From ab350d1e43edef1a39f48d2771a9f136b3c4ae5f Mon Sep 17 00:00:00 2001
From: peterbell10 <peterbell10@live.co.uk>
Date: Sun, 21 Jan 2018 18:45:13 +0000
Subject: cItemGrid: Allocate storage lazily (#4083)

* cItemGrid: Allocate storage lazily
* cItemGrid: Fix spelling, Prioritary -> Priority
---
 src/LazyArray.h | 134 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 134 insertions(+)
 create mode 100644 src/LazyArray.h

(limited to 'src/LazyArray.h')

diff --git a/src/LazyArray.h b/src/LazyArray.h
new file mode 100644
index 000000000..7015d6c47
--- /dev/null
+++ b/src/LazyArray.h
@@ -0,0 +1,134 @@
+
+#pragma once
+
+/** A dynamic array that defers allocation to the first modifying access.
+Const references from the array before allocation will all be to the same default constructed value.
+It is therefore important that default constructed values are indistinguishable from each other. */
+template <typename T>
+class cLazyArray
+{
+	static_assert((!std::is_reference<T>::value && !std::is_array<T>::value),
+		"cLazyArray<T>: T must be a value type");
+	static_assert(std::is_default_constructible<T>::value,
+		"cLazyArray<T>: T must be default constructible");
+public:
+	using value_type = T;
+	using pointer = T *;
+	using const_pointer = const T *;
+	using reference = T &;
+	using const_reference = const T &;
+	using size_type = int;
+	using iterator = pointer;
+	using const_iterator = const_pointer;
+
+	cLazyArray(size_type a_Size) NOEXCEPT:
+		m_Size{ a_Size }
+	{
+		ASSERT(a_Size > 0);
+	}
+
+	cLazyArray(const cLazyArray & a_Other):
+		m_Size{ a_Other.m_Size }
+	{
+		if (a_Other.IsStorageAllocated())
+		{
+			// Note that begin() will allocate the array to copy into
+			std::copy(a_Other.begin(), a_Other.end(), begin());
+		}
+	}
+
+	cLazyArray(cLazyArray && a_Other) NOEXCEPT:
+		m_Array{ std::move(a_Other.m_Array) },
+		m_Size{ a_Other.m_Size }
+	{
+	}
+
+	cLazyArray & operator = (const cLazyArray & a_Other)
+	{
+		cLazyArray(a_Other).swap(*this);
+		return *this;
+	}
+
+	cLazyArray & operator = (cLazyArray && a_Other) NOEXCEPT
+	{
+		m_Array = std::move(a_Other.m_Array);
+		m_Size = a_Other.m_Size;
+		return *this;
+	}
+
+	T & operator [] (size_type a_Idx)
+	{
+		ASSERT((0 <= a_Idx) && (a_Idx < m_Size));
+		return data()[a_Idx];
+	}
+
+	const T & operator [] (size_type a_Idx) const
+	{
+		return GetAt(a_Idx);
+	}
+
+	// STL style interface
+
+	const_iterator cbegin() const { return data(); }
+	iterator        begin()       { return data(); }
+	const_iterator  begin() const { return cbegin(); }
+
+	const_iterator cend() const { return data() + m_Size; }
+	iterator        end()       { return data() + m_Size; }
+	const_iterator  end() const { return cend(); }
+
+	size_type size() const NOEXCEPT { return m_Size; }
+
+	const T * data() const
+	{
+		if (m_Array == nullptr)
+		{
+			m_Array.reset(new T[m_Size]);
+		}
+		return m_Array.get();
+	}
+
+	T * data()
+	{
+		static_assert(!std::is_const<typename decltype(m_Array)::element_type>::value, "");
+		const cLazyArray * const_this = this;
+		return const_cast<T *>(const_this->data());
+	}
+
+	void swap(cLazyArray & a_Other) NOEXCEPT
+	{
+		std::swap(m_Array, a_Other.m_Array);
+		std::swap(m_Size, a_Other.m_Size);
+	}
+
+	friend void swap(cLazyArray & a_Lhs, cLazyArray & a_Rhs) NOEXCEPT
+	{
+		a_Lhs.swap(a_Rhs);
+	}
+
+	// Extra functions to help avoid allocation
+
+	/** A const view of an element of the array. Never causes the array to allocate. */
+	const T & GetAt(size_type a_Idx) const
+	{
+		ASSERT((0 <= a_Idx) && (a_Idx < m_Size));
+		if (IsStorageAllocated())
+		{
+			return data()[a_Idx];
+		}
+		else
+		{
+			static const T DefaultValue;
+			return DefaultValue;
+		}
+	}
+
+	/** Returns true if the array has already been allocated. */
+	bool IsStorageAllocated() const NOEXCEPT { return (m_Array != nullptr); }
+
+private:
+	// Mutable so const data() can allocate the array
+	mutable std::unique_ptr<T[]> m_Array;
+	size_type m_Size;
+};
+
-- 
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