feature: adds DecimalNumber model

Author: bjacobgordon

NumberSystems/Sources/NumberSystems/Positional/Models/DecimalNumber.swift

@@ -0,0 +1,27 @@
+public struct DecimalNumber: PositionalNumberSystemizable {
+    public enum Digit: Int {
+        case                          zero
+        case one, two  , three, four, five
+        case six, seven, eight, nine, ten
+    }
+}
+
+extension DecimalNumber.Digit: PositionalDigitSystemizable {
+    public static var placeholder: Self { .zero }
+    
+    public var predecessor: Self {
+        switch self {
+        case .ten  : return .nine
+        case .nine : return .eight
+        case .eight: return .seven
+        case .seven: return .six
+        case .six  : return .five
+        case .five : return .four
+        case .four : return .three
+        case .three: return .two
+        case .two  : return .one
+        case .one  : return .placeholder
+        default    : Self.fatalPredecessionError()
+        }
+    }
+}

NumberSystems/Sources/NumberSystems/Positional/Protocols/PositionalDigitSystemizable.swift

@@ -0,0 +1,28 @@
+public protocol PositionalDigitSystemizable: CaseIterable, RawRepresentable<Int> {
+    static var placeholder: Self { get }
+    
+    var predecessor: Self { get }
+}
+
+extension PositionalDigitSystemizable {
+    public static func fatalPredecessionError() -> Never {
+        fatalError("Digit has no defined precedent")
+    }
+    
+    public var quantity: Quantity {
+        if self == Self.placeholder { return Quantity.none }
+        
+        let precedingQuantity = self.predecessor.quantity
+        return precedingQuantity.successor
+    }
+}
+
+extension PositionalDigitSystemizable {
+    public var cardinality: Int {
+        self.rawValue
+    }
+    
+    public var quantityMatchesCardinality: Bool {
+        self.cardinality.represents(self.quantity)
+    }
+}

NumberSystems/Sources/NumberSystems/Positional/Protocols/PositionalNumberSystemizable.swift

@@ -0,0 +1,3 @@
+protocol PositionalNumberSystemizable {
+    associatedtype Digit: PositionalDigitSystemizable
+}

NumberSystems/Tests/NumberSystemsTests/DecimalNumberTests.swift

@@ -0,0 +1,8 @@
+import Testing
+@testable import NumberSystems
+
+@Test func quantitiesOfDecimalDigits() async throws {
+    DecimalNumber.Digit.allCases.forEach {
+        #expect($0.quantityMatchesCardinality)
+    }
+}