有一种方法可以生成一个样本子集，该方法是随机的，保证不会重复，使用O(1)存储，并且可以在任何时候重新生成。首先，编写一个函数来根据其词汇索引生成组合。其次，使用第一个Combin(n, m)个整数的伪随机排列以随机顺序遍历这些组合。只需将数字0...100000输入置换中，使用置换的输出作为组合生成器的输入，并处理结果组合即可。
<?PHP
/* (c) 2009 tim - anyone who finds a use for this is very welcome to use it with no restrictions unless they're making a weapon */
//how many sets shall we generate?
$gNumSets = 1000;
//keep track of collisions, just for fun.
$gCollisions = 0;
$starttime = time();
/**
* Generate and return an integer with exactly 15 of the lower 25 bits set (1) and the other 10 unset (0)
*/
function genSetHash(){
$hash = pow(2,25)-1;
$used = array();
for($i=0;$i<10;){
//pick a bit to turn off
$bit = rand(0,24);
if (! in_array($bit,$used)){
$hash = ( $hash & ~pow(2,$bit) );
$i++;
$used[] = $bit;
}
}
return $hash;
}
//we store our solution hashes in here.
$solutions = array();
//generate a bunch of solutions.
for($i=0;$i<$gNumSets;){
$hash = genSetHash();
//ensure no collisions
if (! in_array($hash,$solutions)){
$solutions[] = $hash;
//brag a little.
echo("Generated $i random sets in " . (time()-$starttime) . " seconds.\n");
$i++;
}else {
//there was a collision. There will generally be more the longer the process runs.
echo "thud.\n";
$gCollisions++;
}
}
// okay, we're done with the hard work. $solutions contains a bunch of
// unique, random, ints in the right range. Everything from here on out
// is just output.
//takes an integer with 25 significant digits, and returns an array of 15 numbers between 1 and 25
function hash2set($hash){
$set = array();
for($i=0;$i<24;$i++){
if ($hash & pow(2,$i)){
$set[] = $i+1;
}
}
return $set;
}
//pretty-print our sets.
function formatSet($set){
return "[ " . implode(',',$set) . ']';
}
//if we wanted to print them,
foreach($solutions as $hash){
echo formatSet(hash2set($hash)) . "\n";
}
echo("Generated $gNumSets unique random sets in " . (time()-$starttime) . " seconds.\n");
echo "\n\nDone. $gCollisions collisions.\n";
我认为这些都是正确的，但现在已经很晚了，我已经喝了几瓶很好的啤酒。
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