added FromWei(…)

- added `FromWei(…)` example
- upgraded `github.com/google/go-cmp` v0.5.6 -> v0.5.7

example_test.go

@@ -31,6 +31,14 @@ func ExampleI() {
 	// 31415000000 wei
 }
 
+func ExampleFromWei() {
+	wei := big.NewInt(1_230000000_000000000)
+	fmt.Printf("%s Ether\n", w3.FromWei(wei, 18))
+
+	// Output:
+	// 1.23 Ether
+}
+
 func ExampleNewFunc() {
 	// ABI binding to the balanceOf function of an ERC20 Token.
 	funcBalanceOf, _ := w3.NewFunc("balanceOf(address)", "uint256")

go.mod

@@ -4,7 +4,7 @@ go 1.17
 
 require (
 	github.com/ethereum/go-ethereum v1.10.15
-	github.com/google/go-cmp v0.5.6
+	github.com/google/go-cmp v0.5.7
 )
 
 require (

go.sum

@@ -167,8 +167,8 @@ github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMyw
 github.com/google/go-cmp v0.4.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.4.1/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.4/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
-github.com/google/go-cmp v0.5.6 h1:BKbKCqvP6I+rmFHt06ZmyQtvB8xAkWdhFyr0ZUNZcxQ=
-github.com/google/go-cmp v0.5.6/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+github.com/google/go-cmp v0.5.7 h1:81/ik6ipDQS2aGcBfIN5dHDB36BwrStyeAQquSYCV4o=
+github.com/google/go-cmp v0.5.7/go.mod h1:n+brtR0CgQNWTVd5ZUFpTBC8YFBDLK/h/bpaJ8/DtOE=
 github.com/google/gofuzz v1.1.1-0.20200604201612-c04b05f3adfa/go.mod h1:dBl0BpW6vV/+mYPU4Po3pmUjxk6FQPldtuIdl/M65Eg=
 github.com/google/martian v2.1.0+incompatible/go.mod h1:9I4somxYTbIHy5NJKHRl3wXiIaQGbYVAs8BPL6v8lEs=
 github.com/google/pprof v0.0.0-20181206194817-3ea8567a2e57/go.mod h1:zfwlbNMJ+OItoe0UupaVj+oy1omPYYDuagoSzA8v9mc=

util.go

@@ -1,6 +1,7 @@
 package w3
 
 import (
+	"fmt"
 	"math/big"
 	"strings"
 
@@ -154,3 +155,25 @@ Outer:
 func Keccak(data []byte) common.Hash {
 	return crypto.Keccak256Hash(data)
 }
+
+// FromWei returns the given Wei as decimal with the given number of decimals.
+func FromWei(wei *big.Int, decimals uint8) string {
+	if wei == nil {
+		return fmt.Sprint(nil)
+	}
+
+	d := new(big.Int).Exp(big.NewInt(10), big.NewInt(int64(decimals)), nil)
+
+	sign := ""
+	if wei.Sign() < 0 {
+		sign = "-"
+	}
+	wei = new(big.Int).Abs(wei)
+
+	z, m := new(big.Int).DivMod(wei, d, new(big.Int))
+	if m.Cmp(new(big.Int)) == 0 {
+		return sign + z.String()
+	}
+	s := strings.TrimRight(fmt.Sprintf("%0*s", decimals, m.String()), "0")
+	return sign + z.String() + "." + s
+}

util_test.go

@@ -79,3 +79,46 @@ func BenchmarkI(b *testing.B) {
 		})
 	}
 }
+
+func TestFromWei(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		Wei      *big.Int
+		Decimals uint8
+		Want     string
+	}{
+		{big.NewInt(0), 0, "0"},
+		{big.NewInt(1), 0, "1"},
+		{big.NewInt(0), 18, "0"},
+		{big.NewInt(1), 18, "0.000000000000000001"},
+		{big.NewInt(1000), 18, "0.000000000000001"},
+		{big.NewInt(1000000), 18, "0.000000000001"},
+		{big.NewInt(1000000000), 18, "0.000000001"},
+		{big.NewInt(1000000000000), 18, "0.000001"},
+		{big.NewInt(1000000000000000), 18, "0.001"},
+		{big.NewInt(1000000000000000000), 18, "1"},
+		{big.NewInt(-1), 18, "-0.000000000000000001"},
+		{big.NewInt(-1000), 18, "-0.000000000000001"},
+		{big.NewInt(-1000000), 18, "-0.000000000001"},
+		{big.NewInt(-1000000000), 18, "-0.000000001"},
+		{big.NewInt(-1000000000000), 18, "-0.000001"},
+		{big.NewInt(-1000000000000000), 18, "-0.001"},
+		{big.NewInt(-1000000000000000000), 18, "-1"},
+		{big.NewInt(1000000000000000000), 15, "1000"},
+		{big.NewInt(1000000000000000000), 12, "1000000"},
+		{big.NewInt(1000000000000000000), 9, "1000000000"},
+		{big.NewInt(1000000000000000000), 6, "1000000000000"},
+		{big.NewInt(1000000000000000000), 3, "1000000000000000"},
+		{big.NewInt(1000000000000000000), 0, "1000000000000000000"},
+	}
+
+	for i, test := range tests {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			got := FromWei(test.Wei, test.Decimals)
+			if got != test.Want {
+				t.Fatalf("%q != %q", got, test.Want)
+			}
+		})
+	}
+}