# Grammar for simple arithmetical expressions
require 'rley' # Load the gem

########################################
# Define a grammar for basic arithmetical expressions
builder = Rley::Syntax::GrammarBuilder.new do
  add_terminals('NUMBER')
  add_terminals('LPAREN', 'RPAREN') # For '(', ')' delimiters
  add_terminals('PLUS', 'MINUS') # For '+', '-' operators or sign
  add_terminals('STAR', 'DIVIDE', 'POWER') # For '*', '/', '**' operators
  rule 'expression' => %w[simple_expression]
  rule 'simple_expression' => 'term'
  rule 'simple_expression' => %w[simple_expression add_operator term]
  rule 'term' => 'factor'
  rule 'term' => %w[term mul_operator factor]
  rule 'factor' => 'simple_factor'
  rule 'factor' => %w[simple_factor POWER simple_factor]
  rule 'simple_factor' => %w[sign NUMBER]
  rule 'simple_factor' => %w[LPAREN expression RPAREN]
  rule 'simple_factor' => %w[MINUS LPAREN expression RPAREN]  
  rule 'sign' => 'PLUS'
  rule 'sign' => 'MINUS'
  rule 'sign' => []
  rule 'add_operator' => 'PLUS'
  rule 'add_operator' => 'MINUS'
  rule 'mul_operator' => 'STAR'
  rule 'mul_operator' => 'DIVIDE'
end

# And now build the grammar...
CalcGrammar = builder.grammar