ImFont: Demo, Store Used4kPagesMap[] map in ImFont to facilitate iteration on all codepoints with a large value of IM_UNICODE_CODEPOINT_MAX. (#2815)

Demo uses IsGlyphRangeUnused()

imgui.h

@@ -2257,6 +2257,7 @@ struct ImFont
     float                       Scale;              // 4     // in  // = 1.f      // Base font scale, multiplied by the per-window font scale which you can adjust with SetWindowFontScale()
     float                       Ascent, Descent;    // 4+4   // out //            // Ascent: distance from top to bottom of e.g. 'A' [0..FontSize]
     int                         MetricsTotalSurface;// 4     // out //            // Total surface in pixels to get an idea of the font rasterization/texture cost (not exact, we approximate the cost of padding between glyphs)
+    ImU8                        Used4kPagesMap[(IM_UNICODE_CODEPOINT_MAX+1)/4096/8]; // 2 bytes if ImWchar=ImWchar16, 34 bytes if ImWchar==ImWchar32. Store 1-bit for each block of 4K codepoints that has one active glyph. This is mainly used to facilitate iterations accross all used codepoints.
 
     // Methods
     IMGUI_API ImFont();
@@ -2282,6 +2283,7 @@ struct ImFont
     IMGUI_API void              AddRemapChar(ImWchar dst, ImWchar src, bool overwrite_dst = true); // Makes 'dst' character/glyph points to 'src' character/glyph. Currently needs to be called AFTER fonts have been built.
     IMGUI_API void              SetGlyphVisible(ImWchar c, bool visible);
     IMGUI_API void              SetFallbackChar(ImWchar c);
+    IMGUI_API bool              IsGlyphRangeUnused(unsigned int c_begin, unsigned int c_last);
 };
 
 #if defined(__clang__)

imgui_demo.cpp

@@ -3443,6 +3443,15 @@ void ImGui::ShowStyleEditor(ImGuiStyle* ref)
                         // Display all glyphs of the fonts in separate pages of 256 characters
                         for (unsigned int base = 0; base <= IM_UNICODE_CODEPOINT_MAX; base += 256)
                         {
+                            // Skip ahead if a large bunch of glyphs are not present in the font (test in chunks of 4k)
+                            // This is only a small optimization to reduce the number of iterations when IM_UNICODE_MAX_CODEPOINT is large.
+                            // (if ImWchar==ImWchar32 we will do at least about 272 queries here)
+                            if (!(base & 4095) && font->IsGlyphRangeUnused(base, base + 4095))
+                            {
+                                base += 4096 - 256;
+                                continue;
+                            }
+
                             int count = 0;
                             for (unsigned int n = 0; n < 256; n++)
                                 count += font->FindGlyphNoFallback((ImWchar)(base + n)) ? 1 : 0;

imgui_draw.cpp

@@ -2594,6 +2594,7 @@ ImFont::ImFont()
     Scale = 1.0f;
     Ascent = Descent = 0.0f;
     MetricsTotalSurface = 0;
+    memset(Used4kPagesMap, 0, sizeof(Used4kPagesMap));
 }
 
 ImFont::~ImFont()
@@ -2626,12 +2627,17 @@ void ImFont::BuildLookupTable()
     IndexAdvanceX.clear();
     IndexLookup.clear();
     DirtyLookupTables = false;
+    memset(Used4kPagesMap, 0, sizeof(Used4kPagesMap));
     GrowIndex(max_codepoint + 1);
     for (int i = 0; i < Glyphs.Size; i++)
     {
         int codepoint = (int)Glyphs[i].Codepoint;
         IndexAdvanceX[codepoint] = Glyphs[i].AdvanceX;
         IndexLookup[codepoint] = (ImWchar)i;
+
+        // Mark 4K page as used
+        const int page_n = codepoint / 4096;
+        Used4kPagesMap[page_n >> 3] |= 1 << (page_n & 7);
     }
 
     // Create a glyph to handle TAB
@@ -2660,6 +2666,19 @@ void ImFont::BuildLookupTable()
             IndexAdvanceX[i] = FallbackAdvanceX;
 }
 
+// API is designed this way to avoid exposing the 4K page size
+// e.g. use with IsGlyphRangeUnused(0, 255)
+bool ImFont::IsGlyphRangeUnused(unsigned int c_begin, unsigned int c_last)
+{
+    unsigned int page_begin = (c_begin / 4096);
+    unsigned int page_last = (c_last / 4096);
+    for (unsigned int page_n = page_begin; page_n <= page_last; page_n++)
+        if ((page_n >> 3) < sizeof(Used4kPagesMap))
+            if (Used4kPagesMap[page_n >> 3] & (1 << (page_n & 7)))
+                return false;
+    return true;
+}
+
 void ImFont::SetGlyphVisible(ImWchar c, bool visible)
 {
     if (ImFontGlyph* glyph = (ImFontGlyph*)(void*)FindGlyph((ImWchar)c))