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expr.go
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expr.go
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// Copyright (c) 2013 Couchbase, Inc.
// Package provide a parser to parse basic arithmetic expression based on the
// following rule.
//
// expr -> sum
// prod -> value (mulop value)*
// mulop -> "*"
// | "/"
// sum -> prod (addop prod)*
// addop -> "+"
// | "-"
// value -> num
// | "(" expr ")"
package expr
import "strconv"
import "fmt"
import "github.com/tompaton/goparsec"
var _ = fmt.Sprintf("dummp print")
// Y is root Parser, usually called as `s` in CFG theory.
var Y parsec.Parser
var prod, sum, value parsec.Parser // circular rats
// Terminal rats
var openparan = parsec.Token(`\(`, "OPENPARAN")
var closeparan = parsec.Token(`\)`, "CLOSEPARAN")
var addop = parsec.Token(`\+`, "ADD")
var subop = parsec.Token(`-`, "SUB")
var multop = parsec.Token(`\*`, "MULT")
var divop = parsec.Token(`/`, "DIV")
// NonTerminal rats
// addop -> "+" | "-"
var sumOp = parsec.OrdTokens([]string{`\+`, `-`}, []string{"ADD", "SUB"})
// mulop -> "*" | "/"
var prodOp = parsec.OrdTokens([]string{`\*`, `/`}, []string{"MULT", "DIV"})
// value -> "(" expr ")"
var groupExpr = parsec.And(exprNode, openparan, &sum, closeparan)
// (addop prod)*
var prodK = parsec.Kleene(nil, parsec.And(many2many, sumOp, &prod), nil)
// (mulop value)*
var valueK = parsec.Kleene(nil, parsec.And(many2many, prodOp, &value), nil)
func init() {
// Circular rats come to life
// sum -> prod (addop prod)*
sum = parsec.And(sumNode, &prod, prodK)
// prod-> value (mulop value)*
prod = parsec.And(prodNode, &value, valueK)
// value -> num | "(" expr ")"
value = parsec.OrdChoice(exprValueNode, parsec.Int(), groupExpr)
// expr -> sum
Y = parsec.OrdChoice(one2one, sum)
}
//----------
// Nodifiers
//----------
func sumNode(ns []parsec.ParsecNode) parsec.ParsecNode {
if len(ns) > 0 {
val := ns[0].(int)
for _, x := range ns[1].([]parsec.ParsecNode) {
y := x.([]parsec.ParsecNode)
n := y[1].(int)
switch y[0].(*parsec.Terminal).Name {
case "ADD":
val += n
case "SUB":
val -= n
}
}
return val
}
return nil
}
func prodNode(ns []parsec.ParsecNode) parsec.ParsecNode {
if len(ns) > 0 {
val := ns[0].(int)
for _, x := range ns[1].([]parsec.ParsecNode) {
y := x.([]parsec.ParsecNode)
n := y[1].(int)
switch y[0].(*parsec.Terminal).Name {
case "MULT":
val *= n
case "DIV":
val /= n
}
}
return val
}
return nil
}
func exprNode(ns []parsec.ParsecNode) parsec.ParsecNode {
if len(ns) == 0 {
return nil
}
return ns[1]
}
func exprValueNode(ns []parsec.ParsecNode) parsec.ParsecNode {
if len(ns) == 0 {
return nil
} else if term, ok := ns[0].(*parsec.Terminal); ok {
val, _ := strconv.Atoi(term.Value)
return val
}
return ns[0]
}