Encode Decimals with positive exponents instead of trapping

Sources/RawStructuredFieldValues/PseudoDecimal.swift

@@ -115,7 +115,7 @@ public struct PseudoDecimal: Hashable, Sendable {
         }
     }
 
-    private static func validate(mantissa: Int64, exponent: Int8) throws {
+    package static func validate(mantissa: Int64, exponent: Int8) throws {
         // We compare against our upper and lower bounds. The maximum allowable magnitude of the mantissa is contingent
         // on the exponent.
         switch exponent {

Sources/StructuredFieldValues/Encoder/StructuredFieldValueEncoder.swift

@@ -299,12 +299,7 @@ extension _StructuredFieldEncoder: SingleValueEncodingContainer {
     }
 
     func encode(_ data: Decimal) throws {
-        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 * (data.isSignMinus ? -1 : 1), exponent: Int(exponent))
+        let pd = try PseudoDecimal(data)
         try self.currentStackEntry.storage.insertBareItem(.decimal(pd))
     }
 
@@ -469,12 +464,7 @@ extension _StructuredFieldEncoder {
     }
 
     func append(_ value: Decimal) throws {
-        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 * (value.isSignMinus ? -1 : 1), exponent: Int(exponent))
+        let pd = try PseudoDecimal(value)
         try self.currentStackEntry.storage.appendBareItem(.decimal(pd))
     }
 
@@ -652,12 +642,7 @@ extension _StructuredFieldEncoder {
     }
 
     func encode(_ value: Decimal, forKey key: String) throws {
-        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 * (value.isSignMinus ? -1 : 1), exponent: Int(exponent))
+        let pd = try PseudoDecimal(value)
         try self.currentStackEntry.storage.insertBareItem(.decimal(pd), atKey: key)
     }
 
@@ -994,3 +979,28 @@ extension Item {
         self.init(bareItem: partialItem.bareItem!, parameters: partialItem.parameters)
     }
 }
+
+extension PseudoDecimal {
+    fileprivate init(_ decimal: Decimal) throws {
+        var significand = (decimal.significand.magnitude as NSNumber).intValue  // Yes, really.
+        var decimalExponent = decimal.exponent
+
+        // Foundation stores trailing zeros as a positive exponent (100 is 1e2), so fold them into the significand.
+        while decimalExponent > 0 {
+            let (scaled, overflow) = significand.multipliedReportingOverflow(by: 10)
+            guard !overflow else {
+                throw StructuredHeaderError.invalidIntegerOrDecimal
+            }
+            significand = scaled
+            decimalExponent -= 1
+        }
+
+        guard let exponent = Int8(exactly: decimalExponent) else {
+            throw StructuredHeaderError.invalidIntegerOrDecimal
+        }
+
+        let mantissa = significand * (decimal.isSignMinus ? -1 : 1)
+        try Self.validate(mantissa: Int64(mantissa), exponent: exponent)
+        self.init(mantissa: mantissa, exponent: Int(exponent))
+    }
+}

Tests/StructuredFieldValuesTests/StructuredFieldEncoderTests.swift

@@ -61,6 +61,16 @@ final class StructuredFieldEncoderTests: XCTestCase {
         )
     }
 
+    func testEncodeDecimalWithPositiveExponent() throws {
+        // Foundation stores trailing zeros as a positive exponent, so 100 is 1e2.
+        let encoder = StructuredFieldValueEncoder()
+        XCTAssertEqual(Array("100.0".utf8), try encoder.encode(ItemField(Decimal(100))))
+        XCTAssertEqual(Array("900000000000.0".utf8), try encoder.encode(ItemField(Decimal(string: "9e11")!)))
+        XCTAssertEqual(Array("-100.0, 100.0".utf8), try encoder.encode(List([Decimal(-100), Decimal(100)])))
+        XCTAssertThrowsError(try encoder.encode(ItemField(Decimal(string: "1e12")!)))
+        XCTAssertThrowsError(try encoder.encode(List([Decimal(string: "1e12")!])))
+    }
+
     func testEncodeKeyedItemHeader() throws {
         struct KeyedItem<ItemType: Codable & Equatable>: Equatable, StructuredFieldValue {
             static var structuredFieldType: StructuredFieldType {