Template-Engine/src/character_stream.mjs

import { readFile } from 'node:fs/promises';
import { basename } from 'node:path';

export class CharacterStream {
  constructor() {
    this.reset();
  }

  reset() {
    this._data = '';
    this.fileName = '';
    this._pos = 0;
    this.line = 0;
    this.column = 0;
  }

  async loadFile(fileName) {
    this.reset();
    this.fileName = basename(fileName);
    this._data = await readFile(fileName, 'utf-8');
  }

  peek(length = 1) {
    return this._data.substring(this._pos, this._pos + length);
  }

  peekWhile(predicate) {
    let buffer = '';
    for (
      let offset = 0, c = this._data[this._pos + offset];
      c !== undefined && predicate(c);
      offset++, c = this._data[this._pos + offset]
    ) {
      buffer += c;
    }
    return buffer;
  }

  // Peek first character after any characters that match predicate
  peekAfter(predicate) {
    const skip = this.peekWhile(predicate);
    return this.peek(skip.length + 1).substring(skip.length);
  }

  isNext(str) {
    return this.peek(str.length) === str;
  }

  next(length = 1) {
    const chunk = this.peek(length);

    this._pos += chunk.length;

    this.line += this._countOccurrences(chunk, '\n');

    const lastNewLine = chunk.lastIndexOf('\n');
    if (lastNewLine === -1) {
      this.column += chunk.length;
    } else {
      this.column = chunk.length - lastNewLine - 1;
    }

    return chunk;
  }

  nextWhile(predicate) {
    return this.next(this.peekWhile(predicate).length);
  }

  _countOccurrences = (str, substr) => {
    let count = 0;
    let i = 0;
    while(true) {
      i = str.indexOf(substr, i);
      if (i === -1) break;
      i += substr.length;
      count++;
    }
    return count;
  }

  eof() {
    return this.peek() === '';
  }
}