php 45 lines · 8 steps

Weighted random selection in PHP

Pick a variant at random with each option's odds proportional to its assigned weight.

Explained by highlit

Intermediate weighted-random cumulative-sum sampling validation

1<?php
2 
3namespace App\Experiments;
4 
5use InvalidArgumentException;
6 
7final class WeightedVariantPicker
8{
9    private array $variants;
10    private int $total;
11 
12    public function __construct(array $weights)
13    {
14        if ($weights === []) {
15            throw new InvalidArgumentException('At least one variant is required.');
16        }
17 
18        $this->variants = [];
19        $cumulative = 0;
20 
21        foreach ($weights as $name => $weight) {
22            if ($weight <= 0) {
23                throw new InvalidArgumentException("Weight for \"{$name}\" must be positive.");
24            }
25 
26            $cumulative += $weight;
27            $this->variants[] = ['name' => $name, 'threshold' => $cumulative];
28        }
29 
30        $this->total = $cumulative;
31    }
32 
33    public function pick(): string
34    {
35        $roll = random_int(1, $this->total);
36 
37        foreach ($this->variants as $variant) {
38            if ($roll <= $variant['threshold']) {
39                return $variant['name'];
40            }
41        }
42 
43        return $this->variants[array_key_last($this->variants)]['name'];
44    }
45}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Turning weights into cumulative thresholds lets a single random roll map cleanly to a weighted choice.
2Validating inputs in the constructor keeps the sampling method simple and always correct.
3A final fallback return guards against floating-point or off-by-one gaps in threshold coverage.

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