code review 84230043: net/textproto: simplify common header interning

src/pkg/net/textproto/reader.go

 // Copyright 2010 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package textproto
 
 import (
         "bufio"
         "bytes"
         "io"
@@ skipping 544 matching lines @@
         }
         return s
 }
 
 const toLower = 'a' - 'A'
 
 // canonicalMIMEHeaderKey is like CanonicalMIMEHeaderKey but is
 // allowed to mutate the provided byte slice before returning the
 // string.
 func canonicalMIMEHeaderKey(a []byte) string {
-        // Look for it in commonHeaders , so that we can avoid an
-        // allocation by sharing the strings among all users
-        // of textproto. If we don't find it, a has been canonicalized
-        // so just return string(a).
         upper := true
-        lo := 0
-        hi := len(commonHeaders)
-        for i := 0; i < len(a); i++ {
+        for i, c := range a {
                 // Canonicalize: first letter upper case
                 // and upper case after each dash.
                 // (Host, User-Agent, If-Modified-Since).
                 // MIME headers are ASCII only, so no Unicode issues.
-                c := a[i]
                 if c == ' ' {
                         c = '-'
                 } else if upper && 'a' <= c && c <= 'z' {
                         c -= toLower
                 } else if !upper && 'A' <= c && c <= 'Z' {
                         c += toLower
                 }
                 a[i] = c
                 upper = c == '-' // for next time
-
-                if lo < hi {
-                        for lo < hi && (len(commonHeaders[lo]) <= i || commonHeaders[lo][i] < c) {
-                                lo++
-                        }
-                        for hi > lo && commonHeaders[hi-1][i] > c {
-                                hi--
-                        }
-                }
-        }
-        if lo < hi && len(commonHeaders[lo]) == len(a) {
-                return commonHeaders[lo]
+        }
+        // The compiler recognizes m[string(byteSlice)] as a special
+        // case, so a copy of a's bytes into a new string does not
+        // happen in this map lookup:
+        if v := commonHeader[string(a)]; v != "" {
+                return v
         }
         return string(a)
 }
 
-var commonHeaders = []string{
-        "Accept",
-        "Accept-Charset",
-        "Accept-Encoding",
-        "Accept-Language",
-        "Accept-Ranges",
-        "Cache-Control",
-        "Cc",
-        "Connection",
-        "Content-Id",
-        "Content-Language",
-        "Content-Length",
-        "Content-Transfer-Encoding",
-        "Content-Type",
-        "Cookie",
-        "Date",
-        "Dkim-Signature",
-        "Etag",
-        "Expires",
-        "From",
-        "Host",
-        "If-Modified-Since",
-        "If-None-Match",
-        "In-Reply-To",
-        "Last-Modified",
-        "Location",
-        "Message-Id",
-        "Mime-Version",
-        "Pragma",
-        "Received",
-        "Return-Path",
-        "Server",
-        "Set-Cookie",
-        "Subject",
-        "To",
-        "User-Agent",
-        "Via",
-        "X-Forwarded-For",
-        "X-Imforwards",
-        "X-Powered-By",
-}
+// commonHeader interns common header strings.
+var commonHeader = make(map[string]string)
+
+func init() {
+        for _, v := range []string{
+                "Accept",
+                "Accept-Charset",
+                "Accept-Encoding",
+                "Accept-Language",
+                "Accept-Ranges",
+                "Cache-Control",
+                "Cc",
+                "Connection",
+                "Content-Id",
+                "Content-Language",
+                "Content-Length",
+                "Content-Transfer-Encoding",
+                "Content-Type",
+                "Cookie",
+                "Date",
+                "Dkim-Signature",
+                "Etag",
+                "Expires",
+                "From",
+                "Host",
+                "If-Modified-Since",
+                "If-None-Match",
+                "In-Reply-To",
+                "Last-Modified",
+                "Location",
+                "Message-Id",
+                "Mime-Version",
+                "Pragma",
+                "Received",
+                "Return-Path",
+                "Server",
+                "Set-Cookie",
+                "Subject",
+                "To",
+                "User-Agent",
+                "Via",
+                "X-Forwarded-For",
+                "X-Imforwards",
+                "X-Powered-By",
+        } {
+                commonHeader[v] = v
+        }
+}