PHP:連想配列をキーでグループ化する

例えば、こういう食べ物の連想配列を

<?php
$foods = [
    ['name' => 'Apple', 'category' => 'fruits'],
    ['name' => 'Strawberry', 'category' => 'fruits'],
    ['name' => 'Tomato', 'category' => 'vegetables'],
    ['name' => 'Carot', 'category' => 'vegetables'],
    ['name' => 'water', 'category' => 'drink'],
    ['name' => 'beer', 'category' => 'drink'],
];
?>

カテゴリごとにグループ化して、こんな感じにしたい:

<?php
$expected = [
    'fruits' => [
        ['name' => 'Apple', 'category' => 'fruits'],
        ['name' => 'Strawberry', 'category' => 'fruits'],
    ],
    'vegetables' => [
        ['name' => 'Tomato', 'category' => 'vegetables'],
        ['name' => 'Carot', 'category' => 'vegetables'],
    ],
    'drink' => [
        ['name' => 'water', 'category' => 'drink'],
        ['name' => 'beer', 'category' => 'drink'],
    ],
];
?>

実装案1: foreachを使ったよくある実装

<?php

function array_group_by(array $items, $keyName)
{
    $groups = [];
    foreach ($items as $item) {
        $key = $item[$keyName];
        if (array_key_exists($key, $groups)) {
            $groups[$key][] = $item;
        } else {
            $groups[$key] = [$item];
        }
    }
    return $groups;
}

assert(array_group_by($foods, 'category') === $expected);
?>

更に短縮すると:

function group_by(array $table, string $key): array
{
    $groups = [];
    foreach ($table as $row) {
        $groups[$row[$key]][] = $row;
    }
    return $groups;
}

実装案2: イミュータブル風

<?php

function array_group_by2(array $items, $keyName)
{
    $keys = array_unique(array_column($items, $keyName));
    return array_combine($keys, array_map(function($key) use ($items, $keyName) {
        return array_values(array_filter($items, function ($item) use ($key, $keyName) {
            return $item[$keyName] == $key;
        }));
    }, $keys));
}

assert(array_group_by2($foods, 'category') === $expected);
?>

変数スコープとコレクションのAPI、辛すぎ・・・。やはりPHPはforeachを使うが良さそう。

実装案3: 先に入れ物を作ってforeachでミューテート

<?php

function array_group_by3(array $items, $keyName)
{
    $groups = array_fill_keys(array_column($items, $keyName), []);
    foreach ($items as $item) {
        $groups[$item[$keyName]][] = $item;
    }
    return $groups;
}

assert(array_group_by3($foods, 'category') === $expected);
?>

実装案1と比べて、if-elseが無くなった。まずまずと言った感じ・・・？

実装案4: array_reduce

function group_by(array $table, string $key): array
{
    return array_reduce($table, function (array $groups, array $row) use ($key) {
        $groups[$row[$key]][] = $row;
        return $groups;
    }, []);
}