format-string-interpolation.swift

import Foundation
import Darwin

struct FormatString: ExpressibleByStringInterpolation {
  final class StringInterpolation: StringInterpolationProtocol {
    var formatString = ""
    var args: [CVarArg] = []
    var buffers: [UnsafeMutableRawPointer] = []

    init() {}
    init(literalCapacity: Int, interpolationCount: Int) {}

    deinit {
      for buffer in buffers { free(buffer) }
    }

    func appendLiteral(_ literal: String) {
      formatString += literal.replacingOccurrences(of: "%", with: "%%")
    }

    func appendInterpolation(_ string: String) {
      formatString += "%s"
      string.withCString {
        let buffer = strdup($0)!
        buffers += [UnsafeMutableRawPointer(buffer)]
        args += [buffer]
      }
    }

    func appendInterpolation(_ integer: Int) {
      formatString += "%zd"
      args += [integer]
    }

    func appendInterpolation(hex integer: Int) {
      formatString += "0x%zx"
      args += [integer]
    }

    func appendInterpolation(_ bool: Bool) {
      appendInterpolation(bool ? "true" : "false")
    }
  }

  var interpolation: StringInterpolation

  init(stringInterpolation: StringInterpolation) {
    self.interpolation = stringInterpolation
  }

  init(stringLiteral: String) {
    let interpolation = StringInterpolation()
    interpolation.appendLiteral(stringLiteral)
    self.interpolation = interpolation
  }
}

func withFormatStringAndArgs(_ string: FormatString,
                             call body: (UnsafePointer<CChar>, CVaListPointer) -> ()) {
  string.interpolation.formatString.withCString { stringPtr in
    withVaList(string.interpolation.args) { vaList in
      body(stringPtr, vaList)
    }
  }
}

withFormatStringAndArgs("%hello% \("world"). it's \(true). \(123) \(hex: 0x123)\n") {
  vprintf($0, $1)
}