// Pure validation functions
const cleanCPFDigits = (str: string): string => 
  str.replace(/\D/g, '');

const isValidLength = (cpf: string): boolean => 
  cpf.length === 11;

const hasRepeatedDigits = (cpf: string): boolean => 
  /^(\d)\1{10}$/.test(cpf);

const calculateDigit = (cpf: string, length: number): number => {
  const sum = [...cpf]
    .slice(0, length)
    .reduce((acc, digit, index) => 
      acc + parseInt(digit) * ((length + 1) - index), 0);
    
  const remainder = sum % 11;
  return remainder < 2 ? 0 : 11 - remainder;
};

const validateVerificationDigits = (cpf: string): boolean => {
  const digit1 = calculateDigit(cpf, 9);
  const digit2 = calculateDigit(cpf, 10);
  
  return (
    digit1 === parseInt(cpf.charAt(9)) &&
    digit2 === parseInt(cpf.charAt(10))
  );
};

// Main validation function following Single Responsibility Principle
export function isCPF(str: string): boolean {
  if (!str) return false;

  const cpf = cleanCPFDigits(str);
  
  if (!isValidLength(cpf)) return false;
  if (hasRepeatedDigits(cpf)) return false;
  
  return validateVerificationDigits(cpf);
}