JavaScript immutable two-dimensional array of arrays

immutable-2d-array.js

class ArrayView {
  constructor(array, x, y, width, height) {
    this.array = array;
    this.x = x;
    this.y = y;
    // Automatic bounding
    // x = 30, width of backing array = 50, width of this = 30
    // width = 50 - 30 = 20
    let max_width = array.width - x;
    let max_height = array.width - y;
    /*if(max_width != width || max_height != height) {
      console.log(`Automatically bounding array slice to ${max_width} x ${max_height}`);
    }*/
    this.width = Math.min(max_width, width);
    this.height = Math.min(max_height, height);

    this.rows = (function*() {
      for(let y = 0; y != this.height; y++) {
        const idx = ((this.y + y) * this.array.height) + this.x;
        const tmp = this.array.data.slice(idx, idx + this.width);
        yield [y, tmp];
      }
    }).bind(this);
  }

  get(x, y) {
    return this.array.data[this.y + y][this.x + x];
  }
}

class Immutable2DArray {
  constructor(copy_or_width, height) {
    if(typeof copy_or_width == 'number') {
      this.width = copy_or_width;
      this.height = height;
      this.data = Array(height * copy_or_width);
    } else {
      this.width = copy_or_width.width;
      this.height = copy_or_width.height;

      this.data = copy_or_width.data.slice();
      for(let i = 0; i != this.data.length; i++) {
        if(Array.isArray(this.data[i])) {
          this.data[i] = this.data[i].slice();
        }
      }
    }
  }

  _index(x, y) {
    const idx = (y * this.height) + x;
    if(idx > this.data.length) {
      throw new Error(`Attempt to access ${x},${y} at array of dimensions ${this.width}x${this.height}`);
    }
    return idx;
  }

  get(x, y) {
    const idx = this._index(x, y);
    return this.data[idx];
  }

  occupied(x, y) {
    const val = this.get(x, y);
    return val != null && val.length > 0;
  }

  add(x, y, value) {
    const copy = new Immutable2DArray(this);
    const idx = this._index(x, y);
    if(copy.data[idx]) {
      copy.data[idx] = this.data[idx].slice().push(value);
    } else {
      copy.data[idx] = [value];
    }
    return copy;
  }

  // Mutate in place, for efficiency's sake
  mutateRemove(x, y, value) {
    const idx = this._index(x, y);
    this.data[idx] = this.data[idx].filter((item) => item != value);
    if(this.data[idx].length == 0) this.data[idx] = undefined;
  }

  mutateAdd(x, y, value) {
    const idx = this._index(x, y);
    if(this.data[idx]) {
      this.data[idx].push(value);
    } else {
      this.data[idx] = [value];
    }
    return this;
  }

  view(x, y, width, height) {
    return new ArrayView(this, x, y, width, height);
  }
}