SDL3-TestingSuite/SDL3/FontFind.cs

using Hexa.NET.ImGui;

namespace SDL3_TestingSuite.SDL3;

public struct FontMetrics
{
    public ushort UnitsPerEm;

    public short TypoAscender;
    public short TypoDescender;
    public short TypoLineGap;

    public short WinAscent;
    public short WinDescent;

    public int TypoLineHeight;
    public int WinLineHeight;
}

public static class FontFind
{
    extension(ImGuiIOPtr io)
    {
        public unsafe ImFontPtr AddFont(string fontPath)
        {
            try
            {
                if (fontPath == null || io.Handle == null) return null;
                FileInfo info = new FileInfo(fontPath);
                if (!info.Exists || info.Length <= 0) return null;

                FontMetrics metrics = FontFind.ReadFontMetrics(fontPath);
                float size = FontFind.GetRecommendedPixelSize(metrics);

                ImFontConfigPtr config = ImGui.ImFontConfig();
                config.FontLoaderFlags |= (uint)ImGuiFreeTypeLoaderFlags.LoadColor;
                ImFontPtr font = io.Fonts.AddFontFromFileTTF(fontPath, size, config);
                Program.Logger.Log("Added font: " + Path.GetFileName(fontPath));
                return font;
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
            }
            return null;
        }
        public unsafe bool RemoveFont(string? fontName)
        {
            if (fontName == null || io.Handle == null) return false;

            bool flag = false;
            ImVector<ImFontPtr> fonts = io.Fonts.Fonts;
            List<ImFontPtr> toRemove = new List<ImFontPtr>();
            
            for (int i = 0; i < fonts.Size; i++)
            {
                ImFontPtr font = fonts[i];
                if (font.GetDebugNameS() == fontName)
                {
                    toRemove.Add(font);
                    flag = true;
                }
            }
            
            foreach (ImFontPtr font in toRemove)
            {
                io.Fonts.RemoveFont(font);
                Program.Logger.Log("Removed font: " + fontName);
            }
            
            return flag;
        }
    }

    public static FontMetrics ReadFontMetrics(string fontPath)
    {
        byte[] data = File.ReadAllBytes(fontPath);

        ushort numTables = ReadU16Be(4);
        const int tableDir = 12;

        int headOffset = 0;
        int os2Offset = 0;

        for (int i = 0; i < numTables; i++)
        {
            int entry = tableDir + i * 16;
            string tag = ReadTag(entry);

            uint offset = ReadU32Be(entry + 8);

            if (tag == "head")
                headOffset = (int)offset;

            if (tag == "OS/2")
                os2Offset = (int)offset;
        }

        FontMetrics metrics = new FontMetrics();

        metrics.UnitsPerEm = ReadU16Be(headOffset + 18);

        if (os2Offset != 0)
        {
            metrics.TypoAscender = ReadS16Be(os2Offset + 68);
            metrics.TypoDescender = ReadS16Be(os2Offset + 70);
            metrics.TypoLineGap = ReadS16Be(os2Offset + 72);

            metrics.WinAscent = ReadS16Be(os2Offset + 74);
            metrics.WinDescent = ReadS16Be(os2Offset + 76);
        }

        metrics.TypoLineHeight =
            metrics.TypoAscender - metrics.TypoDescender + metrics.TypoLineGap;

        metrics.WinLineHeight =
            metrics.WinAscent + metrics.WinDescent;

        return metrics;

        ushort ReadU16Be(int o)
        {
            return (ushort)((data[o] << 8) | data[o + 1]);
        }

        short ReadS16Be(int o)
        {
            return (short)ReadU16Be(o);
        }

        uint ReadU32Be(int o)
        {
            return (uint)((data[o] << 24) | (data[o + 1] << 16) | (data[o + 2] << 8) | data[o + 3]);
        }

        string ReadTag(int o)
        {
            char c1 = (char)data[o];
            char c2 = (char)data[o + 1];
            char c3 = (char)data[o + 2];
            char c4 = (char)data[o + 3];
            return new string(new char[] { c1, c2, c3, c4 });
        }
    }
    
    public static int GetRecommendedPixelSize(FontMetrics metrics)
    {
        if (metrics.UnitsPerEm == 0 || metrics.TypoLineHeight == 0)
            return 16;

        const float targetLineHeightPx = 18.0f;

        float scale = targetLineHeightPx * metrics.UnitsPerEm / metrics.TypoLineHeight;

        int size = (int)MathF.Round(scale);

        if (size < 10) size = 10;
        if (size > 72) size = 72;

        return size;
    }
}