// Flexible task card actions.
//
// The main scheduling engine moves tasks through time. This file models the
// small user actions that appear on a flexible task card, such as done, push,
// backlog, and break-up. Keeping these in a service makes UI button handlers
// thin and keeps task-type safety checks in one place.

import 'models.dart';
import 'scheduling_engine.dart';

/// Quick actions available from a flexible task card.
///
/// These are the low-friction card controls the UI can expose directly on a
/// planned flexible task. The service below translates each button into either a
/// direct task update, a scheduling operation, or a follow-up flow.
enum FlexibleTaskQuickAction {
  /// Mark the task completed.
  done,

  /// Ask the user where the task should be pushed.
  push,

  /// Move the task out of today's plan and into backlog.
  backlog,

  /// Start a flow that splits the task into child tasks.
  breakUp,
}

/// Explicit push destinations shown after choosing the push quick action.
///
/// Push starts as a simple quick action, but the actual destination requires one
/// more choice. Keeping destinations as a separate enum prevents the initial card
/// action list from becoming too crowded.
enum PushDestination {
  /// Move the task later within the current planning window.
  nextAvailableSlot,

  /// Move the task to the beginning of the supplied tomorrow/future window.
  tomorrowTopOfQueue,

  /// Remove the task from the active timeline and store it for later.
  backlog,
}

/// Domain result for a flexible task quick action.
///
/// This result deliberately supports three outcomes: the task changed, the user
/// must choose a push destination, or the UI should start the child-task flow.
/// That keeps card code from guessing how to interpret each action.
class FlexibleTaskActionResult {
  const FlexibleTaskActionResult({
    required this.action,
    required this.task,
    this.pushDestinations = const <PushDestination>[],
    this.startsChildTaskFlow = false,
  });

  /// Action the user selected.
  final FlexibleTaskQuickAction action;

  /// Current or updated task, depending on the action.
  final Task task;

  /// Destination choices to show after `push`; empty for direct actions.
  final List<PushDestination> pushDestinations;

  /// Whether the UI should open a child-task creation flow.
  final bool startsChildTaskFlow;

  /// True when the action directly produced an updated [task].
  bool get changedTask => !startsChildTaskFlow && pushDestinations.isEmpty;
}

/// Result from applying a selected push destination.
///
/// The selected destination is included next to the [SchedulingResult] so UI and
/// tests can distinguish "moved later today" from "moved to tomorrow" even if
/// the low-level scheduling change shape is similar.
class PushDestinationResult {
  const PushDestinationResult({
    required this.destination,
    required this.schedulingResult,
  });

  /// Destination that was applied.
  final PushDestination destination;

  /// Full scheduler output: updated tasks, notices, changes, and overlaps.
  final SchedulingResult schedulingResult;

  /// Convenience flag for UI copy or persistence behavior that cares about the
  /// tomorrow queue specifically.
  bool get placesAtTomorrowTopOfQueue {
    return destination == PushDestination.tomorrowTopOfQueue;
  }
}

/// Applies low-friction quick actions for flexible task cards.
///
/// This service is the adapter between small UI button presses and domain logic.
/// It intentionally only accepts flexible tasks; required/locked/surprise items
/// should have their own action rules so the UI cannot accidentally apply a
/// flexible-only behavior to a fixed commitment.
class FlexibleTaskActionService {
  const FlexibleTaskActionService({
    this.schedulingEngine = const SchedulingEngine(),
  });

  /// Scheduling dependency used for actions that need timeline changes.
  final SchedulingEngine schedulingEngine;

  /// Apply the first-stage quick action.
  ///
  /// Direct actions (`done`, `backlog`) return a changed task. `push` returns the
  /// list of destinations the UI should present. `breakUp` signals that the UI
  /// should start a child-task flow rather than changing the task immediately.
  FlexibleTaskActionResult apply({
    required Task task,
    required FlexibleTaskQuickAction action,
    DateTime? updatedAt,
  }) {
    if (!task.isFlexible) {
      throw ArgumentError.value(
          task.type, 'task.type', 'Task must be flexible.');
    }

    switch (action) {
      case FlexibleTaskQuickAction.done:
        return FlexibleTaskActionResult(
          action: action,
          task: task.copyWith(
            status: TaskStatus.completed,
            updatedAt: updatedAt ?? DateTime.now(),
          ),
        );
      case FlexibleTaskQuickAction.push:
        return FlexibleTaskActionResult(
          action: action,
          task: task,
          pushDestinations: const [
            PushDestination.nextAvailableSlot,
            PushDestination.tomorrowTopOfQueue,
            PushDestination.backlog,
          ],
        );
      case FlexibleTaskQuickAction.backlog:
        return FlexibleTaskActionResult(
          action: action,
          task: schedulingEngine.moveToBacklog(task, updatedAt: updatedAt),
        );
      case FlexibleTaskQuickAction.breakUp:
        return FlexibleTaskActionResult(
          action: action,
          task: task,
          startsChildTaskFlow: true,
        );
    }
  }

  /// Apply the second-stage destination selected after the `push` action.
  ///
  /// This needs the full [SchedulingInput] because pushing can shift other
  /// flexible tasks and must avoid locked/required intervals.
  PushDestinationResult applyPushDestination({
    required PushDestination destination,
    required SchedulingInput input,
    required String taskId,
    DateTime? updatedAt,
  }) {
    final result = switch (destination) {
      PushDestination.nextAvailableSlot =>
        schedulingEngine.pushFlexibleTaskToNextAvailableSlot(
          input: input,
          taskId: taskId,
          updatedAt: updatedAt,
        ),
      PushDestination.tomorrowTopOfQueue =>
        schedulingEngine.pushFlexibleTaskToTomorrowTopOfQueue(
          input: input,
          taskId: taskId,
          updatedAt: updatedAt,
        ),
      PushDestination.backlog => _moveTaskToBacklog(
          input: input,
          taskId: taskId,
          updatedAt: updatedAt,
        ),
    };

    return PushDestinationResult(
      destination: destination,
      schedulingResult: result,
    );
  }

  /// Move one planned flexible task to backlog inside a scheduling result.
  ///
  /// This mirrors the shape of other push destination results so callers can
  /// handle every destination through the same `SchedulingResult` interface.
  SchedulingResult _moveTaskToBacklog({
    required SchedulingInput input,
    required String taskId,
    DateTime? updatedAt,
  }) {
    final task = _taskById(input.tasks, taskId);
    if (task == null) {
      return SchedulingResult(
        tasks: input.tasks,
        notices: [
          SchedulingNotice(
            'Task was not found.',
            type: SchedulingNoticeType.noFit,
            taskId: taskId,
          ),
        ],
      );
    }

    if (!task.isFlexible || task.status != TaskStatus.planned) {
      return SchedulingResult(
        tasks: input.tasks,
        notices: [
          SchedulingNotice(
            'Only planned flexible tasks can be moved to backlog.',
            type: SchedulingNoticeType.noFit,
            taskId: task.id,
          ),
        ],
      );
    }

    final movedTask = schedulingEngine.moveToBacklog(
      task,
      updatedAt: updatedAt,
    );

    return SchedulingResult(
      tasks: List<Task>.unmodifiable(
        input.tasks.map((current) {
          if (current.id == task.id) {
            return movedTask;
          }

          return current;
        }),
      ),
      notices: [
        SchedulingNotice(
          'Flexible task moved to backlog.',
          type: SchedulingNoticeType.moved,
          taskId: task.id,
        ),
      ],
      changes: [
        SchedulingChange(
          taskId: task.id,
          previousStart: task.scheduledStart,
          previousEnd: task.scheduledEnd,
          nextStart: null,
          nextEnd: null,
        ),
      ],
    );
  }
}

/// Find one task by id in a list.
Task? _taskById(List<Task> tasks, String taskId) {
  for (final task in tasks) {
    if (task.id == taskId) {
      return task;
    }
  }

  return null;
}