import { Subject } from 'rxjs';

declare const ngDevMode: object | null;

/**
 * Class to be used to power selecting one or more options from a list.
 */
export class SelectionModel<T> {
  /** Currently-selected values. */
  private _selection = new Set<T>();

  /** Keeps track of the deselected options that haven't been emitted by the change event. */
  private _deselectedToEmit: T[] = [];

  /** Keeps track of the selected options that haven't been emitted by the change event. */
  private _selectedToEmit: T[] = [];

  /** Cache for the array value of the selected items. */
  private _selected: T[] | null = null;

  /** Selected values. */
  get selected(): T[] {
    if (!this._selected) {
      this._selected = Array.from(this._selection.values());
    }

    return this._selected;
  }

  /** Event emitted when the value has changed. */
  readonly changed = new Subject<SelectionChange<T>>();

  constructor(
    private _multiple = false,
    initiallySelectedValues?: T[],
    private _emitChanges = true,
    public compareWith?: (o1: T, o2: T) => boolean,
  ) {
    if (initiallySelectedValues && initiallySelectedValues.length) {
      if (_multiple) {
        initiallySelectedValues.forEach(value => this._markSelected(value));
      } else {
        this._markSelected(initiallySelectedValues[0]);
      }

      // Clear the array in order to avoid firing the change event for preselected values.
      this._selectedToEmit.length = 0;
    }
  }

  /**
   * Selects a value or an array of values.
   * @param values The values to select
   * @return Whether the selection changed as a result of this call
   * @breaking-change 16.0.0 make return type boolean
   */
  select(...values: T[]): boolean | void {
    this._verifyValueAssignment(values);
    values.forEach(value => this._markSelected(value));
    const changed = this._hasQueuedChanges();
    this._emitChangeEvent();
    return changed;
  }

  /**
   * Deselects a value or an array of values.
   * @param values The values to deselect
   * @return Whether the selection changed as a result of this call
   * @breaking-change 16.0.0 make return type boolean
   */
  deselect(...values: T[]): boolean | void {
    this._verifyValueAssignment(values);
    values.forEach(value => this._unmarkSelected(value));
    const changed = this._hasQueuedChanges();
    this._emitChangeEvent();
    return changed;
  }

  /**
   * Sets the selected values
   * @param values The new selected values
   * @return Whether the selection changed as a result of this call
   * @breaking-change 16.0.0 make return type boolean
   */
  setSelection(...values: T[]): boolean | void {
    this._verifyValueAssignment(values);
    const oldValues = this.selected;
    const newSelectedSet = new Set(values);
    values.forEach(value => this._markSelected(value));
    oldValues
      .filter(value => !newSelectedSet.has(value))
      .forEach(value => this._unmarkSelected(value));
    const changed = this._hasQueuedChanges();
    this._emitChangeEvent();
    return changed;
  }

  /**
   * Toggles a value between selected and deselected.
   * @param value The value to toggle
   * @return Whether the selection changed as a result of this call
   * @breaking-change 16.0.0 make return type boolean
   */
  toggle(value: T): boolean | void {
    return this.isSelected(value) ? this.deselect(value) : this.select(value);
  }

  /**
   * Clears all of the selected values.
   * @param flushEvent Whether to flush the changes in an event.
   *   If false, the changes to the selection will be flushed along with the next event.
   * @return Whether the selection changed as a result of this call
   * @breaking-change 16.0.0 make return type boolean
   */
  clear(flushEvent = true): boolean | void {
    this._unmarkAll();
    const changed = this._hasQueuedChanges();
    if (flushEvent) {
      this._emitChangeEvent();
    }
    return changed;
  }

  /**
   * Determines whether a value is selected.
   */
  isSelected(value: T): boolean {
    if (this.compareWith) {
      for (const otherValue of this._selection) {
        if (this.compareWith(otherValue, value)) {
          return true;
        }
      }
      return false;
    }
    return this._selection.has(value);
  }

  /**
   * Determines whether the model does not have a value.
   */
  isEmpty(): boolean {
    return this._selection.size === 0;
  }

  /**
   * Determines whether the model has a value.
   */
  hasValue(): boolean {
    return !this.isEmpty();
  }

  /**
   * Sorts the selected values based on a predicate function.
   */
  sort(predicate?: (a: T, b: T) => number): void {
    if (this._multiple && this.selected) {
      this._selected!.sort(predicate);
    }
  }

  /**
   * Gets whether multiple values can be selected.
   */
  isMultipleSelection() {
    return this._multiple;
  }

  /** Emits a change event and clears the records of selected and deselected values. */
  private _emitChangeEvent() {
    // Clear the selected values so they can be re-cached.
    this._selected = null;

    if (this._selectedToEmit.length || this._deselectedToEmit.length) {
      this.changed.next({
        source: this,
        added: this._selectedToEmit,
        removed: this._deselectedToEmit,
      });

      this._deselectedToEmit = [];
      this._selectedToEmit = [];
    }
  }

  /** Selects a value. */
  private _markSelected(value: T) {
    if (!this.isSelected(value)) {
      if (!this._multiple) {
        this._unmarkAll();
      }

      if (!this.isSelected(value)) {
        this._selection.add(value);
      }

      if (this._emitChanges) {
        this._selectedToEmit.push(value);
      }
    }
  }

  /** Deselects a value. */
  private _unmarkSelected(value: T) {
    if (this.isSelected(value)) {
      this._selection.delete(value);

      if (this._emitChanges) {
        this._deselectedToEmit.push(value);
      }
    }
  }

  /** Clears out the selected values. */
  private _unmarkAll() {
    if (!this.isEmpty()) {
      this._selection.forEach(value => this._unmarkSelected(value));
    }
  }

  /**
   * Verifies the value assignment and throws an error if the specified value array is
   * including multiple values while the selection model is not supporting multiple values.
   */
  private _verifyValueAssignment(values: T[]) {
    if (values.length > 1 && !this._multiple && (typeof ngDevMode === 'undefined' || ngDevMode)) {
      throw getMultipleValuesInSingleSelectionError();
    }
  }

  /** Whether there are queued up change to be emitted. */
  private _hasQueuedChanges() {
    return !!(this._deselectedToEmit.length || this._selectedToEmit.length);
  }
}

/**
 * Event emitted when the value of a MatSelectionModel has changed.
 * @docs-private
 */
export interface SelectionChange<T> {
  /** Model that dispatched the event. */
  source: SelectionModel<T>;
  /** Options that were added to the model. */
  added: T[];
  /** Options that were removed from the model. */
  removed: T[];
}

/**
 * Returns an error that reports that multiple values are passed into a selection model
 * with a single value.
 * @docs-private
 */
export function getMultipleValuesInSingleSelectionError() {
  return Error('Cannot pass multiple values into SelectionModel with single-value mode.');
}