fs/io/PeekReader: new utility class

Makefile.am

@@ -507,6 +507,7 @@ FS_LIBS = libfs.a
 
 libfs_a_SOURCES = \
 	src/fs/io/Reader.hxx \
+	src/fs/io/PeekReader.cxx src/fs/io/PeekReader.hxx \
 	src/fs/io/FileReader.cxx src/fs/io/FileReader.hxx \
 	src/fs/io/BufferedReader.cxx src/fs/io/BufferedReader.hxx \
 	src/fs/io/TextFile.cxx src/fs/io/TextFile.hxx \

src/fs/io/PeekReader.cxx

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2003-2014 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 "PeekReader.hxx"
+
+#include <algorithm>
+
+#include <assert.h>
+#include <string.h>
+
+const void *
+PeekReader::Peek(size_t size, Error &error)
+{
+	assert(size > 0);
+	assert(size < sizeof(buffer));
+	assert(buffer_size == 0);
+	assert(buffer_position == 0);
+
+	do {
+		size_t nbytes = next.Read(buffer + buffer_size,
+					size - buffer_size, error);
+		if (nbytes == 0)
+			return nullptr;
+
+		buffer_size += nbytes;
+	} while (buffer_size < size);
+
+	return buffer;
+}
+
+size_t
+PeekReader::Read(void *data, size_t size, Error &error)
+{
+	size_t buffer_remaining = buffer_size - buffer_position;
+	if (buffer_remaining > 0) {
+		size_t nbytes = std::min(buffer_remaining, size);
+		memcpy(data, buffer + buffer_position, nbytes);
+		buffer_position += nbytes;
+		return nbytes;
+	}
+
+	return next.Read(data, size, error);
+}

src/fs/io/PeekReader.hxx

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2003-2014 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.
+ */
+
+#ifndef MPD_PEEK_READER_HXX
+#define MPD_PEEK_READER_HXX
+
+#include "check.h"
+#include "Reader.hxx"
+
+#include <stdint.h>
+
+class AutoGunzipReader;
+
+/**
+ * A filter that allows the caller to peek the first few bytes without
+ * consuming them.  The first call must be Peek(), and the following
+ * Read() will deliver the same bytes again.
+ */
+class PeekReader final : public Reader {
+	Reader &next;
+
+	size_t buffer_size, buffer_position;
+
+	uint8_t buffer[64];
+
+public:
+	PeekReader(Reader &_next)
+		:next(_next), buffer_size(0), buffer_position(0) {}
+
+	const void *Peek(size_t size, Error &error);
+
+	/* virtual methods from class Reader */
+	virtual size_t Read(void *data, size_t size, Error &error) override;
+};
+
+#endif