embolden substrings

embolden.rb

def indices(s,t)
  # indices('abcHellodefHello', 'Hello')
  # => [3,11]
  found = s.index(t)
  if found
    offset = 1 + found
    return [found] + indices(s[offset..], t).map {|e| e + offset}
  else
    return []
  end
end

def insert_at_indices(t, todo)
  # insert_at_indices('abcDEFghi', [[3, '____'], [6, '~~~~~~']])
  #  => "abc____DEF~~~~~~ghi"
  if todo.empty?
    return t
  end
  first_instruction, *rest_instructions = todo.sort_by(&:first)
  idx, value = first_instruction
  t[...idx] + value + insert_at_indices(t[idx..], rest_instructions.map { |instruction_idx, instruction_value| [instruction_idx-idx, instruction_value] })
end

def embolden(candidate, targets)
  # "abc<b>DEF</b>ghi<b>DEF</b>jkl<b>MNO_"
  # => "abc<b>DEF</b>ghi<b>DEF</b>jkl<b>MNO</b>"
  bold_ranges = targets.flat_map { |t| indices(candidate,t).map { |e| e...(e+t.length) }}
  bold_index_set = bold_ranges.flat_map(&:to_a).to_set
  down = (0..candidate.length).select { |e| !bold_index_set.include?(e) && bold_index_set.include?(e-1) }
  up = (0...candidate.length).select { |e| bold_index_set.include?(e) && !bold_index_set.include?(e-1) }
  instructions = up.map { |e| [e,'<b>'] } + down.map { |e| [e, '</b>'] }
  too_long_answer = insert_at_indices("#{candidate}_", instructions)
  too_long_answer[...-1]
end