replace existing query param

lila/src/main/scala/StringOps.scala

@@ -29,10 +29,25 @@ object StringOps:
   def addQueryParams(url: String, params: Map[String, String]): String =
     if params.isEmpty then url
     else
-      val queryString = params // we could encode the key, and we should, but is it really necessary?
-        .map { (key, value) => s"$key=${urlencode(value)}" }
+      val (baseUrl, existingParams) = url.split("\\?", 2) match
+        case Array(base, query) =>
+          val parsed = query
+            .split("&")
+            .map:
+              _.split("=", 2) match
+                case Array(k, v) => k -> Some(v)
+                case Array(k) => k -> None
+            .toMap
+          (base, parsed)
+        case Array(base) => (base, Map.empty[String, Option[String]])
+
+      val mergedParams = existingParams ++ params.map { (k, v) => k -> Some(urlencode(v)) }
+
+      val queryString = mergedParams // we could encode the key, and we should, but is it really necessary?
+        .map: (key, value) =>
+          value.fold(key)(v => s"$key=$v")
         .mkString("&")
-      s"$url${if url.contains("?") then "&" else "?"}$queryString"
+      s"$baseUrl?$queryString"
 
   def removeGarbageChars(str: String) = removeChars(str, isGarbageChar)
 

lila/src/test/scala/StringOpsTest.scala

@@ -87,3 +87,58 @@ class StringTest extends munit.FunSuite:
 line"""),
       "multi\nline"
     )
+
+  test("addQueryParam"):
+    assertEquals(
+      addQueryParam("https://example.com/path", "key", "value"),
+      "https://example.com/path?key=value"
+    )
+    assertEquals(
+      addQueryParam("https://example.com/path?key1", "key2", "value"),
+      "https://example.com/path?key1&key2=value"
+    )
+
+  test("addQueryParams"):
+    assertEquals(
+      addQueryParams("https://example.com/path", Map.empty),
+      "https://example.com/path"
+    )
+    assertEquals(
+      addQueryParams("https://example.com/path", Map("key1" -> "value1", "key2" -> "value2")),
+      "https://example.com/path?key1=value1&key2=value2"
+    )
+    assertEquals(
+      addQueryParams("https://example.com/path?key1=value1", Map("key2" -> "value2")),
+      "https://example.com/path?key1=value1&key2=value2"
+    )
+    assertEquals(
+      addQueryParams("https://example.com/path?key1=value1&key2=value2", Map("key3" -> "value3")),
+      "https://example.com/path?key1=value1&key2=value2&key3=value3"
+    )
+    assertEquals(
+      addQueryParams(
+        "https://example.com/path?encoded1=test%40example.com",
+        Map("encoded2" -> "multiple words")
+      ),
+      "https://example.com/path?encoded1=test%40example.com&encoded2=multiple+words"
+    )
+
+  test("addQueryParams - replace existing param of same key"):
+    assertEquals(
+      addQueryParams("https://example.com/path?key=value1", Map("key" -> "value2")),
+      "https://example.com/path?key=value2"
+    )
+    assertEquals(
+      addQueryParams("https://example.com/path?key", Map("key" -> "value2")),
+      "https://example.com/path?key=value2"
+    )
+
+  test("addQueryParams - parse malformed params"):
+    assertEquals(
+      addQueryParams("https://example.com/path?&&&", Map("key1" -> "value1")),
+      "https://example.com/path?key1=value1"
+    )
+    assertEquals(
+      addQueryParams("https://example.com/path?=value1&key2=&=&&key3=value3", Map("key1" -> "value1")),
+      "https://example.com/path?&key2=&key3=value3&key1=value1"
+    )