Create a list of excluded paths at the base location keeping the specified paths.

Exclude.swift

private func exclude(paths: [String], at basePath: String) -> [String] {
    let slash = CharacterSet(charactersIn: "/")
    let baseURL = URL(fileURLWithPath: basePath, isDirectory: true)
    var exclusions = try! FileManager.default.contentsOfDirectory(atPath: basePath).sorted()
    var inclusions = paths

    // Cleanup paths removing and expanding `/.`, `/..`, etc.
    inclusions = inclusions.reduce(into: [], { inclusions, inclusion in
        let path = URL(fileURLWithPath: inclusion, relativeTo: baseURL).standardized.relativePath.trimmingCharacters(in: slash)
        if !path.isEmpty { inclusions.append(path) }
    })

    // Remove child inclusions, i.e., `["foo/bar", "foo"]` becomes `["foo"]`.
    inclusions = Set(inclusions).reduce(into: [], { inclusions, inclusion in
        inclusions.removeAll(where: { $0.starts(with: "\(inclusion)/") })
        if !inclusions.contains(where: { inclusion.starts(with: "\($0)/") }) { inclusions.append(inclusion) }
    })

    while let inclusion = inclusions.popLast()?.trimmingCharacters(in: slash) {
        exclusions = exclusions.reduce(into: [], { (exclusions, exclusion) in
            // If the inclusion is fully matched then we simply not reduce (add back) it into exclusions
            // list, but examine it deeper if it starts with exclusions.
            if inclusion.starts(with: "\(exclusion)/") {
                exclusions.append(contentsOf: exclude(paths: [String(inclusion.dropFirst(exclusion.count + 1))], at: baseURL.appendingPathComponent(exclusion).path).map({ "\(exclusion)/\($0)" }))
            } else if inclusion != exclusion {
                exclusions.append(exclusion)
            }
        })
    }

    return exclusions
}