letters.rb

require 'rspec/autorun'

def letters(a, b)
  indexes = b.each_char.map do |letter|
    a.each_char.each_with_index.select {|l, _| l == letter }.map {|_, i| i }
  end

  start = indexes.shift

  if indexes.empty?
    [start]
  else
    start.product(*indexes).select {|x| x == x.sort }
  end
end

describe "letters" do
  let(:a) { "hello world" }

  specify "example 1" do
    letters(a, "e").should == [[1]]
  end

  specify "example 2" do
    letters(a, "l").should == [[2,3,9]]
  end

  specify "example 3" do
    letters(a, "el").should == [[1,2], [1,3], [1,9]]
  end

  specify "example 4" do
    letters(a, "lo").should == [[2,4], [2,7], [3,4], [3,7]]
  end

  specify "example 5" do
    letters(a, "lod").should ==  [ [2,4,10], [2,7,10], [3,4,10], [3,7,10] ]
  end
end

Wanted to try my luck, and it turned out very similar to other solutions. It was lots of fun, thanks! :-)

module Refines
  refine Array do
    def strictly_increasing?
      each_cons(2).all? {|x, y| x < y }
    end
  end

  refine String do
    def char_indexes
      each_char
        .with_index
        .with_object(Hash.new { |h, k| h[k] = [] }) { |(char, idx), hsh|
          hsh[char] << idx
        }
    end
  end
end

using Refines

def letter_match(target, str)
  first_set, *rest = target.char_indexes.values_at(*str.split(//))

  first_set.product(*rest).select { |arr| arr.strictly_increasing? }
end

Specs used:

describe "LetterMatch" do
  let(:a) { "hello world" }

  specify "single match" do
    expect(letter_match(a, "e")).to eq [[1]]
  end

  specify "single letter with multiple matches" do
    expect(letter_match(a, "l")).to eq [[2],[3],[9]]
  end

  specify "multiple matches, example 1" do
    expect(letter_match(a, "el")).to eq [[1,2], [1,3], [1,9]]
  end

  specify "multiple matches, example 2" do
    expect(letter_match(a, "lo")).to eq [[2,4], [2,7], [3,4], [3,7]]
  end

  specify "multiple matches, example 3" do
    expect(letter_match(a, "lod")).to eq [[2,4,10], [2,7,10], [3,4,10], [3,7,10]]
  end
end