Add FluentGlyphRangeBuilder and AttachWindowsDefaultFont (#1745)

`FluentGlyphRangeBuilder` can build glyph ranges without having to
allocate native ImGui objects, and supports adding `ReadOnlySpan` and
`IEnumerable` of `char`s and `byte`s, `UnicodeRange`s, and known glyph
ranges for select cultures (CJK for now.)

Added `IFontAtlasBuildToolkitPreBuild.AttachWindowsDefaultFont` which
has a predefined list of default Windows fonts per language.

Dalamud/Interface/ManagedFontAtlas/FluentGlyphRangeBuilder.cs

@@ -0,0 +1,361 @@
+using System.Buffers;
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Globalization;
+using System.Runtime.CompilerServices;
+using System.Text;
+using System.Text.Unicode;
+
+using Dalamud.Interface.ManagedFontAtlas.Internals;
+
+namespace Dalamud.Interface.ManagedFontAtlas;
+
+/// <summary>A fluent ImGui glyph range builder.</summary>
+[SuppressMessage(
+    "StyleCop.CSharp.SpacingRules",
+    "SA1010:Opening square brackets should be spaced correctly",
+    Justification = "No")]
+public struct FluentGlyphRangeBuilder
+{
+    private const int ImUnicodeCodepointMax = char.MaxValue;
+
+    private BitArray? characters;
+
+    /// <summary>Clears the builder.</summary>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>A builder is in cleared state on first use.</remarks>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public FluentGlyphRangeBuilder Clear()
+    {
+        this.characters?.SetAll(false);
+        return this;
+    }
+
+    /// <summary>Adds a single codepoint to the builder.</summary>
+    /// <param name="codepoint">The codepoint to add.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public FluentGlyphRangeBuilder With(char codepoint) => this.With((int)codepoint);
+
+    /// <inheritdoc cref="With(char)"/>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public FluentGlyphRangeBuilder With(uint codepoint) =>
+        codepoint <= char.MaxValue ? this.With((int)codepoint) : this;
+
+    /// <inheritdoc cref="With(char)"/>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public FluentGlyphRangeBuilder With(int codepoint)
+    {
+        if (codepoint <= ImUnicodeCodepointMax)
+            this.EnsureCharacters().Set(codepoint, true);
+        return this;
+    }
+
+    /// <summary>Adds a unicode range to the builder.</summary>
+    /// <param name="range">The unicode range to add.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(UnicodeRange range) =>
+        this.With(range.FirstCodePoint, (range.FirstCodePoint + range.Length) - 1);
+
+    /// <summary>Adds unicode ranges to the builder.</summary>
+    /// <param name="range1">The 1st unicode range to add.</param>
+    /// <param name="range2">The 2st unicode range to add.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(UnicodeRange range1, UnicodeRange range2) =>
+        this.With(range1.FirstCodePoint, (range1.FirstCodePoint + range1.Length) - 1)
+            .With(range2.FirstCodePoint, (range2.FirstCodePoint + range2.Length) - 1);
+
+    /// <summary>Adds unicode ranges to the builder.</summary>
+    /// <param name="range1">The 1st unicode range to add.</param>
+    /// <param name="range2">The 2st unicode range to add.</param>
+    /// <param name="range3">The 3rd unicode range to add.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(UnicodeRange range1, UnicodeRange range2, UnicodeRange range3) =>
+        this.With(range1.FirstCodePoint, (range1.FirstCodePoint + range1.Length) - 1)
+            .With(range2.FirstCodePoint, (range2.FirstCodePoint + range2.Length) - 1)
+            .With(range3.FirstCodePoint, (range3.FirstCodePoint + range3.Length) - 1);
+
+    /// <summary>Adds unicode ranges to the builder.</summary>
+    /// <param name="range1">The 1st unicode range to add.</param>
+    /// <param name="range2">The 2st unicode range to add.</param>
+    /// <param name="range3">The 3rd unicode range to add.</param>
+    /// <param name="range4">The 4th unicode range to add.</param>
+    /// <param name="evenMoreRanges">Even more unicode ranges to add.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(
+        UnicodeRange range1,
+        UnicodeRange range2,
+        UnicodeRange range3,
+        UnicodeRange range4,
+        params UnicodeRange[] evenMoreRanges) =>
+        this.With(range1.FirstCodePoint, (range1.FirstCodePoint + range1.Length) - 1)
+            .With(range2.FirstCodePoint, (range2.FirstCodePoint + range2.Length) - 1)
+            .With(range3.FirstCodePoint, (range3.FirstCodePoint + range3.Length) - 1)
+            .With(range4.FirstCodePoint, (range4.FirstCodePoint + range4.Length) - 1)
+            .With(evenMoreRanges);
+
+    /// <summary>Adds unicode ranges to the builder.</summary>
+    /// <param name="ranges">Unicode ranges to add.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(IEnumerable<UnicodeRange> ranges)
+    {
+        foreach (var range in ranges)
+            this.With(range);
+        return this;
+    }
+
+    /// <summary>Adds a range of characters to the builder.</summary>
+    /// <param name="from">The first codepoint, inclusive.</param>
+    /// <param name="to">The last codepoint, inclusive.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>
+    /// <para>Unsupported codepoints will be ignored.</para>
+    /// <para>If <paramref name="from"/> is more than <paramref name="to"/>, then they will be swapped.</para>
+    /// </remarks>
+    public FluentGlyphRangeBuilder With(char from, char to) =>
+        this.With(Math.Clamp(from, int.MinValue, int.MaxValue), Math.Clamp(to, int.MinValue, int.MaxValue));
+
+    /// <inheritdoc cref="With(char,char)"/>
+    public FluentGlyphRangeBuilder With(uint from, uint to) =>
+        this.With((int)Math.Min(from, int.MaxValue), (int)Math.Min(to, int.MaxValue));
+
+    /// <inheritdoc cref="With(char,char)"/>
+    public FluentGlyphRangeBuilder With(int from, int to)
+    {
+        from = Math.Clamp(from, 1, ImUnicodeCodepointMax);
+        to = Math.Clamp(to, 1, ImUnicodeCodepointMax);
+        if (from > to)
+            (from, to) = (to, from);
+
+        var bits = this.EnsureCharacters();
+        for (; from <= to; from++)
+            bits.Set(from, true);
+        return this;
+    }
+
+    /// <summary>Adds characters from a UTF-8 character sequence.</summary>
+    /// <param name="utf8Sequence">The sequence.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(ReadOnlySpan<byte> utf8Sequence)
+    {
+        var bits = this.EnsureCharacters();
+        while (!utf8Sequence.IsEmpty)
+        {
+            if (Rune.DecodeFromUtf8(utf8Sequence, out var rune, out var len) == OperationStatus.Done
+                && rune.Value < ImUnicodeCodepointMax)
+                bits.Set(rune.Value, true);
+            utf8Sequence = utf8Sequence[len..];
+        }
+
+        return this;
+    }
+
+    /// <summary>Adds characters from a UTF-8 character sequence.</summary>
+    /// <param name="utf8Sequence">The sequence.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(IEnumerable<byte> utf8Sequence)
+    {
+        Span<byte> buf = stackalloc byte[4];
+        var bufp = 0;
+        var bits = this.EnsureCharacters();
+        foreach (var b in utf8Sequence)
+        {
+            buf[bufp++] = b;
+
+            while (Rune.DecodeFromUtf8(buf[..bufp], out var rune, out var len) is var state
+                   && state != OperationStatus.NeedMoreData)
+            {
+                switch (state)
+                {
+                    case OperationStatus.Done when rune.Value <= ImUnicodeCodepointMax:
+                        bits.Set(rune.Value, true);
+                        goto case OperationStatus.InvalidData;
+
+                    case OperationStatus.InvalidData:
+                        bufp -= len;
+                        break;
+
+                    case OperationStatus.NeedMoreData:
+                    case OperationStatus.DestinationTooSmall:
+                    default:
+                        throw new InvalidOperationException($"Unexpected return from {Rune.DecodeFromUtf8}.");
+                }
+            }
+        }
+
+        return this;
+    }
+
+    /// <summary>Adds characters from a UTF-16 character sequence.</summary>
+    /// <param name="utf16Sequence">The sequence.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(ReadOnlySpan<char> utf16Sequence)
+    {
+        var bits = this.EnsureCharacters();
+        while (!utf16Sequence.IsEmpty)
+        {
+            if (Rune.DecodeFromUtf16(utf16Sequence, out var rune, out var len) == OperationStatus.Done
+                && rune.Value <= ImUnicodeCodepointMax)
+                bits.Set(rune.Value, true);
+            utf16Sequence = utf16Sequence[len..];
+        }
+
+        return this;
+    }
+
+    /// <summary>Adds characters from a UTF-16 character sequence.</summary>
+    /// <param name="utf16Sequence">The sequence.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(IEnumerable<char> utf16Sequence)
+    {
+        var bits = this.EnsureCharacters();
+        foreach (var c in utf16Sequence)
+        {
+            if (!char.IsSurrogate(c))
+                bits.Set(c, true);
+        }
+
+        return this;
+    }
+
+    /// <summary>Adds characters from a string.</summary>
+    /// <param name="string">The string.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored.</remarks>
+    public FluentGlyphRangeBuilder With(string @string) => this.With(@string.AsSpan());
+
+    /// <summary>Adds glyphs that are likely to be used in the given culture to the builder.</summary>
+    /// <param name="cultureInfo">A culture info.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>Unsupported codepoints will be ignored. Unsupported culture will do nothing.
+    /// Do make a PR if you need more.</remarks>
+    public FluentGlyphRangeBuilder WithLanguage(CultureInfo cultureInfo)
+    {
+        // Call in chunks of three to avoid allocating arrays.
+        // Avoid adding ranges that goes over BMP; that is, ranges that goes over ImUnicodeCodepointMax.
+        switch (cultureInfo.TwoLetterISOLanguageName)
+        {
+            case "ja":
+                // http://www.rikai.com/library/kanjitables/kanji_codes.unicode.shtml
+                return
+                    this
+                        .With(
+                            UnicodeRanges.CjkSymbolsandPunctuation,
+                            UnicodeRanges.Hiragana,
+                            UnicodeRanges.Katakana)
+                        .With(
+                            UnicodeRanges.HalfwidthandFullwidthForms,
+                            UnicodeRanges.CjkUnifiedIdeographs,
+                            UnicodeRanges.CjkUnifiedIdeographsExtensionA)
+                        // Blame Japanese cell carriers for the below.
+                        .With(
+                            UnicodeRanges.EnclosedCjkLettersandMonths);
+            case "zh":
+                return
+                    this
+                        .With(
+                            UnicodeRanges.CjkUnifiedIdeographs,
+                            UnicodeRanges.CjkUnifiedIdeographsExtensionA);
+            case "ko":
+                return
+                    this
+                        .With(
+                            UnicodeRanges.HangulJamo,
+                            UnicodeRanges.HangulCompatibilityJamo,
+                            UnicodeRanges.HangulSyllables)
+                        .With(
+                            UnicodeRanges.HangulJamoExtendedA,
+                            UnicodeRanges.HangulJamoExtendedB);
+            default:
+                return this;
+        }
+    }
+
+    /// <summary>Adds glyphs that are likely to be used in the given culture to the builder.</summary>
+    /// <param name="languageTag">A language tag that will be used to locate the culture info.</param>
+    /// <returns><c>this</c> for method chaining.</returns>
+    /// <remarks>See <see cref="CultureInfo.GetCultureInfo(string)"/> documentation for supported language tags.
+    /// </remarks>
+    public FluentGlyphRangeBuilder WithLanguage(string languageTag) =>
+        this.WithLanguage(CultureInfo.GetCultureInfo(languageTag));
+
+    /// <summary>Builds the accumulated data into an ImGui glyph range.</summary>
+    /// <param name="addFallbackCodepoints">Whether to add the default fallback codepoints to the range.</param>
+    /// <param name="addEllipsisCodepoints">Whether to add the default ellipsis codepoints to the range.</param>
+    /// <returns>The built ImGui glyph ranges.</returns>
+    public ushort[] Build(bool addFallbackCodepoints = true, bool addEllipsisCodepoints = true)
+    {
+        if (addFallbackCodepoints)
+            this.With(FontAtlasFactory.FallbackCodepoints);
+        if (addEllipsisCodepoints)
+            this.With(FontAtlasFactory.EllipsisCodepoints).With('.');
+        return this.BuildExact();
+    }
+
+    /// <summary>Builds the accumulated data into an ImGui glyph range, exactly as specified.</summary>
+    /// <returns>The built ImGui glyph ranges.</returns>
+    public ushort[] BuildExact()
+    {
+        if (this.characters is null)
+            return [0];
+        var bits = this.characters;
+
+        // Count the number of ranges first.
+        var numRanges = 0;
+        var lastCodepoint = -1;
+        for (var i = 1; i <= ImUnicodeCodepointMax; i++)
+        {
+            if (bits.Get(i))
+            {
+                if (lastCodepoint == -1)
+                    lastCodepoint = i;
+            }
+            else
+            {
+                if (lastCodepoint != -1)
+                {
+                    numRanges++;
+                    lastCodepoint = -1;
+                }
+            }
+        }
+
+        // Handle the final range that terminates on the ending boundary.
+        if (lastCodepoint != -1)
+            numRanges++;
+
+        // Allocate the array and build the range.
+        var res = GC.AllocateUninitializedArray<ushort>((numRanges * 2) + 1);
+        var resp = 0;
+        for (var i = 1; i <= ImUnicodeCodepointMax; i++)
+        {
+            if (bits.Get(i) == ((resp & 1) == 0))
+                res[resp++] = unchecked((ushort)i);
+        }
+
+        // Handle the final range that terminates on the ending boundary.
+        if ((resp & 1) == 1)
+            res[resp++] = ImUnicodeCodepointMax;
+
+        // Add the zero terminator.
+        res[resp] = 0;
+
+        return res;
+    }
+
+    /// <summary>Ensures that <see cref="characters"/> is not null, by creating one as necessary.</summary>
+    /// <returns>An instance of <see cref="BitArray"/>.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private BitArray EnsureCharacters() => this.characters ??= new(ImUnicodeCodepointMax + 1);
+}

Dalamud/Interface/ManagedFontAtlas/FontAtlasBuildToolkitUtilities.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
+using System.Text.Unicode;
 
 using Dalamud.Interface.Utility;
 
@@ -12,6 +13,34 @@ namespace Dalamud.Interface.ManagedFontAtlas;
 /// </summary>
 public static class FontAtlasBuildToolkitUtilities
 {
+    /// <summary>Begins building a new array of <see cref="ushort"/> containing ImGui glyph ranges.</summary>
+    /// <param name="chars">The chars.</param>
+    /// <returns>A new range builder.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static FluentGlyphRangeBuilder BeginGlyphRange(this IEnumerable<char> chars) =>
+        default(FluentGlyphRangeBuilder).With(chars);
+
+    /// <summary>Begins building a new array of <see cref="ushort"/> containing ImGui glyph ranges.</summary>
+    /// <param name="chars">The chars.</param>
+    /// <returns>A new range builder.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static FluentGlyphRangeBuilder BeginGlyphRange(this ReadOnlySpan<char> chars) =>
+        default(FluentGlyphRangeBuilder).With(chars);
+
+    /// <summary>Begins building a new array of <see cref="ushort"/> containing ImGui glyph ranges.</summary>
+    /// <param name="chars">The chars.</param>
+    /// <returns>A new range builder.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static FluentGlyphRangeBuilder BeginGlyphRange(this string chars) =>
+        default(FluentGlyphRangeBuilder).With(chars);
+
+    /// <summary>Begins building a new array of <see cref="ushort"/> containing ImGui glyph ranges.</summary>
+    /// <param name="range">The unicode range.</param>
+    /// <returns>A new range builder.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static FluentGlyphRangeBuilder BeginGlyphRange(this UnicodeRange range) =>
+        default(FluentGlyphRangeBuilder).With(range);
+
     /// <summary>
     /// Compiles given <see cref="char"/>s into an array of <see cref="ushort"/> containing ImGui glyph ranges. 
     /// </summary>
@@ -19,16 +48,12 @@ public static class FontAtlasBuildToolkitUtilities
     /// <param name="addFallbackCodepoints">Add fallback codepoints to the range.</param>
     /// <param name="addEllipsisCodepoints">Add ellipsis codepoints to the range.</param>
     /// <returns>The compiled range.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static ushort[] ToGlyphRange(
         this IEnumerable<char> enumerable,
         bool addFallbackCodepoints = true,
-        bool addEllipsisCodepoints = true)
-    {
-        using var builderScoped = ImGuiHelpers.NewFontGlyphRangeBuilderPtrScoped(out var builder);
-        foreach (var c in enumerable)
-            builder.AddChar(c);
-        return builder.BuildRangesToArray(addFallbackCodepoints, addEllipsisCodepoints);
-    }
+        bool addEllipsisCodepoints = true) =>
+        enumerable.BeginGlyphRange().Build(addFallbackCodepoints, addEllipsisCodepoints);
 
     /// <summary>
     /// Compiles given <see cref="char"/>s into an array of <see cref="ushort"/> containing ImGui glyph ranges. 
@@ -37,16 +62,12 @@ public static class FontAtlasBuildToolkitUtilities
     /// <param name="addFallbackCodepoints">Add fallback codepoints to the range.</param>
     /// <param name="addEllipsisCodepoints">Add ellipsis codepoints to the range.</param>
     /// <returns>The compiled range.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static ushort[] ToGlyphRange(
         this ReadOnlySpan<char> span,
         bool addFallbackCodepoints = true,
-        bool addEllipsisCodepoints = true)
-    {
-        using var builderScoped = ImGuiHelpers.NewFontGlyphRangeBuilderPtrScoped(out var builder);
-        foreach (var c in span)
-            builder.AddChar(c);
-        return builder.BuildRangesToArray(addFallbackCodepoints, addEllipsisCodepoints);
-    }
+        bool addEllipsisCodepoints = true) =>
+        span.BeginGlyphRange().Build(addFallbackCodepoints, addEllipsisCodepoints);
 
     /// <summary>
     /// Compiles given string into an array of <see cref="ushort"/> containing ImGui glyph ranges. 
@@ -55,11 +76,12 @@ public static class FontAtlasBuildToolkitUtilities
     /// <param name="addFallbackCodepoints">Add fallback codepoints to the range.</param>
     /// <param name="addEllipsisCodepoints">Add ellipsis codepoints to the range.</param>
     /// <returns>The compiled range.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static ushort[] ToGlyphRange(
         this string @string,
         bool addFallbackCodepoints = true,
         bool addEllipsisCodepoints = true) =>
-        @string.AsSpan().ToGlyphRange(addFallbackCodepoints, addEllipsisCodepoints);
+        @string.BeginGlyphRange().Build(addFallbackCodepoints, addEllipsisCodepoints);
 
     /// <summary>
     /// Finds the corresponding <see cref="ImFontConfigPtr"/> in

Dalamud/Interface/ManagedFontAtlas/IFontAtlasBuildToolkitPreBuild.cs

@@ -1,4 +1,5 @@
-using System.IO;
+using System.Globalization;
+using System.IO;
 using System.Runtime.InteropServices;
 
 using Dalamud.Interface.FontIdentifier;
@@ -8,6 +9,8 @@ using Dalamud.Utility;
 
 using ImGuiNET;
 
+using TerraFX.Interop.DirectX;
+
 namespace Dalamud.Interface.ManagedFontAtlas;
 
 /// <summary>
@@ -216,10 +219,35 @@ public interface IFontAtlasBuildToolkitPreBuild : IFontAtlasBuildToolkit
     /// <returns>The added font.</returns>
     ImFontPtr AddGameGlyphs(GameFontStyle gameFontStyle, ushort[]? glyphRanges, ImFontPtr mergeFont);
 
+    /// <summary>Adds glyphs from the Windows default font for the given culture info into the provided font.</summary>
+    /// <param name="cultureInfo">The culture info.</param>
+    /// <param name="fontConfig">The font config. If <see cref="SafeFontConfig.MergeFont"/> is not set, then
+    /// <see cref="IFontAtlasBuildToolkit.Font"/> will be used as the target. If that is empty too, then it will do
+    /// nothing.</param>
+    /// <param name="weight">The font weight, in range from <c>1</c> to <c>1000</c>. <c>400</c> is regular(normal).
+    /// </param>
+    /// <param name="stretch">The font stretch, in range from <c>1</c> to <c>9</c>. <c>5</c> is medium(normal).
+    /// </param>
+    /// <param name="style">The font style, in range from <c>0</c> to <c>2</c>. <c>0</c> is normal.</param>
+    /// <remarks>
+    /// <para>May do nothing at all if <paramref name="cultureInfo"/> is unsupported by Dalamud font handler.</para> 
+    /// <para>See
+    /// <a href="https://learn.microsoft.com/en-us/windows/apps/design/globalizing/loc-international-fonts">Microsoft
+    /// Learn</a> for the fonts.</para>
+    /// </remarks>
+    void AttachWindowsDefaultFont(
+        CultureInfo cultureInfo,
+        in SafeFontConfig fontConfig,
+        int weight = (int)DWRITE_FONT_WEIGHT.DWRITE_FONT_WEIGHT_NORMAL,
+        int stretch = (int)DWRITE_FONT_STRETCH.DWRITE_FONT_STRETCH_NORMAL,
+        int style = (int)DWRITE_FONT_STYLE.DWRITE_FONT_STYLE_NORMAL);
+
     /// <summary>
     /// Adds glyphs of extra languages into the provided font, depending on Dalamud Configuration.<br />
     /// <see cref="SafeFontConfig.GlyphRanges"/> will be ignored.
     /// </summary>
-    /// <param name="fontConfig">The font config.</param>
+    /// <param name="fontConfig">The font config. If <see cref="SafeFontConfig.MergeFont"/> is not set, then
+    /// <see cref="IFontAtlasBuildToolkit.Font"/> will be used as the target. If that is empty too, then it will do
+    /// nothing.</param>
     void AttachExtraGlyphsForDalamudLanguage(in SafeFontConfig fontConfig);
 }

Dalamud/Interface/ManagedFontAtlas/Internals/FontAtlasFactory.BuildToolkit.cs

@@ -1,9 +1,9 @@
 using System.Buffers;
 using System.Collections.Generic;
+using System.Globalization;
 using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
-using System.Text.Unicode;
 
 using Dalamud.Configuration.Internal;
 using Dalamud.Interface.FontIdentifier;
@@ -17,6 +17,8 @@ using ImGuiNET;
 
 using SharpDX.DXGI;
 
+using TerraFX.Interop.DirectX;
+
 namespace Dalamud.Interface.ManagedFontAtlas.Internals;
 
 /// <summary>
@@ -433,9 +435,163 @@ internal sealed partial class FontAtlasFactory
         public ImFontPtr AddGameGlyphs(GameFontStyle gameFontStyle, ushort[]? glyphRanges, ImFontPtr mergeFont) =>
             this.gameFontHandleSubstance.AttachGameGlyphs(this, mergeFont, gameFontStyle, glyphRanges);
 
+        /// <inheritdoc/>
+        public void AttachWindowsDefaultFont(
+            CultureInfo cultureInfo,
+            in SafeFontConfig fontConfig,
+            int weight = (int)DWRITE_FONT_WEIGHT.DWRITE_FONT_WEIGHT_NORMAL,
+            int stretch = (int)DWRITE_FONT_STRETCH.DWRITE_FONT_STRETCH_NORMAL,
+            int style = (int)DWRITE_FONT_STYLE.DWRITE_FONT_STYLE_NORMAL)
+        {
+            var targetFont = fontConfig.MergeFont;
+            if (targetFont.IsNull())
+                targetFont = this.Font;
+            if (targetFont.IsNull())
+                return;
+
+            // https://learn.microsoft.com/en-us/windows/apps/design/globalizing/loc-international-fonts
+            var splitTag = cultureInfo.IetfLanguageTag.Split("-");
+            foreach (var test in new[]
+                     {
+                         cultureInfo.IetfLanguageTag,
+                         $"{splitTag[0]}-{splitTag[^1]}",
+                     })
+            {
+                var familyName = test switch
+                {
+                    "af-ZA" => "Segoe UI",
+                    "am-ET" => "Ebrima",
+                    "ar-SA" => "Segoe UI",
+                    "as-IN" => "Nirmala UI",
+                    "az-Latn-AZ" => "Segoe UI",
+                    "be-BY" => "Segoe UI",
+                    "bg-BG" => "Segoe UI",
+                    "bn-BD" => "Nirmala UI",
+                    "bn-IN" => "Nirmala UI",
+                    "bs-Latn-BA" => "Segoe UI",
+                    "ca-ES" => "Segoe UI",
+                    "ca-ES-valencia" => "Segoe UI",
+                    "chr-CHER-US" => "Gadugi",
+                    "cs-CZ" => "Segoe UI",
+                    "cy-GB" => "Segoe UI",
+                    "da-DK" => "Segoe UI",
+                    "de-DE" => "Segoe UI",
+                    "el-GR" => "Segoe UI",
+                    "en-GB" => "Segoe UI",
+                    "es-ES" => "Segoe UI",
+                    "et-EE" => "Segoe UI",
+                    "eu-ES" => "Segoe UI",
+                    "fa-IR" => "Segoe UI",
+                    "fi-FI" => "Segoe UI",
+                    "fil-PH" => "Segoe UI",
+                    "fr-FR" => "Segoe UI",
+                    "ga-IE" => "Segoe UI",
+                    "gd-GB" => "Segoe UI",
+                    "gl-ES" => "Segoe UI",
+                    "gu-IN" => "Nirmala UI",
+                    "ha-Latn-NG" => "Segoe UI",
+                    "he-IL" => "Segoe UI",
+                    "hi-IN" => "Nirmala UI",
+                    "hr-HR" => "Segoe UI",
+                    "hu-HU" => "Segoe UI",
+                    "hy-AM" => "Segoe UI",
+                    "id-ID" => "Segoe UI",
+                    "ig-NG" => "Segoe UI",
+                    "is-IS" => "Segoe UI",
+                    "it-IT" => "Segoe UI",
+                    "ja-JP" => "Yu Gothic UI",
+                    "ka-GE" => "Segoe UI",
+                    "kk-KZ" => "Segoe UI",
+                    "km-KH" => "Leelawadee UI",
+                    "kn-IN" => "Nirmala UI",
+                    "ko-KR" => "Malgun Gothic",
+                    "kok-IN" => "Nirmala UI",
+                    "ku-ARAB-IQ" => "Segoe UI",
+                    "ky-KG" => "Segoe UI",
+                    "lb-LU" => "Segoe UI",
+                    "lt-LT" => "Segoe UI",
+                    "lv-LV" => "Segoe UI",
+                    "mi-NZ" => "Segoe UI",
+                    "mk-MK" => "Segoe UI",
+                    "ml-IN" => "Nirmala UI",
+                    "mn-MN" => "Segoe UI",
+                    "mr-IN" => "Nirmala UI",
+                    "ms-MY" => "Segoe UI",
+                    "mt-MT" => "Segoe UI",
+                    "nb-NO" => "Segoe UI",
+                    "ne-NP" => "Nirmala UI",
+                    "nl-NL" => "Segoe UI",
+                    "nn-NO" => "Segoe UI",
+                    "nso-ZA" => "Segoe UI",
+                    "or-IN" => "Nirmala UI",
+                    "pa-Arab-PK" => "Segoe UI",
+                    "pa-IN" => "Nirmala UI",
+                    "pl-PL" => "Segoe UI",
+                    "prs-AF" => "Segoe UI",
+                    "pt-BR" => "Segoe UI",
+                    "pt-PT" => "Segoe UI",
+                    "qut-GT" => "Segoe UI",
+                    "quz-PE" => "Segoe UI",
+                    "ro-RO" => "Segoe UI",
+                    "ru-RU" => "Segoe UI",
+                    "rw-RW" => "Segoe UI",
+                    "sd-Arab-PK" => "Segoe UI",
+                    "si-LK" => "Nirmala UI",
+                    "sk-SK" => "Segoe UI",
+                    "sl-SI" => "Segoe UI",
+                    "sq-AL" => "Segoe UI",
+                    "sr-Cyrl-BA" => "Segoe UI",
+                    "sr-Cyrl-CS" => "Segoe UI",
+                    "sr-Latn-CS" => "Segoe UI",
+                    "sv-SE" => "Segoe UI",
+                    "sw-KE" => "Segoe UI",
+                    "ta-IN" => "Nirmala UI",
+                    "te-IN" => "Nirmala UI",
+                    "tg-Cyrl-TJ" => "Segoe UI",
+                    "th-TH" => "Leelawadee UI",
+                    "ti-ET" => "Ebrima",
+                    "tk-TM" => "Segoe UI",
+                    "tn-ZA" => "Segoe UI",
+                    "tr-TR" => "Segoe UI",
+                    "tt-RU" => "Segoe UI",
+                    "ug-CN" => "Segoe UI",
+                    "uk-UA" => "Segoe UI",
+                    "ur-PK" => "Segoe UI",
+                    "uz-Latn-UZ" => "Segoe UI",
+                    "vi-VN" => "Segoe UI",
+                    "wo-SN" => "Segoe UI",
+                    "xh-ZA" => "Segoe UI",
+                    "yo-NG" => "Segoe UI",
+                    "zh-CN" => "Microsoft YaHei UI",
+                    "zh-HK" => "Microsoft JhengHei UI",
+                    "zh-TW" => "Microsoft JhengHei UI",
+                    "zh-Hans" => "Microsoft YaHei UI",
+                    "zh-Hant" => "Microsoft YaHei UI",
+                    "zu-ZA" => "Segoe UI",
+                    _ => null,
+                };
+                if (familyName is null)
+                    continue;
+                var family = IFontFamilyId
+                             .ListSystemFonts(false)
+                             .FirstOrDefault(
+                                 x => x.EnglishName.Equals(familyName, StringComparison.InvariantCultureIgnoreCase));
+                if (family?.Fonts[family.FindBestMatch(weight, stretch, style)] is not { } font)
+                    return;
+                font.AddToBuildToolkit(this, fontConfig with { MergeFont = targetFont });
+                return;
+            }
+        }
+
         /// <inheritdoc/>
         public void AttachExtraGlyphsForDalamudLanguage(in SafeFontConfig fontConfig)
         {
+            var targetFont = fontConfig.MergeFont;
+            if (targetFont.IsNull())
+                targetFont = this.Font;
+            if (targetFont.IsNull())
+                return;
+
             var dalamudConfiguration = Service<DalamudConfiguration>.Get();
             if (dalamudConfiguration.EffectiveLanguage == "ko"
                 || Service<DalamudIme>.GetNullable()?.EncounteredHangul is true)
@@ -444,41 +600,24 @@ internal sealed partial class FontAtlasFactory
                     DalamudAsset.NotoSansKrRegular,
                     fontConfig with
                     {
-                        GlyphRanges = ImGuiHelpers.CreateImGuiRangesFrom(
-                            UnicodeRanges.HangulJamo,
-                            UnicodeRanges.HangulCompatibilityJamo,
-                            UnicodeRanges.HangulSyllables,
-                            UnicodeRanges.HangulJamoExtendedA,
-                            UnicodeRanges.HangulJamoExtendedB),
+                        MergeFont = targetFont,
+                        GlyphRanges = default(FluentGlyphRangeBuilder).WithLanguage("ko-kr").BuildExact(),
                     });
             }
 
-            var windowsDir = Environment.GetFolderPath(Environment.SpecialFolder.Windows);
-            var fontPathChs = Path.Combine(windowsDir, "Fonts", "msyh.ttc");
-            if (!File.Exists(fontPathChs))
-                fontPathChs = null;
-
-            var fontPathCht = Path.Combine(windowsDir, "Fonts", "msjh.ttc");
-            if (!File.Exists(fontPathCht))
-                fontPathCht = null;
-
-            if (fontPathCht != null && Service<DalamudConfiguration>.Get().EffectiveLanguage == "tw")
+            if (Service<DalamudConfiguration>.Get().EffectiveLanguage == "tw")
             {
-                this.AddFontFromFile(fontPathCht, fontConfig with
+                this.AttachWindowsDefaultFont(CultureInfo.GetCultureInfo("zh-hant"), fontConfig with
                 {
-                    GlyphRanges = ImGuiHelpers.CreateImGuiRangesFrom(
-                        UnicodeRanges.CjkUnifiedIdeographs,
-                        UnicodeRanges.CjkUnifiedIdeographsExtensionA),
+                    GlyphRanges = default(FluentGlyphRangeBuilder).WithLanguage("zh-hant").BuildExact(),
                 });
             }
-            else if (fontPathChs != null && (Service<DalamudConfiguration>.Get().EffectiveLanguage == "zh"
-                                               || Service<DalamudIme>.GetNullable()?.EncounteredHan is true))
+            else if (Service<DalamudConfiguration>.Get().EffectiveLanguage == "zh"
+                     || Service<DalamudIme>.GetNullable()?.EncounteredHan is true)
             {
-                this.AddFontFromFile(fontPathChs, fontConfig with
+                this.AttachWindowsDefaultFont(CultureInfo.GetCultureInfo("zh-hans"), fontConfig with
                 {
-                    GlyphRanges = ImGuiHelpers.CreateImGuiRangesFrom(
-                        UnicodeRanges.CjkUnifiedIdeographs,
-                        UnicodeRanges.CjkUnifiedIdeographsExtensionA),
+                    GlyphRanges = default(FluentGlyphRangeBuilder).WithLanguage("zh-hans").BuildExact(),
                 });
             }
         }