require 'stringio'

# Calculates SHA-1 message digest of _string_. Returns binary digest.
# For hexadecimal digest, use +*sha1(string).unpack('H*')+.
#--
# This is a simple, pure-Ruby implementation of SHA-1, following
# the algorithm in FIPS 180-1.
#++
def sha1(string)
  # functions and constants
  mask = 0xffffffff
  s = proc{|n, x| ((x << n) & mask) | (x >> (32 - n))}
  f = [
    proc {|b, c, d| (b & c) | (b.^(mask) & d)},
    proc {|b, c, d| b ^ c ^ d},
    proc {|b, c, d| (b & c) | (b & d) | (c & d)},
    proc {|b, c, d| b ^ c ^ d},
  ].freeze
  k = [0x5a827999, 0x6ed9eba1, 0x8f1bbcdc, 0xca62c1d6].freeze

  # initial hash
  h = [0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476, 0xc3d2e1f0]

  bit_len = string.size << 3
  string += "\x80"
  while (string.size % 64) != 56
    string += "\0"
  end
  string = string.force_encoding('ascii-8bit') + [bit_len >> 32, bit_len & mask].pack("N2")

  if string.size % 64 != 0
    fail "failed to pad to correct length"
  end

  io = StringIO.new(string)
  block = ""

  while io.read(64, block)
    w = block.unpack("N16")

    # Process block.
    (16..79).each {|t| w[t] = s[1, w[t-3] ^ w[t-8] ^ w[t-14] ^ w[t-16]]}

    a, b, c, d, e = h
    t = 0
    4.times do |i|
      20.times do
        temp = (s[5, a] + f[i][b, c, d] + e + w[t] + k[i]) & mask
        a, b, c, d, e = temp, a, s[30, b], c, d
        t += 1
      end
    end

    [a,b,c,d,e].each_with_index {|x,i| h[i] = (h[i] + x) & mask}
  end

  h.pack("N5")
end

if __FILE__ == $0
  # Print some example SHA-1 digests.
  # FIPS 180-1 has correct digests for 'abc' and 'abc...opq'.
  [ 'abc',
    'abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq',
    'Rosetta Code',
  ].each {|s| printf("%s:\n  %s\n", s, *sha1(s).unpack('H*'))}
end