lib: uefi: add charset converting helper functions.

The UEFI variable names are in UTF-16. We add several helper
functions to convert US-ASCII to UTF-16.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>

lib/uefi/charset.cpp

@@ -0,0 +1,73 @@
+/*
+ * Copyright (C) 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "charset.h"
+
+size_t utf16_strlen(const char16_t *str) {
+  size_t len = 0;
+
+  for (; *str; str++) {
+    len++;
+  }
+
+  return len;
+}
+
+int utf16_strcmp(const char16_t *s1, const char16_t *s2) {
+  int ret = 0;
+
+  for (; ; s1++, s2++) {
+    ret = *s1 - *s2;
+    if (ret || !*s1)
+      break;
+  }
+
+  return ret;
+}
+
+
+/**
+ * @brief convert utf-8 string to utf-16 string.
+ *
+ * This function converts utf-8 string to utf-16 string. However
+ * it only supports us-ascii input right now.
+ */
+int utf8_to_utf16(char16_t *dest, const char *src, size_t size) {
+  size_t i = 0;
+  for (; i < size - 1 && *src; i++, src++) {
+    dest[i] = *src;
+  }
+
+  dest[i] = 0;
+  return 0;
+}
+
+/**
+ * @brief convert utf-16 string to utf-8 string.
+ *
+ * This function converts utf-16 string to utf-8 string. However
+ * it only supports us-ascii output right now.
+ */
+int utf16_to_utf8(char *dest, const char16_t *src, size_t size) {
+  size_t i = 0;
+  for (; i < size - 1 && *src; i++, src++) {
+    dest[i] = *src;
+  }
+
+  dest[i] = 0;
+  return 0;
+}

lib/uefi/charset.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#ifndef __CHARSET_
+#define __CHARSET_
+
+#include <uefi/types.h>
+
+size_t utf16_strlen(const char16_t *str);
+int utf16_strcmp(const char16_t *s1, const char16_t *s2);
+int utf8_to_utf16(char16_t *dest, const char *src, size_t size);
+int utf16_to_utf8(char *dest, const char16_t *src, size_t size);
+
+#endif

lib/uefi/rules.mk

@@ -21,6 +21,7 @@ MODULE_SRCS += \
 	$(LOCAL_DIR)/events.cpp \
 	$(LOCAL_DIR)/io_stack.cpp \
 	$(LOCAL_DIR)/debug_support.cpp \
+	$(LOCAL_DIR)/charset.cpp \
 
 
 include make/module.mk