util/SparseBuffer: new utility class

src/util/SparseBuffer.hxx

@@ -0,0 +1,141 @@
+/*
+ * Copyright (C) 2013-2018 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef SPARSE_BUFFER_HXX
+#define SPARSE_BUFFER_HXX
+
+#include "HugeAllocator.hxx"
+#include "ConstBuffer.hxx"
+#include "WritableBuffer.hxx"
+
+#include <map>
+
+#include <assert.h>
+
+/**
+ * Helper class for #SparseBuffer which describes which portions of
+ * the buffer have "known" data.
+ */
+class SparseMap {
+	using size_type = std::size_t;
+
+	/**
+	 * Key is start offset, value is end offset.
+	 */
+	using Map = std::map<size_type, size_type>;
+	using Iterator = typename Map::iterator;
+
+	Map map;
+
+public:
+	explicit SparseMap(size_type size) noexcept
+		:map{{size, size}} {
+		assert(size > 0);
+	}
+
+	size_type size() const noexcept {
+		return GetEndOffset();
+	}
+
+	struct CheckResult {
+		size_type undefined_size;
+		size_type defined_size;
+	};
+
+	/**
+	 * Check and classify the given offset.  Returns a structure
+	 * which tells you how much data is undefined, and how much
+	 * data follows which is defined.
+	 */
+	CheckResult Check(size_type offset) const noexcept;
+
+	/**
+	 * Commit a write: mark the given range in the buffer as
+	 * "defined".
+	 */
+	void Commit(size_type start_offset, size_type end_offset) noexcept;
+
+private:
+	size_type GetEndOffset() const noexcept {
+		return std::prev(map.end())->second;
+	}
+
+	Iterator CheckCollapsePrevious(Iterator i) noexcept;
+	Iterator CheckCollapseNext(Iterator i) noexcept;
+};
+
+#endif
+
+/**
+ * A buffer which caches the contents of a "huge" array, and remembers
+ * which chunks are available.
+ */
+template<typename T>
+class SparseBuffer {
+	using Buffer = HugeArray<T>;
+	using size_type = typename Buffer::size_type;
+
+	Buffer buffer;
+
+	SparseMap map;
+
+public:
+	explicit SparseBuffer(size_type size) noexcept
+		:buffer(size), map(size) {
+		buffer.ForkCow(false);
+	}
+
+	size_type size() const noexcept {
+		return map.size();
+	}
+
+	struct ReadResult {
+		size_type undefined_size;
+		ConstBuffer<T> defined_buffer;
+
+		constexpr bool HasData() const noexcept {
+			return undefined_size == 0 &&
+				!defined_buffer.empty();
+		}
+	};
+
+	ReadResult Read(size_type offset) const noexcept {
+		auto c = map.Check(offset);
+		return {c.undefined_size, {&buffer.front() + offset + c.undefined_size, c.defined_size}};
+	}
+
+	WritableBuffer<T> Write(size_type offset) noexcept {
+		auto c = map.Check(offset);
+		return {&buffer.front() + offset, c.undefined_size};
+	}
+
+	void Commit(size_type start_offset, size_type end_offset) noexcept {
+		map.Commit(start_offset, end_offset);
+	}
+};