PHP Iterator 介面實作斐波那契數列：無限序列的優雅輸出

Iterator 介面定義

interface Iterator extends Traversable {
    public function current(): mixed;
    public function key(): mixed;
    public function next(): void;
    public function rewind(): void;
    public function valid(): bool;
}

斐波那契迭代器完整實作

<?php
class Fibonacci implements Iterator {
    private int $previous = 1;
    private int $current  = 0;
    private int $key      = 0;

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

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

    public function next(): void {
        // 核心遞推：F(n) = F(n-1) + F(n-2)
        $newPrevious = $this->current;
        $this->current += $this->previous;
        $this->previous = $newPrevious;
        $this->key++;
    }

    public function rewind(): void {
        $this->previous = 1;
        $this->current  = 0;
        $this->key      = 0;
    }

    public function valid(): bool {
        // 無限序列永遠有效，外部以 break/limit 控制
        return true;
    }
}

// 使用
$fib = new Fibonacci();
$i = 0;
foreach ($fib as $n) {
    echo "$n ";
    if (++$i >= 20) break;
}
// 輸出: 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181

進階：加上限制的有限迭代器

class LimitedFibonacci implements Iterator {
    private int $limit;
    private int $count = 0;
    // ... 同上 current/key/next/rewind ...
    
    public function __construct(int $limit) {
        $this->limit = $limit;
    }
    
    public function valid(): bool {
        return $this->count < $this->limit;
    }
    
    public function next(): void {
        parent::next(); // 假設繼承自 Fibonacci 或複製邏輯
        $this->count++;
    }
    
    public function rewind(): void {
        parent::rewind();
        $this->count = 0;
    }
}

foreach (new LimitedFibonacci(15) as $n) {
    echo "$n ";
}

Generator 寫法 (PHP 5.5+，更簡潔)

function fibonacci(int $limit = INF): Generator {
    $prev = 1;
    $curr = 0;
    for ($i = 0; $i < $limit; $i++) {
        yield $curr;
        [$prev, $curr] = [$curr, $prev + $curr];
    }
}

foreach (fibonacci(20) as $n) {
    echo "$n ";
}

應用場景

• 大數據集流式處理 (避免一次載入陣列)
• 資料庫游標封裝
• 檔案逐行讀取
• API 分頁自動迭代
• 無限數學序列 (質數、階乘、Collatz)

IteratorAggregate 替代方案

class FibonacciCollection implements IteratorAggregate {
    private int $limit;
    public function __construct(int $limit) { $this->limit = $limit; }
    public function getIterator(): Traversable {
        return new LimitedFibonacci($this->limit);
    }
}

$coll = new FibonacciCollection(10);
foreach ($coll as $n) { ... }
// 可直接 count()、array_iterator() 等