[PAN-2946] Add strict short hex strings for retesteth (#1812)

The retesteth program wants byte trimmed UInt256, but our current impl does
nybble trimmed data (which is what the JSON-RPC spec calls for).  Add an API
so we can do both.


Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

util/src/main/java/tech/pegasys/pantheon/util/uint/UInt256Value.java

@@ -166,6 +166,25 @@ public interface UInt256Value<T extends UInt256Value<T>> extends Bytes32Backed,
     return "0x" + hex.substring(i);
   }
 
+  /**
+   * @return This value represented as a byte-minimal hexadecimal string (without leading zero
+   *     pairs).
+   */
+  default String toStrictShortHexString() {
+    final String hex = toHexString();
+    // Skipping '0x'
+    if (hex.charAt(2) != '0') return hex;
+
+    int i = 3;
+    while (i < hex.length() - 1 && hex.charAt(i) == '0') {
+      i++;
+    }
+    // Align the trim so we get full bytes, not stray nybbles.
+    i = i & 0xFFFFFFFE;
+
+    return "0x" + hex.substring(i);
+  }
+
   /** @return This value represented as an hexadecimal string without a 0x prefix. */
   default String toUnprefixedHexString() {
     return toHexString().substring(2);

util/src/test/java/tech/pegasys/pantheon/util/uint/UInt256BytesTest.java

@@ -191,7 +191,7 @@ public class UInt256BytesTest {
   }
 
   @Test
-  public void hexStrings() {
+  public void shortHexStrings() {
     assertThat(UInt256.of(0).toShortHexString()).isEqualTo("0x0");
     assertThat(UInt256.of(1).toShortHexString()).isEqualTo("0x1");
     assertThat(UInt256.fromHexString("0xdeadbeef").toShortHexString()).isEqualTo("0xdeadbeef");
@@ -202,6 +202,33 @@ public class UInt256BytesTest {
         .isEqualTo("0xdecafbad");
     assertThat(UInt256.fromHexString("cafebabe").toShortHexString()).isEqualTo("0xcafebabe");
     assertThat(UInt256.fromHexString("facefeed").toShortHexString()).isEqualTo("0xfacefeed");
+  }
+
+  @Test
+  public void strictShortHexStrings() {
+    assertThat(UInt256.of(0).toStrictShortHexString()).isEqualTo("0x00");
+    assertThat(UInt256.of(1).toStrictShortHexString()).isEqualTo("0x01");
+    assertThat(UInt256.fromHexString("0xdeadbeef").toStrictShortHexString())
+        .isEqualTo("0xdeadbeef");
+    assertThat(
+            UInt256.fromHexString(
+                    "0x00000000000000000000000000000000000000000000000000000000decafbad")
+                .toStrictShortHexString())
+        .isEqualTo("0xdecafbad");
+    assertThat(UInt256.fromHexString("cafebabe").toStrictShortHexString()).isEqualTo("0xcafebabe");
+    assertThat(UInt256.fromHexString("facefeed").toStrictShortHexString()).isEqualTo("0xfacefeed");
+    assertThat(UInt256.fromHexString("0xdedbeef").toStrictShortHexString()).isEqualTo("0x0dedbeef");
+    assertThat(
+            UInt256.fromHexString(
+                    "0x000000000000000000000000000000000000000000000000000000000dcafbad")
+                .toStrictShortHexString())
+        .isEqualTo("0x0dcafbad");
+    assertThat(UInt256.fromHexString("cafebab").toStrictShortHexString()).isEqualTo("0x0cafebab");
+    assertThat(UInt256.fromHexString("facefed").toStrictShortHexString()).isEqualTo("0x0facefed");
+  }
+
+  @Test
+  public void fullHexStrings() {
     assertThat(UInt256.of(0).toHexString())
         .isEqualTo("0x0000000000000000000000000000000000000000000000000000000000000000");
     assertThat(UInt256.of(1).toHexString())