Sort a flat array in recurring ascending sequences - php

Question

I am trying to sort it in a repeating, sequential pattern of numerical order with the largest sets first.
Sample array:
$array = [1,1,1,2,3,2,3,4,5,4,4,4,5,1,2,2,3];
In the above array, I have the highest value of 5 which appears twice so the first two sets would 1,2,3,4,5 then it would revert to the second, highest value set etc.
Desired result:
[1,2,3,4,5,1,2,3,4,5,1,2,3,4,1,2,4]
I am pretty sure I can split the array into chunks of the integer values then cherrypick an item from each subarray sequentially until there are no remaining items, but I just feel that this is going to be poor for performance and I don't want to miss a simple trick that PHP can already handle.

Answer 1

Here's my attempt at a very manual loop using process, the idea is to simply sort the numbers into containers for array_unshifting. I'm sure this is terrible and I'd love someone to do this in five lines or less :)
$array = array(1,1,1,2,3,2,3,4,5,4,4,4,5,1,2,2,3);
sort($array);
// Build the container array
$numbers = array_fill_keys(array_unique($array),array());
// Assignment
foreach( $array as $number )
{
$numbers[ $number ][] = $number;
}
// Worker Loop
$output = array();
while( empty( $numbers ) === false )
{
foreach( $numbers as $outer => $inner )
{
$output[] = array_shift( $numbers[ $outer ] );
if( empty( $numbers[ $outer ] ) )
{
unset( $numbers[ $outer ] );
}
}
}
var_dump( $output );

Answer 2

I think I'd look at this not as a sorting problem, but alternating values from multiple lists, so rather than coming up with sets of distinct numbers I'd make sets of the same number.
Since there's no difference between one 1 and another, all you actually need is to count the number of times each appears. It turns out PHP can do this for you with aaray_count_values.
$sets = array_count_values ($input);
Then we can make sure the sets are in order by sorting by key:
ksort($sets);
Now, we iterate round our sets, counting down how many times we've output each number. Once we've "drained" a set, we remove it from the list, and once we have no sets left, we're all done:
$output = [];
while ( count($sets) > 0 ) {
foreach ( $sets as $number => $count ) {
$output[] = $number;
if ( --$sets[$number] == 0 ) {
unset($sets[$number]);
}
}
}
This algorithm could be adapted for cases where the values are actually distinct but can be put into sets, by having the value of each set be a list rather than a count. Instead of -- you'd use array_shift, and then check if the length of the set was zero.

Answer 3

You can use only linear logic to sort using php functions. Here is optimized way to fill data structures. It can be used for streams, generators or anything else you can iterate and compare.
$array = array(1,1,1,2,3,2,3,4,5,4,4,4,5,1,2,2,3);
sort($array);
$chunks = [];
$index = [];
foreach($array as $i){
if(!isset($index[$i])){
$index[$i]=0;
}
if(!isset($chunks[$index[$i]])){
$chunks[$index[$i]]=[$i];
} else {
$chunks[$index[$i]][] = $i;
}
$index[$i]++;
}
$result = call_user_func_array('array_merge', $chunks);
print_r($result);

Answer 4

<?php
$array = array(1,1,1,2,3,2,3,4,5,4,4,4,5,1,2,2,3);
sort($array);
while($array) {
$n = 0;
foreach($array as $k => $v) {
if($v>$n) {
$result[] = $n = $v;
unset($array[$k]);
}
}
}
echo implode(',', $result);
Output:
1,2,3,4,5,1,2,3,4,5,1,2,3,4,1,2,4

Answer 5

New, more elegant, more performant, more concise answer:
Create a sorting array where each number gets its own independent counter to increment. Then use array_multisort() to sort by this grouping array, then sort by values ascending.
Code: (Demo)
$encounters = [];
foreach ($array as $v) {
$encounters[] = $e[$v] = ($e[$v] ?? 0) + 1;
}
array_multisort($encounters, $array);
var_export($array);
Or with a functional style with no global variable declarations: (Demo)
array_multisort(
array_map(
function($v) {
static $e;
return $e[$v] = ($e[$v] ?? 0) + 1;
},
$array
),
$array
);
var_export($array);
Old answer:
My advice is functionally identical to #El''s snippet, but is implemented in a more concise/modern/attractive fashion.
After ensuring that the input array is sorted, make only one pass over the array and push each re-encountered value into its next row of values. The $counter variable indicates which row (in $grouped) the current value should be pushed into. When finished looping and grouping, $grouped will have unique values in each row. The final step is to merge/flatten the rows (preserving their order).
Code: (Demo)
$grouped = [];
$counter = [];
sort($array);
foreach ($array as $v) {
$counter[$v] = ($counter[$v] ?? -1) + 1;
$grouped[$counter[$v]][] = $v;
}
var_export(array_merge(...$grouped));