//
//  UnorderedSparseSet.swift
//  FirebladeECS
//
//  Created by Christian Treffs on 30.10.17.
//

/// An (unordered) sparse set.
///
/// - `Element`: the element (instance) to store.
/// - `Key`: the unique, hashable datastructure to use as a key to retrieve
///          an element from the sparse set.
///
/// See <https://github.com/bombela/sparseset/blob/master/src/lib.rs> for a reference implementation.
public struct UnorderedSparseSet<Element, Key: Hashable & Codable> {
    /// An index into the dense store.
    public typealias DenseIndex = Int

    /// A sparse store holding indices into the dense mapped to key.
    public typealias SparseStore = [Key: DenseIndex]

    /// A dense store holding all the entries.
    public typealias DenseStore = ContiguousArray<Entry>

    public struct Entry {
        public let key: Key
        public let element: Element
    }

    @usableFromInline var dense: DenseStore
    @usableFromInline var sparse: SparseStore

    public init() {
        self.init(sparse: [:], dense: [])
    }

    init(sparse: SparseStore, dense: DenseStore) {
        self.sparse = sparse
        self.dense = dense
    }

    public var count: Int { dense.count }
    public var isEmpty: Bool { dense.isEmpty }

    @inlinable
    public func contains(_ key: Key) -> Bool {
        find(at: key) != nil
    }

    /// Inset an element for a given key into the set in O(1).
    /// Elements at previously set keys will be replaced.
    ///
    /// - Parameters:
    ///   - element: the element
    ///   - key: the key
    /// - Returns: true if new, false if replaced.
    @discardableResult
    public mutating func insert(_ element: Element, at key: Key) -> Bool {
        if let (denseIndex, _) = find(at: key) {
            dense[denseIndex] = Entry(key: key, element: element)
            return false
        }

        let nIndex = dense.count
        dense.append(Entry(key: key, element: element))
        sparse[key] = nIndex
        return true
    }

    /// Get the element for the given key in O(1).
    ///
    /// - Parameter key: the key
    /// - Returns: the element or nil of key not found.
    @inlinable
    public func get(at key: Key) -> Element? {
        guard let (_, element) = find(at: key) else {
            return nil
        }

        return element
    }

    @inlinable
    public func get(unsafeAt key: Key) -> Element {
        find(at: key).unsafelyUnwrapped.1
    }

    /// Removes the element entry for given key in O(1).
    ///
    /// - Parameter key: the key
    /// - Returns: removed value or nil if key not found.
    @discardableResult
    public mutating func remove(at key: Key) -> Entry? {
        guard let (denseIndex, _) = find(at: key) else {
            return nil
        }

        let removed = swapRemove(at: denseIndex)
        if !dense.isEmpty && denseIndex < dense.count {
            let swappedElement = dense[denseIndex]
            sparse[swappedElement.key] = denseIndex
        }
        sparse[key] = nil
        return removed
    }

    @inlinable
    public mutating func removeAll(keepingCapacity: Bool = false) {
        sparse.removeAll(keepingCapacity: keepingCapacity)
        dense.removeAll(keepingCapacity: keepingCapacity)
    }

    /// Removes an element from the set and returns it in O(1).
    /// The removed element is replaced with the last element of the set.
    ///
    /// - Parameter denseIndex: the dense index
    /// - Returns: the element entry
    private mutating func swapRemove(at denseIndex: Int) -> Entry {
        dense.swapAt(denseIndex, dense.count - 1)
        return dense.removeLast()
    }

    @inlinable
    public func find(at key: Key) -> (Int, Element)? {
        guard let denseIndex = sparse[key], denseIndex < count else {
            return nil
        }
        let entry = self.dense[denseIndex]
        guard entry.key == key else {
            return nil
        }

        return (denseIndex, entry.element)
    }

    @inlinable public var first: Element? {
        dense.first?.element
    }

    @inlinable public var last: Element? {
        dense.last?.element
    }
}

extension UnorderedSparseSet where Key == Int {
    @inlinable
    public subscript(position: DenseIndex) -> Element {
        get {
            get(unsafeAt: position)
        }

        set(newValue) {
            insert(newValue, at: position)
        }
    }
}

// MARK: - Sequence
extension UnorderedSparseSet: Sequence {
    public __consuming func makeIterator() -> ElementIterator {
        ElementIterator(self)
    }

    // MARK: - UnorderedSparseSetIterator
    public struct ElementIterator: IteratorProtocol {
        public private(set) var iterator: IndexingIterator<ContiguousArray<UnorderedSparseSet<Element, Key>.Entry>>

        public init(_ sparseSet: UnorderedSparseSet<Element, Key>) {
            iterator = sparseSet.dense.makeIterator()
        }

        public mutating func next() -> Element? {
            iterator.next()?.element
        }
    }
}

// MARK: - Equatable
extension UnorderedSparseSet.Entry: Equatable where Element: Equatable { }
extension UnorderedSparseSet: Equatable where Element: Equatable {
    public static func == (lhs: UnorderedSparseSet<Element, Key>, rhs: UnorderedSparseSet<Element, Key>) -> Bool {
        lhs.dense == rhs.dense && lhs.sparse == rhs.sparse
    }
}

// MARK: - Codable
extension UnorderedSparseSet.Entry: Codable where Element: Codable { }
extension UnorderedSparseSet: Codable where Element: Codable { }