Kotlin KoansでKotlin入門　第３１回：Sequences

# はじめに
公式の問題集[「Kotlin Koans」](https://play.kotlinlang.org/koans/overview)を解きながらKotlinを学習します。

過去記事はこちら
- Introduction
  - [第０回：Kotlin Koansについて](https://qiita.com/kosuke1/items/41d192af63485bfc8cb8)
  - [第１回：Simple Functions](https://qiita.com/kosuke1/items/5862a3ece96082a62e67)
  - [第２回：Named arguments](https://qiita.com/kosuke1/items/2af11262852ee41b5da7)
  - [第３回：Default arguments](https://qiita.com/kosuke1/items/16cb8ac780df58dc8054)
  - [第４回：Triple-quoted strings](https://qiita.com/kosuke1/items/52cfeed2c49b0252d40e)
  - [第５回：String templates](https://qiita.com/kosuke1/items/0221bbbc5bc70847be6f)
  - [第６回：Nullable types](https://qiita.com/kosuke1/items/aa96e422c5313b6961bc)
  - [第７回：Nothing type](https://qiita.com/kosuke1/items/1ca0e48820d58c9dfe16)
  - [第８回：Lambdas](https://qiita.com/kosuke1/items/8de74f9491e003127f53)
- Classes
  - [第９回：Data classes](https://qiita.com/kosuke1/items/1be2de1b4406b8d914bf) 
  - [第１０回：Smart casts](https://qiita.com/kosuke1/items/50bdecaa0740eb0e1a17)
  - [第１１回：Sealed classes](https://qiita.com/kosuke1/items/71a4da02fe1e99da5d63)
  - [第１２回：Rename on import](https://qiita.com/kosuke1/items/f11465cc15a94e7589a3)
  - [第１３回：Extension functions](https://qiita.com/kosuke1/items/d4a1da00bc142c297221)
- Conventions
  - [第１４回：Comparison](https://qiita.com/kosuke1/items/e9486d044cae32167f36) 
  - [第１５回：Ranges](https://qiita.com/kosuke1/items/797f049ce5f33b1810c9)
  - [第１６回：For loop](https://qiita.com/kosuke1/items/f5f6eeea41fb89b2a20c)
  - [第１７回：Operators overloading](https://qiita.com/kosuke1/items/9845f98a632121f1493b)
  - [第１８回：Invoke](https://qiita.com/kosuke1/items/7879f83341d169374eb3)
- Collections
  - [第１９回：Collections Introduction](https://qiita.com/kosuke1/items/af91a0bd505cd93a3900) 
  - [第２０回：Sort](https://qiita.com/kosuke1/items/ed9a4b36f8573fc65a8f)
  - [第２１回：Filter map](https://qiita.com/kosuke1/items/45e70bac73706643e40c)
  - [第２２回：All, Any, and other predicates](https://qiita.com/kosuke1/items/daa940bf57e93d198ce4)
  - [第２３回：Associate](https://qiita.com/kosuke1/items/a4d8b0701b6848d43473)
  - [第２４回：Group By](https://qiita.com/drafts/d2279add0e79cbe19f01/edit)
  - [第２５回：Partition](https://qiita.com/kosuke1/items/d2279add0e79cbe19f01)
  - [第２６回：FlatMap](https://qiita.com/kosuke1/items/f08f22de98d736df6e12)
  - [第２７回：Max min](https://qiita.com/kosuke1/items/f2af0574bbc594bbe494)
  - [第２８回：Sum](https://qiita.com/kosuke1/items/477c2fd0886231440d06)
  - [第２９回：Fold](https://qiita.com/kosuke1/items/9b6ca3ab409335897adf)
  - [第３０回：Compound tasks](https://qiita.com/kosuke1/items/66949d40ce744bc696cb)

# 問題
## [Sequences](https://play.kotlinlang.org/koans/Collections/Sequences/Task.kt)
[シーケンス](https://kotlinlang.org/docs/sequences.html)について学びましょう。
シーケンスは、eagerlyではなくlazilyに操作を実行することを可能にします。
前のタスクの実装をコピーして、シーケンスに対する操作を使用するように変更してください。

```修正前コード.kt
// Find the most expensive product among all the delivered products
// ordered by the customer. Use `Order.isDelivered` flag.
fun findMostExpensiveProductBy(customer: Customer): Product? {
    TODO()
}

// Count the amount of times a product was ordered.
// Note that a customer may order the same product several times.
fun Shop.getNumberOfTimesProductWasOrdered(product: Product): Int {
    TODO()
}

fun Customer.getOrderedProducts(): Sequence<Product> =
        TODO()
```

```Shop.kt
data class Shop(val name: String, val customers: List<Customer>)

data class Customer(val name: String, val city: City, val orders: List<Order>) {
    override fun toString() = "$name from ${city.name}"
}

data class Order(val products: List<Product>, val isDelivered: Boolean)

data class Product(val name: String, val price: Double) {
    override fun toString() = "'$name' for $price"
}

data class City(val name: String) {
    override fun toString() = name
}
```


# 問題のポイント
シーケンスを作成するには、引数として要素を列挙してsequenceOf()関数を呼び出します。
```Kotlin
val numbersSequence = sequenceOf("four", "three", "two", "one")

```


Iterable オブジェクト (リストやセットなど) をすでに持っている場合は、asSequence() を呼び出すことでシーケンスを作成することができます。
```Kotlin
val numbers = listOf("one", "two", "three", "four")
val numbersSequence = numbers.asSequence()
```

# 解答例
```Kotlin
// Find the most expensive product among all the delivered products
// ordered by the customer. Use `Order.isDelivered` flag.
fun findMostExpensiveProductBy(customer: Customer): Product? {
    return customer
            .orders
            .asSequence()
            .filter(Order::isDelivered)
            .flatMap(Order::products)
            .maxByOrNull(Product::price)
}

// Count the amount of times a product was ordered.
// Note that a customer may order the same product several times.
fun Shop.getNumberOfTimesProductWasOrdered(product: Product): Int {
    return customers
            .asSequence()
            .flatMap(Customer::getOrderedProducts)
            .count { it == product }
}

fun Customer.getOrderedProducts(): Sequence<Product> =
        orders.asSequence().flatMap(Order::products)
```

Sequenceを使用しない前のタスクでの解答
```Kotlin
fun findMostExpensiveProductBy(customer: Customer): Product? {
    return customer
        .orders
        .filter(Order::isDelivered)
        .flatMap(Order::products)
        .maxByOrNull(Product::price)
}

fun Shop.getNumberOfTimesProductWasOrdered(product: Product): Int {
    return customers
        .flatMap(Customer::getOrderedProducts)
        .count { it == product }
}

fun Customer.getOrderedProducts(): List<Product> = 
        orders.flatMap(Order::products)
```