print_hej/dart/src/print_hej.dart

import 'dart:io';

// Calculate arctan(1/x) using Taylor series
// arctan(1/x) = 1/x - 1/(3*x^3) + 1/(5*x^5) - ...
BigInt arctan(int x, int decimals) {
  BigInt scale = BigInt.from(10).pow(decimals + 10);
  BigInt xBig = BigInt.from(x);
  BigInt xSquared = xBig * xBig;
  
  BigInt term = scale ~/ xBig;
  BigInt result = BigInt.zero;
  int n = 0;
  
  while (term != BigInt.zero && n < decimals * 3) {
    BigInt divisor = BigInt.from(2 * n + 1);
    BigInt contrib = term ~/ divisor;
    
    if (n % 2 == 0) {
      result += contrib;
    } else {
      result -= contrib;
    }
    
    term = term ~/ xSquared;
    n++;
  }
  
  return result;
}

BigInt calculatePi(int decimals) {
  BigInt atan1_5 = arctan(5, decimals);
  BigInt atan1_239 = arctan(239, decimals);
  
  // pi = 16*arctan(1/5) - 4*arctan(1/239)
  return atan1_5 * BigInt.from(16) - atan1_239 * BigInt.from(4);
}

void main(List<String> args) {
  int decimals = 100;
  
  if (args.isNotEmpty) {
    try {
      decimals = int.parse(args[0]);
      if (decimals < 1) decimals = 100;
    } catch (e) {
      decimals = 100;
    }
  }
  
  BigInt pi = calculatePi(decimals);
  String piStr = pi.toString();
  
  // Print with decimal point
  stdout.write('3.');
  
  int startIndex = 1;
  int endIndex = startIndex + decimals;
  if (endIndex > piStr.length) {
    endIndex = piStr.length;
  }
  
  stdout.write(piStr.substring(startIndex, endIndex));
  
  // Pad with zeros if needed
  for (int i = 0; i < decimals - (endIndex - startIndex); i++) {
    stdout.write('0');
  }
  
  print('');
}