From fe5670f40b05aab8f00d0624f3deeede37bfef2d Mon Sep 17 00:00:00 2001
From: mizchi <miz404@gmail.com>
Date: Tue, 25 Aug 2026 01:36:01 +0900
Subject: [PATCH 1/2] perf(json): presize maps during FromJson decode

---
 json/from_json.mbt                |  2 +-
 json/from_json_map_bench_test.mbt | 40 +++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)
 create mode 100644 json/from_json_map_bench_test.mbt

diff --git a/json/from_json.mbt b/json/from_json.mbt
index f159f4645..9ff89708e 100644
--- a/json/from_json.mbt
+++ b/json/from_json.mbt
@@ -242,7 +242,7 @@ pub impl[V : FromJson] FromJson for Map[String, V] with fn from_json(json, path)
   guard json is Object(obj) else {
     decode_error(path, "Map::from_json: expected object")
   }
-  let res = Map([])
+  let res = Map([], capacity=obj.length())
   // As above: the test cannot fail, it just makes `key` assignable.
   guard! JsonPath::Key(path, key="") is (Key(_) as new_path)
   // Map::map_with_key
diff --git a/json/from_json_map_bench_test.mbt b/json/from_json_map_bench_test.mbt
new file mode 100644
index 000000000..3064d2b54
--- /dev/null
+++ b/json/from_json_map_bench_test.mbt
@@ -0,0 +1,40 @@
+// Copyright 2026 International Digital Economy Academy
+//
+// 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.
+
+///|
+fn from_json_map_bench_input(size : Int) -> Json {
+  let object : Map[String, Json] = Map([], capacity=size)
+  for i in 0..<size {
+    object[i.to_string()] = Json::number(i.to_double())
+  }
+  Json::object(object)
+}
+
+///|
+test "bench Map::from_json entries=10000" (it : @bench.T) {
+  let input = from_json_map_bench_input(10000)
+  it.bench(fn() {
+    let decoded : Map[String, Int] = try! @json.from_json(input)
+    it.keep(decoded.length())
+  })
+}
+
+///|
+test "bench Map::from_json entries=100000" (it : @bench.T) {
+  let input = from_json_map_bench_input(100000)
+  it.bench(fn() {
+    let decoded : Map[String, Int] = try! @json.from_json(input)
+    it.keep(decoded.length())
+  })
+}

From 8c53a88533aeaacd021ef09f412d8561a42d01bf Mon Sep 17 00:00:00 2001
From: mizchi <miz404@gmail.com>
Date: Tue, 25 Aug 2026 01:44:34 +0900
Subject: [PATCH 2/2] fix(json): reuse source map capacity

---
 json/from_json.mbt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/json/from_json.mbt b/json/from_json.mbt
index 9ff89708e..1bc801736 100644
--- a/json/from_json.mbt
+++ b/json/from_json.mbt
@@ -242,7 +242,7 @@ pub impl[V : FromJson] FromJson for Map[String, V] with fn from_json(json, path)
   guard json is Object(obj) else {
     decode_error(path, "Map::from_json: expected object")
   }
-  let res = Map([], capacity=obj.length())
+  let res = Map([], capacity=obj.capacity())
   // As above: the test cannot fail, it just makes `key` assignable.
   guard! JsonPath::Key(path, key="") is (Key(_) as new_path)
   // Map::map_with_key