fusion/packages/dashboard/app/hooks/useTasks.ts

import { useState, useEffect, useCallback, useRef } from "react";
import type { Task, Column, TaskCreateInput, MergeResult } from "@kb/core";
import * as api from "../api";

/**
 * Compare two ISO timestamp strings.
 * Returns positive if a is newer than b, negative if b is newer, 0 if equal.
 */
function compareTimestamps(a: string | undefined, b: string | undefined): number {
  if (!a && !b) return 0;
  if (!a) return -1; // b is newer if a has no timestamp
  if (!b) return 1;  // a is newer if b has no timestamp
  return a.localeCompare(b);
}

export function useTasks() {
  const [tasks, setTasks] = useState<Task[]>([]);
  const tasksRef = useRef(tasks);
  tasksRef.current = tasks;

  // Fetch initial tasks
  useEffect(() => {
    api.fetchTasks().then(setTasks).catch(() => setTasks([]));
  }, []);

  // SSE live updates
  useEffect(() => {
    const es = new EventSource("/api/events");

    es.addEventListener("task:created", (e) => {
      const task: Task = JSON.parse(e.data);
      setTasks((prev) => [...prev, task]);
    });

    es.addEventListener("task:moved", (e) => {
      // Payload: { task, from, to } - task object includes server-set columnMovedAt
      // We use 'to' as the authoritative column and trust the server's columnMovedAt
      const { task, to }: { task: Task; from: Column; to: Column } = JSON.parse(e.data);
      setTasks((prev) =>
        prev.map((t) =>
          t.id === task.id ? { ...task, column: to } : t
        )
      );
    });

    es.addEventListener("task:updated", (e) => {
      const incoming: Task = JSON.parse(e.data);
      setTasks((prev) =>
        prev.map((t) => {
          if (t.id !== incoming.id) return t;

          // Race condition prevention: If the incoming update has stale column data,
          // preserve the current column. A task:moved event always carries the
          // authoritative column in the 'to' field and updates columnMovedAt.
          // If current state has a newer columnMovedAt, reject the column change.
          const columnTimestampCompare = compareTimestamps(t.columnMovedAt, incoming.columnMovedAt);
          if (columnTimestampCompare > 0 && t.column !== incoming.column) {
            // Current state is newer - preserve column, merge other fields
            return { ...incoming, column: t.column, columnMovedAt: t.columnMovedAt };
          }

          // Edge case: current has columnMovedAt but incoming doesn't (legacy data)
          // Preserve the column information we have
          if (t.columnMovedAt && !incoming.columnMovedAt && t.column !== incoming.column) {
            return { ...incoming, column: t.column, columnMovedAt: t.columnMovedAt };
          }

          return incoming;
        })
      );
    });

    es.addEventListener("task:deleted", (e) => {
      const task: Task = JSON.parse(e.data);
      setTasks((prev) => prev.filter((t) => t.id !== task.id));
    });

    es.addEventListener("task:merged", (e) => {
      // Payload: { task, branch, merged, worktreeRemoved, branchDeleted, ... }
      // The task object has already been moved to 'done' by the server
      const { task }: { task: Task } = JSON.parse(e.data);
      setTasks((prev) =>
        prev.map((t) =>
          // Ensure column is 'done' since that's where merged tasks always go
          t.id === task.id ? { ...task, column: "done" as Column } : t
        )
      );
    });

    es.addEventListener("error", () => {
      setTimeout(() => {
        if (es.readyState === EventSource.CLOSED) {
          // Will reconnect via new effect cycle
        }
      }, 3000);
    });

    return () => es.close();
  }, []);

  const createTask = useCallback(async (input: TaskCreateInput): Promise<Task> => {
    return api.createTask(input);
  }, []);

  const moveTask = useCallback(async (id: string, column: Column): Promise<Task> => {
    return api.moveTask(id, column);
  }, []);

  const deleteTask = useCallback(async (id: string): Promise<Task> => {
    return api.deleteTask(id);
  }, []);

  const mergeTask = useCallback(async (id: string): Promise<MergeResult> => {
    return api.mergeTask(id);
  }, []);

  const retryTask = useCallback(async (id: string): Promise<Task> => {
    return api.retryTask(id);
  }, []);

  return { tasks, createTask, moveTask, deleteTask, mergeTask, retryTask };
}