1 files changed, 80 insertions, 0 deletions
diff --git a/calc/calc.go b/calc/calc.go
new file mode 100644
index 0000000..d24dcea
--- /dev/null
+++ b/calc/calc.go
@@ -0,0 +1,80 @@
+package calc
+
+import (
+	"errors"
+	"fmt"
+
+	. "github.com/vektah/goparsify"
+)
+
+var (
+	value Parser
+
+	sumOp  = Chars("+-", 1, 1)
+	prodOp = Chars("/*", 1, 1)
+
+	groupExpr = Map(And("(", sum, ")"), func(n Node) Node {
+		return Node{Result: n.Child[1].Result}
+	})
+
+	number = Map(NumberLit(), func(n Node) Node {
+		switch i := n.Result.(type) {
+		case int64:
+			return Node{Result: float64(i)}
+		case float64:
+			return Node{Result: i}
+		default:
+			panic(fmt.Errorf("unknown value %#v", i))
+		}
+	})
+
+	sum = Map(And(prod, Kleene(And(sumOp, prod))), func(n Node) Node {
+		i := n.Child[0].Result.(float64)
+
+		for _, op := range n.Child[1].Child {
+			switch op.Child[0].Token {
+			case "+":
+				i += op.Child[1].Result.(float64)
+			case "-":
+				i -= op.Child[1].Result.(float64)
+			}
+		}
+
+		return Node{Result: i}
+	})
+
+	prod = Map(And(&value, Kleene(And(prodOp, &value))), func(n Node) Node {
+		i := n.Child[0].Result.(float64)
+
+		for _, op := range n.Child[1].Child {
+			switch op.Child[0].Token {
+			case "/":
+				i /= op.Child[1].Result.(float64)
+			case "*":
+				i *= op.Child[1].Result.(float64)
+			}
+		}
+
+		return Node{Result: i}
+	})
+
+	Y = Maybe(sum)
+)
+
+func init() {
+	value = Any(number, groupExpr)
+}
+
+func Calc(input string) (float64, error) {
+	result, remaining, err := ParseString(Y, input)
+
+	if err != nil {
+		return 0, err
+	}
+
+	if remaining != "" {
+		return result.(float64), errors.New("left unparsed: " + remaining)
+	}
+
+	return result.(float64), nil
+}