//===----------------------------------------------------------------------===//
// Copyright © 2025 Apple Inc. and the container project authors. All rights reserved.
//
// 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
//
//   https://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 ContainerBuildIR
import ContainerBuildSnapshotter
import Foundation

/// A protocol for computing differences between filesystem snapshots.
public protocol Differ: Sendable {
    /// Compute the difference between two snapshots.
    ///
    /// - Parameters:
    ///   - from: The base snapshot
    ///   - to: The target snapshot
    /// - Returns: The filesystem changes needed to transform `from` into `to`
    func diff(from: Snapshot?, to: Snapshot) async throws -> FilesystemChanges

    /// Compute a digest representing the state of a filesystem path.
    ///
    /// - Parameter path: The filesystem path to digest
    /// - Returns: A digest representing the current state
    func digest(path: String) async throws -> Digest
}

/// A basic in-memory differ implementation.
public struct MemoryDiffer: Differ {
    public init() {}

    public func diff(from base: Snapshot?, to target: Snapshot) async throws -> FilesystemChanges {
        // Stub implementation
        // In a real implementation, this would:
        // 1. Mount or access both snapshots
        // 2. Walk the filesystem trees
        // 3. Compare files, directories, and metadata
        // 4. Return the differences

        FilesystemChanges(
            added: Set<String>(),
            modified: Set<String>(),
            deleted: Set<String>(),
            sizeChange: 0
        )
    }

    public func digest(path: String) async throws -> Digest {
        // Stub implementation
        // In a real implementation, this would compute a merkle tree
        // digest of the filesystem at the given path

        var digestBytes = Data(count: 32)
        digestBytes.withUnsafeMutableBytes { bytes in
            if let baseAddress = bytes.baseAddress {
                memset(baseAddress, 0, 32)
            }
        }

        return try Digest(algorithm: .sha256, bytes: digestBytes)
    }
}