Refactor grammar parsing a bit

lib/propane/grammar.rb

@@ -13,66 +13,90 @@ class Propane
       @tokens = []
       @rules = []
       @code_id = 0
-      input = input.gsub("\r\n", "\n")
-      parse_grammar(input)
+      @line_number = 1
+      @input = input.gsub("\r\n", "\n")
+      parse_grammar!
     end
 
     private
 
-    def parse_grammar(input)
-      line_number = 1
-      while !input.empty?
-        if sliced = input.slice!(/\A\s+/)
-          # Skip white space.
-        elsif sliced = input.slice!(/\A#.*\n/)
-          # Skip comment lines.
-        elsif sliced = input.slice!(/\Amodule\s+(\S+)\s*;/)
-          @modulename = $1
-        elsif sliced = input.slice!(/\Aclass\s+(\S+)\s*;/)
-          @classname = $1
-        elsif sliced = input.slice!(/\Atoken\s+(\S+?)(?:\s+([^\n]+?))?\s*(?:;|<<\n(.*?)^>>\n)/m)
-          name, pattern, code = $1, $2, $3
-          if pattern.nil?
-            pattern = name
-          end
-          unless name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
-            raise Error.new("Invalid token name #{name.inspect}")
-          end
-          token = Token.new(name: name, id: @tokens.size, line_number: line_number)
-          @tokens << token
-          if code
-            code_id = @code_id
-            @code_id += 1
-          else
-            code_id = nil
-          end
-          pattern = Pattern.new(pattern: pattern, token: token, line_number: line_number, code: code, code_id: code_id)
-          @patterns << pattern
-        elsif sliced = input.slice!(/\Atokenid\s+(\S+?)\s*;/m)
-          name = $1
-          unless name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
-            raise Error.new("Invalid token name #{name.inspect}")
-          end
-          token = Token.new(name: name, id: @tokens.size, line_number: line_number)
-          @tokens << token
-        elsif sliced = input.slice!(/\Adrop\s+(\S+)\s*;/)
-          pattern = $1
-          @patterns << Pattern.new(pattern: pattern, line_number: line_number, drop: true)
-        elsif sliced = input.slice!(/\A(\S+)\s*->\s*([^\n]*?)(?:;|<<\n(.*?)^>>\n)/m)
-          rule_name, components, code = $1, $2, $3
-          unless rule_name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
-            raise Error.new("Invalid rule name #{name.inspect}")
-          end
-          components = components.strip.split(/\s+/)
-          # Reserve rule ID 0 for the "real" start rule.
-          @rules << Rule.new(rule_name, components, code, line_number, @rules.size + 1)
-        else
-          if input.size > 25
-            input = input.slice(0..20) + "..."
-          end
-          raise Error.new("Unexpected grammar input at line #{line_number}: #{input.chomp}")
+    def parse_grammar!
+      while @input.size > 0
+        parse_statement!
+      end
+    end
+
+    def parse_statement!
+      @next_line_number = @line_number
+      if consume!(/\A\s+/)
+        # Skip white space.
+      elsif consume!(/\A#.*\n/)
+        # Skip comment lines.
+      elsif md = consume!(/\Amodule\s+(\S+)\s*;/)
+        @modulename = md[1]
+      elsif md = consume!(/\Aclass\s+(\S+)\s*;/)
+        @classname = md[1]
+      elsif md = consume!(/\Atoken\s+(\S+?)(?:\s+([^\n]+?))?\s*(?:;|<<\n(.*?)^>>\n)/m)
+        name, pattern, code = *md[1, 3]
+        if pattern.nil?
+          pattern = name
         end
-        line_number += sliced.count("\n")
+        unless name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
+          raise Error.new("Invalid token name #{name.inspect}")
+        end
+        token = Token.new(name: name, id: @tokens.size, line_number: @line_number)
+        @tokens << token
+        if code
+          code_id = @code_id
+          @code_id += 1
+        else
+          code_id = nil
+        end
+        pattern = Pattern.new(pattern: pattern, token: token, line_number: @line_number, code: code, code_id: code_id)
+        @patterns << pattern
+      elsif md = consume!(/\Atokenid\s+(\S+?)\s*;/m)
+        name = md[1]
+        unless name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
+          raise Error.new("Invalid token name #{name.inspect}")
+        end
+        token = Token.new(name: name, id: @tokens.size, line_number: @line_number)
+        @tokens << token
+      elsif md = consume!(/\Adrop\s+(\S+)\s*;/)
+        pattern = md[1]
+        @patterns << Pattern.new(pattern: pattern, line_number: @line_number, drop: true)
+      elsif md = consume!(/\A(\S+)\s*->\s*([^\n]*?)(?:;|<<\n(.*?)^>>\n)/m)
+        rule_name, components, code = *md[1, 3]
+        unless rule_name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
+          raise Error.new("Invalid rule name #{name.inspect}")
+        end
+        components = components.strip.split(/\s+/)
+        # Reserve rule ID 0 for the "real" start rule.
+        @rules << Rule.new(rule_name, components, code, @line_number, @rules.size + 1)
+      else
+        if @input.size > 25
+          @input = @input.slice(0..20) + "..."
+        end
+        raise Error.new("Unexpected grammar input at line #{@line_number}: #{@input.chomp}")
+      end
+      @line_number = @next_line_number
+    end
+
+    # Check if the input string matches the given regex.
+    #
+    # If so, remove the match from the input string, and update the line
+    # number.
+    #
+    # @param regex [Regexp]
+    #   Regex to attempt to match.
+    #
+    # @return [MatchData, nil]
+    #   MatchData for the given regex if it was matched and removed from the
+    #   input.
+    def consume!(regex)
+      if md = @input.match(regex)
+        @input.slice!(0, md[0].size)
+        @next_line_number += md[0].count("\n")
+        md
       end
     end