PrintStackDepth.swift

import Foundation
public func printStackDepth(
  label: String? = nil,
  fileID: StaticString = #fileID,
  line: UInt = #line
) {
  let thread = pthread_self()
  let stackAddress = UInt(bitPattern: pthread_get_stackaddr_np(thread))
  var used: UInt = 0
  withUnsafeMutablePointer(to: &used) {
    let pointerAddress = UInt(bitPattern: $0)
    // Stack goes down on x86/64 and arm, but we rectify the result in any case this code
    // executes on another architecture using a different convention.
    $0.pointee =
      stackAddress > pointerAddress
      ? stackAddress - pointerAddress
      : pointerAddress - stackAddress
  }
  let stackSize = UInt(pthread_get_stacksize_np(thread))
  let usedFraction = Double(used) / Double(stackSize)
  
  func separated(_ number: UInt) -> String {
    var separated = [String]()
    for (index, char) in "\(number)".reversed().enumerated() {
      if index > 0, index.isMultiple(of: 3) {
        separated.append(",")
      }
      separated.append("\(char)")
    }
    return separated.reversed().joined()
  }
  
  func padded(_ number: UInt) -> String {
    let length = separated(stackSize).count
    let separatedNumber = separated(number)
    let padding = String(repeating: " ", count: max(0, length - separatedNumber.count))
    return padding + separatedNumber
  }

  func filename(_ fileID: StaticString) -> String {
    "\(fileID)".components(separatedBy: "/").last!.replacingOccurrences(of: ".swift", with: "")
  }
  
  let prefix = label.map { "\($0)" } ?? "\(filename(fileID)):l\(line)"

  let message =  """
    \(prefix) Stack Depth: \
    \(padded(used))/\(separated(stackSize)) bytes \
    (\(String(format: "%.2f%%", usedFraction * 100)))
    """
  print(message)
}