! Copyright (C) 2008 Chris Double.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel arrays strings math.parser sequences
peg peg.ebnf peg.parsers memoize math ;
IN: peg.expr

: operator-fold ( lhs seq -- value )
 #! Perform a fold of a lhs, followed by a sequence of pairs being
 #! { operator rhs } in to a tree structure of the correct precedence.
 swap [ first2 swap call ] reduce ;

EBNF: expr 
times    = "*" [[ drop [ * ] ]]
divide   = "/" [[ drop [ / ] ]]
add      = "+" [[ drop [ + ] ]]
subtract = "-" [[ drop [ - ] ]]

digit    = [0-9] [[ digit> ]]
number   = (digit)+ [[ unclip [ swap 10 * + ] reduce ]]

value    = number | ("(" expr ")") [[ second ]] 
product = (value ((times | divide) value)*) [[ first2 operator-fold ]]
sum = (product ((add | subtract) product)*) [[ first2 operator-fold ]]
expr = sum
;EBNF

: eval-expr ( string -- number )
  expr parse-result-ast ;