// convert2postfix converts an infix expression to postfix
func convert2postfix(tokens []string) []string {
var stack stack.Stack
var result []string
for _, token := range tokens {
if isOperator(token) {
OPERATOR:
for {
top, err := stack.Top()
if err == nil && top != "(" {
if opGTE(top.(string), token) {
pop, _ := stack.Pop()
result = append(result, pop.(string))
} else {
break OPERATOR
}
}
break OPERATOR
}
stack.Push(token)
} else if token == "(" {
stack.Push(token)
} else if token == ")" {
PAREN:
for {
top, err := stack.Top()
if err == nil && top != "(" {
pop, _ := stack.Pop()
result = append(result, pop.(string))
} else {
stack.Pop() // pop off "("
break PAREN
}
}
} else if isOperand(token) {
result = append(result, token)
}
}
for !stack.IsEmpty() {
pop, _ := stack.Pop()
result = append(result, pop.(string))
}
return result
}
// evaluatePostfix takes a postfix expression and evaluates it
func evaluatePostfix(postfix []string) (*big.Rat, error) {
var stack stack.Stack
result := new(big.Rat) // note: a new(big.Rat) has value "0/1" ie zero
for _, token := range postfix {
if isOperand(token) {
bigrat := new(big.Rat)
if _, err := fmt.Sscan(token, bigrat); err != nil {
return nil, fmt.Errorf("unable to scan %s", token)
}
stack.Push(bigrat)
} else if isOperator(token) {
op2, err2 := stack.Pop()
if err2 != nil {
return nil, err2
}
var op1 interface{}
if token != "@" {
var err1 error
if op1, err1 = stack.Pop(); err1 != nil {
return nil, err1
}
}
dummy := new(big.Rat)
switch token {
case "**":
float1 := BigratToFloat(op1.(*big.Rat))
float2 := BigratToFloat(op2.(*big.Rat))
float_result := math.Pow(float1, float2)
stack.Push(FloatToBigrat(float_result))
case "*":
result := dummy.Mul(op1.(*big.Rat), op2.(*big.Rat))
stack.Push(result)
case "/":
result := dummy.Quo(op1.(*big.Rat), op2.(*big.Rat))
stack.Push(result)
case "+":
result = dummy.Add(op1.(*big.Rat), op2.(*big.Rat))
stack.Push(result)
case "-":
result = dummy.Sub(op1.(*big.Rat), op2.(*big.Rat))
stack.Push(result)
case "<":
if op1.(*big.Rat).Cmp(op2.(*big.Rat)) <= -1 {
stack.Push(big.NewRat(1, 1))
} else {
stack.Push(new(big.Rat))
}
case ">":
if op1.(*big.Rat).Cmp(op2.(*big.Rat)) >= 1 {
stack.Push(big.NewRat(1, 1))
} else {
stack.Push(new(big.Rat))
}
case "@":
result := dummy.Mul(big.NewRat(-1, 1), op2.(*big.Rat))
stack.Push(result)
}
} else {
return nil, fmt.Errorf("unknown token %v", token)
}
}
retval, err := stack.Pop()
if err != nil {
return nil, err
}
return retval.(*big.Rat), nil
}