import {GL} from '../constants';
import {getPrimitiveModeType} from '../primitives/modes';

function assert(condition: any, message?: string): void {
  if (!condition) {
    throw new Error(message);
  }
}

/**
 * Iteration info for making primitive iterator
 */
type Information = {
  attributes: object;
  type: number | void;
  i1: number;
  i2: number;
  i3: number;
  primitiveIndex?: object;
};
/**
 * Will iterate over each primitive, expanding (dereferencing) indices
 * @param indices
 * @param attributes
 * @param mode
 * @param start
 * @param end
 */
// eslint-disable-next-line complexity
export function* makePrimitiveIterator(
  indices?: any,
  attributes: object = {},
  mode?: number,
  start = 0,
  end?: number
): Iterable<{attributes: object; type: number; i1: number; i2: number; i3: number}> {
  // support indices being an object with a values array
  if (indices) {
    indices = indices.values || indices.value || indices;
  }

  // Autodeduce length from indices
  if (end === undefined) {
    end = indices ? indices.length : start;
  }

  // iteration info
  const info: Information = {
    attributes,
    type: getPrimitiveModeType(mode),
    i1: 0,
    i2: 0,
    i3: 0
  };

  let i = start;
  // @ts-ignore
  while (i < end) {
    switch (mode) {
      case GL.POINTS: // draw single points.
        info.i1 = i;
        i += 1;
        break;
      case GL.LINES: // draw lines. Each set of two vertices is treated as a separate line segment.
        info.i1 = i;
        info.i2 = i + 1;
        i += 2;
        break;
      case GL.LINE_STRIP: // draw lines. Each vertex connects to the one after it.
        info.i1 = i;
        info.i2 = i + 1;
        i += 1;
        break;
      case GL.LINE_LOOP: // draw a connected group of line segments from the first vertex to the last
        info.i1 = i;
        info.i2 = i + 1;
        i += 1;
        break;
      case GL.TRIANGLES: // draw triangles. Each set of three vertices creates a separate triangle.
        info.i1 = i;
        info.i2 = i + 1;
        info.i3 = i + 2;
        i += 3;
        break;
      case GL.TRIANGLE_STRIP: // draw a connected group of triangles.
        info.i1 = i;
        info.i2 = i + 1;
        i += 1;
        break;
      case GL.TRIANGLE_FAN: // draw a connected group of triangles.
        info.i1 = 1;
        info.i2 = i;
        info.i3 = i + 1;
        i += 1;
        break;

      default:
        assert(false);
    }

    // if indices are present, lookup actual vertices in indices
    if (indices) {
      if ('i1' in info) {
        info.i1 = indices[info.i1];
        info.i2 = indices[info.i2];
        info.i3 = indices[info.i3];
      }
    }
    // @ts-ignore
    yield info;
  }
}