From eedad559456b8e62ed3b9bd9e8253e5f8bcc02d6 Mon Sep 17 00:00:00 2001
From: Mikko Rasa <tdb@tdb.fi>
Date: Thu, 8 Nov 2012 21:51:30 +0200
Subject: [PATCH] Add an I/O class to access a restricted range of bytes from
 another object

---
 source/io/slice.cpp | 111 ++++++++++++++++++++++++++++++++++++++++++++
 source/io/slice.h   |  50 ++++++++++++++++++++
 2 files changed, 161 insertions(+)
 create mode 100644 source/io/slice.cpp
 create mode 100644 source/io/slice.h

diff --git a/source/io/slice.cpp b/source/io/slice.cpp
new file mode 100644
index 0000000..423c1a6
--- /dev/null
+++ b/source/io/slice.cpp
@@ -0,0 +1,111 @@
+#include <stdexcept>
+#include "slice.h"
+
+using namespace std;
+
+namespace Msp {
+namespace IO {
+
+Slice::Slice(Seekable &b, SeekOffset s, SeekOffset l):
+	below(b),
+	start_offset(s),
+	length(l),
+	position(0),
+	sync_position(true)
+{
+	if(s<0 || l<0)
+		throw invalid_argument("Slice");
+
+	mode = below.get_mode()&M_RDWR;
+	below.signal_flush_required.connect(sigc::mem_fun(this, &Slice::flush));
+}
+
+void Slice::flush()
+{
+	sync_position = true;
+}
+
+unsigned Slice::prepare_op(unsigned size, Mode m)
+{
+	check_access(m);
+
+	if(sync_position)
+		seek(position, S_BEG);
+
+	SeekOffset remaining = start_offset+length-position;
+	if(size>remaining)
+		size = remaining;
+	if(!size && m==M_READ)
+		set_eof();
+
+	return size;
+}
+
+unsigned Slice::do_write(const char *data, unsigned size)
+{
+	size = prepare_op(size, M_WRITE);
+	if(!size)
+		return 0;
+
+	unsigned len = below.write(data, size);
+	position += len;
+	return len;
+}
+
+unsigned Slice::do_read(char *data, unsigned size)
+{
+	size = prepare_op(size, M_READ);
+	if(!size)
+		return 0;
+
+	unsigned len = below.read(data, size);
+	if(!len && below.eof())
+		set_eof();
+	position += len;
+	return len;
+}
+
+unsigned Slice::put(char c)
+{
+	if(!prepare_op(1, M_WRITE))
+		return 0;
+
+	unsigned len = below.put(c);
+	position += len;
+	return len;
+}
+
+int Slice::get()
+{
+	if(!prepare_op(1, M_READ))
+		return 0;
+
+	int c = below.get();
+	if(c==-1 && below.eof())
+		set_eof();
+	else
+		++position;
+	return c;
+}
+
+SeekOffset Slice::seek(SeekOffset off, SeekType type)
+{
+	if(type==S_BEG)
+		off += start_offset;
+	else if(type==S_CUR)
+		off += position;
+	else if(type==S_END)
+		off += start_offset+length;
+
+	if(off<start_offset || off>start_offset+length)
+		throw bad_seek(off, type);
+
+	signal_flush_required.emit();
+
+	position = below.seek(off, S_BEG)-start_offset;
+	sync_position = false;
+	return position;
+}
+
+} // namespace IO
+} // namespace Msp
diff --git a/source/io/slice.h b/source/io/slice.h
new file mode 100644
index 0000000..f611585
--- /dev/null
+++ b/source/io/slice.h
@@ -0,0 +1,50 @@
+#ifndef MSP_IO_SLICE_H_
+#define MSP_IO_SLICE_H_
+
+#include "seekable.h"
+
+namespace Msp {
+namespace IO {
+
+/**
+Presents a part of seekable I/O object as if it was a complete object.  This
+allows in-place access to embedded resources where the loader expects to
+receive a complete file.
+
+When a Slice is created, its read/write offset is placed at the beginning of
+its range.  If the offset of the underlying object is changed, the Slice will
+restore it before the next access.  This enables multiple Slices to be created
+on top of the same object.
+*/
+class Slice: public Seekable
+{
+private:
+	Seekable &below;
+	SeekOffset start_offset;
+	SeekOffset length;
+	SeekOffset position;
+	bool sync_position;
+
+public:
+	Slice(Seekable &, SeekOffset, SeekOffset);
+
+private:
+	void flush();
+
+	unsigned prepare_op(unsigned, Mode);
+protected:
+	virtual unsigned do_write(const char *, unsigned);
+	virtual unsigned do_read(char *, unsigned);
+
+public:
+	virtual unsigned put(char);
+	virtual int get();
+
+	virtual SeekOffset seek(SeekOffset, SeekType);
+	virtual SeekOffset tell() const { return position; }
+};
+
+} // namespace IO
+} // namespace Msp
+
+#endif
-- 
2.45.2