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Overview
Comment:Completed the description of the delta encoder
Downloads: Tarball | ZIP archive
Timelines: family | ancestors | descendants | both | trunk
Files: files | file ages | folders
SHA1: 59ad045fef5234bfca9e0a83eb1ab98d8214b981
User & Date: aku 2007-08-27 04:35:10.000
Context
2007-08-28
03:18
Fixed typos. ... (check-in: 63564202fd user: aku tags: trunk)
2007-08-27
04:35
Completed the description of the delta encoder ... (check-in: 59ad045fef user: aku tags: trunk)
2007-08-26
22:22
Added section numbers to delta format, labels for linking, navigation bar. Added delta encoder description (incomplete, right now only all the trivial parts). Using TeX for formulas, and mimetex for conversion. ... (check-in: 6f1af23ebe user: aku tags: trunk)
Changes
Unified Diff Ignore Whitespace Patch
Added art/encode10.dia. 




































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































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<?xml version="1.0" encoding="UTF-8"?>
<dia:diagram xmlns:dia="http://www.lysator.liu.se/~alla/dia/">
  <dia:diagramdata>
    <dia:attribute name="background">
      <dia:color val="#ffffff"/>
    </dia:attribute>
    <dia:attribute name="pagebreak">
      <dia:color val="#000099"/>
    </dia:attribute>
    <dia:attribute name="paper">
      <dia:composite type="paper">
        <dia:attribute name="name">
          <dia:string>#Letter#</dia:string>
        </dia:attribute>
        <dia:attribute name="tmargin">
          <dia:real val="2.5399999618530273"/>
        </dia:attribute>
        <dia:attribute name="bmargin">
          <dia:real val="2.5399999618530273"/>
        </dia:attribute>
        <dia:attribute name="lmargin">
          <dia:real val="2.5399999618530273"/>
        </dia:attribute>
        <dia:attribute name="rmargin">
          <dia:real val="2.5399999618530273"/>
        </dia:attribute>
        <dia:attribute name="is_portrait">
          <dia:boolean val="true"/>
        </dia:attribute>
        <dia:attribute name="scaling">
          <dia:real val="1"/>
        </dia:attribute>
        <dia:attribute name="fitto">
          <dia:boolean val="false"/>
        </dia:attribute>
      </dia:composite>
    </dia:attribute>
    <dia:attribute name="grid">
      <dia:composite type="grid">
        <dia:attribute name="width_x">
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        <dia:attribute name="width_y">
          <dia:real val="1"/>
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        <dia:attribute name="visible_x">
          <dia:int val="1"/>
        </dia:attribute>
        <dia:attribute name="visible_y">
          <dia:int val="1"/>
        </dia:attribute>
        <dia:composite type="color"/>
      </dia:composite>
    </dia:attribute>
    <dia:attribute name="color">
      <dia:color val="#d8e5e5"/>
    </dia:attribute>
    <dia:attribute name="guides">
      <dia:composite type="guides">
        <dia:attribute name="hguides"/>
        <dia:attribute name="vguides"/>
      </dia:composite>
    </dia:attribute>
  </dia:diagramdata>
  <dia:layer name="Background" visible="true">
    <dia:group>
      <dia:group>
        <dia:object type="Standard - Box" version="0" id="O0">
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            <dia:point val="6,2"/>
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          <dia:attribute name="obj_bb">
            <dia:rectangle val="5.975,1.975;12.025,22.025"/>
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          <dia:attribute name="elem_corner">
            <dia:point val="6,2"/>
          </dia:attribute>
          <dia:attribute name="elem_width">
            <dia:real val="6"/>
          </dia:attribute>
          <dia:attribute name="elem_height">
            <dia:real val="20"/>
          </dia:attribute>
          <dia:attribute name="border_width">
            <dia:real val="0.050000000000000003"/>
          </dia:attribute>
          <dia:attribute name="show_background">
            <dia:boolean val="true"/>
          </dia:attribute>
          <dia:attribute name="corner_radius">
            <dia:real val="1.1754943508222875e-38"/>
          </dia:attribute>
        </dia:object>
        <dia:group>
          <dia:object type="Standard - Box" version="0" id="O1">
            <dia:attribute name="obj_pos">
              <dia:point val="6,8"/>
            </dia:attribute>
            <dia:attribute name="obj_bb">
              <dia:rectangle val="5.975,7.975;12.025,9.025"/>
            </dia:attribute>
            <dia:attribute name="elem_corner">
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            </dia:attribute>
            <dia:attribute name="elem_width">
              <dia:real val="6"/>
            </dia:attribute>
            <dia:attribute name="elem_height">
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            </dia:attribute>
            <dia:attribute name="border_width">
              <dia:real val="0.050000000000000003"/>
            </dia:attribute>
            <dia:attribute name="inner_color">
              <dia:color val="#ffff00"/>
            </dia:attribute>
            <dia:attribute name="show_background">
              <dia:boolean val="true"/>
            </dia:attribute>
            <dia:attribute name="corner_radius">
              <dia:real val="1.1754943508222875e-38"/>
            </dia:attribute>
          </dia:object>
          <dia:object type="Standard - Text" version="1" id="O2">
            <dia:attribute name="obj_pos">
              <dia:point val="5.95,8.4925"/>
            </dia:attribute>
            <dia:attribute name="obj_bb">
              <dia:rectangle val="5.95,7.95;10.145,8.89"/>
            </dia:attribute>
            <dia:attribute name="text">
              <dia:composite type="text">
                <dia:attribute name="string">
                  <dia:string>#Gap =&gt; Insert#</dia:string>
                </dia:attribute>
                <dia:attribute name="font">
                  <dia:font family="sans" style="0" name="Helvetica"/>
                </dia:attribute>
                <dia:attribute name="height">
                  <dia:real val="0.80000000000000004"/>
                </dia:attribute>
                <dia:attribute name="pos">
                  <dia:point val="5.95,8.4925"/>
                </dia:attribute>
                <dia:attribute name="color">
                  <dia:color val="#000000"/>
                </dia:attribute>
                <dia:attribute name="alignment">
                  <dia:enum val="0"/>
                </dia:attribute>
              </dia:composite>
            </dia:attribute>
            <dia:attribute name="valign">
              <dia:enum val="3"/>
            </dia:attribute>
          </dia:object>
        </dia:group>
        <dia:group>
          <dia:object type="Standard - Box" version="0" id="O3">
            <dia:attribute name="obj_pos">
              <dia:point val="6,2"/>
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            <dia:attribute name="obj_bb">
              <dia:rectangle val="5.975,1.975;12.025,8.025"/>
            </dia:attribute>
            <dia:attribute name="elem_corner">
              <dia:point val="6,2"/>
            </dia:attribute>
            <dia:attribute name="elem_width">
              <dia:real val="6"/>
            </dia:attribute>
            <dia:attribute name="elem_height">
              <dia:real val="6"/>
            </dia:attribute>
            <dia:attribute name="border_width">
              <dia:real val="0.050000000000000003"/>
            </dia:attribute>
            <dia:attribute name="inner_color">
              <dia:color val="#90ee90"/>
            </dia:attribute>
            <dia:attribute name="show_background">
              <dia:boolean val="true"/>
            </dia:attribute>
            <dia:attribute name="corner_radius">
              <dia:real val="1.1754943508222875e-38"/>
            </dia:attribute>
          </dia:object>
          <dia:object type="Standard - Text" version="1" id="O4">
            <dia:attribute name="obj_pos">
              <dia:point val="5.975,2.53456"/>
            </dia:attribute>
            <dia:attribute name="obj_bb">
              <dia:rectangle val="5.975,1.99206;8.91195,2.93206"/>
            </dia:attribute>
            <dia:attribute name="text">
              <dia:composite type="text">
                <dia:attribute name="string">
                  <dia:string>#Processed#</dia:string>
                </dia:attribute>
                <dia:attribute name="font">
                  <dia:font family="sans" style="0" name="Helvetica"/>
                </dia:attribute>
                <dia:attribute name="height">
                  <dia:real val="0.80000000000000004"/>
                </dia:attribute>
                <dia:attribute name="pos">
                  <dia:point val="5.975,2.53456"/>
                </dia:attribute>
                <dia:attribute name="color">
                  <dia:color val="#000000"/>
                </dia:attribute>
                <dia:attribute name="alignment">
                  <dia:enum val="0"/>
                </dia:attribute>
              </dia:composite>
            </dia:attribute>
            <dia:attribute name="valign">
              <dia:enum val="3"/>
            </dia:attribute>
          </dia:object>
        </dia:group>
        <dia:group>
          <dia:object type="Standard - Box" version="0" id="O5">
            <dia:attribute name="obj_pos">
              <dia:point val="6,9"/>
            </dia:attribute>
            <dia:attribute name="obj_bb">
              <dia:rectangle val="5.975,8.975;12.025,18.025"/>
            </dia:attribute>
            <dia:attribute name="elem_corner">
              <dia:point val="6,9"/>
            </dia:attribute>
            <dia:attribute name="elem_width">
              <dia:real val="6"/>
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            <dia:attribute name="elem_height">
              <dia:real val="9"/>
            </dia:attribute>
            <dia:attribute name="border_width">
              <dia:real val="0.050000000000000003"/>
            </dia:attribute>
            <dia:attribute name="inner_color">
              <dia:color val="#ffa500"/>
            </dia:attribute>
            <dia:attribute name="show_background">
              <dia:boolean val="true"/>
            </dia:attribute>
            <dia:attribute name="corner_radius">
              <dia:real val="1.1754943508222875e-38"/>
            </dia:attribute>
          </dia:object>
          <dia:object type="Standard - Text" version="1" id="O6">
            <dia:attribute name="obj_pos">
              <dia:point val="5.975,9.53456"/>
            </dia:attribute>
            <dia:attribute name="obj_bb">
              <dia:rectangle val="5.975,8.99206;11.4275,9.93206"/>
            </dia:attribute>
            <dia:attribute name="text">
              <dia:composite type="text">
                <dia:attribute name="string">
                  <dia:string>#Common =&gt; Copy#</dia:string>
                </dia:attribute>
                <dia:attribute name="font">
                  <dia:font family="sans" style="0" name="Helvetica"/>
                </dia:attribute>
                <dia:attribute name="height">
                  <dia:real val="0.80000000000000004"/>
                </dia:attribute>
                <dia:attribute name="pos">
                  <dia:point val="5.975,9.53456"/>
                </dia:attribute>
                <dia:attribute name="color">
                  <dia:color val="#000000"/>
                </dia:attribute>
                <dia:attribute name="alignment">
                  <dia:enum val="0"/>
                </dia:attribute>
              </dia:composite>
            </dia:attribute>
            <dia:attribute name="valign">
              <dia:enum val="3"/>
            </dia:attribute>
          </dia:object>
        </dia:group>
        <dia:group>
          <dia:object type="Standard - Box" version="0" id="O7">
            <dia:attribute name="obj_pos">
              <dia:point val="5,11"/>
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            <dia:attribute name="obj_bb">
              <dia:rectangle val="4.95,10.95;13.05,15.05"/>
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            <dia:attribute name="elem_corner">
              <dia:point val="5,11"/>
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            <dia:attribute name="elem_width">
              <dia:real val="8"/>
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            <dia:attribute name="elem_height">
              <dia:real val="4"/>
            </dia:attribute>
            <dia:attribute name="inner_color">
              <dia:color val="#bfbfbf"/>
            </dia:attribute>
            <dia:attribute name="show_background">
              <dia:boolean val="true"/>
            </dia:attribute>
            <dia:attribute name="corner_radius">
              <dia:real val="1.1754943508222875e-38"/>
            </dia:attribute>
          </dia:object>
          <dia:object type="Standard - Text" version="1" id="O8">
            <dia:attribute name="obj_pos">
              <dia:point val="4.95,11.9675"/>
            </dia:attribute>
            <dia:attribute name="obj_bb">
              <dia:rectangle val="4.95,10.95;9.26848,12.71"/>
            </dia:attribute>
            <dia:attribute name="text">
              <dia:composite type="text">
                <dia:attribute name="string">
                  <dia:string>#Window#</dia:string>
                </dia:attribute>
                <dia:attribute name="font">
                  <dia:font family="sans" style="0" name="Helvetica"/>
                </dia:attribute>
                <dia:attribute name="height">
                  <dia:real val="1.5"/>
                </dia:attribute>
                <dia:attribute name="pos">
                  <dia:point val="4.95,11.9675"/>
                </dia:attribute>
                <dia:attribute name="color">
                  <dia:color val="#000000"/>
                </dia:attribute>
                <dia:attribute name="alignment">
                  <dia:enum val="0"/>
                </dia:attribute>
              </dia:composite>
            </dia:attribute>
            <dia:attribute name="valign">
              <dia:enum val="3"/>
            </dia:attribute>
          </dia:object>
        </dia:group>
      </dia:group>
      <dia:group>
        <dia:object type="Standard - Box" version="0" id="O9">
          <dia:attribute name="obj_pos">
            <dia:point val="18,2"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="17.975,1.975;24.025,24.025"/>
          </dia:attribute>
          <dia:attribute name="elem_corner">
            <dia:point val="18,2"/>
          </dia:attribute>
          <dia:attribute name="elem_width">
            <dia:real val="6"/>
          </dia:attribute>
          <dia:attribute name="elem_height">
            <dia:real val="22"/>
          </dia:attribute>
          <dia:attribute name="border_width">
            <dia:real val="0.050000000000000003"/>
          </dia:attribute>
          <dia:attribute name="show_background">
            <dia:boolean val="true"/>
          </dia:attribute>
          <dia:attribute name="corner_radius">
            <dia:real val="1.1754943508222875e-38"/>
          </dia:attribute>
        </dia:object>
        <dia:object type="Standard - Box" version="0" id="O10">
          <dia:attribute name="obj_pos">
            <dia:point val="18,5"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="17.975,4.975;24.025,14.025"/>
          </dia:attribute>
          <dia:attribute name="elem_corner">
            <dia:point val="18,5"/>
          </dia:attribute>
          <dia:attribute name="elem_width">
            <dia:real val="6"/>
          </dia:attribute>
          <dia:attribute name="elem_height">
            <dia:real val="9"/>
          </dia:attribute>
          <dia:attribute name="border_width">
            <dia:real val="0.050000000000000003"/>
          </dia:attribute>
          <dia:attribute name="inner_color">
            <dia:color val="#ffa500"/>
          </dia:attribute>
          <dia:attribute name="show_background">
            <dia:boolean val="true"/>
          </dia:attribute>
          <dia:attribute name="corner_radius">
            <dia:real val="1.1754943508222875e-38"/>
          </dia:attribute>
        </dia:object>
      </dia:group>
      <dia:group>
        <dia:object type="Standard - Text" version="1" id="O11">
          <dia:attribute name="obj_pos">
            <dia:point val="14,9"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="14,8.4575;16.8122,9.3975"/>
          </dia:attribute>
          <dia:attribute name="text">
            <dia:composite type="text">
              <dia:attribute name="string">
                <dia:string>#Signature#</dia:string>
              </dia:attribute>
              <dia:attribute name="font">
                <dia:font family="sans" style="0" name="Helvetica"/>
              </dia:attribute>
              <dia:attribute name="height">
                <dia:real val="0.80000000000000004"/>
              </dia:attribute>
              <dia:attribute name="pos">
                <dia:point val="14,9"/>
              </dia:attribute>
              <dia:attribute name="color">
                <dia:color val="#000000"/>
              </dia:attribute>
              <dia:attribute name="alignment">
                <dia:enum val="0"/>
              </dia:attribute>
            </dia:composite>
          </dia:attribute>
          <dia:attribute name="valign">
            <dia:enum val="3"/>
          </dia:attribute>
        </dia:object>
        <dia:object type="Standard - ZigZagLine" version="1" id="O12">
          <dia:attribute name="obj_pos">
            <dia:point val="12,13"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="11.95,9;18.05,13.05"/>
          </dia:attribute>
          <dia:attribute name="orth_points">
            <dia:point val="12,13"/>
            <dia:point val="15,13"/>
            <dia:point val="15,9.5"/>
            <dia:point val="18,9.5"/>
          </dia:attribute>
          <dia:attribute name="orth_orient">
            <dia:enum val="0"/>
            <dia:enum val="1"/>
            <dia:enum val="0"/>
          </dia:attribute>
          <dia:attribute name="autorouting">
            <dia:boolean val="true"/>
          </dia:attribute>
          <dia:attribute name="end_arrow">
            <dia:enum val="22"/>
          </dia:attribute>
          <dia:attribute name="end_arrow_length">
            <dia:real val="0.5"/>
          </dia:attribute>
          <dia:attribute name="end_arrow_width">
            <dia:real val="0.5"/>
          </dia:attribute>
        </dia:object>
      </dia:group>
      <dia:group>
        <dia:object type="Standard - Text" version="1" id="O13">
          <dia:attribute name="obj_pos">
            <dia:point val="2,8"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="2,6.9825;4.6625,8.7425"/>
          </dia:attribute>
          <dia:attribute name="text">
            <dia:composite type="text">
              <dia:attribute name="string">
                <dia:string>#Base#</dia:string>
              </dia:attribute>
              <dia:attribute name="font">
                <dia:font family="sans" style="0" name="Helvetica"/>
              </dia:attribute>
              <dia:attribute name="height">
                <dia:real val="1.5"/>
              </dia:attribute>
              <dia:attribute name="pos">
                <dia:point val="2,8"/>
              </dia:attribute>
              <dia:attribute name="color">
                <dia:color val="#000000"/>
              </dia:attribute>
              <dia:attribute name="alignment">
                <dia:enum val="0"/>
              </dia:attribute>
            </dia:composite>
          </dia:attribute>
          <dia:attribute name="valign">
            <dia:enum val="3"/>
          </dia:attribute>
        </dia:object>
        <dia:object type="Standard - Line" version="0" id="O14">
          <dia:attribute name="obj_pos">
            <dia:point val="5,8"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="4.95,7.5;6.05,8.5"/>
          </dia:attribute>
          <dia:attribute name="conn_endpoints">
            <dia:point val="5,8"/>
            <dia:point val="6,8"/>
          </dia:attribute>
          <dia:attribute name="numcp">
            <dia:int val="1"/>
          </dia:attribute>
          <dia:attribute name="end_arrow">
            <dia:enum val="22"/>
          </dia:attribute>
          <dia:attribute name="end_arrow_length">
            <dia:real val="0.5"/>
          </dia:attribute>
          <dia:attribute name="end_arrow_width">
            <dia:real val="0.5"/>
          </dia:attribute>
        </dia:object>
      </dia:group>
      <dia:group>
        <dia:object type="Standard - Text" version="1" id="O15">
          <dia:attribute name="obj_pos">
            <dia:point val="1,11"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="1,9.9825;3.6725,11.7425"/>
          </dia:attribute>
          <dia:attribute name="text">
            <dia:composite type="text">
              <dia:attribute name="string">
                <dia:string>#Slide#</dia:string>
              </dia:attribute>
              <dia:attribute name="font">
                <dia:font family="sans" style="0" name="Helvetica"/>
              </dia:attribute>
              <dia:attribute name="height">
                <dia:real val="1.5"/>
              </dia:attribute>
              <dia:attribute name="pos">
                <dia:point val="1,11"/>
              </dia:attribute>
              <dia:attribute name="color">
                <dia:color val="#000000"/>
              </dia:attribute>
              <dia:attribute name="alignment">
                <dia:enum val="0"/>
              </dia:attribute>
            </dia:composite>
          </dia:attribute>
          <dia:attribute name="valign">
            <dia:enum val="3"/>
          </dia:attribute>
        </dia:object>
        <dia:object type="Standard - Line" version="0" id="O16">
          <dia:attribute name="obj_pos">
            <dia:point val="4,11"/>
          </dia:attribute>
          <dia:attribute name="obj_bb">
            <dia:rectangle val="3.95,10.5;5.05,11.5"/>
          </dia:attribute>
          <dia:attribute name="conn_endpoints">
            <dia:point val="4,11"/>
            <dia:point val="5,11"/>
          </dia:attribute>
          <dia:attribute name="numcp">
            <dia:int val="1"/>
          </dia:attribute>
          <dia:attribute name="end_arrow">
            <dia:enum val="22"/>
          </dia:attribute>
          <dia:attribute name="end_arrow_length">
            <dia:real val="0.5"/>
          </dia:attribute>
          <dia:attribute name="end_arrow_width">
            <dia:real val="0.5"/>
          </dia:attribute>
        </dia:object>
      </dia:group>
      <dia:object type="Standard - Text" version="1" id="O17">
        <dia:attribute name="obj_pos">
          <dia:point val="6,2"/>
        </dia:attribute>
        <dia:attribute name="obj_bb">
          <dia:rectangle val="6,0.9825;9.38727,2.7425"/>
        </dia:attribute>
        <dia:attribute name="text">
          <dia:composite type="text">
            <dia:attribute name="string">
              <dia:string>#Target#</dia:string>
            </dia:attribute>
            <dia:attribute name="font">
              <dia:font family="sans" style="0" name="Helvetica"/>
            </dia:attribute>
            <dia:attribute name="height">
              <dia:real val="1.5"/>
            </dia:attribute>
            <dia:attribute name="pos">
              <dia:point val="6,2"/>
            </dia:attribute>
            <dia:attribute name="color">
              <dia:color val="#000000"/>
            </dia:attribute>
            <dia:attribute name="alignment">
              <dia:enum val="0"/>
            </dia:attribute>
          </dia:composite>
        </dia:attribute>
        <dia:attribute name="valign">
          <dia:enum val="3"/>
        </dia:attribute>
      </dia:object>
      <dia:object type="Standard - Text" version="1" id="O18">
        <dia:attribute name="obj_pos">
          <dia:point val="18,2"/>
        </dia:attribute>
        <dia:attribute name="obj_bb">
          <dia:rectangle val="18,0.9825;21.3125,2.7425"/>
        </dia:attribute>
        <dia:attribute name="text">
          <dia:composite type="text">
            <dia:attribute name="string">
              <dia:string>#Origin#</dia:string>
            </dia:attribute>
            <dia:attribute name="font">
              <dia:font family="sans" style="0" name="Helvetica"/>
            </dia:attribute>
            <dia:attribute name="height">
              <dia:real val="1.5"/>
            </dia:attribute>
            <dia:attribute name="pos">
              <dia:point val="18,2"/>
            </dia:attribute>
            <dia:attribute name="color">
              <dia:color val="#000000"/>
            </dia:attribute>
            <dia:attribute name="alignment">
              <dia:enum val="0"/>
            </dia:attribute>
          </dia:composite>
        </dia:attribute>
        <dia:attribute name="valign">
          <dia:enum val="3"/>
        </dia:attribute>
      </dia:object>
    </dia:group>
  </dia:layer>
</dia:diagram>
Changes to www/delta_encoder_algorithm.html.
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<li>A hashtable is filled, mapping from the hashes of the chunks to
the list of chunk locations having this hash.
</li>
</ol>

<a name="processing"><h3>2.1 Processing the target</h3>







<b>... to be completed ... </b>





































































<a name="exceptions"><h2>3.0 Exceptions</h2>

<p>If the "original" is at most NHASH bytes long no compression of
changes is possible, and the segment-list of the delta consists of a
single literal which contains the entire "target".</p>





<a name="rollhash"><h2>4.0 The rolling hash</h2>

<p>The rolling hash described below and used to compute content
signatures was chosen not only for good hashing properties, but also
to enable the easy (incremental) recalculation of its value for a
sliding window, i.e. where the oldest character is removed from the
window and a new character is shifted in.<p>

<a name="rhdef"><h2>4.1 Definition</h2>

<p>Assuming an array Z of NHASH bytes (indexing starting at 0) the
hash V is computed via</p>

<p align=center><table><tr><td>
<p><img src="encode1.gif" align="center"></p>
<p><img src="encode2.gif" align="center"></p>
<p><img src="encode3.gif" align="center"></p>
</td></tr></table></p>

where A and B are unsigned 16-bit integers (hence the <u>mod</u>), and
V is a 32-bit unsigned integer with B as MSB, A as LSB.

<a name="rhincr"><h2>4.2 Incremental recalculation</h2>

<p>Assuming an array Z of NHASH bytes (indexing starting at 0) with
hash V (and components A and B), the dropped
byte <img src="encode4.gif" align="center">, and the new byte
<img src="encode5.gif" align="center"> , the new hash can
be computed incrementally via: </p>









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<li>A hashtable is filled, mapping from the hashes of the chunks to
the list of chunk locations having this hash.
</li>
</ol>

<a name="processing"><h3>2.1 Processing the target</h3>

<p>This, the main phase of the encoder, processes the target in a loop
from beginning to end. The state of the encoder is captured by two
locations, the "base" and the "slide". "base" points to the first byte
of the target for which no delta output has been generated yet, and
"slide" is the location of the window used to look in the "origin" for
commonalities. This window is NHASH bytes long.</p>

<p>Initially both "base" and "slide" point to the beginning of the
"target". In each iteration of the loop the encoder decides whether to
<ul>
<li>emit a single instruction
to <a href="delta_format.html#copyrange">copy a range</a>, or
</li>
<li>emit two instructions, first
to <a href="delta_format.html#insertlit">insert a literal</a>, then
to <a href="delta_format.html#copyrange">copy a range</a>, or
</li>
<li>move the window forward one byte.
</li>
</ul>
</p>

<img src="encode10.gif" align="right" hspace="10">
<p>To make this decision the encoder first computes the hash value for
the NHASH bytes in the window and then looks at all the locations in
the "origin" which have the same signature. This part uses the hash
table created by the pre-processing step to effiently find these
locations.</p>

<p>For each of the possible candidates the encoder finds the maximal
range of bytes common to both "origin" and "target", going forward and
backward from "slide" in the "target", and the candidate location in
the "origin". This search is constrained on the side of the "target"
by the "base" (backward search), and the end of the "target" (forward
search), and on the side of the "origin" by the beginning and end of
the "origin", respectively.</p>

<p>From the ranges for all the candidates the best (= largest) common
range is taken and it is determined how many bytes are needed to
encode the bytes between the "base" and the end of that range. If the
range extended back to the "base" then this can be done in a single
copy instruction. Otherwise, i.e if there is a gap between the "base"
and the beginning of the range then two instructions are needed, one
to insert the bytes in the gap as a literal, and a copy instruction
for the range itself. The general situation at this point can be seen
in the picture to the right.</p>

<p>If the number of bytes needed to encode both gap (if present), and
range is less than the number of bytes we are encoding the encoder
will emit the necessary instructions as described above, set "base"
and "slide" to the end of the encoded range and start the next
iteration at that point.</p>

<p>If, on the other hand, the encoder either did not find candidate
locations in the origin, or the best range coming out of the search
needed more bytes to encode the range than there were bytes in the
range, then no instructions are emitted and the window is moved one
byte forward. The "base" is left unchanged in that case.</p>

<p>The processing loop stops at one of two conditions:
<ol>
<li>The encoder decided to move the window forward, but the end of the
window reached the end of the "target". 
</li>
<li>After the emission of instructions the new "base" location is
within NHASH bytes of end of the "target", i.e. there are no more than
at most NHASH bytes left.
</li>
</ol>
</p>

<p>If the processing loop left bytes unencoded, i.e. "base" not
exactly at the end of the "target", as is possible for both end
conditions, then one last insert instruction is emitted to put these
bytes into the delta.<p>

<a name="exceptions"><h2>3.0 Exceptions</h2>

<p>If the "original" is at most NHASH bytes long no compression of
changes is possible, and the segment-list of the delta consists of a
single literal which contains the entire "target".</p>

<p>This is actually equivalent to the second end condition of the
processing loop described in the previous section, just checked before
actually entering the loop.</p>

<a name="rollhash"><h2>4.0 The rolling hash</h2>

<p>The rolling hash described below and used to compute content
signatures was chosen not only for good hashing properties, but also
to enable the easy (incremental) recalculation of its value for a
sliding window, i.e. where the oldest byte is removed from the window
and a new byte is shifted in.<p>

<a name="rhdef"><h3>4.1 Definition</h3>

<p>Assuming an array Z of NHASH bytes (indexing starting at 0) the
hash V is computed via</p>

<p align=center><table><tr><td>
<p><img src="encode1.gif" align="center"></p>
<p><img src="encode2.gif" align="center"></p>
<p><img src="encode3.gif" align="center"></p>
</td></tr></table></p>

where A and B are unsigned 16-bit integers (hence the <u>mod</u>), and
V is a 32-bit unsigned integer with B as MSB, A as LSB.

<a name="rhincr"><h3>4.2 Incremental recalculation</h3>

<p>Assuming an array Z of NHASH bytes (indexing starting at 0) with
hash V (and components A and B), the dropped
byte <img src="encode4.gif" align="center">, and the new byte
<img src="encode5.gif" align="center"> , the new hash can
be computed incrementally via: </p>
︙︙
Added www/encode10.gif.

cannot compute difference between binary files

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