Pulling out a bunch of openssh stuff to dedicated package. (#775)

Sources/Packages/Sources/SSHProtocolKit/OpenSSHReader.swift

@@ -0,0 +1,54 @@
+import Foundation
+
+/// Reads OpenSSH protocol data.
+public final class OpenSSHReader {
+
+    var remaining: Data
+    var done = false
+
+    /// Initialize the reader with an OpenSSH data payload.
+    /// - Parameter data: The data to read.
+    public init(data: Data) {
+        remaining = Data(data)
+    }
+
+    /// Reads the next chunk of data from the playload.
+    /// - Returns: The next chunk of data.
+    public func readNextChunk(convertEndianness: Bool = true) throws(OpenSSHReaderError) -> Data {
+        let length = try readNextBytes(as: UInt32.self, convertEndianness: convertEndianness)
+        guard remaining.count >= length else { throw .beyondBounds }
+        let dataRange = 0..<Int(length)
+        let ret = Data(remaining[dataRange])
+        remaining.removeSubrange(dataRange)
+        if remaining.isEmpty {
+            done = true
+        }
+        return ret
+    }
+
+    public func readNextBytes<T: FixedWidthInteger>(as: T.Type, convertEndianness: Bool = true) throws(OpenSSHReaderError) -> T {
+        let size = MemoryLayout<T>.size
+        guard remaining.count >= size else { throw .beyondBounds }
+        let lengthRange = 0..<size
+        let lengthChunk = remaining[lengthRange]
+        remaining.removeSubrange(lengthRange)
+        if remaining.isEmpty {
+            done = true
+        }
+        let value = unsafe lengthChunk.bytes.unsafeLoad(as: T.self)
+        return convertEndianness ? T(value.bigEndian) : T(value)
+    }
+
+    public func readNextChunkAsString(convertEndianness: Bool = true) throws(OpenSSHReaderError) -> String {
+        try String(decoding: readNextChunk(convertEndianness: convertEndianness), as: UTF8.self)
+    }
+
+    public func readNextChunkAsSubReader(convertEndianness: Bool = true) throws(OpenSSHReaderError) -> OpenSSHReader {
+        OpenSSHReader(data: try readNextChunk(convertEndianness: convertEndianness))
+    }
+
+}
+
+public enum OpenSSHReaderError: Error, Codable {
+    case beyondBounds
+}