From 6dff06130998712209657d26fe612c7dfce7014b Mon Sep 17 00:00:00 2001
From: omar <omarcornut@gmail.com>
Date: Wed, 18 Mar 2020 17:49:13 +0100
Subject: [PATCH] Internals: added ImSpanAllocator<> helper.

---
 imgui_internal.h | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/imgui_internal.h b/imgui_internal.h
index 12e67fdaf..0530130a0 100644
--- a/imgui_internal.h
+++ b/imgui_internal.h
@@ -249,7 +249,7 @@ namespace ImStb
 // - Helper: ImRect
 // - Helper: ImBitArray
 // - Helper: ImBitVector
-// - Helper: ImSpan<>
+// - Helper: ImSpan<>, ImSpanAllocator<>
 // - Helper: ImPool<>
 // - Helper: ImChunkStream<>
 //-----------------------------------------------------------------------------
@@ -531,6 +531,26 @@ struct ImSpan
     inline int  index_from_ptr(const T* it) const   { IM_ASSERT(it >= Data && it < DataEnd); const ptrdiff_t off = it - Data; return (int)off; }
 };
 
+// Helper: ImSpanAllocator<>
+// Facilitate storing multiple chunks into a single large block (the "arena")
+template<int CHUNKS>
+struct ImSpanAllocator
+{
+    char*   BasePtr;
+    int     TotalSize;
+    int     CurrSpan;
+    int     Offsets[CHUNKS];
+
+    ImSpanAllocator()                               { memset(this, 0, sizeof(*this)); }
+    inline void  ReserveBytes(int n, size_t sz)     { IM_ASSERT(n == CurrSpan && n < CHUNKS); Offsets[CurrSpan++] = TotalSize; TotalSize += (int)sz; }
+    inline int   GetArenaSizeInBytes()              { return TotalSize; }
+    inline void  SetArenaBasePtr(void* base_ptr)    { BasePtr = (char*)base_ptr; }
+    inline void* GetSpanPtrBegin(int n)             { IM_ASSERT(n >= 0 && n < CHUNKS && CurrSpan == CHUNKS); return (void*)(BasePtr + Offsets[n]); }
+    inline void* GetSpanPtrEnd(int n)               { IM_ASSERT(n >= 0 && n < CHUNKS && CurrSpan == CHUNKS); return (n + 1 < CHUNKS) ? BasePtr + Offsets[n + 1] : (void*)(BasePtr + TotalSize); }
+    template<typename T> 
+    inline void  GetSpan(int n, ImSpan<T>* span)    { span->set((T*)GetSpanPtrBegin(n), (T*)GetSpanPtrEnd(n)); }
+};
+
 // Helper: ImPool<>
 // Basic keyed storage for contiguous instances, slow/amortized insertion, O(1) indexable, O(Log N) queries by ID over a dense/hot buffer,
 // Honor constructor/destructor. Add/remove invalidate all pointers. Indexes have the same lifetime as the associated object.