Iterators and Generators

<?php

declare(strict_types=1);

/** Iterates pages of a paginated result: an Iterator written by hand. */
final class PageIterator implements Iterator
{
    private int $page = 1;

    public function __construct(private readonly int $totalItems, private readonly int $perPage) {}

    public function current(): string { return sprintf('page %d of %d', $this->page, $this->pages()); }
    public function key(): int { return $this->page; }
    public function next(): void { $this->page++; }
    public function rewind(): void { $this->page = 1; }
    public function valid(): bool { return $this->page <= $this->pages(); }
    private function pages(): int { return (int) ceil($this->totalItems / $this->perPage); }
}

/** A collection: IteratorAggregate with a generator, much shorter. */
final class Shelf implements IteratorAggregate, Countable
{
    public function __construct(private array $titles) {}

    public function getIterator(): Generator
    {
        foreach ($this->titles as $position => $title) {
            yield $position + 1 => $title;
        }
    }

    public function count(): int { return count($this->titles); }
}

foreach (new PageIterator(totalItems: 45, perPage: 20) as $n => $label) {
    echo "{$n}: {$label}\n";
}
foreach (new Shelf(['Solaris', 'Kindred']) as $position => $title) {
    echo "#{$position} {$title}\n";
}

1: page 1 of 3
2: page 2 of 3
3: page 3 of 3
#1 Solaris
#2 Kindred

<?php

declare(strict_types=1);

$titles = new ArrayIterator(['Solaris', 'Kindred', 'Dawn', 'Imago', 'Wild Seed', 'Ariel']);

// Titles with a space or starting with a vowel, first page of 2
$filtered = new CallbackFilterIterator($titles, fn (string $t): bool => str_contains($t, ' ') || preg_match('/^[AEIOU]/', $t) === 1);
$page = new LimitIterator($filtered, offset: 0, limit: 2);
echo implode(', ', iterator_to_array($page, preserve_keys: false)), "\n";      // Imago, Wild Seed

// RegexIterator on the values
echo implode(', ', iterator_to_array(new RegexIterator($titles, '/^[DK]/'), false)), "\n";   // Kindred, Dawn

// CachingIterator: know when the last element comes
$it = new CachingIterator(new ArrayIterator(['Solaris', 'Kindred', 'Dawn']));
foreach ($it as $title) {
    echo $title, $it->hasNext() ? ' -> ' : "\n";       // Solaris -> Kindred -> Dawn
}

// Round-robin assignment of returns to three shelves
$shelves = new LimitIterator(new InfiniteIterator(new ArrayIterator(['A', 'B', 'C'])), 0, 5);
echo implode(' ', iterator_to_array($shelves, false)), "\n";                   // A B C A B

var_dump(iterator_count(new ArrayIterator([1, 2, 3])));                        // int(3)

<?php

declare(strict_types=1);

function loanIds(int $start): Generator
{
    echo "[start]\n";
    for ($id = $start; ; $id++) {          // infinite: the consumer decides when to stop
        yield $id;
    }
}

$ids = loanIds(1000);
echo "generator created, nothing ran yet\n";
foreach ($ids as $id) {
    echo $id, "\n";
    if ($id === 1002) {
        break;
    }
}

generator created, nothing ran yet
[start]
1000
1001
1002

<?php

declare(strict_types=1);

// Create a sample export (in real life: hundreds of megabytes).
$file = sys_get_temp_dir() . '/loans.csv';
$out = new SplFileObject($file, 'w');
$out->fputcsv(['isbn', 'member', 'days_late'], escape: '');
foreach ([['978-0-00-000001-1', 'M-1', 3], ['978-0-00-000003-5', 'M-2', 0], ['978-0-00-000002-8', 'M-1', 12]] as $row) {
    $out->fputcsv($row, escape: '');
}
$out = null;                                         // close the file

/** @return Generator<int, array<string, string>> */
function readCsv(string $path): Generator
{
    $f = new SplFileObject($path);
    $f->setFlags(SplFileObject::READ_CSV | SplFileObject::SKIP_EMPTY | SplFileObject::READ_AHEAD);
    $f->setCsvControl(escape: '');
    $header = null;
    foreach ($f as $row) {
        if ($header === null) {
            $header = $row;
            continue;
        }
        yield array_combine($header, $row);
    }
}

function overdue(iterable $loans): Generator
{
    foreach ($loans as $loan) {
        if ((int) $loan['days_late'] > 0) {
            yield $loan;
        }
    }
}

function finesByMember(iterable $loans, int $centsPerDay): array
{
    $fines = [];
    foreach ($loans as $loan) {
        $fines[$loan['member']] = ($fines[$loan['member']] ?? 0) + (int) $loan['days_late'] * $centsPerDay;
    }
    return $fines;
}

print_r(finesByMember(overdue(readCsv($file)), 25));   // ["M-1" => 375]
echo 'peak memory stays flat: ', memory_get_peak_usage() < 4_000_000 ? 'yes' : 'no', "\n";