[6.1] Fix ISO Latin 1 Encoding/Decoding issues

Sources/FoundationEssentials/String/String+IO.swift

@@ -18,33 +18,6 @@ internal import _FoundationCShims
 
 fileprivate let stringEncodingAttributeName = "com.apple.TextEncoding"
 
-private struct ExtendingToUTF16Sequence<Base: Sequence<UInt8>> : Sequence {
-    typealias Element = UInt16
-
-    struct Iterator : IteratorProtocol {
-        private var base: Base.Iterator
-
-        init(_ base: Base.Iterator) {
-            self.base = base
-        }
-
-        mutating func next() -> Element? {
-            guard let value = base.next() else { return nil }
-            return UInt16(value)
-        }
-    }
-
-    private let base: Base
-
-    init(_ base: Base) {
-        self.base = base
-    }
-
-    func makeIterator() -> Iterator {
-        Iterator(base.makeIterator())
-    }
-}
-
 
 @available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
 extension String {
@@ -181,12 +154,9 @@ extension String {
             }
         #if !FOUNDATION_FRAMEWORK
         case .isoLatin1:
-            guard bytes.allSatisfy(\.isValidISOLatin1) else {
-                return nil
-            }
-            // isoLatin1 is an 8-bit encoding that represents a subset of UTF-16
-            // Map to 16-bit values and decode as UTF-16
-            self.init(_validating: ExtendingToUTF16Sequence(bytes), as: UTF16.self)
+            // ISO Latin 1 bytes are always valid since it's an 8-bit encoding that maps scalars 0x0 through 0xFF
+            // Simply extend each byte to 16 bits and decode as UTF-16
+            self.init(decoding: bytes.lazy.map { UInt16($0) }, as: UTF16.self)
         case .macOSRoman:
             func buildString(_ bytes: UnsafeBufferPointer<UInt8>) -> String {
                 String(unsafeUninitializedCapacity: bytes.count * 3) { buffer in

Sources/FoundationEssentials/String/StringProtocol+Essentials.swift

@@ -21,12 +21,6 @@ import Darwin
 
 internal import _FoundationCShims
 
-extension BinaryInteger {
-    var isValidISOLatin1: Bool {
-        (0x20 <= self && self <= 0x7E) || (0xA0 <= self && self <= 0xFF)
-    }
-}
-
 extension UInt8 {
     private typealias UTF8Representation = (UInt8, UInt8, UInt8)
     private static func withMacRomanMap<R>(_ body: (UnsafeBufferPointer<UTF8Representation>) -> R) -> R {
@@ -228,12 +222,14 @@ extension String {
             return data + swapped
         #if !FOUNDATION_FRAMEWORK
         case .isoLatin1:
-            return try? Data(capacity: self.utf16.count) { buffer in
-                for scalar in self.utf16 {
-                    guard scalar.isValidISOLatin1 else {
+            // ISO Latin 1 encodes code points 0x0 through 0xFF (a maximum of 2 UTF-8 scalars per ISO Latin 1 Scalar)
+            // The UTF-8 count is a cheap, reasonable starting capacity as it is precise for the all-ASCII case and it will only over estimate by 1 byte per non-ASCII character
+            return try? Data(capacity: self.utf8.count) { buffer in
+                for scalar in self.unicodeScalars {
+                    guard let valid = UInt8(exactly: scalar.value) else {
                         throw CocoaError(.fileWriteInapplicableStringEncoding)
                     }
-                    buffer.appendElement(UInt8(scalar & 0xFF))
+                    buffer.appendElement(valid)
                 }
             }
         case .macOSRoman:

Tests/FoundationEssentialsTests/StringTests.swift

@@ -1336,7 +1336,9 @@ final class StringTests : XCTestCase {
             "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
             "0123456789",
             "!\"#$%&'()*+,-./",
-            "¡¶ÅÖæöÿ\u{00A0}~"
+            "¡¶ÅÖæöÿ\u{0080}\u{00A0}~",
+            "Hello\nworld",
+            "Hello\r\nworld"
         ], invalid: [
             "🎺",
             "מ",