try implementing a command that can search the index for packages

Sources/apk/Index/ApkIndexReading.swift

@@ -0,0 +1,45 @@
+/*
+ * darwin-apk © 2024 Gay Pizza Specifications
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import Foundation
+
+public extension ApkIndex {
+  init(readFrom indexURL: URL) throws {
+    let file = try FileInputStream(indexURL)
+    var gzip = GZipReader()
+    var tarRecords = [TarReader.Entry]()
+    for tarData in try (0..<2).map({ _ in try gzip.read(inStream: file) }) {
+      let tarStream = MemoryInputStream(buffer: tarData)
+      tarRecords += try TarReader.read(tarStream)
+    }
+
+    guard case .file(let signatureName, _) = tarRecords.first else {
+      throw ApkIndexReadingError.missingSignature
+    }
+    guard let apkIndexFile = tarRecords.firstFile(name: "APKINDEX") else {
+      throw ApkIndexReadingError.missingIndex
+    }
+    guard let description = tarRecords.firstFile(name: "DESCRIPTION") else {
+      throw ApkIndexReadingError.missingDescription
+    }
+
+    try self.init(raw:
+      try ApkRawIndex(lines: MemoryInputStream(buffer: apkIndexFile).lines))
+  }
+}
+
+public enum ApkIndexReadingError: Error, LocalizedError {
+  case missingSignature
+  case missingIndex
+  case missingDescription
+
+  public var errorDescription: String? {
+    switch self {
+    case .missingSignature:   "Missing signature"
+    case .missingIndex:       "APKINDEX missing"
+    case .missingDescription: "DESCRIPTION missing"
+    }
+  }
+}