Added PrimUnreserve() API. Obsoleted calling ImDrawList::PrimReserve() with a negative count.

docs/CHANGELOG.txt

@@ -36,6 +36,12 @@ HOW TO UPDATE?
 Breaking Changes:
 - Removed implicit default parameter to IsMouseDragging(int button = 0) to be consistent
   with other mouse functions (none of the other functions have it).
+- Obsoleted calling ImDrawList::PrimReserve() with a negative count (which was vaguely
+  documented and rarely if ever used). Instead we added an explicit PrimUnreserve() API
+  which can be implemented faster. Also clarified pre-existing constraints which weren't
+  documented (can only unreserve from the last reserve call). If you suspect you ever
+  used that feature before, #define IMGUI_DEBUG_PARANOID in imconfig.h to catch existing
+  calls. [@ShironekoBen]
 
 Other Changes:
 - Inputs: Added ImGuiMouseButton enum for convenience (e.g. ImGuiMouseButton_Right=1).

imgui.cpp

@@ -353,6 +353,7 @@ CODE
  When you are not sure about a old symbol or function name, try using the Search/Find function of your IDE to look for comments or references in all imgui files.
  You can read releases logs https://github.com/ocornut/imgui/releases for more details.
 
+ - 2019/12/08 (1.75) - obsoleted calling ImDrawList::PrimReserve() with a negative count (which was the vaguely documented and rarely if ever used). Instead we added an explicit PrimUnreserve() API.
  - 2019/12/06 (1.75) - removed implicit default parameter to IsMouseDragging(int button = 0) to be consistent with other mouse functions (none of the other functions have it).
  - 2019/11/21 (1.74) - ImFontAtlas::AddCustomRectRegular() now requires an ID larger than 0x110000 (instead of 0x10000) to conform with supporting Unicode planes 1-16 in a future update. ID below 0x110000 will now assert.
  - 2019/11/19 (1.74) - renamed IMGUI_DISABLE_FORMAT_STRING_FUNCTIONS to IMGUI_DISABLE_DEFAULT_FORMAT_FUNCTIONS for consistency.

imgui.h

@@ -1293,7 +1293,7 @@ struct ImVector
     inline int          _grow_capacity(int sz) const        { int new_capacity = Capacity ? (Capacity + Capacity/2) : 8; return new_capacity > sz ? new_capacity : sz; }
     inline void         resize(int new_size)                { if (new_size > Capacity) reserve(_grow_capacity(new_size)); Size = new_size; }
     inline void         resize(int new_size, const T& v)    { if (new_size > Capacity) reserve(_grow_capacity(new_size)); if (new_size > Size) for (int n = Size; n < new_size; n++) memcpy(&Data[n], &v, sizeof(v)); Size = new_size; }
-    inline void         shrink(int new_size)                { IM_ASSERT(new_size <= Size); Size = new_size; }
+    inline void         shrink(int new_size)                { IM_ASSERT(new_size <= Size); Size = new_size; } // Resize a vector to a smaller size, guaranteed not to cause a reallocation
     inline void         reserve(int new_capacity)           { if (new_capacity <= Capacity) return; T* new_data = (T*)IM_ALLOC((size_t)new_capacity * sizeof(T)); if (Data) { memcpy(new_data, Data, (size_t)Size * sizeof(T)); IM_FREE(Data); } Data = new_data; Capacity = new_capacity; }
 
     // NB: It is illegal to call push_back/push_front/insert with a reference pointing inside the ImVector data itself! e.g. v.push_back(v[10]) is forbidden.
@@ -1990,6 +1990,7 @@ struct ImDrawList
     IMGUI_API void  Clear();
     IMGUI_API void  ClearFreeMemory();
     IMGUI_API void  PrimReserve(int idx_count, int vtx_count);
+    IMGUI_API void  PrimUnreserve(int idx_count, int vtx_count);
     IMGUI_API void  PrimRect(const ImVec2& a, const ImVec2& b, ImU32 col);      // Axis aligned rectangle (composed of two triangles)
     IMGUI_API void  PrimRectUV(const ImVec2& a, const ImVec2& b, const ImVec2& uv_a, const ImVec2& uv_b, ImU32 col);
     IMGUI_API void  PrimQuadUV(const ImVec2& a, const ImVec2& b, const ImVec2& c, const ImVec2& d, const ImVec2& uv_a, const ImVec2& uv_b, const ImVec2& uv_c, const ImVec2& uv_d, ImU32 col);

imgui_draw.cpp

@@ -516,10 +516,13 @@ void ImDrawList::PopTextureID()
     UpdateTextureID();
 }
 
-// NB: this can be called with negative count for removing primitives (as long as the result does not underflow)
+// Reserve space for a number of vertices and indices.
+// You must finish filling your reserved data before calling PrimReserve() again, as it may reallocate or 
+// submit the intermediate results. PrimUnreserve() can be used to release unused allocations.
 void ImDrawList::PrimReserve(int idx_count, int vtx_count)
 {
     // Large mesh support (when enabled)
+    IM_ASSERT_PARANOID(idx_count >= 0 && vtx_count >= 0);
     if (sizeof(ImDrawIdx) == 2 && (_VtxCurrentIdx + vtx_count >= (1 << 16)) && (Flags & ImDrawListFlags_AllowVtxOffset))
     {
         _VtxCurrentOffset = VtxBuffer.Size;
@@ -527,7 +530,7 @@ void ImDrawList::PrimReserve(int idx_count, int vtx_count)
         AddDrawCmd();
     }
 
-    ImDrawCmd& draw_cmd = CmdBuffer.Data[CmdBuffer.Size-1];
+    ImDrawCmd& draw_cmd = CmdBuffer.Data[CmdBuffer.Size - 1];
     draw_cmd.ElemCount += idx_count;
 
     int vtx_buffer_old_size = VtxBuffer.Size;
@@ -539,6 +542,17 @@ void ImDrawList::PrimReserve(int idx_count, int vtx_count)
     _IdxWritePtr = IdxBuffer.Data + idx_buffer_old_size;
 }
 
+// Release the a number of reserved vertices/indices from the end of the last reservation made with PrimReserve().
+void ImDrawList::PrimUnreserve(int idx_count, int vtx_count)
+{
+    IM_ASSERT_PARANOID(idx_count >= 0 && vtx_count >= 0);
+
+    ImDrawCmd& draw_cmd = CmdBuffer.Data[CmdBuffer.Size - 1];
+    draw_cmd.ElemCount -= idx_count;
+    VtxBuffer.shrink(VtxBuffer.Size - vtx_count);
+    IdxBuffer.shrink(IdxBuffer.Size - idx_count);
+}
+
 // Fully unrolled with inline call to keep our debug builds decently fast.
 void ImDrawList::PrimRect(const ImVec2& a, const ImVec2& c, ImU32 col)
 {

imgui_internal.h

@@ -847,7 +847,7 @@ struct IMGUI_API ImDrawListSharedData
     ImVec2          TexUvWhitePixel;            // UV of white pixel in the atlas
     ImFont*         Font;                       // Current/default font (optional, for simplified AddText overload)
     float           FontSize;                   // Current/default font size (optional, for simplified AddText overload)
-    float           CurveTessellationTol;
+    float           CurveTessellationTol;       // Tessellation tolerance when using PathBezierCurveTo()
     ImVec4          ClipRectFullscreen;         // Value for PushClipRectFullscreen()
     ImDrawListFlags InitialFlags;               // Initial flags at the beginning of the frame (it is possible to alter flags on a per-drawlist basis afterwards)