util: optimize MarshalJSON allocations

name                  old time/op    new time/op    delta
Uint256MarshalJSON-8     230ns ± 6%      83ns ±13%  -63.78%  (p=0.000 n=8+10)

name                  old alloc/op   new alloc/op   delta
Uint256MarshalJSON-8      320B ± 0%       80B ± 0%  -75.00%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
Uint256MarshalJSON-8      5.00 ± 0%      1.00 ± 0%  -80.00%  (p=0.000 n=10+10)

pkg/util/bench_test.go

@@ -0,0 +1,13 @@
+package util
+
+import (
+	"testing"
+)
+
+func BenchmarkUint256MarshalJSON(b *testing.B) {
+	v := Uint256{0x01, 0x02, 0x03}
+
+	for i := 0; i < b.N; i++ {
+		_, _ = v.MarshalJSON()
+	}
+}

pkg/util/uint160.go

@@ -132,7 +132,12 @@ func (u *Uint160) UnmarshalJSON(data []byte) (err error) {
 
 // MarshalJSON implements the json marshaller interface.
 func (u Uint160) MarshalJSON() ([]byte, error) {
-	return []byte(`"0x` + u.StringLE() + `"`), nil
+	r := make([]byte, 3+Uint160Size*2+1)
+	copy(r, `"0x`)
+	r[len(r)-1] = '"'
+	slice.Reverse(u[:]) // u is a copy, so we can mangle it in any way.
+	hex.Encode(r[3:], u[:])
+	return r, nil
 }
 
 // UnmarshalYAML implements the YAML Unmarshaler interface.

pkg/util/uint256.go

@@ -109,7 +109,12 @@ func (u *Uint256) UnmarshalJSON(data []byte) (err error) {
 
 // MarshalJSON implements the json marshaller interface.
 func (u Uint256) MarshalJSON() ([]byte, error) {
-	return []byte(`"0x` + u.StringLE() + `"`), nil
+	r := make([]byte, 3+Uint256Size*2+1)
+	copy(r, `"0x`)
+	r[len(r)-1] = '"'
+	slice.Reverse(u[:]) // u is a copy, so we can mangle it in any way.
+	hex.Encode(r[3:], u[:])
+	return r, nil
 }
 
 // CompareTo compares two Uint256 with each other. Possible output: 1, -1, 0