src/lib/agent/TaskGraph.ts

/**
 * TaskGraph.ts — Directed Acyclic Graph for agent task orchestration (FASE 4)
 *
 * Replaces the monolithic prompt with:
 *   Planner → Task Graph → Executors → Verifier → Memory Update
 *
 * Supports:
 * - Dependency-based execution ordering (topological sort)
 * - Parallel execution of independent nodes
 * - Node status tracking (pending → ready → running → done/failed/skipped)
 */

export type NodeType =
  | "plan"       // Meta-node representing the overall goal
  | "tool_call"  // Execute a specific tool
  | "verify"     // Verify a tool result
  | "synthesize" // Aggregate results into final answer
  | "memory";    // Persist findings to memory

export type NodeStatus =
  | "pending"   // Waiting for deps
  | "ready"     // All deps done, can execute
  | "running"   // Currently executing
  | "done"      // Completed successfully
  | "failed"    // Execution failed
  | "skipped";  // Skipped due to upstream failure

export interface TaskNode {
  id: string;
  type: NodeType;
  label: string;
  tool?: string;
  input?: Record<string, unknown>;
  deps: string[];      // IDs of nodes that must complete before this
  status: NodeStatus;
  result?: string;
  error?: string;
  startedAt?: number;
  finishedAt?: number;
  priority?: number;   // Fix 7 (S421): posizione nel piano strutturato (basso = prima)
}

export interface TaskEdge {
  from: string;
  to: string;
}

export interface TaskGraph {
  id: string;
  goal: string;
  nodes: Map<string, TaskNode>;
  edges: TaskEdge[];
  createdAt: number;
}

// ─── Graph builder ────────────────────────────────────────────────────────────

let _nodeCounter = 0;
function nodeId(prefix = "n"): string {
  return `${prefix}-${Date.now()}-${++_nodeCounter}`;
}

export interface GraphSpec {
  goal: string;
  steps: Array<{
    tool: string;
    label: string;
    input?: Record<string, unknown>;
    dependsOn?: number[]; // 0-based indices of earlier steps
    verify?: boolean;
    priority?: number;    // Fix 7 (S421): posizione nel piano strutturato (basso = prima)
  }>;
  synthesize?: boolean;
  memorize?: boolean;
}

export function buildGraph(spec: GraphSpec): TaskGraph {
  const graph: TaskGraph = {
    id: nodeId("graph"),
    goal: spec.goal,
    nodes: new Map(),
    edges: [],
    createdAt: Date.now(),
  };

  // Plan node (root)
  const planNode: TaskNode = {
    id: "plan-root",
    type: "plan",
    label: `Goal: ${spec.goal.slice(0, 80)}`,
    deps: [],
    status: "done", // plan is already done (we are the planner)
    result: spec.goal,
  };
  graph.nodes.set(planNode.id, planNode);

  // Tool call nodes
  const stepIds: string[] = [];
  for (const [i, step] of spec.steps.entries()) {
    const id = nodeId("tool");
    stepIds.push(id);

    const deps = step.dependsOn
      ? step.dependsOn.map(idx => stepIds[idx]).filter(Boolean)
      : i === 0
        ? ["plan-root"]
        : [stepIds[i - 1]];

    const node: TaskNode = {
      id,
      type: "tool_call",
      label: step.label,
      tool: step.tool,
      input: step.input ?? {},
      deps,
      status: "pending",
      priority: step.priority,  // Fix 7 (S421): propaga dal piano strutturato
    };
    graph.nodes.set(id, node);

    for (const dep of deps) {
      graph.edges.push({ from: dep, to: id });
    }

    // Optional verify node
    if (step.verify) {
      const vid = nodeId("verify");
      const vnode: TaskNode = {
        id: vid,
        type: "verify",
        label: `Verifica: ${step.label}`,
        tool: step.tool,
        deps: [id],
        status: "pending",
      };
      graph.nodes.set(vid, vnode);
      graph.edges.push({ from: id, to: vid });
      // Replace the tool step id in stepIds so subsequent deps point to the verify node
      stepIds[i] = vid;
    }
  }

  // Memory node (optional)
  let lastLayerIds = stepIds.slice(-3);
  if (spec.memorize) {
    const mid = nodeId("mem");
    const mnode: TaskNode = {
      id: mid,
      type: "memory",
      label: "Memorizza risultati",
      deps: lastLayerIds,
      status: "pending",
    };
    graph.nodes.set(mid, mnode);
    for (const dep of lastLayerIds) {
      graph.edges.push({ from: dep, to: mid });
    }
    lastLayerIds = [mid];
  }

  // Synthesize node (always last)
  if (spec.synthesize !== false) {
    const sid = nodeId("synth");
    const snode: TaskNode = {
      id: sid,
      type: "synthesize",
      label: "Sintetizza e rispondi",
      deps: lastLayerIds,
      status: "pending",
    };
    graph.nodes.set(sid, snode);
    for (const dep of lastLayerIds) {
      graph.edges.push({ from: dep, to: sid });
    }
  }

  return graph;
}

// ─── Graph utilities ──────────────────────────────────────────────────────────

/** Returns nodes in parallel layers (each layer can execute concurrently) */
export function topologicalLayers(graph: TaskGraph): string[][] {
  const inDegree = new Map<string, number>();
  const adjList = new Map<string, string[]>();

  for (const node of graph.nodes.values()) {
    if (!inDegree.has(node.id)) inDegree.set(node.id, 0);
    adjList.set(node.id, []);
  }
  for (const edge of graph.edges) {
    adjList.get(edge.from)?.push(edge.to);
    inDegree.set(edge.to, (inDegree.get(edge.to) ?? 0) + 1);
  }

  const layers: string[][] = [];
  let current = [...inDegree.entries()].filter(([, d]) => d === 0).map(([id]) => id);

  while (current.length > 0) {
    layers.push(current);
    const next: string[] = [];
    for (const id of current) {
      for (const child of adjList.get(id) ?? []) {
        const deg = (inDegree.get(child) ?? 1) - 1;
        inDegree.set(child, deg);
        if (deg === 0) next.push(child);
      }
    }
    current = next;
  }

  return layers;
}

/** Returns all nodes whose deps are done and status is still pending */
export function getReadyNodes(graph: TaskGraph): TaskNode[] {
  const ready: TaskNode[] = [];
  for (const node of graph.nodes.values()) {
    if (node.status !== "pending") continue;
    const depsOk = node.deps.every(dep => {
      const d = graph.nodes.get(dep);
      return d?.status === "done" || d?.status === "skipped";
    });
    if (depsOk) ready.push(node);
  }
  return ready;
}

/** Mark a node and propagate skip to dependents if it failed */
export function markNode(graph: TaskGraph, id: string, status: NodeStatus, result?: string, error?: string) {
  const node = graph.nodes.get(id);
  if (!node) return;
  node.status = status;
  if (result !== undefined) node.result = result;
  if (error !== undefined) node.error = error;
  node.finishedAt = Date.now();

  if (status === "failed") {
    // Skip all transitive dependents
    const skip = (nid: string) => {
      for (const n of graph.nodes.values()) {
        if (n.deps.includes(nid) && (n.status === "pending" || n.status === "ready")) {
          n.status = "skipped";
          n.error = `Saltato: dipendenza "${nid}" fallita`;
          skip(n.id);
        }
      }
    };
    skip(id);
  }
}

export function graphDone(graph: TaskGraph): boolean {
  return [...graph.nodes.values()].every(
    n => n.status === "done" || n.status === "failed" || n.status === "skipped",
  );
}

export function collectResults(graph: TaskGraph): string {
  const results: string[] = [];
  for (const node of graph.nodes.values()) {
    if (node.type === "tool_call" && node.result) {
      results.push(`[${node.label}]\n${node.result}`);
    }
  }
  return results.join("\n\n---\n\n");
}