Add "sum" function for Sequences with AnyCurrency elements

Motivation:

It is common for developers to want a quick sum of values with currencies, to display totals.

To mirror functionality found on Sequence protocols with `max` and `min`, a `sum` with a predicate overload should
be provided.

Modifications:

- Add a `sum` function that starts with a zero value and adds all the elements together
- Add a `sum(where:)` overload that filters the elements by a predicate before summing

Result:

It should be more natural and easy for developers to quickly get a sum of currencies.

Sources/Currency/AnyCurrency+Sequence.swift

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Currency open source project
+//
+// Copyright (c) 2020 Currency project authors
+// Licensed under MIT License
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Currency project authors
+//
+// SPDX-License-Identifier: MIT
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+extension Sequence where Element: AnyCurrency {
+  /// Returns the sum total of all amounts in the sequence.
+  ///
+  /// For example:
+  ///
+  ///     let amounts: [USD] = [304.98, 19.02]
+  ///     let sumTotal = amounts.sum()
+  ///     print(sumTotal)
+  ///     // prints "324"
+  ///
+  /// If the sequence has no elements, you will receive a currency with a value of "0".
+  /// - Complexity: O(*n*) , where *n* is the length of the sequence.
+  /// - Returns: A currency value representing the sum total of all the amounts in the sequence.
+  public func sum() -> Element {
+    return self.reduce(into: .init(.zero), { $0 += $1 })
+  }
+
+  /// Returns the sum total of all amounts in the sequence that satify the given predicate.
+  /// For example:
+  ///
+  ///     let amounts: [USD] = [304.98, 19.02, 30.21]
+  ///     let sumTotal = amounts.sum(where: { $0.roundedAmount > 20 })
+  ///     print(sumTotal)
+  ///     // prints "335.19"
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the sequence.
+  /// - Parameter predicate: A closure that takes a currency element as its argument
+  ///   and returns a Boolean value that indicates whether the passed element should be included in the sum.
+  /// - Returns:A currency value representing the sum total of all the amounts in the sequence that satisfies `predicate`.
+  public func sum(where predicate: (Element) throws -> Bool) rethrows -> Element {
+    return try self.filter(predicate).sum()
+  }
+}

Tests/CurrencyTests/AnyCurrencyTests.swift

@@ -159,3 +159,19 @@ final class AnyCurrencyTests: XCTestCase {
     XCTAssertEqual("\(pounds)", "£398.98")
   }
 }
+
+// MARK: -
+// MARK: Sequence<AnyCurrency>
+
+extension AnyCurrencyTests {
+  func testSequenceSum() {
+    let amounts = [USD(30.47), -107.8239, 1_203.9832, -504.3982]
+    XCTAssertEqual(amounts.sum().roundedAmount, 622.23)
+  }
+
+  func testSequenceSum_withPredicate() {
+    let amounts: [USD] = [304.98, 19.02, 30.21]
+    let sumTotal = amounts.sum(where: { $0.roundedAmount > 20 })
+    XCTAssertEqual(sumTotal.roundedAmount, 335.19)
+  }
+}

Tests/CurrencyTests/XCTestManifests.swift

@@ -1,25 +1,11 @@
 #if !canImport(ObjectiveC)
 import XCTest
 
-extension CurrencyMintTests {
-    // DO NOT MODIFY: This is autogenerated, use:
-    //   `swift test --generate-linuxmain`
-    // to regenerate.
-    static let __allTests__CurrencyMintTests = [
-        ("testLookupByNum_fails", testLookupByNum_fails),
-        ("testLookupByNum_passes", testLookupByNum_passes),
-        ("testLookupByNum_withAmount", testLookupByNum_withAmount),
-        ("testLookupByString_fails", testLookupByString_fails),
-        ("testLookupByString_passes", testLookupByString_passes),
-        ("testLookupByString_withAmount", testLookupByString_withAmount),
-    ]
-}
-
-extension MoneyTests {
+extension AnyCurrencyTests {
     // DO NOT MODIFY: This is autogenerated, use:
     //   `swift test --generate-linuxmain`
     // to regenerate.
-    static let __allTests__MoneyTests = [
+    static let __allTests__AnyCurrencyTests = [
         ("testAddition", testAddition),
         ("testComparable", testComparable),
         ("testDescription", testDescription),
@@ -29,6 +15,8 @@ extension MoneyTests {
         ("testInit", testInit),
         ("testMinorUnits", testMinorUnits),
         ("testMultiplication", testMultiplication),
+        ("testSequenceSum", testSequenceSum),
+        ("testSequenceSum_withPredicate", testSequenceSum_withPredicate),
         ("testStringInterpolation_customFormatter", testStringInterpolation_customFormatter),
         ("testStringInterpolation_customLocale", testStringInterpolation_customLocale),
         ("testStringInterpolation_defaultFormatter", testStringInterpolation_defaultFormatter),
@@ -38,10 +26,24 @@ extension MoneyTests {
     ]
 }
 
+extension CurrencyMintTests {
+    // DO NOT MODIFY: This is autogenerated, use:
+    //   `swift test --generate-linuxmain`
+    // to regenerate.
+    static let __allTests__CurrencyMintTests = [
+        ("testLookupByNum_fails", testLookupByNum_fails),
+        ("testLookupByNum_passes", testLookupByNum_passes),
+        ("testLookupByNum_withAmount", testLookupByNum_withAmount),
+        ("testLookupByString_fails", testLookupByString_fails),
+        ("testLookupByString_passes", testLookupByString_passes),
+        ("testLookupByString_withAmount", testLookupByString_withAmount),
+    ]
+}
+
 public func __allTests() -> [XCTestCaseEntry] {
     return [
+        testCase(AnyCurrencyTests.__allTests__AnyCurrencyTests),
         testCase(CurrencyMintTests.__allTests__CurrencyMintTests),
-        testCase(MoneyTests.__allTests__MoneyTests),
     ]
 }
 #endif