Use PercentCodec and MessageSupport in ALPN header codec

Drop local percent-decoding logic in AlpnHeaderSupport and delegate to core5 PercentCodec.
Use MessageSupport#headerOfTokens to format the ALPN header value.

Author: arturobernalg

httpclient5/src/main/java/org/apache/hc/client5/http/impl/AlpnHeaderSupport.java

@@ -26,7 +26,6 @@
  */
 package org.apache.hc.client5.http.impl;
 
-
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -35,8 +34,10 @@
 import org.apache.hc.core5.annotation.Contract;
 import org.apache.hc.core5.annotation.Internal;
 import org.apache.hc.core5.annotation.ThreadingBehavior;
+import org.apache.hc.core5.http.Header;
 import org.apache.hc.core5.http.message.MessageSupport;
 import org.apache.hc.core5.http.message.ParserCursor;
+import org.apache.hc.core5.net.PercentCodec;
 import org.apache.hc.core5.util.Args;
 
 /**
@@ -48,6 +49,7 @@
 @Internal
 public final class AlpnHeaderSupport {
 
+    private static final String HEADER_NAME = "ALPN";
     private static final char[] HEXADECIMAL = "0123456789ABCDEF".toCharArray();
 
     private AlpnHeaderSupport() {
@@ -58,16 +60,12 @@ private AlpnHeaderSupport() {
      */
     public static String formatValue(final List<String> protocolIds) {
         Args.notEmpty(protocolIds, "protocolIds");
-        final StringBuilder sb = new StringBuilder();
-        boolean first = true;
+        final List<String> tokens = new ArrayList<>(protocolIds.size());
         for (final String id : protocolIds) {
-            if (!first) {
-                sb.append(", ");
-            }
-            sb.append(encodeId(id));
-            first = false;
+            tokens.add(encodeId(id));
         }
-        return sb.toString();
+        final Header header = MessageSupport.headerOfTokens(HEADER_NAME, tokens);
+        return header.getValue();
     }
 
     /**
@@ -107,27 +105,10 @@ public static String encodeId(final String id) {
 
     /**
      * Decodes percent-encoded token to raw ID using UTF-8.
-     * Accepts lowercase hex; malformed/incomplete sequences are left literal.
      */
     public static String decodeId(final String token) {
         Args.notBlank(token, "token");
-        final byte[] buf = new byte[token.length()];
-        int bi = 0;
-        for (int i = 0; i < token.length(); ) {
-            final char c = token.charAt(i);
-            if (c == '%' && i + 2 < token.length()) {
-                final int hi = hexVal(token.charAt(i + 1));
-                final int lo = hexVal(token.charAt(i + 2));
-                if (hi >= 0 && lo >= 0) {
-                    buf[bi++] = (byte) ((hi << 4) | lo);
-                    i += 3;
-                    continue;
-                }
-            }
-            buf[bi++] = (byte) c;
-            i++;
-        }
-        return new String(buf, 0, bi, StandardCharsets.UTF_8);
+        return PercentCodec.decode(token, StandardCharsets.UTF_8);
     }
 
     // RFC7230 tchar minus '%' (RFC7639 requires '%' be percent-encoded)
@@ -167,17 +148,4 @@ private static void appendPctEncoded(final int b, final StringBuilder sb) {
         sb.append(HEXADECIMAL[(b >>> 4) & 0x0F]);
         sb.append(HEXADECIMAL[b & 0x0F]);
     }
-
-    private static int hexVal(final char c) {
-        if (c >= '0' && c <= '9') {
-            return c - '0';
-        }
-        if (c >= 'A' && c <= 'F') {
-            return 10 + (c - 'A');
-        }
-        if (c >= 'a' && c <= 'f') {
-            return 10 + (c - 'a');
-        }
-        return -1;
-    }
 }