Add streaming layout visitors and TextMetrics

src/SixLabors.Fonts/TextLayout.Visitors.cs

@@ -0,0 +1,107 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.Fonts;
+
+/// <content>
+/// Visitor types for streaming laid-out glyphs through <see cref="LayoutText{TVisitor}"/>.
+/// </content>
+internal static partial class TextLayout
+{
+    /// <summary>
+    /// Receives laid-out glyphs streamed from <see cref="LayoutText{TVisitor}(TextBox, TextOptions, ref TVisitor)"/>.
+    /// Implementations are value types so the generic dispatch is specialized by the JIT and no boxing or
+    /// delegate allocation is required.
+    /// </summary>
+    internal interface IGlyphLayoutVisitor
+    {
+        /// <summary>
+        /// Invoked once for each laid-out glyph in layout order.
+        /// </summary>
+        /// <param name="glyph">The laid-out glyph.</param>
+        public void Visit(in GlyphLayout glyph);
+    }
+
+    /// <summary>
+    /// Collects streamed glyphs into a <see cref="List{T}"/>.
+    /// </summary>
+    internal readonly struct GlyphLayoutCollector : IGlyphLayoutVisitor
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="GlyphLayoutCollector"/> struct.
+        /// </summary>
+        /// <param name="glyphs">The list to collect streamed glyphs into.</param>
+        public GlyphLayoutCollector(List<GlyphLayout> glyphs) => this.Glyphs = glyphs;
+
+        /// <summary>
+        /// Gets the accumulated glyphs.
+        /// </summary>
+        public List<GlyphLayout> Glyphs { get; }
+
+        /// <inheritdoc/>
+        public readonly void Visit(in GlyphLayout glyph) => this.Glyphs.Add(glyph);
+    }
+
+    /// <summary>
+    /// Accumulates the union of glyph ink bounds as glyphs are streamed, avoiding the allocation
+    /// of a <see cref="List{T}"/> and a second iteration pass.
+    /// </summary>
+    internal struct GlyphBoundsAccumulator : IGlyphLayoutVisitor
+    {
+        private readonly float dpi;
+        private float left;
+        private float top;
+        private float right;
+        private float bottom;
+        private bool any;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="GlyphBoundsAccumulator"/> struct.
+        /// </summary>
+        /// <param name="dpi">The device-independent pixels per unit for the containing <see cref="TextOptions"/>.</param>
+        public GlyphBoundsAccumulator(float dpi)
+        {
+            this.dpi = dpi;
+            this.left = float.MaxValue;
+            this.top = float.MaxValue;
+            this.right = float.MinValue;
+            this.bottom = float.MinValue;
+            this.any = false;
+        }
+
+        /// <inheritdoc/>
+        public void Visit(in GlyphLayout glyph)
+        {
+            FontRectangle box = glyph.BoundingBox(this.dpi);
+
+            if (box.Left < this.left)
+            {
+                this.left = box.Left;
+            }
+
+            if (box.Top < this.top)
+            {
+                this.top = box.Top;
+            }
+
+            if (box.Right > this.right)
+            {
+                this.right = box.Right;
+            }
+
+            if (box.Bottom > this.bottom)
+            {
+                this.bottom = box.Bottom;
+            }
+
+            this.any = true;
+        }
+
+        /// <summary>
+        /// Returns the accumulated ink bounds, or <see cref="FontRectangle.Empty"/> if no glyphs were visited.
+        /// </summary>
+        /// <returns>The union of the ink bounds of all visited glyphs.</returns>
+        public readonly FontRectangle Result()
+            => this.any ? FontRectangle.FromLTRB(this.left, this.top, this.right, this.bottom) : FontRectangle.Empty;
+    }
+}