InputText: amends: now can use memchr()/strchr() for line counting. Much faster! (#7925)

docs/CHANGELOG.txt

@@ -63,6 +63,10 @@ Other changes:
      #6223, #6364, #6387, #6567, #6692, #6724, #6939, #6984, #7246, #7270, #7375, #7421, #7434,
      #7472, #7581, #7724, #7926, #7937 and probably more..)
 - Nav: pressing any keyboard key while holding Alt disable toggling nav layer on Alt release. (#4439)
+- InputText: internal refactoring to simplify and optimize the code. The ImWchar buffer has been
+  removed. Simplifications allowed to implement new optimizations for handling very large text buffers
+  (e.g. in our testing, handling of a 1 MB text buffer is now 3 times faster in VS2022 Debug build).
+  This is the first step toward more refactorig. (#7925) [@alektron, @ocornut]
 - InputText: added CJK double-width punctuation to list of separators considered for CTRL+Arrow.
 - TextLinkOpenURL(): modified tooltip to display a verb "Open 'xxxx'". (#7885, #7660)
 - Backends: SDL2: use SDL_Vulkan_GetDrawableSize() when available. (#7967, #3190) [@scribam]

docs/TODO.txt

@@ -65,7 +65,6 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - selectable: generic BeginSelectable()/EndSelectable() mechanism. (work out alongside range-select branch)
  - selectable: a way to visualize partial/mixed selection (e.g. parent tree node has children with mixed selection)
 
- - input text: clean up the mess caused by converting UTF-8 <> wchar. the code is rather inefficient right now and super fragile. (WIP branch)
  - input text: preserve scrolling when unfocused?
  - input text: reorganize event handling, allow CharFilter to modify buffers, allow multiple events? (#541)
  - input text: expose CursorPos in char filter event (#816)

imgui_widgets.cpp

@@ -5119,9 +5119,7 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
             // In multi-line mode, we never exit the loop until all lines are counted, so add one extra to the searches_remaining counter.
             searches_remaining += is_multiline ? 1 : 0;
             int line_count = 0;
-            //for (const ImWchar* s = text_begin; (s = (const ImWchar*)wcschr((const wchar_t*)s, (wchar_t)'\n')) != NULL; s++)  // FIXME-OPT: Could use this when wchar_t are 16-bit
-            for (const char* s = text_begin; *s != 0; s++)
-                if (*s == '\n')
+            for (const char* s = text_begin; (s = strchr(s, '\n')) != NULL; s++) // FIXME-OPT: memchr() would be faster?
             {
                 line_count++;
                 if (searches_result_line_no[0] == -1 && s >= searches_input_ptr[0]) { searches_result_line_no[0] = line_count; if (--searches_remaining <= 0) break; }
@@ -5199,11 +5197,8 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
                     break;
                 if (rect_pos.y < clip_rect.y)
                 {
-                    //p = (const ImWchar*)wmemchr((const wchar_t*)p, '\n', text_selected_end - p);  // FIXME-OPT: Could use this when wchar_t are 16-bit
-                    //p = p ? p + 1 : text_selected_end;
-                    while (p < text_selected_end)
-                        if (*p++ == '\n')
-                            break;
+                    p = (const char*)memchr((void*)p, '\n', text_selected_end - p);
+                    p = p ? p + 1 : text_selected_end;
                 }
                 else
                 {
@@ -5213,13 +5208,14 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
                     rect.ClipWith(clip_rect);
                     if (rect.Overlaps(clip_rect))
                         draw_window->DrawList->AddRectFilled(rect.Min, rect.Max, bg_color);
-                }
                     rect_pos.x = draw_pos.x - draw_scroll.x;
+                }
                 rect_pos.y += g.FontSize;
             }
         }
 
         // We test for 'buf_display_max_length' as a way to avoid some pathological cases (e.g. single-line 1 MB string) which would make ImDrawList crash.
+        // FIXME-OPT: Multiline could submit a smaller amount of contents to AddText() since we already iterated through it.
         if (is_multiline || (buf_display_end - buf_display) < buf_display_max_length)
         {
             ImU32 col = GetColorU32(is_displaying_hint ? ImGuiCol_TextDisabled : ImGuiCol_Text);