import { Cursor } from "./cursor";
import { StrandPath } from "./path";

export class SelectionRange {
  readonly anchor: Cursor;
  readonly head: Cursor;

  constructor(anchor = new Cursor(), head = anchor.clone()) {
    this.anchor = anchor;
    this.head = head;
  }

  get commonStrandPath(): StrandPath {
    const minLength = Math.min(
      this.anchor.strandPath.length,
      this.head.strandPath.length
    );
    const commonPath: StrandPath = [];
    for (let i = 0; i < minLength; i++) {
      const anchorLevel = this.anchor.strandPath[i];
      const headLevel = this.head.strandPath[i];
      if (
        anchorLevel.tokenIndex === headLevel.tokenIndex &&
        anchorLevel.childIndex === headLevel.childIndex
      ) {
        commonPath.push(anchorLevel);
      } else {
        break;
      }
    }
    return commonPath;
  }

  get commonStartPos(): number {
    const anchorPos = this.projectCursorPosOnCommonStrand(this.anchor, "left");
    const headPos = this.projectCursorPosOnCommonStrand(this.head, "left");
    return Math.min(anchorPos, headPos);
  }

  get commonEndPos(): number {
    const anchorPos = this.projectCursorPosOnCommonStrand(this.anchor, "right");
    const headPos = this.projectCursorPosOnCommonStrand(this.head, "right");
    return Math.max(anchorPos, headPos);
  }

  private projectCursorPosOnCommonStrand(
    cursor: Cursor,
    bias: "left" | "right"
  ): number {
    const commonStrandPath = this.commonStrandPath;
    for (let i = 0; i < commonStrandPath.length; i++) {
      const { tokenIndex, childIndex } = cursor.strandPath[i];
      const commonLevel = commonStrandPath[i];
      if (
        tokenIndex !== commonLevel.tokenIndex ||
        childIndex !== commonLevel.childIndex
      ) {
        throw new Error("Cursor is not on or within the common strand");
      }
    }

    if (cursor.strandPath.length === commonStrandPath.length) {
      return cursor.pos;
    }

    const { tokenIndex } = cursor.strandPath[commonStrandPath.length];
    if (bias === "left") {
      return tokenIndex;
    } else {
      return tokenIndex + 1;
    }
  }

  isCollapsed(): boolean {
    return this.anchor.equals(this.head);
  }
}