带最小金额的随机红包分配方法

RandomRedPacket.swift

/// 随机红包（仅带最小金额）
///
/// - Parameters:
///   - amount: 总额（分）
///   - slice: 分成几份，至少一份
///   - minimun: 每个红包的最小金额（分），默认1分（这和微信目前业务场景是一致的）
func redPacket(ammount: Int, slice: Int = 1, minimum: Int = 1) throws -> [Int] {
    guard minimum > 0, slice > 0, ammount >= minimum * slice else {
        throw NSError(domain: "com.tencent.wechat.redpacket", code: -1, userInfo: [NSLocalizedDescriptionKey: "无法按要求分配红包"])
        return [ammount]
    }
    if slice == 1 {
        return [ammount]
    }
    // 插板法：总共 ammount - minimum * slice + 1个空，需插入 slice - 1 块板
    // 每个空均可重插，空与空之间可以是0间距
    let numberOfSlot = ammount - minimum * slice + 1
    // arc4random_uniform 的结果是 less than 给定的参数
    let arr = Array(0..<(slice-1)).map({ _ in Int(arc4random_uniform(UInt32(numberOfSlot))) }).sorted()
    var result: [Int] = [arr[0] + minimum]
    for i in 1..<arr.count {
        result.append(arr[i] - arr[i - 1] + minimum)
    }
    result.append(ammount - result.reduce(0, +))
    return result
}


// 测试代码：
print("开始发红包：")
for _ in 0..<100 { // 执行100次
    do {
        // 这儿的值可随意修改。比如，现在可解读为10块钱，9个红包，每个红包不少于1块钱。
        let t = 1000, s = 9, m = 100
        let result = try redPacket(ammount: t, slice: s, minimum: m)
        let total = result.reduce(0, +)
        print(result, total == t)
    } catch {
        print(error)
    }
}

RedPacketsWithInferiorAndSupereme.swift

// 带上下限的红包分配方案

// 一次分发
func deliverRedPacket(total: Int, min: Int, max: Int, num: Int) throws -> Int {
    guard total > 0, min > 0, max >= min, total >= min * num, total <= max * num else {
        throw NSError.init(domain: "redpacket", code: -1, userInfo: nil)
    }
    guard num > 1 else {
        return total
    }
    let leastLastRedPacket = total - (num - 1) * max
    let maxLastRedPacket = total - (num - 1) * min
    let low = leastLastRedPacket < min ? min: leastLastRedPacket
    var high = maxLastRedPacket > max ? max : maxLastRedPacket
    let ave = total / num > 1 ? total / num : 1
    if high > 2 * ave {
        print(high, ave)
//        high = 2 * ave // 不允许单个红包大于剩余红包平均值的2倍
    }
    return low + Int(arc4random_uniform(UInt32(high - low + 1)))
}


// 整个分发过程
func redPacket(total: Int, min: Int, max: Int, num: Int) throws -> [Int] {
    var t = total
    var result = [Int]()
    for i in 0..<num {
        let packet = try deliverRedPacket(total: t, min: min, max: max, num: num - i)
        result.append(packet)
        t -= packet
    }
    return result
}


// 测试代码

let t = 200000
let min = 1000
let max = 20000
let num = 50


for _ in 0..<30 {
    do {
        print(try redPacket(total: t, min: min, max: max, num: num))
    } catch {
        print(error)
    }
}

目前，微信随机红包不支持设定最小金额（实为1分），我给出一种带最小金额的随机红包分配方法，且分配结果公平公正。 本质是排列组合里的插板法。 仅需19行代码，算法复杂度O(nn)，n为红包个数（sorted和reduce是nn的复杂度）。