Module: Baseh::Checksum

Defined in:
lib/baseh/checksum.rb

Overview

Version 1 checksum, spec section 6.2. Rolling polynomial over symbol values, then modulus conversion into the checksum alphabet.

Constant Summary collapse

INITIAL_STATE = 
17
MULTIPLIER = 
37

Class Method Summary collapse

Class Method Details

.calculate_checksum(prepared, body, body_index = nil, length = prepared.checksum_length) ⇒ Object

Expected checksum string for a normalized body.



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# File 'lib/baseh/checksum.rb', line 43

def calculate_checksum(prepared, body, body_index = nil, length = prepared.checksum_length)
  return "" if length.zero?

  body_index ||= BaseN.alphabet_index(prepared.body_alphabet)
  value = checksum_value(prepared, body, body_index, length)
  BaseN.encode_base_n(value, prepared.checksum_alphabet, length)
end

.checksum_value(prepared, body, body_index = nil, length = prepared.checksum_length) ⇒ Object

Returns the checksum value in [0, modulus). Spec 22: expandable generations may pass a shorter effective checksum length; the modulus is then S^length instead of the profile default.



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# File 'lib/baseh/checksum.rb', line 15

def checksum_value(prepared, body, body_index = nil, length = prepared.checksum_length)
  body_index ||= BaseN.alphabet_index(prepared.body_alphabet)
  modulus =
    if length == prepared.checksum_length
      prepared.checksum_modulus
    else
      [prepared.checksum_alphabet.length, 1].max**length
    end

  state = INITIAL_STATE
  prepared.profile_id.each_byte do |byte|
    state = (state * MULTIPLIER + byte + 1) % modulus
  end
  state = (state * MULTIPLIER) % modulus
  body.each_char.each_with_index do |ch, pos|
    symbol_value = body_index[ch]
    unless symbol_value
      raise BasehError.new(
        "INVALID_CHARACTER",
        "Body symbol #{ch.inspect} is not in the body alphabet"
      )
    end
    state = (state * MULTIPLIER + symbol_value + pos + 1) % modulus
  end
  state
end