useful property wrapper for prefix sums

Versioned.swift

@propertyWrapper
struct Versioned<Value>: RandomAccessCollection {
    private var history: [Value]
    
    var startIndex: Int { 0 }
    var endIndex: Int { allValues.endIndex }
    subscript(position: Int) -> Value {
        get { allValues[position] }
        set { allValues[position] = newValue }
    }

    init(wrappedValue: Value) {
        self.history = [wrappedValue]
    }

    var wrappedValue: Value {
        get { allValues[allValues.endIndex - 1] }
        set { allValues[allValues.endIndex - 1] = newValue }
    }
    
    var projectedValue: Versioned<Value> {
        get { self }
        set { self = newValue }
    }

    mutating func commit() {
        allValues.append(wrappedValue)
    }
    
    mutating func commit(_ newValue: Value) {
        allValues.append(newValue)
    }
    
    mutating func commit(_ updateValue: (inout Value) -> Void) {
        allValues.append(updateValue(&wrappedValue))
    }
}