Resolved an arithmetic overflow error in Decimal division caused by improper upcasting (#850)

- Upcast numbers to higher bitwidth before performing calculations.

resolves: rdar://133458109
resolves: #595

Author: iCharlesHu

Sources/FoundationEssentials/Decimal/Decimal+Math.swift

@@ -949,7 +949,7 @@ extension Decimal {
 
         // D1: Normalize
         // Calculate d such that `d*highest_dight_of_divisor >= b/2 (0x8000)
-        let d = (1 << 16) / UInt32(divisor[divisor.count - 1] + 1)
+        let d: UInt32 = (1 << 16) / (UInt32(divisor[divisor.count - 1]) + 1)
         // This is to make the whole algorithm work and
         // (dividend * d) / (divisor * d) == dividend / divisor
         var normalizedDividend = try self._integerMultiplyByShort(

Tests/FoundationEssentialsTests/DecimalTests.swift

@@ -613,6 +613,32 @@ final class DecimalTests : XCTestCase {
         XCTAssertTrue(Decimal._compare(lhs: expected, rhs: result) == .orderedSame)
     }
 
+    func testCrashingDivision() throws {
+        // This test makes sure the following division
+        // does not crash
+        let first: Decimal = Decimal(1147858867)
+        let second: Decimal = Decimal(4294967295)
+        let result = first / second
+        let expected: Decimal = Decimal(
+            _exponent: -38,
+            _length: 8,
+            _isNegative: 0,
+            _isCompact: 1,
+            _reserved: 0,
+            _mantissa: (
+                58076,
+                13229,
+                12316,
+                25502,
+                15252,
+                32996,
+                11611,
+                5147
+            )
+        )
+        XCTAssertEqual(result, expected)
+    }
+
     func testPower() throws {
         var a = Decimal(1234)
         var result = try a._power(exponent: 0, roundingMode: .plain)
@@ -1272,5 +1298,4 @@ final class DecimalTests : XCTestCase {
         XCTAssertEqual(length, 3)
     }
     #endif
-
 }