From bb852bf59fa2320d2de16f66644c916f3da8818c Mon Sep 17 00:00:00 2001
From: Adwait Kumar Singh <adwsingh@amazon.com>
Date: Fri, 14 Aug 2026 21:29:47 +0530
Subject: [PATCH] Use compact strings when writing CBOR text

---
 .../smithy/java/cbor/CborSerializer.java      | 35 ++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/codecs/cbor-codec/src/main/java/software/amazon/smithy/java/cbor/CborSerializer.java b/codecs/cbor-codec/src/main/java/software/amazon/smithy/java/cbor/CborSerializer.java
index 34367ee625..0d56b34516 100644
--- a/codecs/cbor-codec/src/main/java/software/amazon/smithy/java/cbor/CborSerializer.java
+++ b/codecs/cbor-codec/src/main/java/software/amazon/smithy/java/cbor/CborSerializer.java
@@ -37,6 +37,7 @@
 import java.time.Instant;
 import java.util.Arrays;
 import java.util.function.BiConsumer;
+import software.amazon.smithy.java.codecs.commons.CompactStringAccess;
 import software.amazon.smithy.java.codecs.commons.StripedPool;
 import software.amazon.smithy.java.core.schema.Schema;
 import software.amazon.smithy.java.core.schema.SerializableStruct;
@@ -369,6 +370,19 @@ private void writeStringValue(String value) {
         }
         ensureCapacity(5 + charLen * 3);
         int headerStart = pos;
+        byte[] latin1 = CompactStringAccess.latin1Bytes(value);
+        if (latin1 != null) {
+            for (byte current : latin1) {
+                if (current < 0) {
+                    encodeLatin1TextStringRewind(latin1, headerStart);
+                    return;
+                }
+            }
+            tagAndLengthUnchecked(TYPE_TEXTSTRING, charLen);
+            System.arraycopy(latin1, 0, buf, pos, charLen);
+            pos += charLen;
+            return;
+        }
         //Don't scan if the string is too long.
         if (charLen < 1000) {
             int orAccum = 0;
@@ -385,6 +399,21 @@ private void writeStringValue(String value) {
         encodeUtf8TextStringRewind(value, charLen, headerStart);
     }
 
+    private void encodeLatin1TextStringRewind(byte[] value, int headerStart) {
+        int writeStart = headerStart + 5;
+        int p = writeStart;
+        for (byte current : value) {
+            int c = current & 0xff;
+            if (c < 0x80) {
+                buf[p++] = current;
+            } else {
+                buf[p++] = (byte) (0xC0 | (c >> 6));
+                buf[p++] = (byte) (0x80 | (c & 0x3F));
+            }
+        }
+        finishTextStringRewind(headerStart, p - writeStart);
+    }
+
     /**
      * Encodes {@code value} as a CBOR text string starting at {@code headerStart}. Caller must have
      * reserved {@code 5 + charLen * 3} bytes from {@code headerStart}. Writes data into a 5-byte-header
@@ -426,7 +455,11 @@ private void encodeUtf8TextStringRewind(String value, int charLen, int headerSta
                 buf[p++] = (byte) (0x80 | (c & 0x3F));
             }
         }
-        int byteLen = p - writeStart;
+        finishTextStringRewind(headerStart, p - writeStart);
+    }
+
+    private void finishTextStringRewind(int headerStart, int byteLen) {
+        int writeStart = headerStart + 5;
         pos = headerStart;
         tagAndLengthUnchecked(TYPE_TEXTSTRING, byteLen);
         int actualHeaderLen = pos - headerStart;