Start on a test lexer for lexer specs

spec/imbecile_spec.rb

@@ -1,5 +1,51 @@
-RSpec.describe Imbecile do
+class TestLexer
-  it "has a version number" do
+  def initialize(token_dfa)
-    expect(Imbecile::VERSION).not_to be nil
+    @token_dfa = token_dfa
+  end
+
+  def lex(input)
+    input_chars = input.chars
+    output = []
+    while lexed_token = lex_token(input_chars)
+      output << lexed_token
+      input_chars.slice!(0, lexed_token[1].size)
+    end
+    unless input_chars.empty?
+      raise "Unmatched input"
+    end
+    output
+  end
+
+  def lex_token(input_chars)
+    return nil if input_chars.empty?
+    s = ""
+    current_state = @token_dfa.start_state
+    last_accepts = nil
+    input_chars.each_with_index do |input_char, index|
+      if next_state = transition(current_state, input_char)
+        current_state = next_state
+        if current_state.accepts
+          last_accepts = current_state.accepts
+        end
+        s += input_char
+      else
+        break
+      end
+    end
+    if last_accepts
+      [last_accepts, s]
+    end
+  end
+
+  def transition(state, input_char)
+    state.transitions.each do |transition|
+      if transition.code_point_range.include?(input_char.ord)
+        return transition.destination
+      end
+    end
+    nil
   end
 end
+
+describe Imbecile do
+end