ArgParse.swift

//===--- ArgParse.swift ---------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

#if canImport(Glibc)
import Glibc
#elseif canImport(Musl)
import Musl
#elseif os(Windows)
import MSVCRT
#else
import Darwin
#endif

enum ArgumentError: Error {
  case missingValue(String)
  case invalidType(value: String, type: String, argument: String?)
  case unsupportedArgument(String)
}

extension ArgumentError: CustomStringConvertible {
  public var description: String {
    switch self {
    case let .missingValue(key):
      return "missing value for '\(key)'"
    case let .invalidType(value, type, argument):
      return (argument == nil)
        ? "'\(value)' is not a valid '\(type)'"
        : "'\(value)' is not a valid '\(type)' for '\(argument!)'"
    case let .unsupportedArgument(argument):
      return "unsupported argument '\(argument)'"
    }
  }
}

/// Type-checked parsing of the argument value.
///
/// - Returns: Typed value of the argument converted using the `parse` function.
///
/// - Throws: `ArgumentError.invalidType` when the conversion fails.
func checked<T>(
  _ parse: (String) throws -> T?,
  _ value: String,
  argument: String? = nil
) throws -> T {
  if let t = try parse(value)  { return t }
  var type = "\(T.self)"
  if type.starts(with: "Optional<") {
      let s = type.index(after: type.firstIndex(of: "<")!)
      let e = type.index(before: type.endIndex) // ">"
      type = String(type[s ..< e]) // strip Optional< >
  }
  throw ArgumentError.invalidType(
    value: value, type: type, argument: argument)
}

/// Parser that converts the program's command line arguments to typed values
/// according to the parser's configuration, storing them in the provided
/// instance of a value-holding type.
class ArgumentParser<U> {
    private var result: U
    private var validOptions: [String] {
      return arguments.compactMap { $0.name }
    }
    private var arguments: [Argument] = []
    private let programName: String = {
      // Strip full path from the program name.
      let r = CommandLine.arguments[0].reversed()
      let ss = r[r.startIndex ..< (r.firstIndex(of: "/") ?? r.endIndex)]
      return String(ss.reversed())
    }()
    private var positionalArgs = [String]()
    private var optionalArgsMap = [String : String]()

    /// Argument holds the name of the command line parameter, its help
    /// description and a rule that's applied to process it.
    ///
    /// The rule is typically a value processing closure used to convert it
    /// into given type and storing it in the parsing result.
    ///
    /// See also: addArgument, parseArgument
    struct Argument {
      let name: String?
      let help: String?
      let apply: () throws -> ()
    }

    /// ArgumentParser is initialized with an instance of a type that holds
    /// the results of the parsing of the individual command line arguments.
    init(into result: U) {
      self.result = result
      self.arguments += [
        Argument(name: "--help", help: "show this help message and exit",
                 apply: printUsage)
      ]
    }

    private func printUsage() {
      guard let _ = optionalArgsMap["--help"] else { return }
      let space = " "
      let maxLength = arguments.compactMap({ $0.name?.count }).max()!
      let padded = { (s: String) in
        " \(s)\(String(repeating:space, count: maxLength - s.count))  " }
      let f: (String, String) -> String = {
        "\(padded($0))\($1)"
          .split(separator: "\n")
          .joined(separator: "\n" + padded(""))
       }
      let positional = f("TEST", "name or number of the benchmark to measure;\n"
                         + "use +/- prefix to filter by substring match")
      let optional =  arguments.filter { $0.name != nil }
                                .map { f($0.name!, $0.help ?? "") }
                                .joined(separator: "\n")
      print(
      """
      usage: \(programName) [--argument=VALUE] [TEST [TEST ...]]

      positional arguments:
      \(positional)

      optional arguments:
      \(optional)
      """)
      exit(0)
    }

    /// Parses the command line arguments, returning the result filled with
    /// specified argument values or report errors and exit the program if
    /// the parsing fails.
    public func parse() -> U {
      do {
        try parseArgs() // parse the argument syntax
        try arguments.forEach { try $0.apply() } // type-check and store values
        return result
      } catch let error as ArgumentError {
        fputs("error: \(error)\n", stderr)
        exit(1)
      } catch {
        fflush(stdout)
        fatalError("\(error)")
      }
    }

    /// Using CommandLine.arguments, parses the structure of optional and
    /// positional arguments of this program.
    ///
    /// We assume that optional switch args are of the form:
    ///
    ///     --opt-name[=opt-value]
    ///
    /// with `opt-name` and `opt-value` not containing any '=' signs. Any
    /// other option passed in is assumed to be a positional argument.
    ///
    /// - Throws: `ArgumentError.unsupportedArgument` on failure to parse
    ///     the supported argument syntax.
    private func parseArgs() throws {

      // For each argument we are passed...
      for arg in CommandLine.arguments[1..<CommandLine.arguments.count] {
        // If the argument doesn't match the optional argument pattern. Add
        // it to the positional argument list and continue...
        if !arg.starts(with: "--") {
          positionalArgs.append(arg)
          continue
        }
        // Attempt to split it into two components separated by an equals sign.
        let components = arg.split(separator: "=")
        let optionName = String(components[0])
        guard validOptions.contains(optionName) else {
          throw ArgumentError.unsupportedArgument(arg)
        }
        var optionVal : String
        switch components.count {
          case 1: optionVal = ""
          case 2: optionVal = String(components[1])
          default:
          // If we do not have two components at this point, we can not have
          // an option switch. This is an invalid argument. Bail!
          throw ArgumentError.unsupportedArgument(arg)
        }
        optionalArgsMap[optionName] = optionVal
      }
    }

    /// Add a rule for parsing the specified argument.
    ///
    /// Stores the type-erased invocation of the `parseArgument` in `Argument`.
    ///
    /// Parameters:
    ///   - name: Name of the command line argument. E.g.: `--opt-arg`.
    ///       `nil` denotes positional arguments.
    ///   - property: Property on the `result`, to store the value into.
    ///   - defaultValue: Value used when the command line argument doesn't
    ///       provide one.
    ///   - help: Argument's description used when printing usage with `--help`.
    ///   - parser: Function that converts the argument value to given type `T`.
    public func addArgument<T>(
      _ name: String?,
      _ property: WritableKeyPath<U, T>,
      defaultValue: T? = nil,
      help: String? = nil,
      parser: @escaping (String) throws -> T? = { _ in nil }
    ) {
      arguments.append(Argument(name: name, help: help)
        { try self.parseArgument(name, property, defaultValue, parser) })
    }

    /// Process the specified command line argument.
    ///
    /// For optional arguments that have a value we attempt to convert it into
    /// given type using the supplied parser, performing the type-checking with
    /// the `checked` function.
    /// If the value is empty the `defaultValue` is used instead.
    /// The typed value is finally stored in the `result` into the specified
    /// `property`.
    ///
    /// For the optional positional arguments, the [String] is simply assigned
    /// to the specified property without any conversion.
    ///
    /// See `addArgument` for detailed parameter descriptions.
    private func parseArgument<T>(
      _ name: String?,
      _ property: WritableKeyPath<U, T>,
      _ defaultValue: T?,
      _ parse: (String) throws -> T?
    ) throws {
      if let name = name, let value = optionalArgsMap[name] {
        guard !value.isEmpty || defaultValue != nil
          else { throw ArgumentError.missingValue(name) }

        result[keyPath: property] = (value.isEmpty)
          ? defaultValue!
          : try checked(parse, value, argument: name)
      } else if name == nil {
        result[keyPath: property] = positionalArgs as! T
      }
    }
}