Fossil

   *   <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.


<a name="structure"></a><h1>2.0 Structure</h1>
<verbatim type="pikchr">
    box invisible width 0.2 ""
    box height 50% "Header"
    box same "Segments"
    box same "Trailer"
</verbatim>

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

<p>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.</p>

<a name="header"></a><h2>2.1 Header</h2>
<verbatim type="pikchr">
    box invisible width 0.2 ""
    box height 50% "Size"
    box same "\"\n\""
</verbatim>

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

<p>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.</p>

<a name="trailer"></a><h2>2.2 Trailer</h2>
<verbatim type="pikchr">
    box invisible width 0.2 ""
    box height 50% "Checksum"
    box same "\":\""
</verbatim>

<p>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.</p>

<p>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.</p>

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

<a name="slist"></a><h2>2.3 Segment-List</h2>
<verbatim type="pikchr">
    box invisible width 0.2 ""
    PART1: [
        B1: box height 50% width 15% ""
        B2: box same ""
        B3: box same ""
            move 0.1 ; box same invisible "***" ; move 0.1
            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
    box invisible "" height 0.1
    box "Length" height 50% with .nw at (PART1.w, previous.s)
    right
    box "\":\"" same
    box "Bytes" same
</verbatim>

<p>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.</p>

<p>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.</p>

<a name="insertlit"></a><h3>2.3.1 Insert Literal</h3>

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

<verbatim type="pikchr">
    box invisible width 0.2 ""
    box "Length" height 50%
    box "\":\"" same
    box "Bytes" same
</verbatim>

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

<a name="copyrange"></a><h3>2.3.2 Copy Range</h3>

<p>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.</p>

<verbatim type="pikchr">
    box invisible width 0.2 ""
    box "Length" height 50%
    box "\"@\"" same
    box "Offset" same
    box "\",\"" same
</verbatim>


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

<a name="intcoding"></a><h1>3.0 Encoding of integers</h1>

<p>
The format currently handles only 32 bit integer numbers. They are

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

<p>This can be taken apart into the following parts:</p>

<table border=1>
<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>

<p>The unified diff behind the above delta is</p>