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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head><meta http-equiv="content-type" content="text/html; charset=UTF-8" /><title>FPGenerator xref</title><link type="text/css" rel="stylesheet" href="../../../stylesheet.css" /></head><body><div id="overview"><a href="../../../../apidocs/st/ata/util/FPGenerator.html">View Javadoc</a></div><pre><a name="1" href="#1">1</a> <a name="2" href="#2">2</a> <strong>package</strong> <a href="../../../st/ata/util/package-summary.html">st.ata.util</a>;<a name="3" href="#3">3</a> <a name="4" href="#4">4</a> <strong>import</strong> java.util.Hashtable;<a name="5" href="#5">5</a> <a name="6" href="#6">6</a> <em>/**<em>*</em></em><a name="7" href="#7">7</a> <a name="8" href="#8">8</a> <em><p> This class provides methods that construct fingerprints of strings</em><a name="9" href="#9">9</a> <em>of bytes via operations in <i>GF[2^d]</i> for <i>0 < d <= 64</i>.</em><a name="10" href="#10">10</a> <em><i>GF[2^d]</i> is represented as the set of polynomials of degree</em><a name="11" href="#11">11</a> <em><i>d</i> with coefficients in <i>Z(2)</i>, modulo an irreducible</em><a name="12" href="#12">12</a> <em>polynomial <i>P</i> of degree <i>d</i>. The representation of</em><a name="13" href="#13">13</a> <em>polynomials is as an unsigned binary number in which the least</em><a name="14" href="#14">14</a> <em>significant exponent is kept in the most significant bit.</em><a name="15" href="#15">15</a> <a name="16" href="#16">16</a> <em><p> Let S be a string of bytes and <i>g(S)</i> the string obtained by</em><a name="17" href="#17">17</a> <em>taking the byte <code>0x01</code> followed by eight <code>0x00</code></em><a name="18" href="#18">18</a> <em>bytes followed by <code>S</code>. Let <i>f(S)</i> be the polynomial</em><a name="19" href="#19">19</a> <em>associated to the string <i>S</i> viewed as a polynomial with</em><a name="20" href="#20">20</a> <em>coefficients in the field <i>Z(2)</i>. The fingerprint of S is simply</em><a name="21" href="#21">21</a> <em>the value <i>f(g(S))</i> modulo <i>P</i>. Because polynomials are</em><a name="22" href="#22">22</a> <em>represented with the least significant coefficient in the most</em><a name="23" href="#23">23</a> <em>significant bit, fingerprints of degree <i>d</i> are stored in the</em><a name="24" href="#24">24</a> <em><code>d</code> <strong>most</code> significant bits of a long word.</em><a name="25" href="#25">25</a> <a name="26" href="#26">26</a> <em><p> Fingerprints can be used as a probably unique id for the input</em><a name="27" href="#27">27</a> <em>string. More precisely, if <i>P</i> is chosen at random among</em><a name="28" href="#28">28</a> <em>irreducible polynomials of degree <i>d</i>, then the probability that</em><a name="29" href="#29">29</a> <em>any two strings <i>A</i> and <i>B</i> have the same fingerprint is</em><a name="30" href="#30">30</a> <em>less than <i>max(|A|,|B|)/2^(d+1)</i> where <i>|A|</i> is the length</em><a name="31" href="#31">31</a> <em>of A in bits.</em><a name="32" href="#32">32</a> <a name="33" href="#33">33</a> <em><p> The routines named <code>extend[8]</code> and <code>fp[8]</code></em><a name="34" href="#34">34</a> <em>return reduced results, while <code>extend_[byte/char/int/long]</code></em><a name="35" href="#35">35</a> <em>do not. An <em>un</em>reduced result is a number that is equal (mod</em><a name="36" href="#36">36</a> <em></code>polynomial</code> to the desired fingerprint but may have</em><a name="37" href="#37">37</a> <em>degree <code>degree</code> or higher. The method <code>reduce</code></em><a name="38" href="#38">38</a> <em>reduces such a result to a polynomial of degree less than</em><a name="39" href="#39">39</a> <em><code>degree</code>. Obtaining reduced results takes longer than</em><a name="40" href="#40">40</a> <em>obtaining unreduced results; thus, when fingerprinting long strings,</em><a name="41" href="#41">41</a> <em>it's better to obtain irreduced results inside the fingerprinting loop</em><a name="42" href="#42">42</a> <em>and use <code>reduce</code> to reduce to a fingerprint after the loop.</em><a name="43" href="#43">43</a> <a name="44" href="#44">44</a> <em>*/</em><a name="45" href="#45">45</a> <a name="46" href="#46">46</a> <em class="comment">// Tested by: TestFPGenerator</em><a name="47" href="#47">47</a> <a name="48" href="#48">48</a> <strong>public</strong> <strong>final</strong> <strong>class</strong> <a href="../../../st/ata/util/FPGenerator.html">FPGenerator</a> {<a name="49" href="#49">49</a> <a name="50" href="#50">50</a> <em>/**<em>* Return a fingerprint generator. The fingerprints generated</em></em><a name="51" href="#51">51</a> <em> will have degree <code>degree</code> and will be generated by</em><a name="52" href="#52">52</a> <em> <code>polynomial</code>. If a generator based on</em><a name="53" href="#53">53</a> <em> <code>polynomial</code> has already been created, it will be</em><a name="54" href="#54">54</a> <em> returned. Requires that <code>polynomial</code> is an</em><a name="55" href="#55">55</a> <em> irreducible polynomial of degree <code>degree</code> (the</em><a name="56" href="#56">56</a> <em> array <code>polynomials</code> contains some irreducible</em><a name="57" href="#57">57</a> <em> polynomials). */</em><a name="58" href="#58">58</a> <strong>public</strong> <strong>static</strong> <a href="../../../st/ata/util/FPGenerator.html">FPGenerator</a> make(<strong>long</strong> polynomial, <strong>int</strong> degree) {<a name="59" href="#59">59</a> Long l = <strong>new</strong> Long(polynomial);<a name="60" href="#60">60</a> <a href="../../../st/ata/util/FPGenerator.html">FPGenerator</a> fpgen = (FPGenerator) generators.get(l);<a name="61" href="#61">61</a> <strong>if</strong> (fpgen == <strong>null</strong>) {<a name="62" href="#62">62</a> fpgen = <strong>new</strong> <a href="../../../st/ata/util/FPGenerator.html">FPGenerator</a>(polynomial, degree);<a name="63" href="#63">63</a> generators.put(l, fpgen);<a name="64" href="#64">64</a> }<a name="65" href="#65">65</a> <strong>return</strong> fpgen;<a name="66" href="#66">66</a> }<a name="67" href="#67">67</a> <strong>private</strong> <strong>static</strong> <strong>final</strong> Hashtable generators = <strong>new</strong> Hashtable(10);<a name="68" href="#68">68</a> <a name="69" href="#69">69</a> <strong>private</strong> <strong>static</strong> <strong>final</strong> <strong>long</strong> zero = 0;<a name="70" href="#70">70</a> <strong>private</strong> <strong>static</strong> <strong>final</strong> <strong>long</strong> one = 0x8000000000000000L;<a name="71" href="#71">71</a> <a name="72" href="#72">72</a> <a name="73" href="#73">73</a> <em>/**<em>* Return a value equal (mod <code>polynomial</code>) to</em></em><a name="74" href="#74">74</a> <em> <code>fp</code> and of degree less than <code>degree</code>. */</em><a name="75" href="#75">75</a> <strong>public</strong> <strong>long</strong> reduce(<strong>long</strong> fp) {<a name="76" href="#76">76</a> <strong>int</strong> N = (8 - degree/8);<a name="77" href="#77">77</a> <strong>long</strong> local = (N == 8 ? 0 : fp & (-1L << 8*N));<a name="78" href="#78">78</a> <strong>long</strong> temp = zero;<a name="79" href="#79">79</a> <strong>for</strong> (<strong>int</strong> i = 0; i < N; i++) {<a name="80" href="#80">80</a> temp ^= ByteModTable[8+i][((<strong>int</strong>)fp) & 0xff];<a name="81" href="#81">81</a> fp >>>= 8;<a name="82" href="#82">82</a> };<a name="83" href="#83">83</a> <strong>return</strong> local ^ temp;<a name="84" href="#84">84</a> }<a name="85" href="#85">85</a> <a name="86" href="#86">86</a> <em>/**<em>* Extends <code>f</code> with lower eight bits of <code>v</code></em></em><a name="87" href="#87">87</a> <em> with<em>out</em> full reduction. In other words, returns a</em><a name="88" href="#88">88</a> <em> polynomial that is equal (mod <code>polynomial</code>) to the</em><a name="89" href="#89">89</a> <em> desired fingerprint but may be of higher degree than the</em><a name="90" href="#90">90</a> <em> desired fingerprint. */</em><a name="91" href="#91">91</a> <strong>public</strong> <strong>long</strong> extend_byte(<strong>long</strong> f, <strong>int</strong> v) {<a name="92" href="#92">92</a> f ^= (0xff & v);<a name="93" href="#93">93</a> <strong>int</strong> i = (<strong>int</strong>)f;<a name="94" href="#94">94</a> <strong>long</strong> result = (f>>>8);<a name="95" href="#95">95</a> result ^= ByteModTable[7][i & 0xff];<a name="96" href="#96">96</a> <strong>return</strong> result;<a name="97" href="#97">97</a> }<a name="98" href="#98">98</a> <a name="99" href="#99">99</a> <em>/**<em>* Extends <code>f</code> with lower sixteen bits of <code>v</code>.</em></em><a name="100" href="#100">100</a> <em> Does not reduce. */</em><a name="101" href="#101">101</a> <strong>public</strong> <strong>long</strong> extend_<strong>char</strong>(<strong>long</strong> f, <strong>int</strong> v) {<a name="102" href="#102">102</a> f ^= (0xffff & v);<a name="103" href="#103">103</a> <strong>int</strong> i = (<strong>int</strong>)f;<a name="104" href="#104">104</a> <strong>long</strong> result = (f>>>16);<a name="105" href="#105">105</a> result ^= ByteModTable[6][i & 0xff]; i >>>= 8;<a name="106" href="#106">106</a> result ^= ByteModTable[7][i & 0xff];<a name="107" href="#107">107</a> <strong>return</strong> result;<a name="108" href="#108">108</a> }<a name="109" href="#109">109</a> <a name="110" href="#110">110</a> <em>/**<em>* Extends <code>f</code> with (all bits of) <code>v</code>.</em></em><a name="111" href="#111">111</a> <em> Does not reduce. */</em><a name="112" href="#112">112</a> <strong>public</strong> <strong>long</strong> extend_<strong>int</strong>(<strong>long</strong> f, <strong>int</strong> v) {<a name="113" href="#113">113</a> f ^= (0xffffffffL & v);<a name="114" href="#114">114</a> <strong>int</strong> i = (<strong>int</strong>)f;<a name="115" href="#115">115</a> <strong>long</strong> result = (f>>>32);<a name="116" href="#116">116</a> result ^= ByteModTable[4][i & 0xff]; i >>>= 8;<a name="117" href="#117">117</a> result ^= ByteModTable[5][i & 0xff]; i >>>= 8;<a name="118" href="#118">118</a> result ^= ByteModTable[6][i & 0xff]; i >>>= 8;<a name="119" href="#119">119</a> result ^= ByteModTable[7][i & 0xff];<a name="120" href="#120">120</a> <strong>return</strong> result;<a name="121" href="#121">121</a> }<a name="122" href="#122">122</a> <a name="123" href="#123">123</a> <em>/**<em>* Extends <code>f</code> with <code>v</code>.</em></em><a name="124" href="#124">124</a> <em> Does not reduce. */</em><a name="125" href="#125">125</a> <strong>public</strong> <strong>long</strong> extend_<strong>long</strong>(<strong>long</strong> f, <strong>long</strong> v) {<a name="126" href="#126">126</a> f ^= v;<a name="127" href="#127">127</a> <strong>long</strong> result = ByteModTable[0][(<strong>int</strong>)(f & 0xff)]; f >>>= 8;<a name="128" href="#128">128</a> result ^= ByteModTable[1][(<strong>int</strong>)(f & 0xff)]; f >>>= 8;<a name="129" href="#129">129</a> result ^= ByteModTable[2][(<strong>int</strong>)(f & 0xff)]; f >>>= 8;<a name="130" href="#130">130</a> result ^= ByteModTable[3][(<strong>int</strong>)(f & 0xff)]; f >>>= 8;<a name="131" href="#131">131</a> result ^= ByteModTable[4][(<strong>int</strong>)(f & 0xff)]; f >>>= 8;<a name="132" href="#132">132</a> result ^= ByteModTable[5][(<strong>int</strong>)(f & 0xff)]; f >>>= 8;<a name="133" href="#133">133</a> result ^= ByteModTable[6][(<strong>int</strong>)(f & 0xff)]; f >>>= 8;<a name="134" href="#134">134</a> result ^= ByteModTable[7][(<strong>int</strong>)(f & 0xff)];<a name="135" href="#135">135</a> <strong>return</strong> result;<a name="136" href="#136">136</a> }<a name="137" href="#137">137</a> <a name="138" href="#138">138</a> <a name="139" href="#139">139</a> <em>/**<em>* Compute fingerprint of "n" bytes of "buf" starting from</em></em><a name="140" href="#140">140</a> <em> "buf[start]". Requires "[start, start+n)" is in bounds. */</em><a name="141" href="#141">141</a> <strong>public</strong> <strong>long</strong> fp(byte[] buf, <strong>int</strong> start, <strong>int</strong> n) {<a name="142" href="#142">142</a> <strong>return</strong> extend(empty, buf, start, n);<a name="143" href="#143">143</a> }<a name="144" href="#144">144</a> <a name="145" href="#145">145</a> <em>/**<em>* Compute fingerprint of (all bits of) "n" characters of "buf"</em></em><a name="146" href="#146">146</a> <em> starting from "buf[i]". Requires "[i, i+n)" is in bounds. */</em>⌨️ 快捷键说明
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