Fix implementation of Decimal.nextUp.

Author: xwu

Darwin/Foundation-swiftoverlay-Tests/TestDecimal.swift

@@ -617,6 +617,18 @@ class TestDecimal : XCTestCase {
         XCTAssertNotEqual(x.nextDown, x)
         XCTAssertNotEqual(x.nextUp, x)
 
+        // For similar reasons, '3.40282366920938463463374607431768211455',
+        // which has the same significand as 'Decimal.greatestFiniteMagnitude',
+        // is an important value to test because the distance to the next
+        // representable value is more than 'ulp' and instead requires
+        // incrementing '_exponent'.
+        x = Decimal(string: "3.40282366920938463463374607431768211455")!
+        XCTAssertEqual(x.ulp, Decimal(string: "0.00000000000000000000000000000000000001")!)
+        XCTAssertEqual(x.nextUp, Decimal(string: "3.4028236692093846346337460743176821146")!)
+        x = Decimal(string: "3.4028236692093846346337460743176821146")!
+        XCTAssertEqual(x.ulp, Decimal(string: "0.0000000000000000000000000000000000001")!)
+        XCTAssertEqual(x.nextDown, Decimal(string: "3.40282366920938463463374607431768211455")!)
+
         x = 1
         XCTAssertEqual(x.ulp, Decimal(string: "1e-38")!)
         XCTAssertEqual(x.nextDown, x - Decimal(string: "1e-38")!)

Darwin/Foundation-swiftoverlay/Decimal.swift

@@ -620,6 +620,23 @@ extension Decimal {
     }
 
     public var nextUp: Decimal {
+        if _isNegative == 1 {
+            if _exponent > -128
+                && (_mantissa.0, _mantissa.1, _mantissa.2, _mantissa.3) == (0x999a, 0x9999, 0x9999, 0x9999)
+                && (_mantissa.4, _mantissa.5, _mantissa.6, _mantissa.7) == (0x9999, 0x9999, 0x9999, 0x1999) {
+                return Decimal(
+                    _exponent: _exponent &- 1, _length: 8, _isNegative: 1, _isCompact: 1,
+                    _reserved: 0, _mantissa: (0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff))
+            }
+        } else {
+            if _exponent < 127
+            && (_mantissa.0, _mantissa.1, _mantissa.2, _mantissa.3) == (0xffff, 0xffff, 0xffff, 0xffff)
+            && (_mantissa.4, _mantissa.5, _mantissa.6, _mantissa.7) == (0xffff, 0xffff, 0xffff, 0xffff) {
+                return Decimal(
+                    _exponent: _exponent &+ 1, _length: 8, _isNegative: 0, _isCompact: 1,
+                    _reserved: 0, _mantissa: (0x999a, 0x9999, 0x9999, 0x9999, 0x9999, 0x9999, 0x9999, 0x1999))
+            }
+        }
         return self + ulp
     }
 

Sources/Foundation/Decimal.swift

@@ -831,6 +831,23 @@ extension Decimal {
     }
 
     public var nextUp: Decimal {
+        if _isNegative == 1 {
+            if _exponent > -128
+                && (_mantissa.0, _mantissa.1, _mantissa.2, _mantissa.3) == (0x999a, 0x9999, 0x9999, 0x9999)
+                && (_mantissa.4, _mantissa.5, _mantissa.6, _mantissa.7) == (0x9999, 0x9999, 0x9999, 0x1999) {
+                return Decimal(
+                    _exponent: _exponent &- 1, _length: 8, _isNegative: 1, _isCompact: 1,
+                    _reserved: 0, _mantissa: (0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff, 0xffff))
+            }
+        } else {
+            if _exponent < 127
+            && (_mantissa.0, _mantissa.1, _mantissa.2, _mantissa.3) == (0xffff, 0xffff, 0xffff, 0xffff)
+            && (_mantissa.4, _mantissa.5, _mantissa.6, _mantissa.7) == (0xffff, 0xffff, 0xffff, 0xffff) {
+                return Decimal(
+                    _exponent: _exponent &+ 1, _length: 8, _isNegative: 0, _isCompact: 1,
+                    _reserved: 0, _mantissa: (0x999a, 0x9999, 0x9999, 0x9999, 0x9999, 0x9999, 0x9999, 0x1999))
+            }
+        }
         return self + ulp
     }
 

Tests/Foundation/Tests/TestDecimal.swift

@@ -869,6 +869,18 @@ class TestDecimal: XCTestCase {
         XCTAssertNotEqual(x.nextDown, x)
         XCTAssertNotEqual(x.nextUp, x)
 
+        // For similar reasons, '3.40282366920938463463374607431768211455',
+        // which has the same significand as 'Decimal.greatestFiniteMagnitude',
+        // is an important value to test because the distance to the next
+        // representable value is more than 'ulp' and instead requires
+        // incrementing '_exponent'.
+        x = Decimal(string: "3.40282366920938463463374607431768211455")!
+        XCTAssertEqual(x.ulp, Decimal(string: "0.00000000000000000000000000000000000001")!)
+        XCTAssertEqual(x.nextUp, Decimal(string: "3.4028236692093846346337460743176821146")!)
+        x = Decimal(string: "3.4028236692093846346337460743176821146")!
+        XCTAssertEqual(x.ulp, Decimal(string: "0.0000000000000000000000000000000000001")!)
+        XCTAssertEqual(x.nextDown, Decimal(string: "3.40282366920938463463374607431768211455")!)
+
         x = 1
         XCTAssertEqual(x.ulp, Decimal(string: "1e-38")!)
         XCTAssertEqual(x.nextDown, x - Decimal(string: "1e-38")!)