SR-11150: NSDecimalNumber .multiplying(byPowerOf10:) produces incorrect results.

- Call NSDecimalMultiplyByPowerOf10() instead of NSDecimalPower() to
  multiply the number by 10 raised to the given power, not raise
  the number to a given power.

Author: spevans

Foundation/NSDecimalNumber.swift

@@ -303,12 +303,13 @@ open class NSDecimalNumber : NSNumber {
     open func multiplying(byPowerOf10 power: Int16) -> NSDecimalNumber {
         return multiplying(byPowerOf10: power, withBehavior: nil)
     }
+
     open func multiplying(byPowerOf10 power: Int16, withBehavior b: NSDecimalNumberBehaviors?) -> NSDecimalNumber {
         var result = Decimal()
         var input = self.decimal
         let behavior = b ?? NSDecimalNumber.defaultBehavior
         let roundingMode = behavior.roundingMode()
-        let error = NSDecimalPower(&result, &input, Int(power), roundingMode)
+        let error = NSDecimalMultiplyByPowerOf10(&result, &input, power, roundingMode)
         handle(error,behavior)
         return NSDecimalNumber(decimal: result)
     }

TestFoundation/TestDecimal.swift

@@ -1143,6 +1143,25 @@ class TestDecimal: XCTestCase {
         XCTAssertEqual(NSDecimalString(&decimal, NSDictionary(dictionary: d4)), "-123456.789")
     }
 
+    func test_multiplyingByPowerOf10() {
+        let decimalNumber = NSDecimalNumber(string: "0.022829306361065572")
+        let d1 = decimalNumber.multiplying(byPowerOf10: 18)
+        XCTAssertEqual(d1.stringValue, "22829306361065572")
+        let d2 = d1.multiplying(byPowerOf10: -18)
+        XCTAssertEqual(d2.stringValue, "0.022829306361065572")
+
+        XCTAssertEqual(NSDecimalNumber(string: "0.01").multiplying(byPowerOf10: 0).stringValue, "0.01")
+        XCTAssertEqual(NSDecimalNumber(string: "0.01").multiplying(byPowerOf10: 1).stringValue, "0.1")
+        XCTAssertEqual(NSDecimalNumber(string: "0.01").multiplying(byPowerOf10: -1).stringValue, "0.001")
+        XCTAssertEqual(NSDecimalNumber(value: 0).multiplying(byPowerOf10: 0).stringValue, "0")
+        XCTAssertEqual(NSDecimalNumber(value: 0).multiplying(byPowerOf10: -1).stringValue, "0")
+        XCTAssertEqual(NSDecimalNumber(value: 0).multiplying(byPowerOf10: 1).stringValue, "0")
+
+        XCTAssertEqual(NSDecimalNumber(value: 1).multiplying(byPowerOf10: 128).stringValue, "NaN")
+        XCTAssertEqual(NSDecimalNumber(value: 1).multiplying(byPowerOf10: 127).stringValue, "10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000")
+        XCTAssertEqual(NSDecimalNumber(value: 1).multiplying(byPowerOf10: -128).stringValue, "0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001")
+        XCTAssertEqual(NSDecimalNumber(value: 1).multiplying(byPowerOf10: -129).stringValue, "NaN")
+    }
 
     static var allTests : [(String, (TestDecimal) -> () throws -> Void)] {
         return [
@@ -1171,6 +1190,7 @@ class TestDecimal: XCTestCase {
             ("test_bridging", test_bridging),
             ("test_stringWithLocale", test_stringWithLocale),
             ("test_NSDecimalString", test_NSDecimalString),
+            ("test_multiplyingByPowerOf10", test_multiplyingByPowerOf10),
         ]
     }
 }