import Foundation

/// One current file and one bounded archive. The owning process serializes writes.
struct BoundedLog {
    let url: URL
    var maxBytes = 5 * 1024 * 1024

    func append(_ line: String) throws {
        let files = FileManager.default
        try files.createDirectory(at: url.deletingLastPathComponent(), withIntermediateDirectories: true)
        if !files.fileExists(atPath: url.path) {
            guard files.createFile(atPath: url.path, contents: nil, attributes: [.posixPermissions: 0o600]) else {
                throw CocoaError(.fileWriteUnknown)
            }
        }
        try files.setAttributes([.posixPermissions: 0o600], ofItemAtPath: url.path)
        let handle = try FileHandle(forUpdating: url)
        defer { try? handle.close() }
        let size = try handle.seekToEnd()
        let data = Data(line.utf8).suffix(maxBytes)
        let archive = url.appendingPathExtension("1")
        var archiveSize: UInt64 = 0
        if files.fileExists(atPath: archive.path) {
            try files.setAttributes([.posixPermissions: 0o600], ofItemAtPath: archive.path)
            archiveSize = (try files.attributesOfItem(atPath: archive.path)[.size] as? NSNumber)?.uint64Value ?? 0
        }
        if size + UInt64(data.count) > UInt64(maxBytes) || archiveSize > UInt64(maxBytes) {
            try handle.seek(toOffset: size > UInt64(maxBytes) ? size - UInt64(maxBytes) : 0)
            let tail = try handle.read(upToCount: maxBytes) ?? Data()
            guard files.createFile(atPath: archive.path, contents: tail, attributes: [.posixPermissions: 0o600]) else {
                throw CocoaError(.fileWriteUnknown)
            }
            try handle.truncate(atOffset: 0)
            try handle.seek(toOffset: 0)
        }
        try handle.write(contentsOf: data)
    }
}