Fix issue with partial UTF-8 string (#6317)

Summary:
It will cause JNI exception if we don't pass in UTF-8 string.

Alternative 1 (this): wait until we have complete UTF-8 tokens.
Alternative 2 (?): Fix this from runner layer
Alternative 3 (no): Change the API to use uint8_t array, but if we want to display on app in real time, this is still an issue.

Pull Request resolved: #6317

Reviewed By: Riandy

Differential Revision: D64580932

Pulled By: kirklandsign

fbshipit-source-id: 341ea906097707fae0f97d32dea974ad44425083

extension/android/jni/jni_layer_llama.cpp

@@ -6,11 +6,9 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-#include <cassert>
 #include <chrono>
-#include <iostream>
+#include <cstdint>
 #include <memory>
-#include <sstream>
 #include <string>
 #include <unordered_map>
 #include <vector>
@@ -33,6 +31,43 @@
 namespace llm = ::executorch::extension::llm;
 using ::executorch::runtime::Error;
 
+namespace {
+bool utf8_check_validity(const char* str, size_t length) {
+  for (size_t i = 0; i < length; ++i) {
+    uint8_t byte = static_cast<uint8_t>(str[i]);
+    if (byte >= 0x80) { // Non-ASCII byte
+      if (i + 1 >= length) { // Incomplete sequence
+        return false;
+      }
+      uint8_t next_byte = static_cast<uint8_t>(str[i + 1]);
+      if ((byte & 0xE0) == 0xC0 &&
+          (next_byte & 0xC0) == 0x80) { // 2-byte sequence
+        i += 2;
+      } else if (
+          (byte & 0xF0) == 0xE0 && (next_byte & 0xC0) == 0x80 &&
+          (i + 2 < length) &&
+          (static_cast<uint8_t>(str[i + 2]) & 0xC0) ==
+              0x80) { // 3-byte sequence
+        i += 3;
+      } else if (
+          (byte & 0xF8) == 0xF0 && (next_byte & 0xC0) == 0x80 &&
+          (i + 2 < length) &&
+          (static_cast<uint8_t>(str[i + 2]) & 0xC0) == 0x80 &&
+          (i + 3 < length) &&
+          (static_cast<uint8_t>(str[i + 3]) & 0xC0) ==
+              0x80) { // 4-byte sequence
+        i += 4;
+      } else {
+        return false; // Invalid sequence
+      }
+    }
+  }
+  return true; // All bytes were valid
+}
+
+std::string token_buffer;
+} // namespace
+
 namespace executorch_jni {
 
 class ExecuTorchLlamaCallbackJni
@@ -45,6 +80,15 @@ class ExecuTorchLlamaCallbackJni
     static auto cls = ExecuTorchLlamaCallbackJni::javaClassStatic();
     static const auto method =
         cls->getMethod<void(facebook::jni::local_ref<jstring>)>("onResult");
+
+    token_buffer += result;
+    if (!utf8_check_validity(token_buffer.c_str(), token_buffer.size())) {
+      ET_LOG(
+          Info, "Current token buffer is not valid UTF-8. Waiting for more.");
+      return;
+    }
+    result = token_buffer;
+    token_buffer = "";
     facebook::jni::local_ref<jstring> s = facebook::jni::make_jstring(result);
     method(self(), s);
   }