require 'digest'

# Constants
ITOA64 = "./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"

def to64(value, length)
  result = ""
  length.times do
    result << ITOA64[value & 0x3f]
    value >>= 6
  end
  result
end

def gossamer_md5_crypt(password, legacy_hash)
  # Extract the salt from the legacy hash
  parts = legacy_hash.split('$')
  salt = parts[2]

  # Limit the salt to 8 characters
  salt = salt[0, 8]

  magic = "$GT$"

  ctx = Digest::MD5.new
  ctx.update(password)
  ctx.update(magic)
  ctx.update(salt)

  final = Digest::MD5.new
  final.update(password)
  final.update(salt)
  final.update(password)
  final_digest = final.digest

  password_length = password.length

  while password_length > 0
    ctx.update(final_digest[0, [password_length, 16].min])
    password_length -= 16
  end

  password_length = password.length
  while password_length > 0
    if password_length & 1 != 0
      ctx.update("\x00")
    else
      ctx.update(password[0, 1])
    end
    password_length >>= 1
  end

  final_digest = ctx.digest

  1000.times do |i|
    ctx1 = Digest::MD5.new
    if i & 1 != 0
      ctx1.update(password)
    else
      ctx1.update(final_digest)
    end
    ctx1.update(salt) if i % 3 != 0
    ctx1.update(password) if i % 7 != 0
    if i & 1 != 0
      ctx1.update(final_digest)
    else
      ctx1.update(password)
    end
    final_digest = ctx1.digest
  end

  result = ''
  result << to64((final_digest[0].ord << 16) | (final_digest[6].ord << 8) | final_digest[12].ord, 4)
  result << to64((final_digest[1].ord << 16) | (final_digest[7].ord << 8) | final_digest[13].ord, 4)
  result << to64((final_digest[2].ord << 16) | (final_digest[8].ord << 8) | final_digest[14].ord, 4)
  result << to64((final_digest[3].ord << 16) | (final_digest[9].ord << 8) | final_digest[15].ord, 4)
  result << to64((final_digest[4].ord << 16) | (final_digest[10].ord << 8) | final_digest[5].ord, 4)
  result << to64(final_digest[11].ord, 2)

  "#{magic}#{salt}$#{result}"
end

def verify_gossamer_password(password, legacy_hash)
  generated_hash = gossamer_md5_crypt(password, legacy_hash)
  generated_hash == legacy_hash
end

# Example usage
legacy_hash = "$GT$i5ZNZdfX$077FK6JU70HIr2pR1/uLP1"
password = "polyflex80"

resulting_hash = gossamer_md5_crypt(password, legacy_hash)
puts "Generated hash: #{resulting_hash}"
puts "Legacy hash: #{legacy_hash}"
puts "Password verification: #{verify_gossamer_password(password, legacy_hash)}"