added check for overflow/underflow

examples/gno.land/p/demo/grc/grc20/banker.gno

@@ -1,6 +1,7 @@
 package grc20
 
 import (
+	"errors"
 	"std"
 	"strconv"
 
@@ -24,6 +25,8 @@ type Banker struct {
 	token       *token   // to share the same pointer
 }
 
+const uint64_max = 1<<64 - 1
+
 func NewBanker(name, symbol string, decimals uint) *Banker {
 	if name == "" {
 		panic("name should not be empty")
@@ -55,7 +58,11 @@ func (b *Banker) Mint(address std.Address, amount uint64) error {
 		return ErrInvalidAddress
 	}
 
-	// TODO: check for overflow
+	// check for overflow
+	if uint64_max - b.totalSupply < amount   {
+		err := ufmt.Sprintf("you can't mint more than %d tokens", uint64_max - b.totalSupply)
+		return errors.New(err)
+	}
 
 	b.totalSupply += amount
 	currentBalance := b.BalanceOf(address)

examples/gno.land/p/demo/grc/grc20/types.gno

@@ -12,6 +12,7 @@ var (
 	ErrInsufficientAllowance = errors.New("insufficient allowance")
 	ErrInvalidAddress        = errors.New("invalid address")
 	ErrCannotTransferToSelf  = errors.New("cannot send transfer to self")
+	ErrUnderflow			 = errors.New("the a;ount is higher than the total supply")
 )
 
 type Token interface {

examples/gno.land/p/demo/grc/grc20/z_0_filetest.gno

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"std"
+	"gno.land/p/demo/testutils"
+
+	banker "gno.land/p/demo/grc/grc20"
+)
+const uint64_max = 1<<64 - 1
+
+func main() {
+	b := banker.NewBanker("Dummy", "DUMMY", 6)
+	var owner   = testutils.TestAddress("owner")
+	// Mint uint64_max - 1000 tokens for owner
+	b.Mint(owner, uint64_max - 1000)
+	// Try to mint 1001 tokens for owner, should fail because it exceeds the limit
+	println(b.Mint(owner, 1001).Error())
+
+}
+
+// Output:
+// you can't mint more than 1000 tokens