util/SparseBuffer: new utility class

src/util/SparseBuffer.cxx

@@ -0,0 +1,113 @@
+/*
+ * 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.
+ */
+
+#include "SparseBuffer.hxx"
+
+SparseMap::CheckResult
+SparseMap::Check(size_type offset) const noexcept
+{
+	assert(!map.empty());
+	assert(offset < GetEndOffset());
+
+	auto i = map.lower_bound(offset);
+	if (i == map.end())
+		/* from here up until the end of the file is
+		   defined */
+		return {0, std::prev(i)->second - offset};
+
+	assert(i->first >= offset);
+
+	if (offset == i->first)
+		/* at the beginning of this chunk: the whole chunk can
+		   be read */
+		return {0, i->second - offset};
+
+	if (i == map.begin())
+		/* before the very first chunk: there's a hole, and
+		   after the hole, the whole chunk can be read */
+		return {i->first - offset, i->second - i->first};
+
+	auto p = std::prev(i);
+	assert(p->first < offset);
+
+	if (offset >= p->second)
+		/* between two chunks */
+		return {i->first - offset, i->second - i->first};
+
+	/* inside a chunk: the rest of the chunk can be read */
+	return {0, p->second - offset};
+}
+
+void
+SparseMap::Commit(size_type start_offset, size_type end_offset) noexcept
+{
+	assert(start_offset < end_offset);
+
+	auto e = map.emplace(start_offset, end_offset);
+	if (!e.second && end_offset > e.first->second)
+		e.first->second = end_offset;
+
+	CheckCollapseNext(CheckCollapsePrevious(e.first));
+}
+
+inline SparseMap::Iterator
+SparseMap::CheckCollapsePrevious(Iterator i) noexcept
+{
+	assert(i != map.end());
+
+	while (i != map.begin()) {
+		auto previous = std::prev(i);
+		if (previous->second < i->first)
+			break;
+
+		if (i->second > previous->second)
+			previous->second = i->second;
+
+		map.erase(i);
+		i = previous;
+	}
+
+	return i;
+}
+
+inline SparseMap::Iterator
+SparseMap::CheckCollapseNext(Iterator i) noexcept
+{
+	assert(i != map.end());
+
+	for (auto next = std::next(i);
+	     next != map.end() && i->second >= next->first;) {
+		if (next->second > i->second)
+			i->second = next->second;
+
+		next = map.erase(next);
+	}
+
+	return i;
+}