Optimize normalizeValue for headers

Author: mtdowling

http/http-api/src/main/java/software/amazon/smithy/java/http/api/HeaderUtils.java

@@ -12,9 +12,6 @@ public final class HeaderUtils {
     // Header name lookup table: 0=invalid, 1=valid lower, 2=valid upper.
     private static final byte[] HEADER_NAME_TABLE = new byte[128];
 
-    // Header value lookup table: true = allowed character
-    private static final boolean[] VALID_VALUE_CHAR = new boolean[256];
-
     static {
         for (char c = 'a'; c <= 'z'; c++) {
             HEADER_NAME_TABLE[c] = 1;
@@ -26,16 +23,6 @@ public final class HeaderUtils {
         for (int i = 0; i < validChars.length(); i++) {
             HEADER_NAME_TABLE[validChars.charAt(i)] = 1;
         }
-
-        // Valid value chars per RFC 7230 field-content: SP, HTAB, VCHAR, obs-text
-        VALID_VALUE_CHAR[' '] = true;
-        VALID_VALUE_CHAR['\t'] = true;
-        for (int c = 0x21; c <= 0x7E; c++) {
-            VALID_VALUE_CHAR[c] = true;
-        }
-        for (int c = 0x80; c <= 0xFF; c++) {
-            VALID_VALUE_CHAR[c] = true;
-        }
     }
 
     private HeaderUtils() {}
@@ -102,25 +89,36 @@ static String normalizeName(String name) {
      * @throws IllegalArgumentException if the value contains invalid characters
      */
     public static String normalizeValue(String value) {
-        // Simulate String.trim(), but we only want to trim leading and trailing ' ' and '\t'.
         int len = value.length();
-        int end = len - 1;
-        int start = trimStart(value, end);
-        end = trimEnd(value, start);
-        if (start > end) {
-            return "";
+        if (len == 0) {
+            return value;
         }
 
-        for (int i = start; i <= end; i++) {
+        // Peek at the boundaries to detect whether trimming is needed.
+        char first = value.charAt(0);
+        char last = value.charAt(len - 1);
+        boolean trimNeeded = first == ' ' || first == '\t' || last == ' ' || last == '\t';
+
+        // Ensure valid chars match obs-text
+        for (int i = 0; i < len; i++) {
             char c = value.charAt(i);
-            if (c > 255 || !VALID_VALUE_CHAR[c]) {
+            if (c > 0xFF || (c < 0x20 && c != '\t') || c == 0x7F) {
                 throw invalidHeaderValueChar(value);
             }
         }
 
-        return (start == 0 && end == len - 1)
-                ? value
-                : value.substring(start, end + 1);
+        if (!trimNeeded) {
+            return value;
+        }
+
+        int end = len - 1;
+        int start = trimStart(value, end);
+        end = trimEnd(value, start);
+        if (start > end) {
+            return "";
+        }
+
+        return value.substring(start, end + 1);
     }
 
     private static int trimStart(String s, int end) {