DictArray.swift

public struct DictArray<Key: Hashable, Value> : CustomDebugStringConvertible {
    private typealias Bucket = [Value]
    private var buckets: [ Key: Bucket ]
    
    init() {
        buckets = [:]
    }
    
    public mutating func add(value:Value, key:Key) {
        var bucket:Bucket
        if let existingBucket = buckets[key] {
            bucket = existingBucket
        } else {
            bucket = Bucket()
        }
        
        bucket.append(value)
        buckets[key] = bucket
    }
    
    @discardableResult public mutating func pop(at key:Key) -> Value? {
        guard var existingBucket = buckets[key],
              let element:Value = existingBucket.removeLast() else {
            return nil
        }
        
        if existingBucket.count == 0 {
            // remove bucket
            buckets.removeValue(forKey: key)
        } else {
            buckets[key] = existingBucket
        }
        return element
    }
    
    public func hasValue(at key:Key) -> Bool {
        return buckets[key] != nil
    }
    
    public var debugDescription: String {
        var str = "["
        for k in buckets.keys {
            str += "\n   \(k) => ["
            for e in buckets[k]! {
                str += "\(e), "
            }
            str += "]"
        }
        str += "\n]"
        return str
    }
}