Remove dependency on JDK 17 HexFormat

Add ByteArray, ByteUtils, and HexFormat classes
to do it.

secp-api/src/main/java/module-info.java

@@ -18,6 +18,7 @@
     requires org.jspecify;
 
     exports org.bitcoinj.secp.api;
+    exports org.bitcoinj.secp.api.internal; /* TEMPORARY */
 
     uses org.bitcoinj.secp.api.Secp256k1Provider;
 }

secp-api/src/main/java/org/bitcoinj/secp/api/ByteArray.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2023-2024 secp256k1-jdk Developers.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.bitcoinj.secp.api;
+
+import org.bitcoinj.secp.api.internal.ByteUtils;
+import org.bitcoinj.secp.api.internal.HexFormat;
+
+import java.util.Arrays;
+
+/**
+ * An effectively-immutable byte array.
+ */
+public interface ByteArray extends Comparable<ByteArray> {
+    HexFormat HEX_FORMAT = new HexFormat();
+
+    /**
+     * @return the bytes as an array
+     */
+    byte[] bytes();
+
+    /**
+     * @return the bytes as a hex-formatted string
+     */
+    default String formatHex() {
+        return HEX_FORMAT.formatHex(bytes());
+    }
+
+    /**
+     * {@inheritDoc}
+     * <p>For {@link ByteArray} this is a byte-by-byte, unsigned comparison.
+     * @param o {@inheritDoc}
+     * @return {@inheritDoc}
+     */
+    @Override
+    default int compareTo(ByteArray o) {
+        return ByteUtils.arrayUnsignedComparator().compare(bytes(), o.bytes());
+    }
+
+    /**
+     * Utility method to format hex bytes as string
+     * @param bytes bytes to format
+     * @return hex-formatted String
+     */
+    static String toHexString(byte[] bytes) {
+        return HEX_FORMAT.formatHex(bytes);
+    }
+}

secp-api/src/main/java/org/bitcoinj/secp/api/P256K1XOnlyPubKey.java

@@ -16,7 +16,6 @@
 package org.bitcoinj.secp.api;
 
 import java.math.BigInteger;
-import java.util.HexFormat;
 
 /**
  *
@@ -56,7 +55,6 @@ static P256K1XOnlyPubKey of(BigInteger x) {
      * Default implementation. Currently used by all known implementations
      */
     class P256K1XOnlyPubKeyImpl implements P256K1XOnlyPubKey {
-        private static final HexFormat formatter = HexFormat.of();
         private final BigInteger x;
 
         public P256K1XOnlyPubKeyImpl(P256k1PubKey pubKey) {
@@ -86,7 +84,7 @@ public byte[] getSerialized() {
          */
         @Override
         public String toString() {
-            return formatter.formatHex(getSerialized());
+            return ByteArray.toHexString(getSerialized());
         }
     }
 }

secp-api/src/main/java/org/bitcoinj/secp/api/P256k1PubKey.java

@@ -19,14 +19,11 @@
 import java.security.interfaces.ECPublicKey;
 import java.security.spec.ECParameterSpec;
 import java.security.spec.ECPoint;
-import java.util.HexFormat;
 
 /**
  *
  */
 public interface P256k1PubKey extends ECPublicKey {
-    HexFormat hf = HexFormat.of();
-
     @Override
     default String getAlgorithm() {
         return "Secp256k1";

secp-api/src/main/java/org/bitcoinj/secp/api/SignatureData.java

@@ -18,6 +18,6 @@
 /**
  *
  */
-public interface SignatureData {
-    public byte[] bytes();
+public interface SignatureData extends ByteArray {
+    byte[] bytes();
 }

secp-api/src/main/java/org/bitcoinj/secp/api/internal/ByteUtils.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2023-2024 secp256k1-jdk Developers.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.bitcoinj.secp.api.internal;
+
+import java.util.Comparator;
+
+/**
+ *
+ */
+public class ByteUtils {
+    /**
+     * Provides a byte array comparator.
+     * @return A comparator for byte[]
+     */
+    public static Comparator<byte[]> arrayUnsignedComparator() {
+        return ARRAY_UNSIGNED_COMPARATOR;
+    }
+
+    // In Java 9, this can be replaced with Arrays.compareUnsigned()
+    private static final Comparator<byte[]> ARRAY_UNSIGNED_COMPARATOR = (a, b) -> {
+        int minLength = Math.min(a.length, b.length);
+        for (int i = 0; i < minLength; i++) {
+            int result = compareUnsigned(a[i], b[i]);
+            if (result != 0) {
+                return result;
+            }
+        }
+        return a.length - b.length;
+    };
+
+    private static int compareUnsigned(byte a, byte b) {
+        return Byte.toUnsignedInt(a) - Byte.toUnsignedInt(b);
+    }
+}

secp-api/src/main/java/org/bitcoinj/secp/api/internal/HexFormat.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2023-2024 secp256k1-jdk Developers.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.bitcoinj.secp.api.internal;
+
+/**
+ * This class implements a subset of the functionality of the {@code HexFormat} class available in Java 17 and later.
+ * Its behavior is the same as an instance of Java 17 {@code HexFormat} created with {@code HexFormat.of()}.
+ * It is an internal class and may be removed in the future when and if we have a Java 17 baseline. This is a copy of
+ * HexFormat.java in `bitcoinj-base/org.bitcoinj.base.internal`.
+ * <p>Thanks to this <a href="https://www.baeldung.com/java-byte-arrays-hex-strings">Baeldung article</a>.
+ */
+public class HexFormat {
+    public String formatHex(byte[] bytes) {
+        StringBuilder stringBuilder = new StringBuilder(bytes.length * 2);
+        for (byte aByte : bytes) {
+            stringBuilder.append(byteToHex(aByte));
+        }
+        return stringBuilder.toString();
+    }
+
+    public byte[] parseHex(String hexString) {
+        if (hexString.length() % 2 == 1) {
+            throw new IllegalArgumentException("Invalid hexadecimal String supplied.");
+        }
+
+        byte[] bytes = new byte[hexString.length() / 2];
+        for (int i = 0; i < hexString.length(); i += 2) {
+            bytes[i / 2] = hexToByte(hexString.substring(i, i + 2));
+        }
+        return bytes;
+    }
+    private String byteToHex(byte num) {
+        char[] hexDigits = new char[2];
+        hexDigits[0] = Character.forDigit((num >> 4) & 0xF, 16);
+        hexDigits[1] = Character.forDigit((num & 0xF), 16);
+        return new String(hexDigits);
+    }
+
+    private byte hexToByte(String hexString) {
+        int firstDigit = toDigit(hexString.charAt(0));
+        int secondDigit = toDigit(hexString.charAt(1));
+        return (byte) ((firstDigit << 4) + secondDigit);
+    }
+
+    private int toDigit(char hexChar) {
+        int digit = Character.digit(hexChar, 16);
+        if (digit == -1) {
+            throw new IllegalArgumentException("Invalid Hexadecimal Character: "+ hexChar);
+        }
+        return digit;
+    }
+}

secp-bouncy/src/main/java/org/bitcoinj/secp/bouncy/BouncyPubKey.java

@@ -16,6 +16,7 @@
 package org.bitcoinj.secp.bouncy;
 
 
+import org.bitcoinj.secp.api.ByteArray;
 import org.bitcoinj.secp.api.P256k1PubKey;
 import org.bouncycastle.math.ec.ECPoint;
 
@@ -49,7 +50,7 @@ public byte[] getEncoded() {
 
     @Override
     public String toString() {
-        return hf.formatHex(bytes()) ;
+        return ByteArray.HEX_FORMAT.formatHex(bytes());
     }
 
     @Override

secp-ffm/src/main/java/org/bitcoinj/secp/ffm/SignaturePojo.java

@@ -15,6 +15,7 @@
  */
 package org.bitcoinj.secp.ffm;
 
+import org.bitcoinj.secp.api.ByteArray;
 import org.bitcoinj.secp.api.SignatureData;
 
 import java.lang.foreign.MemorySegment;