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

diff --git a/imgui.h b/imgui.h
index 7ab1792..9418182 100644
--- a/imgui.h
+++ b/imgui.h

@@ -2201,6 +2201,7 @@
     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_MAX_CODEPOINT/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.
     bool                        DirtyLookupTables;  // 1     // out //
 
     // Methods
@@ -2226,6 +2227,7 @@
     IMGUI_API void              AddGlyph(ImWchar c, float x0, float y0, float x1, float y1, float u0, float v0, float u1, float v1, float advance_x);
     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              SetFallbackChar(ImWchar c);
+    IMGUI_API bool              IsGlyphRangeUnused(unsigned int c_begin, unsigned int c_last);
 };
 
 #if defined(__clang__)

diff --git a/imgui_demo.cpp b/imgui_demo.cpp
index 17d2927..19902ac 100644
--- a/imgui_demo.cpp
+++ b/imgui_demo.cpp

@@ -3401,10 +3401,18 @@
                     if (ImGui::TreeNode("Glyphs", "Glyphs (%d)", font->Glyphs.Size))
                     {
                         // Display all glyphs of the fonts in separate pages of 256 characters
-                        for (int base = 0; base < IM_UNICODE_MAX_CODEPOINT; base += 256)
+                        for (unsigned int base = 0; base < IM_UNICODE_MAX_CODEPOINT; 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 (!(base & 4095) && font->IsGlyphRangeUnused(base, base + 4095))
+                            {
+                                base += 4096 - 256;
+                                continue;
+                            }
+
                             int count = 0;
-                            for (int n = 0; n < 256; n++)
+                            for (unsigned int n = 0; n < 256; n++)
                                 count += font->FindGlyphNoFallback((ImWchar)(base + n)) ? 1 : 0;
                             if (count > 0 && ImGui::TreeNode((void*)(intptr_t)base, "U+%04X..U+%04X (%d %s)", base, base + 255, count, count > 1 ? "glyphs" : "glyph"))
                             {
@@ -3412,7 +3420,7 @@
                                 float cell_spacing = style.ItemSpacing.y;
                                 ImVec2 base_pos = ImGui::GetCursorScreenPos();
                                 ImDrawList* draw_list = ImGui::GetWindowDrawList();
-                                for (int n = 0; n < 256; n++)
+                                for (unsigned int n = 0; n < 256; n++)
                                 {
                                     ImVec2 cell_p1(base_pos.x + (n % 16) * (cell_size + cell_spacing), base_pos.y + (n / 16) * (cell_size + cell_spacing));
                                     ImVec2 cell_p2(cell_p1.x + cell_size, cell_p1.y + cell_size);

diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index ad3cf1f..078c662 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp

@@ -2499,6 +2499,7 @@
     Scale = 1.0f;
     Ascent = Descent = 0.0f;
     MetricsTotalSurface = 0;
+    memset(Used4kPagesMap, 0, sizeof(Used4kPagesMap));
 }
 
 ImFont::~ImFont()
@@ -2530,12 +2531,17 @@
     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
@@ -2559,6 +2565,19 @@
             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::SetFallbackChar(ImWchar c)
 {
     FallbackChar = c;