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# Taken from http://lukaszwrobel.pl/blog/math-parser-part-3-implementation
module ExpressionParser
class Lexer
def initialize(input)
@input = input
@return_previous_token = false
end
def get_next_token
if @return_previous_token
@return_previous_token = false
return @previous_token
end
token = Token.new
@input.lstrip!
case @input
when /\A\+/ then
token.kind = Token::Plus
when /\A-/ then
token.kind = Token::Minus
when /\A\*/ then
token.kind = Token::Multiply
when /\Adiv/ then
token.kind = Token::Divide
when /\A\// then
token.kind = Token::Divide
when /\A\d+(\.\d+)?/
token.kind = Token::Number
token.value = $&.to_f
when /\A\(/
token.kind = Token::LParen
when /\A\)/
token.kind = Token::RParen
when ''
token.kind = Token::End
when /\Ae/
token.kind = Token::Number
token.value = 2.718281828459
when /\Api/
token.kind = Token::Number
token.value = 3.1415926535898
when /\Amod/
token.kind = Token::MOD
when /\A!=/
token.kind = Token::NotEqual
when /\A<>/
token.kind = Token::NotEqual
when /\A>=/
token.kind = Token::GThanE
when /\A>/
token.kind = Token::GThan
when /\A<=/
token.kind = Token::LThanE
when /\A</
token.kind = Token::LThan
when /\A=/
token.kind = Token::Equal
end
raise "Unknown token #{@input}" if token.unknown?
@input = $'
@previous_token = token
token
end
def revert
@return_previous_token = true
end
end
end
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