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<div id="Layer1" style="position:absolute; width:711px; height:21px; z-index:1; top: 10px; background-color: #CCCCCC; layer-background-color: #CCCCCC; border: 1px none #000000; left: 26px"><b>|</b><font face="宋体" size="2"><a href="../text1/text1-0.htm">第一章</a></font><b>|</b><font face="宋体" size="2"><a href="../text2/text2-0.htm">第二章</a></font><b>|</b><font face="宋体" size="2"><a href="../text3/text3-0.htm">第三章</a></font><b>|</b><font face="宋体" size="2"><a href="../text4/text4-0.htm">第四章</a></font><b>|</b><font face="宋体" size="2"><a href="../text5/text5-0.htm">第五章</a></font><b>|</b><font face="宋体" size="2"><a href="../text6/text6-0.htm">第六章</a></font><b>|</b><font face="宋体" size="2"><a href="../text7/text7-0.htm">第七章</a></font><b>|</b><font face="宋体" size="2">第八章</font><b>|</b><font face="宋体" size="2"><a href="../text9/text9-0.htm">第九章</a></font><b>|</b><font face="宋体" size="2"><a href="../text10/text10-0.htm">第十章</a></font><b>|</b><font size="2" face="宋体"><a href="../textA/textA-0.htm">算法分析</a><b><font color="#000000">|</font></b>
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<b><font face="Arial, Helvetica, sans-serif" size="4" color="#000000">(2</font></b><b><font face="Arial, Helvetica, sans-serif" size="4" color="#000000">)<font color="#FF0000"> hashing function</font>
<i><font color="#0033CC">ideal hashing function :</font></i>
both easy to compute and produces few collisions
<font color="#FF0000">uniform hash function</font> -- a random x has an equal chance of
hashing into any of the b buckets ( f(x) = i is 1/b )
① <i><font size="3" color="#CC0099">Mid-square</font></i> ( <i>f<font size="2">m</font></i> ) <i><font size="3" color="#CC0099">*/ 平方中值*/</font></i>
squaring the identifier and then using an appropriate number
of bits from the middle of the square to obtain the bucket address.
r bits --> the range of the value = <img src="image/2r.gif" width="19" height="17" align="middle">
--> the size of the hash table should be <img src="image/2r.gif" width="19" height="17" align="middle">
② <i><font size="3" color="#CC0099">Division</font></i> (<i>f <font size="2">D</font></i> ) <i><font size="3" color="#CC0099">/*模除*/</font></i>
divide the identifier x by some M (the table size )and use the
remainder as the hash address ( 0 ~ M-1) for x
<i>f <font size="2">D</font>(x)</i> = x % M
</font></b></pre>
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