Convert number into decimal representation via packed BCD

bcd.rb

# Convert number into decimal representation intermediate packed BCD.
# Uses two 64-bit integers

def assert_eq(a, b)
  raise "#{a} != #{b}" unless a == b
end

def number_to_packed_bcd_int(n)
  raise if n > (10**32) - 1
  # Use two u64
  bcd_high, bcd_low = 0, 0
  ndigits = 0
  loop do
    ndigits += 1
    digit = n % 10
    n /= 10
    bcd_high = (bcd_high << 4) & 0xFFFFFFFF_FFFFFFFF
    bcd_high = bcd_high | (bcd_low >> 60) & 0xF
    bcd_low = (bcd_low << 4) & 0xFFFFFFFF_FFFFFFFF
    bcd_low = bcd_low | digit
    break if n == 0
  end
  [ndigits, bcd_high, bcd_low]
end

def packed_bcd_int_to_string(ndigits, bcd_high, bcd_low)
  raise if ndigits == 0
  s = String.new("", capacity: ndigits)
  ndigits.times do
    digit = bcd_low & 0xF
    s << ('0'.ord + digit).chr
    bcd_low = (bcd_low >> 4) | (bcd_high & 0xF) << 60
    bcd_high = bcd_high >> 4
  end
  s
end

def to_decimal_s(n)
  packed_bcd_int_to_string(*number_to_packed_bcd_int(n))
end

assert_eq(number_to_packed_bcd_int(0), [1, 0, 0])
assert_eq(number_to_packed_bcd_int(9), [1, 0, 9])
assert_eq(number_to_packed_bcd_int(10), [2, 0, 1])
assert_eq(number_to_packed_bcd_int(56), [2, 0, 5 | (6 << 4)])
assert_eq(number_to_packed_bcd_int(100), [3, 0, 1])
assert_eq(number_to_packed_bcd_int(12345), [5, 0, 1 | (2 << 4) | (3 << 8) | (4 << 12) | (5 << 16)])

assert_eq(packed_bcd_int_to_string(2, 0, 5 | (6 << 4)), "56")

assert_eq(to_decimal_s(12345), "12345")

1000.times do
  n = rand(1_000_000_000_000)
  assert_eq(to_decimal_s(n), n.to_s(10))
end