common: Add unused UrlEncode function

diff --git a/src/common/url.cpp b/src/common/url.cpp
index 30ac26a1..19db4e99 100644
--- a/src/common/url.cpp
+++ b/src/common/url.cpp
@@ -37,3 +37,25 @@ std::string UrlDecode(std::string_view url_encoded)
 
     return res;
 }
+
+std::string UrlEncode(std::string_view str)
+{
+    std::string res;
+    res.reserve(str.size() * 3); // worst case: every char needs encoding
+
+    for (char ch : str) {
+        auto c = static_cast<unsigned char>(ch);
+        // Unreserved characters per RFC 3986, Section 2.3
+        if ((c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') ||
+            (c >= '0' && c <= '9') || c == '-' || c == '_' || c == '.' || c == '~') {
+            res += ch;
+        } else {
+            // Percent-encode all other characters
+            res += '%';
+            constexpr char hex_chars[] = "0123456789ABCDEF";
+            res += hex_chars[(c >> 4) & 0xF];
+            res += hex_chars[c & 0xF];
+        }
+    }
+    return res;
+}
diff --git a/src/common/url.h b/src/common/url.h
index 718a607f..3defaf98 100644
--- a/src/common/url.h
+++ b/src/common/url.h
@@ -14,4 +14,7 @@
  */
 std::string UrlDecode(std::string_view url_encoded);
 
+/* Encode a URL. */
+std::string UrlEncode(std::string_view str);
+
 #endif // BITCOIN_COMMON_URL_H
diff --git a/src/test/common_url_tests.cpp b/src/test/common_url_tests.cpp
index 065c7d97..33e80c82 100644
--- a/src/test/common_url_tests.cpp
+++ b/src/test/common_url_tests.cpp
@@ -5,6 +5,7 @@
 #include <common/url.h>
 
 #include <string>
+#include <string_view>
 
 #include <boost/test/unit_test.hpp>
 
@@ -72,4 +73,44 @@ BOOST_AUTO_TEST_CASE(decode_internal_nulls_test) {
     BOOST_CHECK_EQUAL(UrlDecode("abc%00%00"), result2);
 }
 
+BOOST_AUTO_TEST_CASE(url_encode) {
+    auto to_encode = std::string_view{
+        "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f"
+        "\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f"
+        "\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f"
+        "\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f"
+        "\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f"
+        "\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f"
+        "\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f"
+        "\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f"
+        "\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f"
+        "\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f"
+        "\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf"
+        "\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf"
+        "\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf"
+        "\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf"
+        "\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef"
+        "\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff",
+        256};
+    auto expected_encoded =
+        "%00%01%02%03%04%05%06%07%08%09%0A%0B%0C%0D%0E%0F"
+        "%10%11%12%13%14%15%16%17%18%19%1A%1B%1C%1D%1E%1F"
+        "%20%21%22%23%24%25%26%27%28%29%2A%2B%2C-.%2F"
+        "0123456789%3A%3B%3C%3D%3E%3F"
+        "%40ABCDEFGHIJKLMNO"
+        "PQRSTUVWXYZ%5B%5C%5D%5E_"
+        "%60abcdefghijklmno"
+        "pqrstuvwxyz%7B%7C%7D~%7F"
+        "%80%81%82%83%84%85%86%87%88%89%8A%8B%8C%8D%8E%8F"
+        "%90%91%92%93%94%95%96%97%98%99%9A%9B%9C%9D%9E%9F"
+        "%A0%A1%A2%A3%A4%A5%A6%A7%A8%A9%AA%AB%AC%AD%AE%AF"
+        "%B0%B1%B2%B3%B4%B5%B6%B7%B8%B9%BA%BB%BC%BD%BE%BF"
+        "%C0%C1%C2%C3%C4%C5%C6%C7%C8%C9%CA%CB%CC%CD%CE%CF"
+        "%D0%D1%D2%D3%D4%D5%D6%D7%D8%D9%DA%DB%DC%DD%DE%DF"
+        "%E0%E1%E2%E3%E4%E5%E6%E7%E8%E9%EA%EB%EC%ED%EE%EF"
+        "%F0%F1%F2%F3%F4%F5%F6%F7%F8%F9%FA%FB%FC%FD%FE%FF";
+    BOOST_CHECK_EQUAL(UrlEncode(to_encode), expected_encoded);
+    BOOST_CHECK_EQUAL(UrlDecode(expected_encoded), to_encode);
+}
+
 BOOST_AUTO_TEST_SUITE_END()