From c05b68b64d607d4f9088211609b0b5c73d6a3cf3 Mon Sep 17 00:00:00 2001
From: patrick-immometrica <patrick@immometrica.com>
Date: Thu, 5 Nov 2026 12:01:35 +0100
Subject: [PATCH] perf(header): skip the 12-slot step when a HeaderMap first
 grows

A map from HeaderMap::new() starts with 6 usable slots and doubles
from there, so a header block of 13-24 entries, which is common for
browser requests, rehashes twice: 6 -> 12 -> 24. Going straight from
8 to 32 raw slots on the first grow makes that a single rehash and
saves a pair of allocations. Maps of up to 6 entries are untouched.
---
 src/header/map.rs   |  5 ++++-
 tests/header_map.rs | 10 ++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/header/map.rs b/src/header/map.rs
index 6547114b..de17882d 100644
--- a/src/header/map.rs
+++ b/src/header/map.rs
@@ -1846,7 +1846,10 @@ impl<T> HeaderMap<T> {
                 self.entries = Vec::with_capacity(usable_capacity(new_raw_cap));
             } else {
                 let raw_cap = self.indices.len();
-                self.try_grow(raw_cap << 1)?;
+                // Go from 6 to 24 usable slots in one step, so a header block of
+                // 7 to 24 entries is rehashed once instead of twice.
+                let new_raw_cap = if raw_cap == 8 { 32 } else { raw_cap << 1 };
+                self.try_grow(new_raw_cap)?;
             }
         }
 
diff --git a/tests/header_map.rs b/tests/header_map.rs
index 77ab138c..786a5011 100644
--- a/tests/header_map.rs
+++ b/tests/header_map.rs
@@ -35,6 +35,16 @@ fn smoke() {
     }
 }
 
+#[test]
+fn first_grow_goes_to_24() {
+    let mut headers = HeaderMap::new();
+    for i in 0..7 {
+        let name = HeaderName::from_bytes(format!("x-{}", i).as_bytes()).unwrap();
+        headers.insert(name, HeaderValue::from_static("v"));
+    }
+    assert_eq!(headers.capacity(), 24);
+}
+
 #[test]
 #[should_panic]
 fn reserve_over_capacity() {