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authorJonathan Nieder <jrnieder@gmail.com>2011-01-03 03:54:58 (GMT)
committerJonathan Nieder <jrnieder@gmail.com>2011-03-28 01:23:32 (GMT)
commit9d2f5ddfe56fcc228a36dd079f0897e0f474eb4e (patch)
treeac08de7bff81eb1d33395d8cb1b2998a60475afb /vcs-svn/sliding_window.c
parent3371f9b3fb13a508c447e484618b9b5e2b9e45a7 (diff)
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vcs-svn: learn to maintain a sliding view of a file
Each section of a Subversion-format delta only requires examining (and keeping in random-access memory) a small portion of the preimage. At any moment, this portion starts at a certain file offset and has a well-defined length, and as the delta is applied, the portion advances from the beginning to the end of the preimage. Add a move_window function to keep track of this view into the preimage. You can use it like this: buffer_init(f, NULL); struct sliding_view window = SLIDING_VIEW_INIT(f); move_window(&window, 3, 7); /* (1) */ move_window(&window, 5, 5); /* (2) */ move_window(&window, 12, 2); /* (3) */ strbuf_release(&window.buf); buffer_deinit(f); The data structure is called sliding_view instead of _window to prevent confusion with svndiff0 Windows. 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. When move_window returns, the file position indicator is at position window->off + window->width and the data from positions window->off to the current file position are stored in window->buf. This function performs only sequential access from the input file and never seeks, so it can be safely used on pipes and sockets. On end-of-file, move_window silently reads less than the caller requested. On other errors, it prints a message and returns -1. Helped-by: David Barr <david.barr@cordelta.com> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Diffstat (limited to 'vcs-svn/sliding_window.c')
-rw-r--r--vcs-svn/sliding_window.c77
1 files changed, 77 insertions, 0 deletions
diff --git a/vcs-svn/sliding_window.c b/vcs-svn/sliding_window.c
new file mode 100644
index 0000000..1b8d987
--- /dev/null
+++ b/vcs-svn/sliding_window.c
@@ -0,0 +1,77 @@
+/*
+ * 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 int input_error(struct line_buffer *file)
+{
+ if (!buffer_ferror(file))
+ return error("delta preimage ends early");
+ return error("cannot read delta preimage: %s", strerror(errno));
+}
+
+static int skip_or_whine(struct line_buffer *file, off_t gap)
+{
+ if (buffer_skip_bytes(file, gap) != gap)
+ return input_error(file);
+ return 0;
+}
+
+static int read_to_fill_or_whine(struct line_buffer *file,
+ struct strbuf *buf, size_t width)
+{
+ buffer_read_binary(file, buf, width - buf->len);
+ if (buf->len != width)
+ return input_error(file);
+ return 0;
+}
+
+static int check_overflow(off_t a, size_t b)
+{
+ if (b > maximum_signed_value_of_type(off_t))
+ return error("unrepresentable length in delta: "
+ "%"PRIuMAX" > OFF_MAX", (uintmax_t) b);
+ if (signed_add_overflows(a, (off_t) b))
+ return error("unrepresentable offset in delta: "
+ "%"PRIuMAX" + %"PRIuMAX" > OFF_MAX",
+ (uintmax_t) a, (uintmax_t) b);
+ return 0;
+}
+
+int move_window(struct sliding_view *view, off_t off, size_t width)
+{
+ off_t file_offset;
+ assert(view);
+ assert(view->width <= view->buf.len);
+ assert(!check_overflow(view->off, view->buf.len));
+
+ if (check_overflow(off, width))
+ return -1;
+ if (off < view->off || off + width < view->off + view->width)
+ 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(&view->buf, 0, off - view->off);
+ } else {
+ /* Seek ahead to skip the gap. */
+ if (skip_or_whine(view->file, off - file_offset))
+ return -1;
+ strbuf_setlen(&view->buf, 0);
+ }
+
+ if (view->buf.len > width)
+ ; /* Already read. */
+ else if (read_to_fill_or_whine(view->file, &view->buf, width))
+ return -1;
+
+ view->off = off;
+ view->width = width;
+ return 0;
+}