1 files changed, 107 insertions, 0 deletions
diff --git a/parser.go b/parser.go
new file mode 100644
index 0000000..014624f
--- /dev/null
+++ b/parser.go
@@ -0,0 +1,107 @@
+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())
+}