Compare header names case-insensitively without allocating

http.carp

@@ -260,29 +260,45 @@ attribute, whatever order the two appear in.")
         c
         (parse-set-prop c (Array.unsafe-last &max-ages))))))
 
+(private ascii-lower-byte)
+(hidden ascii-lower-byte)
+; the byte at `i` of `s`, A–Z folded to a–z, as `tolower` does in the C locale
+(defn ascii-lower-byte [s i]
+  (let [c (Char.to-int (String.char-at s i))]
+    (if (and (>= c 65) (<= c 90)) (+ c 32) c)))
+
+(private header-name=)
+(hidden header-name=)
+; whether header names `a` and `b` are equal, ignoring ASCII case; `len` is the
+; length of `b`. It allocates nothing, unlike comparing `String.ascii-to-lower`s.
+(defn header-name= [a b len]
+  (and (= (String.length a) len)
+       (let-do [i 0
+                same true]
+         (while-do (and same (< i len))
+           (set! same (= (ascii-lower-byte a i) (ascii-lower-byte b i)))
+           (set! i (Int.inc i)))
+         same)))
+
 (private header-lookup)
 (hidden header-lookup)
 (defn header-lookup [hdrs name]
-  (let [lower-name &(String.ascii-to-lower name)]
+  (let [len (String.length name)]
     (Map.kv-reduce
       &(fn [acc k v]
         (cond
           (Maybe.just? &acc) acc
-          (= &(String.ascii-to-lower k) lower-name)
-            (Maybe.Just @(Array.unsafe-first v))
+          (header-name= k name len) (Maybe.Just @(Array.unsafe-first v))
           acc))
       (the (Maybe String) (Maybe.Nothing))
       hdrs)))
 
 (private header-values)
 (hidden header-values)
 (defn header-values [hdrs name]
-  (let [lower-name &(String.ascii-to-lower name)]
+  (let [len (String.length name)]
     (Map.kv-reduce
-      &(fn [acc k v]
-        (if (= &(String.ascii-to-lower k) lower-name)
-          (Array.concat &[acc @v])
-          acc))
+      &(fn [acc k v] (if (header-name= k name len) (Array.concat &[acc @v]) acc))
       (the (Array String) [])
       hdrs)))