Swift: Swiftの高階関数：map、filter、reduceと関数チェーン

var activeUsers: [String] = []
for user in users {
    if user.isActive {
        activeUsers.append(user.name)
    }
}
activeUsers.sort()
var result: [String] = []
for name in activeUsers {
    result.append("User: \(name)")
}

let result = users
    .filter { $0.isActive }
    .map { $0.name }
    .sorted()
    .map { "User: \($0)" }

// ============================================
// map: Apply transformation to each element
// ============================================
let numbers = [1, 2, 3, 4, 5]
// for-loop approach (old style)
var doubled: [Int] = []
for n in numbers { doubled.append(n * 2) }
// map approach (modern style)
let doubledMap = numbers.map { $0 * 2 }
print("doubledMap: \(doubledMap)")
// Extract string lengths
let words = ["Swift", "Python", "Go"]
let lengths = words.map { $0.count }
print("lengths: \(lengths)")

// ============================================
// filter: Keep elements that satisfy a condition
// ============================================
let scores = [45, 82, 91, 63, 77, 55, 88]
// Filter passing scores (>= 60)
let passed = scores.filter { $0 >= 60 }
print("Passed: \(passed)")
// Filter excellent scores (>= 85)
let excellent = scores.filter { $0 >= 85 }
print("Excellent: \(excellent)")
// Chaining: filter failing scores then double them
let failedDoubled = scores
    .filter { $0 < 60 }
    .map { $0 * 2 }
print("Failed scores doubled: \(failedDoubled)")

// ============================================
// reduce: Reduction operations
// ============================================
let numbers = [1, 2, 3, 4, 5]
// Sum
let sum = numbers.reduce(0) { $0 + $1 }
print("sum: \(sum)")
// Product
let product = numbers.reduce(1) { $0 * $1 }
print("product: \(product)")
// Concatenate strings
let words = ["Swift", "is", "awesome"]
let sentence = words.reduce("") { $0.isEmpty ? $1 : "\($0) \($1)" }
print("sentence: \(sentence)")
// Named parameters for clarity
let sumNamed = numbers.reduce(0) { total, next in total + next }
print("sumNamed: \(sumNamed)")

// ============================================
// compactMap and flatMap
// ============================================
// compactMap: Filter nil
let strings = ["1", "two", "3", "four", "5"]
let numbers2 = strings.compactMap { Int($0) }
print("numbers: \(numbers2)")
// flatMap: Flatten nested arrays
let nested = [[1, 2], [3, 4, 5], [6]]
let flattened = nested.flatMap { $0 }
print("flattened: \(flattened)")

// ============================================
// Chaining: User data analysis pipeline
// ============================================
struct User {
    let name: String
    let age: Int
    let isActive: Bool
    let scores: [Int]
}
let users = [
    User(name: "Alice", age: 28, isActive: true, scores: [85, 90, 78]),
    User(name: "Bob", age: 22, isActive: false, scores: [70, 75]),
    User(name: "Charlie", age: 35, isActive: true, scores: [95, 92, 98, 100]),
    User(name: "Diana", age: 17, isActive: true, scores: [60, 65]),
    User(name: "Eve", age: 30, isActive: false, scores: [88, 82])
]
// Chain pipeline: active users → adults → average score → sort by score → format
let report = users
    .filter { $0.isActive }
    .filter { $0.age >= 18 }
    .map { user -> (name: String, avgScore: Double) in
        let avg = Double(user.scores.reduce(0, +)) / Double(user.scores.count)
        return (user.name, avg)
    }
    .sorted { $0.avgScore > $1.avgScore }
    .map { "\($0.name): \(String(format: "%.1f", $0.avgScore))" }
print("=== Active Adult User Report ===")
report.forEach { print($0) }

// ============================================
// Complete example: Shopping cart price calculator
// Features: map + filter + reduce + compactMap combined
// ============================================
import Foundation
struct CartItem {
    let name: String
    let price: Double
    let quantity: Int
    let isDiscounted: Bool
}
let cart = [
    CartItem(name: "Laptop", price: 1299, quantity: 1, isDiscounted: false),
    CartItem(name: "Mouse", price: 29.99, quantity: 2, isDiscounted: true),
    CartItem(name: "Keyboard", price: 99.99, quantity: 1, isDiscounted: true),
    CartItem(name: "Cable", price: 9.99, quantity: 3, isDiscounted: false),
    CartItem(name: "Monitor", price: 399, quantity: 0, isDiscounted: false)
]
// 1. Filter out zero-quantity items
let validItems = cart.filter { $0.quantity > 0 }
// 2. Calculate total per item (15% off for discounted items)
let itemTotals = validItems.map { item -> (String, Double) in
    let unitPrice = item.isDiscounted ? item.price * 0.85 : item.price
    let total = unitPrice * Double(item.quantity)
    return (item.name, total)
}
// 3. Calculate grand total
let grandTotal = itemTotals.reduce(0) { $0 + $1.1 }
// 4. Format output
print("=== Shopping Cart ===")
itemTotals.forEach { print("  \($0): $\(String(format: "%.2f", $1))") }
print("  ----------")
print("  Total: $\(String(format: "%.2f", grandTotal))")