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package main
import (
"fmt"
"math/big"
)
type Op int
const (
OpInvalid Op = iota
OpAdd
OpSub
OpMul
OpDiv
OpExp
)
type Expr interface {
Eval() *big.Int
}
type NumExpr struct {
N *big.Int
}
func (n *NumExpr) Eval() *big.Int {
return n.N
}
type NopExpr struct {
}
func (e *NopExpr) Eval() *big.Int {
return &big.Int{}
}
type OpExpr struct {
Op Op
L Expr
R Expr
}
func (o *OpExpr) Eval() *big.Int {
l := o.L.Eval()
r := o.R.Eval()
n := new(big.Int)
switch o.Op {
case OpAdd:
n.Add(l, r)
case OpSub:
n.Sub(l, r)
case OpMul:
n.Mul(l, r)
case OpDiv:
n.Div(l, r)
case OpExp:
n.Exp(l, r, nil)
}
return n
}
func Parse(d []byte) (e Expr, err error) {
sc := newScanner(d)
t, d := sc.Scan()
switch t {
case NUM:
s := string(d)
n, ok := big.NewInt(0).SetString(s, 10)
if !ok {
return nil, fmt.Errorf("invalid number %s at %d", s, sc.Pos())
}
l := &NumExpr{N: n}
var op Op
t, d = sc.Scan()
switch t {
case EOF:
return l, nil
case ADD:
op = OpAdd
case SUB:
op = OpSub
case MUL:
op = OpMul
case DIV:
op = OpDiv
case EXP:
op = OpExp
default:
return nil, fmt.Errorf("expected operator at %d", sc.Pos())
}
t, d = sc.Scan()
if t != NUM {
return nil, fmt.Errorf("expected number at %d", sc.Pos())
}
s = string(d)
n2, ok := big.NewInt(0).SetString(s, 10)
if !ok {
return nil, fmt.Errorf("invalid nummber %s at %d", s, sc.Pos())
}
r := &NumExpr{N: n2}
return &OpExpr{Op: op, L: l, R: r}, nil
case EOF:
return &NopExpr{}, nil
}
return nil, fmt.Errorf("invalid token %s at index %d", t, sc.Pos())
}
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