FileManager+ReduceDirectory

//
// FileManager+ReduceDirectory.swift
// Copyright (c) 2023 BrightDigit.
//

import Foundation

private extension Error where Self == NSError {
  static func fileNotFound(at url: URL) -> NSError {
    NSError(
      domain: NSCocoaErrorDomain,
      code: NSFileNoSuchFileError,
      userInfo: [
        NSFilePathErrorKey: url.path,
        NSUnderlyingErrorKey: POSIXError(.ENOENT)
      ]
    )
  }
}

extension FileManager {
  func reduce<T: Sendable>(
    _ key: URLResourceKey,
    directoryAt url: URL,
    _ initial: T,
    _ nextPartialResult: @escaping (T, T) async throws -> T
  ) async throws -> T {
    try await self.reduce(key, directoryAt: url) { sequence in
      try await sequence.reduce(initial, nextPartialResult)
    }
  }

  func reduce<T: Sendable>(
    _ key: URLResourceKey,
    directoryAt url: URL,
    by reduce: @escaping (ThrowingTaskGroup<T, Error>) async throws -> T
  ) async throws -> T {
    try await self.reduce([key], directoryAt: url, fromValues: { values in
      values.allValues[key] as! T
    }, by: reduce)
  }

  func reduce<T: Sendable>(
    _ keys: [URLResourceKey],
    directoryAt url: URL,
    fromValues valuesFrom: @escaping @Sendable (URLResourceValues) -> T,
    _ initial: T,
    _ nextPartialResult: @escaping (T, T) async throws -> T
  ) async throws -> T {
    try await self.reduce(keys, directoryAt: url, fromValues: valuesFrom) { sequence in
      try await sequence.reduce(initial, nextPartialResult)
    }
  }

  func reduce<T: Sendable>(
    _ keys: [URLResourceKey],
    directoryAt url: URL,
    fromValues valuesFrom: @escaping (URLResourceValues) -> T,
    by reduce: @escaping (ThrowingTaskGroup<T, Error>) async throws -> T
  ) async throws -> T {
    let allKeys: Set<URLResourceKey> = Set([.isDirectoryKey, .isRegularFileKey] + keys)

    guard let enumerator = enumerator(at: url, includingPropertiesForKeys: Array(allKeys)) else {
      throw .fileNotFound(at: url)
    }

    return try await withThrowingTaskGroup(of: T.self) { taskGroup in
      for case let url as URL in enumerator {
        taskGroup.addTask {
          try await Task {
            let values = try url.resourceValues(forKeys: allKeys)
            if values.isDirectory == true {
              return try await self.reduce(
                keys,
                directoryAt: url,
                fromValues: valuesFrom,
                by: reduce
              )
            } else {
              return valuesFrom(values)
            }
          }.value
        }
      }
      return try await reduce(taskGroup)
    }
  }

  func accumulateSizeFromDirectory(at url: URL) async throws -> Int {
    try await self.reduce(.fileSizeKey, directoryAt: url, 0, +)
  }
}