Fix GZip.swift not being the actual struct name

Sources/apk/Utility/GZipReader.swift

@@ -0,0 +1,82 @@
+/*
+ * darwin-apk © 2024 Gay Pizza Specifications
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import Foundation
+import zlib
+
+struct GZipReader: ~Copyable {
+  private static let bufferSize = 0x8000
+
+  private var zstream = z_stream()
+  private var inputBuffer = [UInt8](repeating: 0, count: Self.bufferSize)
+  private var outputBuffer = [UInt8](repeating: 0, count: Self.bufferSize)
+
+  deinit {
+    var zstream = self.zstream
+    inflateEnd(&zstream)
+  }
+
+  mutating func read(inStream stream: InputStream) throws(GZipError) -> Data {
+    // Initialise zlib if this is the first time we're called
+    // otherwise reset the stream in anticipation of reading the next concatenated stream
+    var zerr = if self.zstream.state == nil {
+      inflateInit2_(&self.zstream, 16 + MAX_WBITS, ZLIB_VERSION, Int32(MemoryLayout<z_stream>.size))
+    } else {
+      inflateReset(&self.zstream)
+    }
+    guard zerr == Z_OK else {
+      throw .zlib(zerr)
+    }
+
+    var payload = Data()
+    repeat {
+      if self.zstream.avail_in == 0 {
+        // Zlib has asked for more input, fill the input buffer
+        let read: Int
+        do {
+          read = try stream.read(&inputBuffer, maxLength: inputBuffer.count)
+        } catch {
+          throw .streamError(error)
+        }
+        guard read > 0 else {
+          throw .truncatedStream
+        }
+
+        // Reset input buffer read state
+        self.zstream.avail_in = UInt32(read)
+        self.zstream.next_in = inputBuffer.withUnsafeMutableBufferPointer(\.baseAddress!)
+      }
+
+      // Inflate next chunk of stream
+      self.zstream.avail_out = UInt32(outputBuffer.count)
+      self.zstream.next_out = outputBuffer.withUnsafeMutableBufferPointer(\.baseAddress!)
+      zerr = inflate(&self.zstream, Z_NO_FLUSH)
+
+      // Copy output bytes to payload
+      let decodedBytes = outputBuffer.count - Int(self.zstream.avail_out)
+      payload += Data(outputBuffer[..<decodedBytes])
+
+    } while zerr == Z_OK
+    guard zerr == Z_STREAM_END else {
+      throw .zlib(zerr)
+    }
+
+    return payload
+  }
+}
+
+enum GZipError: LocalizedError {
+  case truncatedStream
+  case streamError(_ err: StreamError)
+  case zlib(_ err: Int32)
+
+  var errorDescription: String? {
+    switch self {
+    case .truncatedStream:      "Reached end-of-stream before decoding finished"
+    case .streamError(let err): "Underlying stream error: \(err.localizedDescription)"
+    case .zlib(let err):        "zlib error \(err)"
+    }
+  }
+}