Step by Step solution about How to resolve the algorithm Best shuffle step by step in the Ruby programming language

The provided Ruby code defines a method called best_shuffle that attempts to find the best possible shuffle of a given string s while preserving the original character frequencies. It returns the shuffled string and a score indicating how many characters in the shuffled string are in the same position as in the original string.

Here's a detailed breakdown of the code:

1. Initialization:

   • An empty array pos is created to store positions in the desired order for filling the shuffled string.
   • A hash g is created to group characters in s by their positions. For example, if s = "abracadabra", then g["a"] = [2, 4] because characters at positions 2 and 4 are both 'a'.

2. Sorting:

   • An array k is created by sorting the keys of g (i.e., the unique letters) in ascending order based on how many times each letter appears in s.

3. Filling Positions:

   • The code iterates through the sorted letters in k until all letters have been processed.
   • For each letter, it checks if its entry exists in g. If it does, it pushes the position of the first occurrence of that letter in s onto the pos array.
   • If the entry for the letter becomes empty (i.e., all positions for that letter have been filled), its key is deleted from g.

4. Shuffling and Scoring:

   • A copy of the input string s is created in letters.
   • An empty string new of the same length as s is created, initially filled with a placeholder character ('?').
   • The code iterates through the positions in pos in reverse order.
   • For each position, it finds the first character in letters that is the same as the character at that position in s.
   • The found character is placed in the corresponding position in new and is removed from letters.

5. Scoring:

   • Finally, the score is calculated by comparing new with the original string s, counting the number of character matches between the two strings.

6. Example Usage:

   • The code demonstrates the best_shuffle method on several different input strings and prints the original string, the shuffled string, and the calculated score.

In summary, the best_shuffle method aims to find a shuffled string that preserves the original character frequencies while also maximizing the number of characters that remain in the same position as in the original string. It does this by carefully filling the positions in the shuffled string based on the occurrence counts of each letter in the input string.

def best_shuffle(s)
  # Fill _pos_ with positions in the order
  # that we want to fill them.
  pos = []
  # g["a"] = [2, 4] implies that s[2] == s[4] == "a"
  g = s.length.times.group_by { |i| s[i] }
  
  # k sorts letters from low to high count
  k = g.sort_by { |k, v| v.length }.map { |k, v| k }
  
  until g.empty?
    k.each do |letter|
      g[letter] or next
      pos.push(g[letter].pop)
      g[letter].empty? and g.delete letter
    end
  end
  
  # Now fill in _new_ with _letters_ according to each position
  # in _pos_, but skip ahead in _letters_ if we can avoid
  # matching characters that way.
  letters = s.dup
  new = "?" * s.length
  until letters.empty?
    i, p = 0, pos.pop
    i += 1 while letters[i] == s[p] and i < (letters.length - 1)
    new[p] = letters.slice! i
  end
  
  score = new.chars.zip(s.chars).count { |c, d| c == d }
  [new, score]
end

%w(abracadabra seesaw elk grrrrrr up a).each do |word|
  puts "%s, %s, (%d)" % [word, *best_shuffle(word)]
end