More basic version of Horspool pattern matching algorithm, that jump uses a simplified version of the bad character table, in ruby.

alex_basic_horspool.rb

#First iteration moving towards Harspool algorithm simply jumps forward by
# pattern_length if char not in pattern is found
# function accepts a string and a pattern and returns the index in the string where
# the pattern first occurs, or "not found"

def brute_search_2 string, pattern
  pattern_length = pattern.length
  bad_match_table = Hash.new

  # Generates hash table with keys as all chars in pattern, and values as true
  for i in (0...pattern_length)
      bad_match_table[pattern[i]] = true
  end

  # Iterate through the string starting at string[pattern_length-1]
  # We are starting there because we are looking for matches in the pattern starting
  # at the last char.
  # we cant do a for loop, because we need to add pattern_length to string_index
  # when the char is not in our good_characters hash
  # therefore we must define string_index as pattern_length -1 and iterate up ourselves
  string_index = pattern_length - 1
  while string_index < string.length
    match_count = 0
    loop do
      # if a non-match is found, then break.
      break if string[string_index - match_count] != pattern[(pattern_length - 1 - match_count)]
      # if it wasn't a non-match, it must be a match!
      match_count += 1
      # if that match_count reaches the lenght of the pattern, you found the pattern!
      # return the index in string where the pattern begins
      return string_index - (pattern_length - 1) if match_count == pattern_length
    end
    if bad_match_table[string[string_index]]
      string_index += 1
    else
      string_index += pattern_length
    end
  end
  return "not found"
end

# tests
puts " "
puts "Brute Force Right to Left Test:"
puts " "

# match
test_string     = "let them eat cake"
test_pattern = "cake"
should_be    = 13
result       = brute_search_2 test_string, test_pattern

puts "Pattern at index #{result} "
raise "This is wrong" unless result == should_be

# no match
test_string2     = "let them eat cake"
test_pattern2 = "potato"
should_be2    = "not found"
result2       = brute_search_2 test_string2, test_pattern2

puts "Pattern at index #{result2} "

raise "This is wrong" unless result == should_be