caret / packages / core / src / cursor.ts

import type { EditorState } from "./editorState";
import {
  StrandPath,
  strandPathsEqual,
  strandPathIsAncestor,
  TokenPath,
} from "./path";

export type RawCursor = {
  strandPath: StrandPath;
  pos: number;
};

export class Cursor {
  strandPath: StrandPath;
  pos: number;

  constructor(strandPath: StrandPath = [], pos: number = 0) {
    this.strandPath = strandPath;
    this.pos = pos;
  }

  equals(other: Cursor): boolean {
    return (
      this.pos === other.pos &&
      strandPathsEqual(this.strandPath, other.strandPath)
    );
  }

  toRaw(): RawCursor {
    return {
      strandPath: this.strandPath,
      pos: this.pos,
    };
  }

  static fromRaw(raw: RawCursor): Cursor {
    return new Cursor(raw.strandPath, raw.pos);
  }

  clone(): Cursor {
    return new Cursor(
      this.strandPath.map((level) => ({ ...level })),
      this.pos
    );
  }

  moveRight(editorState: EditorState) {
    let currentStrand = editorState.content;
    for (const level of this.strandPath) {
      const token = currentStrand.tokens[level.tokenIndex];
      currentStrand = token.children[level.childIndex];
    }

    // If the next token has children, move into its first child strand
    const nextToken = currentStrand.tokens[this.pos];
    if (nextToken && nextToken.children.length > 0) {
      this.strandPath.push({
        tokenIndex: this.pos,
        childIndex: 0,
      });
      this.pos = 0;
      return;
    }

    if (this.pos < currentStrand.tokens.length) {
      this.pos += 1;
      return;
    }

    // We are at the end of the current strand. Try to move into a sibling strand or up to the parent.
    let parentStrand = editorState.content;
    for (let i = 0; i < this.strandPath.length - 1; i++) {
      const level = this.strandPath[i];
      const token = parentStrand.tokens[level.tokenIndex];
      parentStrand = token.children[level.childIndex];
    }

    const lastStrandPathPart = this.strandPath[this.strandPath.length - 1];
    if (!lastStrandPathPart) {
      // Already at the top-level strand; cannot move right
      return;
    }

    const { tokenIndex, childIndex } = lastStrandPathPart;
    const parentToken = parentStrand.tokens[tokenIndex];
    const parentChildIndex = childIndex;

    if (parentChildIndex + 1 < parentToken.children.length) {
      // Move to the next sibling child strand
      this.strandPath[this.strandPath.length - 1].childIndex += 1;
      this.pos = 0;
      return;
    } else {
      // Move up to the parent strand
      this.strandPath.pop();
      this.pos = tokenIndex + 1; // Move to the position after the parent token
      return;
    }
  }

  moveLeft(editorState: EditorState) {
    let currentStrand = editorState.content;
    for (const level of this.strandPath) {
      const token = currentStrand.tokens[level.tokenIndex];
      currentStrand = token.children[level.childIndex];
    }

    // If the previous token has children, move into its last child strand
    if (this.pos > 0) {
      const prevToken = currentStrand.tokens[this.pos - 1];
      if (prevToken && prevToken.children.length > 0) {
        this.strandPath.push({
          tokenIndex: this.pos - 1,
          childIndex: prevToken.children.length - 1,
        });
        const lastChildStrand =
          prevToken.children[prevToken.children.length - 1];
        this.pos = lastChildStrand.tokens.length;
        return;
      }
    }

    if (this.pos > 0) {
      this.pos -= 1;
      return;
    }

    // We are at the start of the current strand. Try to move into a sibling strand or up to the parent.
    let parentStrand = editorState.content;
    for (let i = 0; i < this.strandPath.length - 1; i++) {
      const level = this.strandPath[i];
      const token = parentStrand.tokens[level.tokenIndex];
      parentStrand = token.children[level.childIndex];
    }

    const lastStrandPathPart = this.strandPath[this.strandPath.length - 1];
    if (!lastStrandPathPart) {
      // Already at the top-level strand; cannot move left
      return;
    }

    const { tokenIndex, childIndex } = lastStrandPathPart;
    const parentToken = parentStrand.tokens[tokenIndex];
    const parentChildIndex = childIndex;

    if (parentChildIndex > 0) {
      // Move to the previous sibling child strand
      this.strandPath[this.strandPath.length - 1].childIndex -= 1;
      const siblingStrand =
        parentToken.children[
          this.strandPath[this.strandPath.length - 1].childIndex
        ];
      this.pos = siblingStrand.tokens.length;
      return;
    } else {
      // Move up to the parent strand
      this.strandPath.pop();
      this.pos = tokenIndex; // Move to the position before the parent token
      return;
    }
  }

  /**
   * Adjust the cursor to account for an insertion at a given location.
   * @param insertCursor The location where the insertion occurred (referencing a cursor in the old document before insertion).
   * @param insertCount The number of tokens inserted.
   * @returns void The cursor is updated in place.
   */
  updateAfterInsertion(insertCursor: Cursor, insertCount: number) {
    if (strandPathsEqual(this.strandPath, insertCursor.strandPath)) {
      if (insertCursor.pos <= this.pos) {
        this.pos += insertCount;
      }
      return;
    }

    if (strandPathIsAncestor(insertCursor.strandPath, this.strandPath)) {
      const relativeLevel = this.strandPath[insertCursor.strandPath.length];
      if (relativeLevel.tokenIndex >= insertCursor.pos) {
        relativeLevel.tokenIndex += insertCount;
      }
      return;
    }
  }

  /**
   * Adjust the cursor to account for a token deletion at a given path.
   * @param deletedTokenPath The path of the token that was deleted.
   * @returns void The cursor is updated in place.
   */
  updateAfterTokenDeletion(deletedTokenPath: TokenPath) {
    if (strandPathsEqual(this.strandPath, deletedTokenPath.strandPath)) {
      if (deletedTokenPath.tokenIndex < this.pos) {
        this.pos -= 1;
      }
      return;
    }

    if (strandPathIsAncestor(deletedTokenPath.strandPath, this.strandPath)) {
      const relativeLevel = this.strandPath[deletedTokenPath.strandPath.length];
      if (relativeLevel.tokenIndex > deletedTokenPath.tokenIndex) {
        relativeLevel.tokenIndex -= 1;
      } else if (relativeLevel.tokenIndex === deletedTokenPath.tokenIndex) {
        this.strandPath = this.strandPath.slice(
          0,
          deletedTokenPath.strandPath.length
        );
        this.pos = deletedTokenPath.tokenIndex;
      }
      return;
    }
  }
}