// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

using System;
using System.Numerics;
using System.Runtime.CompilerServices;
using SixLabors.Fonts;
using SixLabors.Fonts.Rendering;

namespace SixLabors.ImageSharp.Drawing.Text {
  /// <summary>
  /// A rendering surface that Fonts can use to generate shapes by following a path.
  /// Each glyph is positioned along the path and rotated to match the path tangent
  /// at the glyph's horizontal center.
  /// </summary>
  internal sealed class PathGlyphBuilder : GlyphBuilder
  {
      /// <summary>
      /// The path that glyphs are laid out along. Exposed as <see cref="IPathInternals"/>
      /// to access the <see cref="IPathInternals.PointAlongPath"/> method for efficient
      /// position + tangent queries.
      /// </summary>
      private readonly IPathInternals path;

      /// <summary>
      /// Initializes a new instance of the <see cref="PathGlyphBuilder"/> class.
      /// </summary>
      /// <param name="path">The path to render the glyphs along.</param>
      public PathGlyphBuilder(IPath path)
      {
          if (path is IPathInternals internals)
          {
              this.path = internals;
          }
          else
          {
              // Wrap in ComplexPolygon to gain IPathInternals.
              this.path = new ComplexPolygon(path);
          }
      }

      /// <inheritdoc/>
      protected override bool BeginGlyph(in FontRectangle bounds, in GlyphRendererParameters parameters)
      {
          // Translate + rotate the glyph to follow the path. Always returns true because
          // path-based glyphs are never cached (each has a unique per-position transform).
          this.TransformGlyph(in bounds);
          return true;
      }

      /// <summary>
      /// Computes the translation + rotation matrix that places a glyph along the path.
      /// The glyph's horizontal center is mapped to the path distance, and the glyph
      /// is rotated to match the path tangent at that point.
      /// </summary>
      /// <param name="bounds">The font-metric bounding rectangle of the glyph.</param>
      [MethodImpl(MethodImplOptions.AggressiveInlining)]
      private void TransformGlyph(in FontRectangle bounds)
      {
          // Query the path at the glyph's horizontal center.
          Vector2 half = new(bounds.Width * .5F, 0);
          SegmentInfo pathPoint = this.path.PointAlongPath(bounds.Left + half.X);

          // Translate so the glyph's top-left aligns with the path point,
          // then rotate around the path point to follow the tangent.
          Vector2 translation = (Vector2)pathPoint.Point - bounds.Location - half + new Vector2(0, bounds.Top);
          Matrix4x4 matrix = Matrix4x4.CreateTranslation(translation.X, translation.Y, 0) * new Matrix4x4(Matrix3x2.CreateRotation(pathPoint.Angle - MathF.PI, (Vector2)pathPoint.Point));

          this.Builder.SetTransform(matrix);
      }
  }
}