Subversion's delta format has the convenient property that applying
each section of the delta only requires examining (and keeping in
memory) a small portion of the preimage. At any moment, this portion
begins at a well-defined file offset and has a well-defined length,
and as the delta is applied, it moves from the beginning to the end
of the file. Add a move_window() function to keep track of such a
window into a file.
You can use it like this:
struct line_buffer preimage = LINE_BUFFER_INIT;
buffer_init(&preimage, NULL);
struct view window = {&preimage, 0, STRBUF_INIT};
move_window(&window, 3, 7); /* (1) */
move_window(&window, 5, 5); /* (2) */
move_window(&window, 12, 2); /* (3) */
strbuf_release(&window.buf);
buffer_deinit(&preimage);
In this example: (1) reads 10 bytes and discards the first 3;
(2) discards the first 2, which are not needed any more; and (3)
skips 2 bytes and reads 2 new bytes to work with.
Whenever move_window() returns, the file position indicator is at
position window->off + window->buf.len and the data from positions
window->off to the current file position are stored in window->buf.
This function does only sequential access and never seeks, so it
can be safely used on pipes and sockets.
On end-of-file, move_window() just silently reads less than the
caller requested. On other errors, it prints a message to stderr
and returns -1.
Helped-by: David Barr <david.barr@xxxxxxxxxxxx>
Signed-off-by: Jonathan Nieder <jrnieder@xxxxxxxxx>
---
Makefile | 5 ++-
vcs-svn/sliding_window.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++
vcs-svn/sliding_window.h | 14 ++++++++++
vcs-svn/LICENSE | 2 +
4 files changed, 84 insertions(+), 2 deletions(-)
create mode 100644 vcs-svn/sliding_window.c
create mode 100644 vcs-svn/sliding_window.h
diff --git a/Makefile b/Makefile
index 1f1ce04..d99da33 100644
--- a/Makefile
+++ b/Makefile
@@ -1765,7 +1765,8 @@ endif
XDIFF_OBJS = xdiff/xdiffi.o xdiff/xprepare.o xdiff/xutils.o xdiff/xemit.o \
xdiff/xmerge.o xdiff/xpatience.o
VCSSVN_OBJS = vcs-svn/string_pool.o vcs-svn/line_buffer.o \
- vcs-svn/repo_tree.o vcs-svn/fast_export.o vcs-svn/svndump.o
+ vcs-svn/repo_tree.o vcs-svn/fast_export.o vcs-svn/svndump.o \
+ vcs-svn/sliding_window.o
OBJECTS := $(GIT_OBJS) $(XDIFF_OBJS) $(VCSSVN_OBJS)
dep_files := $(foreach f,$(OBJECTS),$(dir $f).depend/$(notdir $f).d)
@@ -1892,7 +1893,7 @@ xdiff-interface.o $(XDIFF_OBJS): \
$(VCSSVN_OBJS): \
vcs-svn/obj_pool.h vcs-svn/trp.h vcs-svn/string_pool.h \
vcs-svn/line_buffer.h vcs-svn/repo_tree.h vcs-svn/fast_export.h \
- vcs-svn/svndump.h
+ vcs-svn/svndump.h vcs-svn/sliding_window.h
endif
exec_cmd.s exec_cmd.o: EXTRA_CPPFLAGS = \
diff --git a/vcs-svn/sliding_window.c b/vcs-svn/sliding_window.c
new file mode 100644
index 0000000..8273970
--- /dev/null
+++ b/vcs-svn/sliding_window.c
@@ -0,0 +1,65 @@
+/*
+ * Licensed under a two-clause BSD-style license.
+ * See LICENSE for details.
+ */
+
+#include "git-compat-util.h"
+#include "sliding_window.h"
+#include "line_buffer.h"
+#include "strbuf.h"
+
+static void strbuf_remove_from_left(struct strbuf *sb, size_t nbytes)
+{
+ assert(nbytes <= sb->len);
+ memmove(sb->buf, sb->buf + nbytes, sb->len - nbytes);
+ strbuf_setlen(sb, sb->len - nbytes);
+}
+
+static int check_overflow(off_t a, size_t b)
+{
+ if ((off_t) b < 0)
+ return error("Unrepresentable length: "
+ "%"PRIu64" > OFF_MAX", (uint64_t) b);
+ if (signed_add_overflows(a, (off_t) b))
+ return error("Unrepresentable offset: "
+ "%"PRIu64" + %"PRIu64" > OFF_MAX",
+ (uint64_t) a, (uint64_t) b);
+ return 0;
+}
+
+int move_window(struct view *view, off_t off, size_t len)
+{
+ off_t file_offset;
+ assert(view && view->file);
+ assert(!check_overflow(view->off, view->buf.len));
+
+ if (check_overflow(off, len))
+ return -1;
+ if (off < view->off || off + len < view->off + view->buf.len)
+ return error("Invalid delta: window slides left");
+
+ file_offset = view->off + view->buf.len;
+ if (off < file_offset)
+ /* Move the overlapping region into place. */
+ strbuf_remove_from_left(&view->buf, off - view->off);
+ else
+ strbuf_setlen(&view->buf, 0);
+ if (off > file_offset) {
+ /* Seek ahead to skip the gap. */
+ const off_t gap = off - file_offset;
+ const off_t nread = buffer_skip_bytes(view->file, gap);
+ if (nread != gap) {
+ if (!buffer_ferror(view->file)) /* View ends early. */
+ goto done;
+ return error("Cannot seek forward in input: %s",
+ strerror(errno));
+ }
+ file_offset += gap;
+ }
+ buffer_read_binary(&view->buf, len - view->buf.len, view->file);
+ if (buffer_ferror(view->file))
+ return error("Cannot read preimage: %s", strerror(errno));
+ done:
+ view->off = off;
+ return 0;
+}
diff --git a/vcs-svn/sliding_window.h b/vcs-svn/sliding_window.h
new file mode 100644
index 0000000..b9f0552
--- /dev/null
+++ b/vcs-svn/sliding_window.h
@@ -0,0 +1,14 @@
+#ifndef SLIDING_WINDOW_H_
+#define SLIDING_WINDOW_H_
+
+#include "strbuf.h"
+
+struct view {
+ struct line_buffer *file;
+ off_t off;
+ struct strbuf buf;
+};
+
+extern int move_window(struct view *view, off_t off, size_t len);
+
+#endif
diff --git a/vcs-svn/LICENSE b/vcs-svn/LICENSE
index 0a5e3c4..805882c 100644
--- a/vcs-svn/LICENSE
+++ b/vcs-svn/LICENSE
@@ -1,6 +1,8 @@
Copyright (C) 2010 David Barr <david.barr@xxxxxxxxxxxx>.
All rights reserved.
+Copyright (C) 2010 Jonathan Nieder <jrnieder@xxxxxxxxx>.
+
Copyright (C) 2008 Jason Evans <jasone@xxxxxxxxxxxxx>.
All rights reserved.
--
1.7.2.3
--
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