input/rewind: move sources out of the "plugins" directory

This is not a plugin.

src/input/RewindInputStream.cxx

@@ -0,0 +1,151 @@
+/*
+ * Copyright 2003-2017 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "RewindInputStream.hxx"
+#include "ProxyInputStream.hxx"
+
+#include <assert.h>
+#include <string.h>
+
+class RewindInputStream final : public ProxyInputStream {
+	/**
+	 * The read position within the buffer.  Undefined as long as
+	 * ReadingFromBuffer() returns false.
+	 */
+	size_t head;
+
+	/**
+	 * The write/append position within the buffer.
+	 */
+	size_t tail = 0;
+
+	/**
+	 * The size of this buffer is the maximum number of bytes
+	 * which can be rewinded cheaply without passing the "seek"
+	 * call to CURL.
+	 *
+	 * The origin of this buffer is always the beginning of the
+	 * stream (offset 0).
+	 */
+	char buffer[64 * 1024];
+
+public:
+	explicit RewindInputStream(InputStreamPtr _input)
+		:ProxyInputStream(std::move(_input)) {}
+
+	/* virtual methods from InputStream */
+
+	void Update() noexcept override {
+		if (!ReadingFromBuffer())
+			ProxyInputStream::Update();
+	}
+
+	bool IsEOF() noexcept override {
+		return !ReadingFromBuffer() && ProxyInputStream::IsEOF();
+	}
+
+	size_t Read(void *ptr, size_t size) override;
+	void Seek(offset_type offset) override;
+
+private:
+	/**
+	 * Are we currently reading from the buffer, and does the
+	 * buffer contain more data for the next read operation?
+	 */
+	bool ReadingFromBuffer() const noexcept {
+		return tail > 0 && offset < input->GetOffset();
+	}
+};
+
+size_t
+RewindInputStream::Read(void *ptr, size_t read_size)
+{
+	if (ReadingFromBuffer()) {
+		/* buffered read */
+
+		assert(head == (size_t)offset);
+		assert(tail == (size_t)input->GetOffset());
+
+		if (read_size > tail - head)
+			read_size = tail - head;
+
+		memcpy(ptr, buffer + head, read_size);
+		head += read_size;
+		offset += read_size;
+
+		return read_size;
+	} else {
+		/* pass method call to underlying stream */
+
+		size_t nbytes = input->Read(ptr, read_size);
+
+		if (input->GetOffset() > (offset_type)sizeof(buffer))
+			/* disable buffering */
+			tail = 0;
+		else if (tail == (size_t)offset) {
+			/* append to buffer */
+
+			memcpy(buffer + tail, ptr, nbytes);
+			tail += nbytes;
+
+			assert(tail == (size_t)input->GetOffset());
+		}
+
+		CopyAttributes();
+
+		return nbytes;
+	}
+}
+
+void
+RewindInputStream::Seek(offset_type new_offset)
+{
+	assert(IsReady());
+
+	if (tail > 0 && new_offset <= (offset_type)tail) {
+		/* buffered seek */
+
+		assert(!ReadingFromBuffer() ||
+		       head == (size_t)offset);
+		assert(tail == (size_t)input->GetOffset());
+
+		head = (size_t)new_offset;
+		offset = new_offset;
+	} else {
+		/* disable the buffer, because input has left the
+		   buffered range now */
+		tail = 0;
+
+		ProxyInputStream::Seek(new_offset);
+	}
+}
+
+InputStreamPtr
+input_rewind_open(InputStreamPtr is)
+{
+	assert(is != nullptr);
+	assert(!is->IsReady() || is->GetOffset() == 0);
+
+	if (is->IsReady() && is->IsSeekable())
+		/* seekable resources don't need this plugin */
+		return is;
+
+	return std::make_unique<RewindInputStream>(std::move(is));
+}