From 07a4b400b5f3f3a5be47abe4e87c415cb7f79de1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?G=C3=BCrg=C3=BCn=20Day=C4=B1o=C4=9Flu?= <hey@gurgun.day>
Date: Sun, 15 Mar 2026 16:51:29 +0100
Subject: [PATCH] url: optimize URLSearchParams set/delete duplicate handling

---
 benchmark/url/url-searchparams-mutation.js | 43 +++++++++++++++++++
 lib/internal/url.js                        | 49 +++++++++++++++-------
 2 files changed, 78 insertions(+), 14 deletions(-)
 create mode 100644 benchmark/url/url-searchparams-mutation.js

diff --git a/benchmark/url/url-searchparams-mutation.js b/benchmark/url/url-searchparams-mutation.js
new file mode 100644
index 00000000000000..cb01793037437e
--- /dev/null
+++ b/benchmark/url/url-searchparams-mutation.js
@@ -0,0 +1,43 @@
+'use strict';
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  method: ['set', 'delete'],
+  type: ['unique', 'duplicates'],
+  count: [10, 1000],
+  n: [1e4],
+});
+
+function buildSeed(type, count) {
+  const parts = new Array(count);
+
+  if (type === 'duplicates') {
+    for (let i = 0; i < count; i++) {
+      parts[i] = `dup=${i}`;
+    }
+  } else {
+    for (let i = 0; i < count; i++) {
+      parts[i] = `k${i}=${i}`;
+    }
+  }
+
+  return new URLSearchParams(parts.join('&'));
+}
+
+function main({ method, type, count, n }) {
+  const seed = buildSeed(type, count);
+  const key = type === 'duplicates' ? 'dup' : 'k0';
+  const mutate = method === 'set' ?
+    (params) => params.set(key, 'updated') :
+    (params) => params.delete(key);
+
+  for (let i = 0; i < 1e3; i++) {
+    mutate(new URLSearchParams(seed));
+  }
+
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    mutate(new URLSearchParams(seed));
+  }
+  bench.end(n);
+}
diff --git a/lib/internal/url.js b/lib/internal/url.js
index 813208f39e6622..328f8c683f7393 100644
--- a/lib/internal/url.js
+++ b/lib/internal/url.js
@@ -495,26 +495,37 @@ class URLSearchParams {
 
     const list = this.#searchParams;
     name = StringPrototypeToWellFormed(`${name}`);
+    const len = list.length;
+    let write = 0;
 
     if (value !== undefined) {
       value = StringPrototypeToWellFormed(`${value}`);
-      for (let i = 0; i < list.length;) {
+      for (let i = 0; i < len; i += 2) {
         if (list[i] === name && list[i + 1] === value) {
-          list.splice(i, 2);
-        } else {
-          i += 2;
+          continue;
+        }
+        if (write !== i) {
+          list[write] = list[i];
+          list[write + 1] = list[i + 1];
         }
+        write += 2;
       }
     } else {
-      for (let i = 0; i < list.length;) {
+      for (let i = 0; i < len; i += 2) {
         if (list[i] === name) {
-          list.splice(i, 2);
-        } else {
-          i += 2;
+          continue;
+        }
+        if (write !== i) {
+          list[write] = list[i];
+          list[write + 1] = list[i + 1];
         }
+        write += 2;
       }
     }
 
+    if (write !== len)
+      list.length = write;
+
     if (this.#context) {
       setURLSearchParamsModified(this.#context);
     }
@@ -594,24 +605,34 @@ class URLSearchParams {
     const list = this.#searchParams;
     name = StringPrototypeToWellFormed(`${name}`);
     value = StringPrototypeToWellFormed(`${value}`);
+    const len = list.length;
 
     // If there are any name-value pairs whose name is `name`, in `list`, set
     // the value of the first such name-value pair to `value` and remove the
     // others.
     let found = false;
-    for (let i = 0; i < list.length;) {
+    let write = 0;
+    for (let i = 0; i < len; i += 2) {
       const cur = list[i];
+      let keep = true;
       if (cur === name) {
         if (!found) {
-          list[i + 1] = value;
+          list[write] = cur;
+          list[write + 1] = value;
           found = true;
-          i += 2;
         } else {
-          list.splice(i, 2);
+          keep = false;
         }
-      } else {
-        i += 2;
+      } else if (write !== i) {
+        list[write] = cur;
+        list[write + 1] = list[i + 1];
       }
+      if (keep)
+        write += 2;
+    }
+
+    if (found && write !== len) {
+      list.length = write;
     }
 
     // Otherwise, append a new name-value pair whose name is `name` and value