Debug Tools: Debug Log: add and use ImGuiTextLineIndex.

This is needed for next commit where we want to parse hovered Debug Log line.

imgui.cpp

@@ -65,7 +65,7 @@ CODE
 // [SECTION] MISC HELPERS/UTILITIES (Color functions)
 // [SECTION] ImGuiStorage
 // [SECTION] ImGuiTextFilter
-// [SECTION] ImGuiTextBuffer
+// [SECTION] ImGuiTextBuffer, ImGuiTextIndex
 // [SECTION] ImGuiListClipper
 // [SECTION] STYLING
 // [SECTION] RENDER HELPERS
@@ -2457,7 +2457,7 @@ bool ImGuiTextFilter::PassFilter(const char* text, const char* text_end) const
 }
 
 //-----------------------------------------------------------------------------
-// [SECTION] ImGuiTextBuffer
+// [SECTION] ImGuiTextBuffer, ImGuiTextIndex
 //-----------------------------------------------------------------------------
 
 // On some platform vsnprintf() takes va_list by reference and modifies it.
@@ -2525,6 +2525,20 @@ void ImGuiTextBuffer::appendfv(const char* fmt, va_list args)
     va_end(args_copy);
 }
 
+void ImGuiTextIndex::append(const char* base, int old_size, int new_size)
+{
+    IM_ASSERT(old_size >= 0 && new_size >= old_size && new_size >= EndOffset);
+    if (old_size == new_size)
+        return;
+    if (EndOffset == 0 || base[EndOffset - 1] == '\n')
+        LineOffsets.push_back(EndOffset);
+    const char* base_end = base + new_size;
+    for (const char* p = base + old_size; (p = (const char*)memchr(p, '\n', base_end - p)) != 0; )
+        if (++p < base_end) // Don't push a trailing offset on last \n
+            LineOffsets.push_back((int)(intptr_t)(p - base));
+    EndOffset = ImMax(EndOffset, new_size);
+}
+
 //-----------------------------------------------------------------------------
 // [SECTION] ImGuiListClipper
 // This is currently not as flexible/powerful as it should be and really confusing/spaghetti, mostly because we changed
@@ -4798,6 +4812,7 @@ void ImGui::Shutdown()
     }
     g.LogBuffer.clear();
     g.DebugLogBuf.clear();
+    g.DebugLogIndex.clear();
 
     g.Initialized = false;
 }
@@ -13430,6 +13445,7 @@ void ImGui::DebugLogV(const char* fmt, va_list args)
     g.DebugLogBuf.appendfv(fmt, args);
     if (g.DebugLogFlags & ImGuiDebugLogFlags_OutputToTTY)
         IMGUI_DEBUG_PRINTF("%s", g.DebugLogBuf.begin() + old_size);
+    g.DebugLogIndex.append(g.DebugLogBuf.c_str(), old_size, g.DebugLogBuf.size());
 }
 
 void ImGui::ShowDebugLogWindow(bool* p_open)
@@ -13454,12 +13470,24 @@ void ImGui::ShowDebugLogWindow(bool* p_open)
     SameLine(); CheckboxFlags("IO", &g.DebugLogFlags, ImGuiDebugLogFlags_EventIO);
 
     if (SmallButton("Clear"))
+    {
         g.DebugLogBuf.clear();
+        g.DebugLogIndex.clear();
+    }
     SameLine();
     if (SmallButton("Copy"))
         SetClipboardText(g.DebugLogBuf.c_str());
     BeginChild("##log", ImVec2(0.0f, 0.0f), true, ImGuiWindowFlags_AlwaysVerticalScrollbar | ImGuiWindowFlags_AlwaysHorizontalScrollbar);
-    TextUnformatted(g.DebugLogBuf.begin(), g.DebugLogBuf.end()); // FIXME-OPT: Could use a line index, but TextUnformatted() has a semi-decent fast path for large text.
+
+    ImGuiListClipper clipper;
+    clipper.Begin(g.DebugLogIndex.size());
+    while (clipper.Step())
+        for (int line_no = clipper.DisplayStart; line_no < clipper.DisplayEnd; line_no++)
+        {
+            const char* line_begin = g.DebugLogIndex.get_line_begin(g.DebugLogBuf.c_str(), line_no);
+            const char* line_end = g.DebugLogIndex.get_line_end(g.DebugLogBuf.c_str(), line_no);
+            TextUnformatted(line_begin, line_end);
+        }
     if (GetScrollY() >= GetScrollMaxY())
         SetScrollHereY(1.0f);
     EndChild();

imgui_internal.h

@@ -302,6 +302,7 @@ namespace ImStb
 // - Helper: ImSpan<>, ImSpanAllocator<>
 // - Helper: ImPool<>
 // - Helper: ImChunkStream<>
+// - Helper: ImGuiTextIndex
 //-----------------------------------------------------------------------------
 
 // Helpers: Hashing
@@ -694,6 +695,20 @@ struct ImChunkStream
 
 };
 
+// Helper: ImGuiTextIndex<>
+// Maintain a line index for a text buffer. This is a strong candidate to be moved into the public API.
+struct ImGuiTextIndex
+{
+    ImVector<int>   LineOffsets;
+    int             EndOffset = 0;                          // Because we don't own text buffer we need to maintain EndOffset (may bake in LineOffsets?)
+
+    void            clear()                                 { LineOffsets.clear(); EndOffset = 0; }
+    int             size()                                  { return LineOffsets.Size; }
+    const char*     get_line_begin(const char* base, int n) { return base + LineOffsets[n]; }
+    const char*     get_line_end(const char* base, int n)   { return base + (n + 1 < LineOffsets.Size ? (LineOffsets[n + 1] - 1) : EndOffset); }
+    void            append(const char* base, int old_size, int new_size);
+};
+
 //-----------------------------------------------------------------------------
 // [SECTION] ImDrawList support
 //-----------------------------------------------------------------------------
@@ -1850,6 +1865,7 @@ struct ImGuiContext
     // Debug Tools
     ImGuiDebugLogFlags      DebugLogFlags;
     ImGuiTextBuffer         DebugLogBuf;
+    ImGuiTextIndex          DebugLogIndex;
     bool                    DebugItemPickerActive;              // Item picker is active (started with DebugStartItemPicker())
     ImU8                    DebugItemPickerMouseButton;
     ImGuiID                 DebugItemPickerBreakId;             // Will call IM_DEBUG_BREAK() when encountering this ID