bulls_and_cows.rb

# https://leetcode.com/problems/bulls-and-cows/
#
# You are playing the following Bulls and Cows game with your friend: You write
# a 4-digit secret number and ask your friend to guess it, each time your friend
# guesses a number, you give a hint, the hint tells your friend how many digits
# are in the correct positions (called "bulls") and how many digits are in the
# wrong positions (called "cows"), your friend will use those hints to find out
# the secret number.
#
# For example:
#
#     Secret number:  1807
#     Friend's guess: 7810
#     Hint: 1 bull and 3 cows. (The bull is 8, the cows are 0, 1 and 7.)
#
# According to Wikipedia: "Bulls and Cows (also known as Cows and Bulls or Pigs
# and Bulls or Bulls and Cleots) is an old code-breaking mind or paper and
# pencil game for two or more players, predating the similar commercially
# marketed board game Mastermind. The numerical version of the game is usually
# played with 4 digits, but can also be played with 3 or any other number of
# digits."
#
# Write a function to return a hint according to the secret number and friend's
# guess, use A to indicate the bulls and B to indicate the cows, in the above
# example, your function should return 1A3B.
#
# You may assume that the secret number and your friend's guess only contain
# digits, and their lengths are always equal.
#
# Credits:
#
#     Special thanks to @jeantimex for adding this problem and creating all
#     test cases.


# @param {String} secret
# @param {String} guess
# @return {String}
def get_hint(secret, guess)
  c1, c2, m1, m2 = 0, 0, {}, {}

  0.upto(secret.size - 1) do |idx|
    ch1, ch2 = secret[idx], guess[idx]

    if ch1 == ch2
      c1 += 1; next
    end

    m1[ch1] ||= 0; m1[ch1] += 1
    m2[ch2] ||= 0; m2[ch2] += 1
  end

  m1.each do |k, v|
    c2 += [v, m2[k]].min if m2.key?(k)
  end

  "#{c1}A#{c2}B"
end