// Copyright (c) 2015, Google Inc. Please see the AUTHORS file for details.
// All rights reserved. Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

part of vector_math_geometry;

/// Generate vertex normals based on a list of [positions] and [indices].
/// [normals] is assumed to be zeroed out, and much match [positions] in length.
/// [indices] is assumed to represent a triangle list.
void generateNormals(
    Vector3List normals, Vector3List positions, Uint16List indices) {
  final Vector3 p0 = new Vector3.zero(),
      p1 = new Vector3.zero(),
      p2 = new Vector3.zero(),
      norm = new Vector3.zero();

  // Loop through every polygon, find it's normal, and add that to the vertex
  // normals.
  for (int i = 0; i < indices.length; i += 3) {
    final int i0 = indices[i], i1 = indices[i + 1], i2 = indices[i + 2];
    positions..load(i0, p0)..load(i1, p1)..load(i2, p2);

    p1.sub(p0);
    p2.sub(p0);

    // Store the normalized cross product of p1 and p2 in p0.
    p1.crossInto(p2, p0).normalize();

    // Add the face normal to each vertex normal.
    normals.load(i0, norm);
    normals[i0] = norm..add(p0);

    normals.load(i1, norm);
    normals[i1] = norm..add(p0);

    normals.load(i2, norm);
    normals[i2] = norm..add(p0);
  }

  // Loop through all the normals and normalize them.
  for (int i = 0; i < normals.length; ++i) {
    normals.load(i, norm);
    normals[i] = norm..normalize();
  }
}

/// Generate vertex tangents based on a list of [positions], [normals],
/// [texCoords] and [indices].
/// [tangents] is assumed to be zeroed out, and much match [positions],
/// [normals], and [texCoords] in length.
/// [indices] is assumed to represent a triangle list.
/// Tangents are returned as Vector4s. The X, Y, and Z component represent
/// the tangent and the W component represents the direction of the bitangent
/// which can be generated as:
/// vec4 bitangent = cross(normal, tangent.xyz) * tangent.w;
/// Derived from the granddaddy of all tangent generation functions:
/// http://www.terathon.com/code/tangent.html
void generateTangents(Vector4List tangents, Vector3List positions,
    Vector3List normals, Vector2List texCoords, Uint16List indices) {
  final Vector3 p0 = new Vector3.zero(),
      p1 = new Vector3.zero(),
      p2 = new Vector3.zero(),
      n = new Vector3.zero(),
      t = new Vector3.zero(),
      udir = new Vector3.zero(),
      vdir = new Vector3.zero();

  final Vector2 uv0 = new Vector2.zero(),
      uv1 = new Vector2.zero(),
      uv2 = new Vector2.zero();

  final Vector4 tan = new Vector4.zero();

  final Vector3List tan0 = new Vector3List(positions.length),
      tan1 = new Vector3List(positions.length);

  for (int i = 0; i < indices.length; i += 3) {
    final int i0 = indices[i], i1 = indices[i + 1], i2 = indices[i + 2];
    positions..load(i0, p0)..load(i1, p1)..load(i2, p2);

    texCoords..load(i0, uv0)..load(i1, uv1)..load(i2, uv2);

    p1.sub(p0);
    p2.sub(p0);

    uv1.sub(uv0);
    uv2.sub(uv0);

    final double r = 1.0 / (uv1.x * uv2.y - uv2.x * uv1.y);

    udir.setValues((uv2.y * p1.x - uv1.y * p2.x) * r,
        (uv2.y * p1.y - uv1.y * p2.y) * r, (uv2.y * p1.z - uv1.y * p2.z) * r);
    vdir.setValues((uv1.x * p2.x - uv2.x * p1.x) * r,
        (uv1.x * p2.y - uv2.x * p1.y) * r, (uv1.x * p2.z - uv2.x * p1.z) * r);

    tan0.load(i0, p0);
    tan0[i0] = p0..add(udir);
    tan0.load(i1, p0);
    tan0[i1] = p0..add(udir);
    tan0.load(i2, p0);
    tan0[i2] = p0..add(udir);

    tan1.load(i0, p0);
    tan1[i0] = p0..add(vdir);
    tan1.load(i1, p0);
    tan1[i1] = p0..add(vdir);
    tan1.load(i2, p0);
    tan1[i2] = p0..add(vdir);
  }

  for (int i = 0; i < tangents.length; ++i) {
    normals.load(i, n);
    tan0.load(i, t);

    p1
      ..setFrom(n)
      ..scale(n.dot(t));
    p0
      ..setFrom(t)
      ..sub(p1)
      ..normalize();

    tan1.load(i, p1);
    n.crossInto(t, p2);
    final double sign = (p2.dot(p1) < 0.0) ? -1.0 : 1.0;

    tangents.load(i, tan);
    tangents[i] = tan..setValues(p0.x, p0.y, p0.z, sign);
  }
}
