<title>Fossil Delta Format</title>
<h1>1.0 Overview</h1>

Fossil achieves efficient storage and low-bandwidth synchronization
through the use of delta-compression.  Instead of storing
or transmitting the complete content of an artifact, Fossil stores or
transmits only the changes relative to a related artifact.

This document describes the delta-encoding format used by Fossil.
The intended audience is developers working on either
<a href="index.wiki">Fossil</a> itself, or on tools compatible with
Fossil. Understanding of this document is <em>not</em> required for
ordinary users of Fossil.  This document is an implementation detail.

This document only describes the delta file format.  A 
[./delta_encoder_algorithm.wiki|separate document] describes one possible
algorithm for generating deltas in this format.

<h2>1.1 Sample Software And Analysis Tools</h2>

The core routines used to generate and apply deltas in this format
are contained in the [../src/delta.c|delta.c] source file.  Interface
logic, including "test-*" commands to directly manipulate deltas are
contained in the [../src/deltacmd.c|deltacmd.c] source file.  SQL functions
to create, apply, and analyze deltas are implemented by code in the
[../src/deltafunc.c|deltafunc.c] source file.

The following command-line tools are available to create and apply
deltas and to test the delta logic:

   *   [/help?cmd=test-delta|fossil test-delta] &rarr; Run self-tests of
       the delta logic

   *   [/help?cmd=test-delta-create|fossil test-delta-create X Y] &rarr; compute
       a delta that converts file X into file Y.  Output that delta.

   *   [/help?cmd=test-delta-apply|fossil test-delta-apply X D] &rarr; apply
       delta D to input file X and output the result.

   *   [/help?cmd=test-delta-analyze|fossil test-delta-analyze X Y] &rarr; compute
       and delta that converts file X into file Y but instead of writing the
       delta to output, write performance information about the delta.

When running the [/help?cmd=sqlite3|fossil sql] command to get an
interactive SQL session connected to the repository, the following
additional SQL functions are provided:

   *   <b>delta_create(</b><i>X</i><b>,</b><i>Y</i><b>)</b> &rarr;
       Compute a data that carries blob X into blob Y and return that delta
       as a blob.

   *   <b>delta_apply(</b><i>X</i><b>,</b><i>D</i><b>)</b> &rarr;
       Apply delta D to input blob X return a new blob which is the result.


   *   <b>delta_output_size(</b><i>D</i>)</b> &rarr;
       Return the size of the output that would result from applying delta D.

   *   <b>delta_parse(</b><i>D</i>)</b> &rarr; This is a table-valued function
       that returns one row for the header, for the trailer, and for each segment
       in delta D.


<h1 id="structure">2.0 Structure</h1>
<verbatim type="pikchr">
    leftmargin = 0.1
    box height 50% "Header"
    box same "Segments"
    box same "Trailer"
</verbatim>

A delta consists of three parts, a "header", a "trailer", and a
"segment-list" between them.

Both header and trailer provide information about the target
helping the decoder, and the segment-list describes how the target can
be constructed from the original.

<h2 id="header">2.1 Header</h2>
<verbatim type="pikchr">
    leftmargin = 0.1
    box height 50% "Size"
    box same "\"\\n\""
</verbatim>

The header consists of a single number followed by a newline
character (ASCII 0x0a). The number is the length of the target in
bytes.

This means that, given a delta, the decoder can compute the size of
the target (and allocate any necessary memory based on that) by simply
reading the first line of the delta and decoding the number found
there. In other words, before it has to decode everything else.

<h2 id="trailer">2.2 Trailer</h2>
<verbatim type="pikchr">
    leftmargin = 0.1
    box height 50% "Checksum"
    box same "\";\""
</verbatim>

The trailer consists of a single number followed by a semicolon (ASCII
0x3b). This number is a checksum of the target and can be used by a
decoder to verify that the delta applied correctly, reconstructing the
target from the original.

The checksum is computed by treating the target as a series of
32-bit integer numbers (MSB first), and summing these up, modulo
2^32-1. A target whose length is not a multiple of 4 is padded with
0-bytes (ASCII 0x00) at the end.

By putting this information at the end of the delta a decoder has
it available immediately after the target has been reconstructed
fully.

<h2 id="slist">2.3 Segment-List</h2>
<verbatim type="pikchr">
    leftmargin = 0.1
    PART1: [
        B1: box height 50% width 15% ""
        B2: box same ""
        B3: box same ""
            "***"
            box height 50% width 15% ""
        I1: line down 50% from B2 .s
            arrow right until even with B3.e
            box "Insert Literal" height 50%
            line down 75% from I1 .s
            arrow right until even with B3.e
            box "Copy Range" height 50%
    ]
    down
    PART2: [
        ""
        box "Length" height 50%
        right
        box "\":\"" same
        box "Bytes" same
    ] with .nw at previous.sw
</verbatim>

The segment-list of a delta describes how to create the target from
the original by a combination of inserting literal byte-sequences and
copying ranges of bytes from the original. This is where the
compression takes place, by encoding the large common parts of
original and target in small copy instructions.

The target is constructed from beginning to end, with the data
generated by each instruction appended after the data of all previous
instructions, with no gaps.

<h3 id="insertlit">2.3.1 Insert Literal</h3>

A literal is specified by two elements, the size of the literal in
bytes, and the bytes of the literal itself.

<verbatim type="pikchr">
    leftmargin = 0.1
    box "Length" height 50%
    box "\":\"" same
    box "Bytes" same
</verbatim>

The length is written first, followed by a colon character (ASCII
0x3a), followed by the bytes of the literal.

<h3 id="copyrange">2.3.2 Copy Range</h3>

A range to copy is specified by two numbers, the offset of the
first byte in the original to copy, and the size of the range, in
bytes. The size zero is special, its usage indicates that the range
extends to the end of the original.

<verbatim type="pikchr">
    leftmargin = 0.1
    box "Length" height 50%
    box "\"@\"" same
    box "Offset" same
    box "\",\"" same
</verbatim>


The length is written first, followed by an "at" character (ASCII
0x40), then the offset, followed by a comma (ASCII 0x2c).

<h1 id="intcoding">3.0 Encoding of integers</h1>

The format currently handles only 32 bit integer numbers. They are
written base-64 encoded, MSB first, and without leading
"0"-characters, except if they are significant (i.e. 0 => "0").

The base-64 encoding uses one character for each 6 bits of
the integer to be encoded.  The encoding characters are:

<pre>
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz~
</pre>

The least significant 6 bits of the integer are encoded by
the first character, followed by
the next 6 bits, and so on until all non-zero bits of the integer
are encoded.  The minimum number of encoding characters is used.
Note that for integers less than 10, the base-64 coding is a
ASCII decimal rendering of the number itself.

<h1 id="examples">4.0 Examples</h1>

<h2 id="examplesint">4.1 Integer encoding</h2>

<table>
<tr>
<th>Value</th>
<th>Encoding</th>
</tr>
<tr>
<td>0</td>
<td>0</td>
</tr>
<tr>
<td>6246</td>
<td>1Xb</td>
</tr>
<tr>
<td>-1101438770</td>
<td>2zMM3E</td>
</tr>
</table>

<h2 id="examplesdelta">4.2 Delta encoding</h2>

An example of a delta using the specified encoding is:

<pre>
1Xb
4E@0,2:thFN@4C,6:scenda1B@Jd,6:scenda5x@Kt,6:pieces79@Qt,F: Example: eskil~E@Y0,2zMM3E;</pre>
</pre>

This can be taken apart into the following parts:

<table>
<tr><th>What  </th> <th>Encoding         </th><th>Meaning </th><th>Details</th></tr>
<tr><td>Header</td> <td>1Xb              </td><td>Size    </td><td> 6246         </td></tr>
<tr><td>S-List</td> <td>4E@0,            </td><td>Copy    </td><td> 270 @ 0      </td></tr>
<tr><td>&nbsp;</td> <td>2:th             </td><td>Literal </td><td> 2 'th'       </td></tr>
<tr><td>&nbsp;</td> <td>FN@4C,           </td><td>Copy    </td><td> 983 @ 268        </td></tr>
<tr><td>&nbsp;</td> <td>6:scenda         </td><td>Literal </td><td> 6 'scenda'       </td></tr>
<tr><td>&nbsp;</td> <td>1B@Jd,           </td><td>Copy    </td><td> 75 @ 1256        </td></tr>
<tr><td>&nbsp;</td> <td>6:scenda         </td><td>Literal </td><td> 6 'scenda'       </td></tr>
<tr><td>&nbsp;</td> <td>5x@Kt,           </td><td>Copy    </td><td> 380 @ 1336       </td></tr>
<tr><td>&nbsp;</td> <td>6:pieces     </td><td>Literal </td><td> 6 'pieces'       </td></tr>
<tr><td>&nbsp;</td> <td>79@Qt,           </td><td>Copy    </td><td> 457 @ 1720     </td></tr>
<tr><td>&nbsp;</td> <td>F: Example: eskil</td><td>Literal </td><td> 15 ' Example: eskil'</td></tr>
<tr><td>&nbsp;</td> <td>~E@Y0,           </td><td>Copy    </td><td>  4046 @ 2176        </td></tr>
<tr><td>Trailer</td><td>2zMM3E           </td><td>Checksum</td><td> -1101438770         </td></tr>
</table>

The unified diff behind the above delta is

<verbatim>
bluepeak:(761) ~/Projects/Tcl/Fossil/Devel/devel > diff -u ../DELTA/old ../DELTA/new
--- ../DELTA/old        2007-08-23 21:14:40.000000000 -0700
+++ ../DELTA/new        2007-08-23 21:14:33.000000000 -0700
@@ -5,7 +5,7 @@

  *  If the server does not have write permission on the database
     file, or on the directory containing the database file (and
-    it is thus unable to update database because it cannot create
+    it is thus unable to update the database because it cannot create
     a rollback journal) then it currently fails silently on a push.
     It needs to return a helpful error.

@@ -27,8 +27,8 @@
  *  Additional information displayed for the "vinfo" page:

      +  All leaves of this version that are not included in the
-        descendant list.  With date, user, comment, and hyperlink.
-        Leaves in the descendant table should be marked as such.
+        descendant list.  With date, user, comment, and hyperlink.
+        Leaves in the descendant table should be marked as such.
         See the compute_leaves() function to see how to find all
         leaves.
      +  Add file diff links to the file change list.
@@ -37,7 +37,7 @@

  *  The /xfer handler (for push, pull, and clone) does not do
     delta compression.  This results in excess bandwidth usage.
-    There are some code in xfer.c that are sketches of ideas on
+    There are some pieces in xfer.c that are sketches of ideas on
     how to do delta compression, but nothing has been implemented.

  *  Enhancements to the diff and tkdiff commands in the cli.
@@ -45,7 +45,7 @@
     single file.  Allow diffs against any two arbitrary versions,
     not just diffs against the current check-out.  Allow
     configuration options to replace tkdiff with some other
-    visual differ of the users choice.
+    visual differ of the users choice. Example: eskil.

  *  Ticketing interface (expand this bullet)
</verbatim>



<h1 id="notes">Notes</h1>

<ul>
<li>Pure text files generate a pure text delta.
</li>
<li>Binary files generate a delta that may contain some binary data.
</li>
<li>The delta encoding does not attempt to compress the content.
It was considered to be much
more sensible to do compression using a separate general-purpose
compression library, like <a href="http://www.zlib.net">zlib</a>.
</li>
</ul>
