): void;
  /** What feeds an input, emitting whatever it depends on first. */
  input(node: GraphNode, port: PortDef): T | null;
  /** Whether the walk reached the node. */
  reached(node: GraphNode): boolean;
  /** Reports property names that are invalid (`invalid` returns the text) or taken twice. */
  checkNames(
    nodes: GraphNode[],
    nameOf: (node: GraphNode) => string,
    invalid: (name: string) => string,
  ): void;
  /** The problem of the first node in graph order, or null when there is none. */
  firstProblem(): string | null;
}

/**
 * The walk shared by the graph compilers: every node reachable from where `input` is first asked
 * is emitted once, before anything that reads it, and a node feeding itself is a problem.
 */
export function graphEmitter<D extends NodeDef, T>(graph: Graph, rules: EmitRules<D, T>): GraphEmitter<T> {
  const problems: Record<string, string> = {};
  const emitted = new Map<string, Record<string, T>>();
  const visiting = new Set<string>();

  const problem = (node: GraphNode, text: string) => {
    if (!(node.id in problems)) problems[node.id] = text;
  };

  const emitNode = (node: GraphNode): Record<string, T> | null => {
    const done = emitted.get(node.id);
    if (done) return done;
    if (visiting.has(node.id)) {
      problem(node, 'Feeds itself');
      return null;
    }
    const def = nodeDef(node.type);
    if (!def || !rules.isDef(def)) {
      problem(node, `Not a ${rules.noun} node`);
      return null;
    }
    visiting.add(node.id);
    const inputs: Record<string, T> = {};
    let complete = true;
    for (const port of def.inputs) {
      const value = input(node, port);
      if (value) inputs[port.key] = value;
      else complete = false;
    }
    visiting.delete(node.id);
    if (!complete) return null;
    const outputs = rules.emit(def, inputs, node);
    emitted.set(node.id, outputs);
    return outputs;
  };

  const input = (node: GraphNode, port: PortDef): T | null => {
    const edge = edgesInto(graph, node.id, port.key)[0];
    if (edge) {
      const source = findNode(graph, edge.from.node);
      const value = (source && emitNode(source))?.[edge.from.port];
      if (!value) return null;
      return rules.conform ? rules.conform(node, port, value) : value;
    }
    const def = nodeDef(node.type);
    return rules.unfed(node, port, def && rules.isDef(def) ? def : undefined);
  };

  return {
    problems,
    problem,
    input,
    reached: (node) => emitted.has(node.id),
    checkNames(nodes, nameOf, invalid) {
      const names = new Set<string>();
      for (const node of nodes) {
        const name = nameOf(node);
        const text = invalid(name);
        if (text) problem(node, text);
        else if (names.has(name)) problem(node, `Another property is named "${name}"`);
        names.add(name);
      }
    },
    firstProblem() {
      const first = graph.nodes.find((n) => n.id in problems);
      return first ? problems[first.id]! : null;
    },
  };
}