78 lines
2.3 KiB
PHP
78 lines
2.3 KiB
PHP
<?php
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namespace App\Services\Stats;
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/**
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* Which keys occur inside a date window that only ever moves forward.
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*
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* Built for the network time-lapse: its layers are windows over a day-ordered
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* list of [day, key] events - "traded in the month ending on this step",
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* "traded before this date" - and each step moves them on by a week or a
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* month. Two pointers walk the events once in total, so all steps together
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* cost one pass, and each step reports only the keys that entered or left.
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*
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* Days are 'Y-m-d' strings, which compare correctly as strings.
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*/
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class SlidingWindow
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{
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/** @var array<string,int> Occurrences of each key inside the window. */
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private array $count = [];
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private int $enter = 0;
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private int $leave = 0;
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private int $size;
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/** @param array<array{0:string, 1:string}> $events [day, key], ordered by day */
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public function __construct(private array $events)
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{
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$this->size = count($events);
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}
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/**
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* Move the window to [$start, $end] (inclusive; null start = no lower
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* bound) and return what entered and left since the previous call.
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*
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* Both bounds may only move forward.
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*
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* @return array{added: array<string>, removed: array<string>}
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*/
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public function slideTo(?string $start, string $end): array
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{
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$before = [];
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while ($this->enter < $this->size && $this->events[$this->enter][0] <= $end) {
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$key = $this->events[$this->enter++][1];
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$before[$key] ??= ($this->count[$key] ?? 0) > 0;
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$this->count[$key] = ($this->count[$key] ?? 0) + 1;
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}
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if ($start !== null) {
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while ($this->leave < $this->enter && $this->events[$this->leave][0] < $start) {
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$key = $this->events[$this->leave++][1];
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$before[$key] ??= ($this->count[$key] ?? 0) > 0;
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if (--$this->count[$key] === 0) {
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unset($this->count[$key]);
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}
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}
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}
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$added = [];
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$removed = [];
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foreach ($before as $key => $was) {
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$now = isset($this->count[$key]);
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if ($now && !$was) {
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$added[] = $key;
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} elseif (!$now && $was) {
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$removed[] = $key;
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}
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}
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return ['added' => $added, 'removed' => $removed];
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}
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}
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