Update for open-source Decimal fixes (#16)

* Update to future-proof usage of Decimal.significand

* Update all remaining uses of Decimal.significand

* Update usage of `Decimal(sign:exponent:significand:)`

* Apply suggestions from code review

Co-authored-by: Cory Benfield <lukasa@apple.com>

Co-authored-by: Cory Benfield <lukasa@apple.com>

Author: xwu

Sources/StructuredFieldValues/Decoder/BareItemDecoder.swift

@@ -116,7 +116,7 @@ extension BareItemDecoder: SingleValueDecodingContainer {
 
         return Decimal(sign: pseudoDecimal.mantissa > 0 ? .plus : .minus,
                        exponent: Int(pseudoDecimal.exponent),
-                       significand: Decimal(pseudoDecimal.mantissa))
+                       significand: Decimal(pseudoDecimal.mantissa.magnitude))
     }
 
     func decodeNil() -> Bool {

Sources/StructuredFieldValues/Encoder/StructuredFieldValueEncoder.swift

@@ -281,12 +281,12 @@ extension _StructuredFieldEncoder: SingleValueEncodingContainer {
     }
 
     func encode(_ data: Decimal) throws {
-        let significand = (data.significand as NSNumber).intValue // Yes, really.
+        let significand = (data.significand.magnitude as NSNumber).intValue // Yes, really.
         guard let exponent = Int8(exactly: data.exponent) else {
             throw StructuredHeaderError.invalidIntegerOrDecimal
         }
 
-        let pd = PseudoDecimal(mantissa: significand, exponent: Int(exponent))
+        let pd = PseudoDecimal(mantissa: significand * (data.isSignMinus ? -1 : 1), exponent: Int(exponent))
         try self.currentStackEntry.storage.insertBareItem(.decimal(pd))
     }
 
@@ -438,12 +438,12 @@ extension _StructuredFieldEncoder {
     }
 
     func append(_ value: Decimal) throws {
-        let significand = (value.significand as NSNumber).intValue // Yes, really.
+        let significand = (value.significand.magnitude as NSNumber).intValue // Yes, really.
         guard let exponent = Int8(exactly: value.exponent) else {
             throw StructuredHeaderError.invalidIntegerOrDecimal
         }
 
-        let pd = PseudoDecimal(mantissa: significand, exponent: Int(exponent))
+        let pd = PseudoDecimal(mantissa: significand * (value.isSignMinus ? -1 : 1), exponent: Int(exponent))
         try self.currentStackEntry.storage.appendBareItem(.decimal(pd))
     }
 
@@ -605,12 +605,12 @@ extension _StructuredFieldEncoder {
     }
 
     func encode(_ value: Decimal, forKey key: String) throws {
-        let significand = (value.significand as NSNumber).intValue // Yes, really.
+        let significand = (value.significand.magnitude as NSNumber).intValue // Yes, really.
         guard let exponent = Int8(exactly: value.exponent) else {
             throw StructuredHeaderError.invalidIntegerOrDecimal
         }
 
-        let pd = PseudoDecimal(mantissa: significand, exponent: Int(exponent))
+        let pd = PseudoDecimal(mantissa: significand * (value.isSignMinus ? -1 : 1), exponent: Int(exponent))
         try self.currentStackEntry.storage.insertBareItem(.decimal(pd), atKey: key)
     }