Fix Data Base64 length encoding bug

### Motivation

We have an issue in encodeComputeCapacity. If we add lineBreaks, we assumed to add line breaks for lines that ended at the max capacity.

### Changes

- encodeComputeCapacity checks if the last line uses the full length and removes unnecessary seperatorBytes if needed

rdar://155204772

Author: fabianfett

Sources/FoundationEssentials/Data/Data+Base64.swift

@@ -395,7 +395,12 @@ extension Base64 {
 
         let lineLength = options.contains(.lineLength64Characters) ? 64 : 76
         let lineBreaks = capacityWithoutBreaks / lineLength
-        let lineBreakCapacity = lineBreaks * seperatorBytes
+        var lineBreakCapacity = lineBreaks * seperatorBytes
+        // in case the last row uses all available space, we don't need to add line breaks
+        // but we can't remove bytes if we have an empty input
+        if capacityWithoutBreaks % lineLength == 0 && capacityWithoutBreaks > seperatorBytes {
+            lineBreakCapacity -= seperatorBytes
+        }
         return capacityWithoutBreaks + lineBreakCapacity
     }
 

Tests/FoundationEssentialsTests/DataTests.swift

@@ -2276,6 +2276,62 @@ extension DataTests {
         #expect("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" == base64, "medium base64 conversion should work")
     }
 
+    @Test func testBase64LineLengthOptions() {
+        let expected46 = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=="
+        let length46String = Data(repeating:0, count: 46).base64EncodedString(options: .lineLength64Characters)
+        #expect(length46String == expected46)
+        let length46Data = Data(repeating:0, count: 46).base64EncodedData(options: .lineLength64Characters)
+        #expect(length46Data.count == 64)
+        #expect(String(decoding: length46Data, as: Unicode.UTF8.self) == expected46)
+
+        let expected47 = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="
+        let length47String = Data(repeating:0, count: 47).base64EncodedString(options: .lineLength64Characters)
+        #expect(length47String == expected47)
+        let length47Data = Data(repeating:0, count: 47).base64EncodedData(options: .lineLength64Characters)
+        #expect(length47Data.count == 64)
+        #expect(String(decoding: length47Data, as: Unicode.UTF8.self) == expected47)
+
+        let expected48 = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+        let length48String = Data(repeating:0, count: 48).base64EncodedString(options: .lineLength64Characters)
+        #expect(length48String == expected48)
+        let length48Data = Data(repeating:0, count: 48).base64EncodedData(options: .lineLength64Characters)
+        #expect(length48Data.count == 64)
+        #expect(String(decoding: length48Data, as: Unicode.UTF8.self) == expected48)
+
+        let length49 = Data(repeating:0, count: 49).base64EncodedString(options: .lineLength64Characters)
+        #expect(length49 == #"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\#r\#nAA=="#)
+        #expect(Array(length49.utf8)[64] == 13)
+        #expect(Array(length49.utf8)[65] == 10)
+    }
+
+    // we have more encodeToStringTests than we have encodeToDataTests.
+    // lets fix this by ensuring data output matches string output.
+    @Test func testBase64DataOutputMatchesStingOutput() {
+        for count in 0..<10_000 {
+            let data = Data(repeating: 0, count: count)
+            let stringBase64 = data.base64EncodedString(options: .lineLength64Characters)
+            let dataBase64 = data.base64EncodedData(options: .lineLength64Characters)
+
+            #expect(stringBase64 == String(decoding: dataBase64, as: Unicode.UTF8.self))
+        }
+
+        for count in 0..<10_000 {
+            let data = Data(repeating: 0, count: count)
+            let stringBase64 = data.base64EncodedString(options: .lineLength76Characters)
+            let dataBase64 = data.base64EncodedData(options: .lineLength76Characters)
+
+            #expect(stringBase64 == String(decoding: dataBase64, as: Unicode.UTF8.self))
+        }
+
+        for count in 0..<10_000 {
+            let data = Data(repeating: 0, count: count)
+            let stringBase64 = data.base64EncodedString()
+            let dataBase64 = data.base64EncodedData()
+
+            #expect(stringBase64 == String(decoding: dataBase64, as: Unicode.UTF8.self))
+        }
+    }
+
     @Test func anyHashableContainingData() {
         let values: [Data] = [
             Data(base64Encoded: "AAAA")!,