excalidraw/src/zindex.ts

import { AppState } from "./types";
import { ExcalidrawElement } from "./element/types";
import { getElementsInGroup } from "./groups";
import { findLastIndex, findIndex } from "./utils";

/**
 * Returns indices of elements to move based on selected elements.
 * Includes contiguous deleted elements that are between two selected elements,
 *  e.g.: [0 (selected), 1 (deleted), 2 (deleted), 3 (selected)]
 */
const getIndicesToMove = (
  elements: readonly ExcalidrawElement[],
  appState: AppState,
) => {
  let selectedIndices: number[] = [];
  let deletedIndices: number[] = [];
  let includeDeletedIndex = null;
  let i = -1;
  while (++i < elements.length) {
    if (appState.selectedElementIds[elements[i].id]) {
      if (deletedIndices.length) {
        selectedIndices = selectedIndices.concat(deletedIndices);
        deletedIndices = [];
      }
      selectedIndices.push(i);
      includeDeletedIndex = i + 1;
    } else if (elements[i].isDeleted && includeDeletedIndex === i) {
      includeDeletedIndex = i + 1;
      deletedIndices.push(i);
    } else {
      deletedIndices = [];
    }
  }
  return selectedIndices;
};

const toContiguousGroups = (array: number[]) => {
  let cursor = 0;
  return array.reduce((acc, value, index) => {
    if (index > 0 && array[index - 1] !== value - 1) {
      cursor = ++cursor;
    }
    (acc[cursor] || (acc[cursor] = [])).push(value);
    return acc;
  }, [] as number[][]);
};

/**
 * Returns next candidate index that's available to be moved to. Currently that
 *  is a non-deleted element, and not inside a group (unless we're editing it).
 */
const getTargetIndex = (
  appState: AppState,
  elements: ExcalidrawElement[],
  boundaryIndex: number,
  direction: "left" | "right",
) => {
  const sourceElement = elements[boundaryIndex];

  const indexFilter = (element: ExcalidrawElement) => {
    if (element.isDeleted) {
      return false;
    }
    // if we're editing group, find closest sibling irrespective of whether
    // there's a different-group element between them (for legacy reasons)
    if (appState.editingGroupId) {
      return element.groupIds.includes(appState.editingGroupId);
    }
    return true;
  };

  const candidateIndex =
    direction === "left"
      ? findLastIndex(elements, indexFilter, Math.max(0, boundaryIndex - 1))
      : findIndex(elements, indexFilter, boundaryIndex + 1);

  const nextElement = elements[candidateIndex];

  if (!nextElement) {
    return -1;
  }

  if (appState.editingGroupId) {
    if (
      // candidate element is a sibling in current editing group → return
      sourceElement?.groupIds.join("") === nextElement?.groupIds.join("")
    ) {
      return candidateIndex;
    } else if (!nextElement?.groupIds.includes(appState.editingGroupId)) {
      // candidate element is outside current editing group → prevent
      return -1;
    }
  }

  if (!nextElement.groupIds.length) {
    return candidateIndex;
  }

  const siblingGroupId = appState.editingGroupId
    ? nextElement.groupIds[
        nextElement.groupIds.indexOf(appState.editingGroupId) - 1
      ]
    : nextElement.groupIds[nextElement.groupIds.length - 1];

  const elementsInSiblingGroup = getElementsInGroup(elements, siblingGroupId);

  if (elementsInSiblingGroup.length) {
    // assumes getElementsInGroup() returned elements are sorted
    // by zIndex (ascending)
    return direction === "left"
      ? elements.indexOf(elementsInSiblingGroup[0])
      : elements.indexOf(
          elementsInSiblingGroup[elementsInSiblingGroup.length - 1],
        );
  }

  return candidateIndex;
};

const shiftElements = (
  appState: AppState,
  elements: ExcalidrawElement[],
  direction: "left" | "right",
) => {
  const indicesToMove = getIndicesToMove(elements, appState);
  let groupedIndices = toContiguousGroups(indicesToMove);

  if (direction === "right") {
    groupedIndices = groupedIndices.reverse();
  }

  groupedIndices.forEach((indices, i) => {
    const leadingIndex = indices[0];
    const trailingIndex = indices[indices.length - 1];
    const boundaryIndex = direction === "left" ? leadingIndex : trailingIndex;

    const targetIndex = getTargetIndex(
      appState,
      elements,
      boundaryIndex,
      direction,
    );

    if (targetIndex === -1 || boundaryIndex === targetIndex) {
      return;
    }

    const leadingElements =
      direction === "left"
        ? elements.slice(0, targetIndex)
        : elements.slice(0, leadingIndex);
    const targetElements = elements.slice(leadingIndex, trailingIndex + 1);
    const displacedElements =
      direction === "left"
        ? elements.slice(targetIndex, leadingIndex)
        : elements.slice(trailingIndex + 1, targetIndex + 1);
    const trailingElements =
      direction === "left"
        ? elements.slice(trailingIndex + 1)
        : elements.slice(targetIndex + 1);

    elements =
      direction === "left"
        ? [
            ...leadingElements,
            ...targetElements,
            ...displacedElements,
            ...trailingElements,
          ]
        : [
            ...leadingElements,
            ...displacedElements,
            ...targetElements,
            ...trailingElements,
          ];
  });

  return elements;
};

const shiftElementsToEnd = (
  elements: readonly ExcalidrawElement[],
  appState: AppState,
  direction: "left" | "right",
) => {
  const indicesToMove = getIndicesToMove(elements, appState);
  const targetElements: ExcalidrawElement[] = [];
  const displacedElements: ExcalidrawElement[] = [];

  let leadingIndex, trailingIndex;
  if (direction === "left") {
    if (appState.editingGroupId) {
      const groupElements = getElementsInGroup(
        elements,
        appState.editingGroupId,
      );
      if (!groupElements.length) {
        return elements;
      }
      leadingIndex = elements.indexOf(groupElements[0]);
    } else {
      leadingIndex = 0;
    }

    trailingIndex = indicesToMove[indicesToMove.length - 1];
  } else {
    if (appState.editingGroupId) {
      const groupElements = getElementsInGroup(
        elements,
        appState.editingGroupId,
      );
      if (!groupElements.length) {
        return elements;
      }
      trailingIndex = elements.indexOf(groupElements[groupElements.length - 1]);
    } else {
      trailingIndex = elements.length - 1;
    }

    leadingIndex = indicesToMove[0];
  }

  for (let index = leadingIndex; index < trailingIndex + 1; index++) {
    if (indicesToMove.includes(index)) {
      targetElements.push(elements[index]);
    } else {
      displacedElements.push(elements[index]);
    }
  }

  const leadingElements = elements.slice(0, leadingIndex);
  const trailingElements = elements.slice(trailingIndex + 1);

  return direction === "left"
    ? [
        ...leadingElements,
        ...targetElements,
        ...displacedElements,
        ...trailingElements,
      ]
    : [
        ...leadingElements,
        ...displacedElements,
        ...targetElements,
        ...trailingElements,
      ];
};

// public API
// -----------------------------------------------------------------------------

export const moveOneLeft = (
  elements: readonly ExcalidrawElement[],
  appState: AppState,
) => {
  return shiftElements(appState, elements.slice(), "left");
};

export const moveOneRight = (
  elements: readonly ExcalidrawElement[],
  appState: AppState,
) => {
  return shiftElements(appState, elements.slice(), "right");
};

export const moveAllLeft = (
  elements: readonly ExcalidrawElement[],
  appState: AppState,
) => {
  return shiftElementsToEnd(elements, appState, "left");
};

export const moveAllRight = (
  elements: readonly ExcalidrawElement[],
  appState: AppState,
) => {
  return shiftElementsToEnd(elements, appState, "right");
};