emboldener.rb

class Emboldener
  def initialize(text:, targets:)
    @text = text
    @targets = targets
  end

  def run
    cursor = 0
    result = ""

    merged_indexes.each do |range|
      result << @text[cursor...range.begin]
      result << embolden(@text[range])

      cursor = range.end + 1
    end

    result
  end

  private def embolden(s) = "<b>#{s}</b>"

  private def merged_indexes
    all_matching_indexes.reduce([]) do |merged, range|
      next [range] if merged.empty?

      last_range = merged.pop
      overlapped_range = merge_range(range, last_range)
      
      if overlapped_range
        merged.push(overlapped_range)
      else
        merged.push(last_range, range)
      end
    end
  end

  private def all_matching_indexes
    @targets.flat_map { index_all(_1) }.sort_by(&:begin)
  end

  private def index_all(target)
    ranges = []
    cursor = 0

    loop do
      found_index = @text.index(target, cursor) or break

      ranges.push(found_index..found_index + target.size)
      cursor = found_index + target.size
    end

    ranges
  end

  private def merge_range(a, b)
    return Range.new(a.begin, b.end) if a.cover?(b.begin)
    return Range.new(b.begin, a.end) if b.cover?(a.begin)

    nil
  end
end

Emboldener.new(text: "aaabbcc", targets: ["aaa","aab","bc"]).run
# => "<b>aaabbcc</b>"

Emboldener.new(text: "abcxyz123", targets: ["abc", "bcx", "123"]).run
# => "<b>abcxy</b>z<b>123</b>"