Add ApplicationProtobuf (#233)

Author: fboemer

Package.swift

@@ -55,6 +55,7 @@ let package = Package(
         .library(
             name: "PrivateNearestNeighborSearchProtobuf",
             targets: ["PrivateNearestNeighborSearchProtobuf"]),
+        .library(name: "ApplicationProtobuf", targets: ["ApplicationProtobuf"]),
         .library(name: "_TestUtilities", targets: ["_TestUtilities"]),
         .executable(name: "PIRGenerateDatabase", targets: ["PIRGenerateDatabase"]),
         .executable(name: "PIRProcessDatabase", targets: ["PIRProcessDatabase"]),
@@ -132,6 +133,14 @@ let package = Package(
                            .product(name: "SwiftProtobuf", package: "swift-protobuf")],
             exclude: ["generated/README.md", "protobuf_module_mappings.txtpb"],
             swiftSettings: librarySettings),
+        .target(
+            name: "ApplicationProtobuf",
+            dependencies: ["HomomorphicEncryptionProtobuf",
+                           "PrivateInformationRetrieval",
+                           "PrivateNearestNeighborSearch",
+                           .product(name: "SwiftProtobuf", package: "swift-protobuf")],
+            exclude: ["generated/README.md", "protobuf_module_mappings.txtpb"],
+            swiftSettings: librarySettings),
         .target(
             name: "_TestUtilities",
             dependencies: [
@@ -238,6 +247,15 @@ let package = Package(
                 "PrivateNearestNeighborSearch",
                 "PrivateNearestNeighborSearchProtobuf",
             ], swiftSettings: executableSettings),
+        .testTarget(
+            name: "ApplicationProtobufTests",
+            dependencies: [
+                "ApplicationProtobuf",
+                "PrivateNearestNeighborSearch",
+                "PrivateInformationRetrieval",
+                "_TestUtilities",
+                .product(name: "SwiftProtobuf", package: "swift-protobuf"),
+            ], swiftSettings: executableSettings),
     ])
 
 // MARK: - Benchmarks

Sources/ApplicationProtobuf/ApplicationProtobuf.docc/ApplicationProtobuf.md

@@ -0,0 +1,12 @@
+# ``ApplicationProtobuf``
+
+Protocol buffer support for HE applications, including `PrivateInformationRetrieval` and `PrivateNearestNeighborSearch`
+
+## Overview
+`ApplicationProtobuf` contains supports for using [PrivateInformationRetrieval](https://swiftpackageindex.com/apple/swift-homomorphic-encryption/main/documentation/privateinformationretrieval) and [PrivateNearestNeighborSearch](https://swiftpackageindex.com/apple/swift-homomorphic-encryption/main/documentation/privatenearestneighborsearch) with [protocol buffers](https://protobuf.dev/), commonly referred to as `protobuf`.
+This module contains:
+* the generated Swift code to use the protobuf types.
+* conversion between protobuf types and [PrivateInformationRetrieval](https://swiftpackageindex.com/apple/swift-homomorphic-encryption/main/documentation/privateinformationretrieval) runtime types.
+* conversion between protobuf types and [PrivateNearestNeighborSearch](https://swiftpackageindex.com/apple/swift-homomorphic-encryption/main/documentation/privatenearestneighborsearch) runtime types.
+
+The protobuf schemas are defined at [https://github.com/apple/swift-homomorphic-encryption-protobuf](https://github.com/apple/swift-homomorphic-encryption-protobuf).

Sources/ApplicationProtobuf/ConversionError.swift

@@ -0,0 +1,49 @@
+// Copyright 2024-2025 Apple Inc. and the Swift Homomorphic Encryption project authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Foundation
+import SwiftProtobuf
+
+/// Error type when converting between protobuf and native objects.
+public enum ConversionError: Error {
+    case unrecognizedEnumValue(enum: any Enum.Type, value: Int)
+    case unsetField(field: String, message: any Message.Type)
+    case unsetOneof(oneof: any Message.Type, field: String)
+    case unspecifiedEnumValue(enum: any Enum.Type)
+}
+
+extension ConversionError {
+    static func unsetOneof(oneof: any Message.Type, field: AnyKeyPath) -> Self {
+        .unsetOneof(oneof: oneof, field: String(reflecting: field))
+    }
+
+    static func unsetField(_ field: AnyKeyPath, in message: any Message.Type) -> Self {
+        .unsetField(field: String(reflecting: field), message: message)
+    }
+}
+
+extension ConversionError: LocalizedError {
+    public var errorDescription: String? {
+        switch self {
+        case let .unrecognizedEnumValue(enum: enumeration, value):
+            "Unrecognized value \(value) in enum \(enumeration)"
+        case let .unsetField(field, message):
+            "Unset field \(field) in message \(message)"
+        case let .unsetOneof(oneof, field):
+            "Unset oneof in message \(oneof) for field \(field)"
+        case let .unspecifiedEnumValue(enum: enumeration):
+            "Unspecified value for enum \(enumeration)"
+        }
+    }
+}