Decimal: Add more tests for addition and subtraction

- integerAdd(), integerSubtract(): Use more meaningful variable names.

Author: spevans

Foundation/Decimal.swift

@@ -1191,57 +1191,57 @@ public func NSDecimalAdd(_ result: UnsafeMutablePointer<Decimal>, _ leftOperand:
 }
 
 fileprivate func integerAdd(_ result: inout WideDecimal, _ left: inout Decimal, _ right: inout Decimal) -> NSDecimalNumber.CalculationError {
-    var i: UInt32 = 0
+    var idx: UInt32 = 0
     var carry: UInt16 = 0
-    let c: UInt32 = min(left._length, right._length)
+    let maxIndex: UInt32 = min(left._length, right._length) // The highest index with bits set in both values
 
-    while i < c {
-        let li = UInt32(left[i])
-        let ri = UInt32(right[i])
-        let accumulator = li + ri + UInt32(carry)
-        carry = UInt16(truncatingIfNeeded:accumulator >> 16)
-        result[i] = UInt16(truncatingIfNeeded:accumulator)
-        i += 1
+    while idx < maxIndex {
+        let li = UInt32(left[idx])
+        let ri = UInt32(right[idx])
+        let sum = li + ri + UInt32(carry)
+        carry = UInt16(truncatingIfNeeded: sum >> 16)
+        result[idx] = UInt16(truncatingIfNeeded: sum)
+        idx += 1
     }
 
-    while i < left._length {
+    while idx < left._length {
         if carry != 0 {
-            let li = UInt32(left[i])
-            let accumulator = li + UInt32(carry)
-            carry = UInt16(truncatingIfNeeded:accumulator >> 16)
-            result[i] = UInt16(truncatingIfNeeded:accumulator)
-            i += 1
+            let li = UInt32(left[idx])
+            let sum = li + UInt32(carry)
+            carry = UInt16(truncatingIfNeeded: sum >> 16)
+            result[idx] = UInt16(truncatingIfNeeded: sum)
+            idx += 1
         } else {
-            while i < left._length {
-                result[i] = left[i]
-                i += 1
+            while idx < left._length {
+                result[idx] = left[idx]
+                idx += 1
             }
             break
         }
     }
-    while i < right._length {
+    while idx < right._length {
         if carry != 0 {
-            let ri = UInt32(right[i])
-            let accumulator = ri + UInt32(carry)
-            carry = UInt16(truncatingIfNeeded:accumulator >> 16)
-            result[i] = UInt16(truncatingIfNeeded:accumulator)
-            i += 1
+            let ri = UInt32(right[idx])
+            let sum = ri + UInt32(carry)
+            carry = UInt16(truncatingIfNeeded: sum >> 16)
+            result[idx] = UInt16(truncatingIfNeeded: sum)
+            idx += 1
         } else {
-            while i < right._length {
-                result[i] = right[i]
-                i += 1
+            while idx < right._length {
+                result[idx] = right[idx]
+                idx += 1
             }
             break
         }
     }
-    result._length = i
+    result._length = idx
 
     if carry != 0 {
-        result[i] = carry
-        i += 1
-        result._length = i
+        result[idx] = carry
+        idx += 1
+        result._length = idx
     }
-    if i > Decimal.maxSize {
+    if idx > Decimal.maxSize {
         return .overflow
     }
 
@@ -1256,46 +1256,48 @@ fileprivate func integerAdd(_ result: inout WideDecimal, _ left: inout Decimal,
 //    give b-a...
 //
 fileprivate func integerSubtract(_ result: inout Decimal, _ left: inout Decimal, _ right: inout Decimal) -> NSDecimalNumber.CalculationError {
-    var i: UInt32 = 0
+    var idx: UInt32 = 0
+    let maxIndex: UInt32 = min(left._length, right._length) // The highest index with bits set in both values
     var borrow: UInt16 = 0
-    let c: UInt32 = min(left._length, right._length)
 
-    while i < c {
-        let li = UInt32(left[i])
-        let ri = UInt32(right[i])
-        let accumulator: UInt32 = (0x10000 + li) - UInt32(borrow) - ri
-        result[i] = UInt16(truncatingIfNeeded:accumulator)
-        borrow = 1 - UInt16(truncatingIfNeeded:accumulator >> 16)
-        i += 1
+    while idx < maxIndex {
+        let li = UInt32(left[idx])
+        let ri = UInt32(right[idx])
+        // 0x10000 is to borrow in advance to avoid underflow.
+        let difference: UInt32 = (0x10000 + li) - UInt32(borrow) - ri
+        result[idx] = UInt16(truncatingIfNeeded: difference)
+        // borrow = 1 if the borrow was used.
+        borrow = 1 - UInt16(truncatingIfNeeded: difference >> 16)
+        idx += 1
     }
 
-    while i < left._length {
+    while idx < left._length {
         if borrow != 0 {
-            let li = UInt32(left[i])
-            let accumulator = 0xffff + li // + no carry
-            borrow = 1 - UInt16(truncatingIfNeeded:accumulator >> 16)
-            result[i] = UInt16(truncatingIfNeeded:accumulator)
-            i += 1
+            let li = UInt32(left[idx])
+            let sum = 0xffff + li // + no carry
+            borrow = 1 - UInt16(truncatingIfNeeded: sum >> 16)
+            result[idx] = UInt16(truncatingIfNeeded: sum)
+            idx += 1
         } else {
-            while i < left._length {
-                result[i] = left[i]
-                i += 1
+            while idx < left._length {
+                result[idx] = left[idx]
+                idx += 1
             }
             break
         }
     }
-    while i < right._length {
-        let ri = UInt32(right[i])
-        let accumulator = 0xffff - ri + UInt32(borrow)
-        borrow = 1 - UInt16(truncatingIfNeeded:accumulator >> 16)
-        result[i] = UInt16(truncatingIfNeeded:accumulator)
-        i += 1
+    while idx < right._length {
+        let ri = UInt32(right[idx])
+        let difference = 0xffff - ri + UInt32(borrow)
+        borrow = 1 - UInt16(truncatingIfNeeded: difference >> 16)
+        result[idx] = UInt16(truncatingIfNeeded: difference)
+        idx += 1
     }
 
     if borrow != 0 {
         return .overflow
     }
-    result._length = i;
+    result._length = idx;
     result.trimTrailingZeros()
 
     return .noError;

TestFoundation/TestDecimal.swift

@@ -288,18 +288,28 @@ class TestDecimal: XCTestCase {
 
         XCTAssertEqual(Decimal(186243 * 15673 as Int64), Decimal(186243) * Decimal(15673))
 
-        XCTAssertEqual(Decimal(5538) + Decimal(2880.4), Decimal(5538 + 2880.4))
+        XCTAssertEqual(Decimal(string: "5538")! + Decimal(string: "2880.4")!, Decimal(string: "8418.4")!)
         XCTAssertEqual(NSDecimalNumber(floatLiteral: 5538).adding(NSDecimalNumber(floatLiteral: 2880.4)), NSDecimalNumber(floatLiteral: 5538 + 2880.4))
 
-        XCTAssertEqual(Decimal(5538) - Decimal(2880.4), Decimal(5538 - 2880.4))
-        XCTAssertEqual(Decimal(2880.4) - Decimal(5538), Decimal(2880.4 - 5538))
+        XCTAssertEqual(Decimal(string: "5538.0")! - Decimal(string: "2880.4")!, Decimal(string: "2657.6")!)
+        XCTAssertEqual(Decimal(string: "2880.4")! - Decimal(5538), Decimal(string: "-2657.6")!)
         XCTAssertEqual(Decimal(0x10000) - Decimal(0x1000), Decimal(0xf000))
         XCTAssertEqual(Decimal(0x1_0000_0000) - Decimal(0x1000), Decimal(0xFFFFF000))
         XCTAssertEqual(Decimal(0x1_0000_0000_0000) - Decimal(0x1000), Decimal(0xFFFFFFFFF000))
         XCTAssertEqual(Decimal(1234_5678_9012_3456_7899 as UInt64) - Decimal(1234_5678_9012_3456_7890 as UInt64), Decimal(9))
         XCTAssertEqual(Decimal(0xffdd_bb00_8866_4422 as UInt64) - Decimal(0x7777_7777), Decimal(0xFFDD_BB00_10EE_CCAB as UInt64))
         XCTAssertEqual(NSDecimalNumber(floatLiteral: 5538).subtracting(NSDecimalNumber(floatLiteral: 2880.4)), NSDecimalNumber(floatLiteral: 5538 - 2880.4))
         XCTAssertEqual(NSDecimalNumber(floatLiteral: 2880.4).subtracting(NSDecimalNumber(floatLiteral: 5538)), NSDecimalNumber(floatLiteral: 2880.4 - 5538))
+
+        XCTAssertEqual(Decimal.greatestFiniteMagnitude - Decimal.greatestFiniteMagnitude, Decimal(0))
+        XCTAssertEqual(Decimal.leastFiniteMagnitude - Decimal(1), Decimal.leastFiniteMagnitude)
+        let overflowed = Decimal.greatestFiniteMagnitude + Decimal.greatestFiniteMagnitude
+        XCTAssertTrue(overflowed.isNaN)
+
+        let highBit = Decimal(_exponent: 0, _length: 8, _isNegative: 0, _isCompact: 1, _reserved: 0, _mantissa: (0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x8000))
+        let otherBits = Decimal(_exponent: 0, _length: 8, _isNegative: 0, _isCompact: 1, _reserved: 0, _mantissa: (0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0x7fff))
+        XCTAssertEqual(highBit - otherBits, Decimal(1))
+        XCTAssertEqual(otherBits + Decimal(1), highBit)
     }
 
     func test_Misc() {