Fix bug #13: addition/subtraction of negative BigInts and zero.

Author: objecthub

NumberKit.xcodeproj/xcshareddata/xcschemes/NumberKit iOS.xcscheme

@@ -15,7 +15,7 @@
             <BuildableReference
                BuildableIdentifier = "primary"
                BlueprintIdentifier = "CCD2603823F20F7200DFE5A4"
-               BuildableName = "NumberKit iOS.framework"
+               BuildableName = "NumberKit.framework"
                BlueprintName = "NumberKit iOS"
                ReferencedContainer = "container:NumberKit.xcodeproj">
             </BuildableReference>
@@ -61,7 +61,7 @@
          <BuildableReference
             BuildableIdentifier = "primary"
             BlueprintIdentifier = "CCD2603823F20F7200DFE5A4"
-            BuildableName = "NumberKit iOS.framework"
+            BuildableName = "NumberKit.framework"
             BlueprintName = "NumberKit iOS"
             ReferencedContainer = "container:NumberKit.xcodeproj">
          </BuildableReference>

Sources/NumberKit/BigInt.swift

@@ -260,7 +260,7 @@ public struct BigInt: Hashable,
     // Determine base
     let radix = UInt32(base.radix)
     // Shortcut handling of zero
-    if isZero {
+    if self.isZero {
       return "0"
     }
     var radixPow: UInt32 = 1
@@ -428,6 +428,9 @@ public struct BigInt: Hashable,
 
   /// Returns the sum of `self` and `rhs` as a `BigInt`.
   public func plus(_ rhs: BigInt) -> BigInt {
+    guard !rhs.isZero else {
+      return self
+    }
     guard self.negative == rhs.negative else {
       return self.minus(rhs.negate)
     }
@@ -454,6 +457,9 @@ public struct BigInt: Hashable,
 
   /// Returns the difference between `self` and `rhs` as a `BigInt`.
   public func minus(_ rhs: BigInt) -> BigInt {
+    guard !rhs.isZero else {
+      return self
+    }
     guard self.negative == rhs.negative else {
       return self.plus(rhs.negate)
     }
@@ -606,7 +612,7 @@ public struct BigInt: Hashable,
   /// Computes the square root; this is the largest `BigInt` value `x` such that `x * x` is
   /// smaller than `self`.
   public var sqrt: BigInt {
-    guard !self.isNegative else {
+    guard !self.negative else {
       preconditionFailure("cannot compute square root of negative number")
     }
     guard !self.isZero && !self.isOne else {

Tests/NumberKitTests/BigIntTests.swift

@@ -356,6 +356,15 @@ class BigIntTests: XCTestCase {
     XCTAssertEqual(x3n.description, x3s)
   }
 
+  func testBasicOpWithZero() {
+    let x1: BigInt = "0"
+    let x2: BigInt = "-1"
+    XCTAssertFalse(x1.isNegative)
+    XCTAssertFalse(x1.negate.isNegative)
+    XCTAssertEqual(x2.plus(x1), x2)
+    XCTAssertEqual(x2.minus(x1), x2)
+  }
+  
   static let allTests = [
     ("testConstructors", testConstructors),
     ("testPlus", testPlus),