Terrace/packages/js/src/document.ts

import type { Reader } from "./readers/reader.js";
import { createLineData, parseLine, type LineData } from "./parser.js";

// Represents a single node/line in a Terrace document
export class TerraceNode {
  private _lineData: LineData;
  private _content: string;
  private _lineNumber: number;
  private _document: TerraceDocument;

  constructor(
    lineData: LineData,
    content: string,
    lineNumber: number,
    document: TerraceDocument
  ) {
    this._lineData = { ...lineData }; // Copy to avoid mutations
    this._content = content;
    this._lineNumber = lineNumber;
    this._document = document;
  }

  // Current line properties (zero-allocation - just slice references)
  get head(): string {
    return this._content.slice(this._lineData.offsetHead, this._lineData.offsetTail);
  }

  get tail(): string {
    return this._content.slice(this._lineData.offsetTail + 1);
  }

  get content(): string {
    return this._content.slice(this._lineData.offsetHead);
  }

  get level(): number {
    return this._lineData.level;
  }

  get lineNumber(): number {
    return this._lineNumber;
  }

  // Convenience methods
  is(value: string): boolean {
    return this.head === value;
  }

  isEmpty(): boolean {
    return this._content.trim() === '';
  }

  // Content access with different indent handling
  raw(offset?: number): string {
    return this._content.slice(offset ?? 0);
  }

  // Navigation (streaming-compatible)
  async* children(): AsyncIterableIterator<TerraceNode> {
    const parentLevel = this.level;

    while (true) {
      const node = await this._document._getNextNode();
      if (node === null) break;

      // If we encounter a node at or below parent level, it's not a child
      if (node.level <= parentLevel) {
        this._document._pushBack(node);
        break;
      }

      // Yield any node that is deeper than the parent
      // This supports arbitrary nesting as per Terrace spec
      yield node;
    }
  }

  async* siblings(): AsyncIterableIterator<TerraceNode> {
    const currentLevel = this.level;

    while (true) {
      const node = await this._document._getNextNode();
      if (node === null) break;

      if (node.level < currentLevel) {
        this._document._pushBack(node);
        break;
      }

      if (node.level === currentLevel) {
        yield node;
      }
    }
  }
}

// Main document iterator
export class TerraceDocument {
  private _reader: Reader;
  private _indent: string;
  private _lineData: LineData;
  private _currentLineNumber: number;
  private _pushedBackNodes: TerraceNode[] = [];
  private _isExhausted: boolean = false;

  constructor(reader: Reader, indent: string = " ") {
    this._reader = reader;
    this._indent = indent;
    this._lineData = createLineData(indent);
    this._currentLineNumber = 0;
  }

  async*[Symbol.asyncIterator](): AsyncIterableIterator<TerraceNode> {
    while (true) {
      const node = await this._getNextNode();
      if (node === null) break;
      yield node;
    }
  }

  async _getNextNode(): Promise<TerraceNode | null> {
    // Check for pushed back nodes first (LIFO order)
    if (this._pushedBackNodes.length > 0) {
      return this._pushedBackNodes.pop()!;
    }

    // If we've exhausted the reader, return null
    if (this._isExhausted) {
      return null;
    }

    const line = await this._reader();
    if (line == null) {
      this._isExhausted = true;
      return null;
    }

    this._currentLineNumber++;
    parseLine(line, this._lineData);

    return new TerraceNode(
      this._lineData,
      line,
      this._currentLineNumber,
      this
    );
  }

  _pushBack(node: TerraceNode): void {
    this._pushedBackNodes.push(node);
  }

  // Utility methods for functional chaining
  async filter(predicate: (node: TerraceNode) => boolean): Promise<TerraceNode[]> {
    const results: TerraceNode[] = [];
    for await (const node of this) {
      if (predicate(node)) {
        results.push(node);
      }
    }
    return results;
  }

  async find(predicate: (node: TerraceNode) => boolean): Promise<TerraceNode | undefined> {
    for await (const node of this) {
      if (predicate(node)) {
        return node;
      }
    }
    return undefined;
  }

  async map<T>(mapper: (node: TerraceNode) => T | Promise<T>): Promise<T[]> {
    const results: T[] = [];
    for await (const node of this) {
      results.push(await mapper(node));
    }
    return results;
  }

  async toArray(): Promise<TerraceNode[]> {
    const results: TerraceNode[] = [];
    for await (const node of this) {
      results.push(node);
    }
    return results;
  }
}

// Legacy Document type for backwards compatibility references
export type Document = TerraceDocument;

/**
 * Creates a new Terrace document iterator
 *
 * @param {Reader} reader When called, resolves to a string containing the next line in the document
 * @param {String} indent The character used for indentation in the document. Only a single character is permitted
 * @returns {TerraceDocument} An iterable document that can be used with for-await-of loops
 */
export function useDocument(reader: Reader, indent: string = " "): TerraceDocument {
  return new TerraceDocument(reader, indent);
}