# usage:
#   ruby rx.rb                          # run the unit test
#   ruby -rrx -e 'p matchstr(...)'      # use rx as library.  don't run the unit test.

def count_try(exp, str)
  $try_count = 0
  matchstr(exp, str)
  $try_count
end

def matchstr(exp, str)
  result = []
  try(exp, str.split(//), 0) {|pos|
    result << pos
  }
  result
end

def try(exp, seq, pos, &block)
  $try_count += 1 if defined? $try_count
  case exp[0]
  when :empseq
    try_empseq(seq, pos, &block)
  when :lit
    _, sym = exp
    try_lit(sym, seq, pos, &block)
  when :cat
    _, e1, e2 = exp
    try_cat(e1, e2, seq, pos, &block)
  when :alt
    _, e1, e2 = exp
    try_alt(e1, e2, seq, pos, &block)
  when :rep
    _, e = exp
    try_rep(e, seq, pos, &block)
  else
    raise "unexpected AST: #{exp.inspect}"
  end
end

def try_empseq(seq, pos)
  yield pos
end

def try_lit(sym, seq, pos)
  if pos < seq.length && seq[pos] == sym
    yield pos + 1
  end
end

def try_cat(e1, e2, seq, pos, &block)
  try(e1, seq, pos) {|pos2|
    try(e2, seq, pos2, &block)
  }
end

def try_alt(e1, e2, seq, pos, &block)
  try(e1, seq, pos, &block)
  try(e2, seq, pos, &block)
end

def try_rep(exp, seq, pos, &block)
  try(exp, seq, pos) {|pos2|
    try_rep(exp, seq, pos2, &block) if pos < pos2
  }
  yield pos
end

if $0 == __FILE__       # The trick to run the unit test only for non-library execution.
  require 'test/unit'
  class TestRX < Test::Unit::TestCase
    def test_empseq
      assert_equal([0], matchstr([:empseq], ""))
    end

    def test_lit
      assert_equal([], matchstr([:lit, "a"], ""))
      assert_equal([1], matchstr([:lit, "a"], "a"))
      assert_equal([1], matchstr([:lit, "a"], "aa"))
      assert_equal([], matchstr([:lit, "a"], "b"))
    end

    def test_cat
      assert_equal([], matchstr([:cat, [:lit, "a"], [:lit, "b"]], ""))
      assert_equal([], matchstr([:cat, [:lit, "a"], [:lit, "b"]], "a"))
      assert_equal([2], matchstr([:cat, [:lit, "a"], [:lit, "b"]], "ab"))
      assert_equal([2], matchstr([:cat, [:lit, "a"], [:lit, "b"]], "abc"))
    end

    def test_alt
      assert_equal([], matchstr([:alt, [:lit, "a"], [:lit, "b"]], ""))
      assert_equal([1], matchstr([:alt, [:lit, "a"], [:lit, "b"]], "a"))
      assert_equal([1], matchstr([:alt, [:lit, "a"], [:lit, "b"]], "b"))
    end

    def test_rep
      assert_equal([0], matchstr([:rep, [:lit, "a"]], ""))
      assert_equal([5,4,3,2,1,0], matchstr([:rep, [:lit, "a"]], "aaaaa"))
    end

  end
end