git/vcs-svn/sliding_window.c
Jonathan Nieder 9d2f5ddfe5 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>
2011-03-27 20:23:32 -05:00

78 lines
1.9 KiB
C

/*
* 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;
}