1 files changed, 69 insertions, 0 deletions
diff --git a/ext/js/data/sandbox/string-util.js b/ext/js/data/sandbox/string-util.js
new file mode 100644
index 00000000..65d73eef
--- /dev/null
+++ b/ext/js/data/sandbox/string-util.js
@@ -0,0 +1,69 @@
+/*
+ * Copyright (C) 2022  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Class containing generic string utility functions.
+ */
+class StringUtil {
+    /**
+     * Reads code points from a string in the forward direction.
+     * @param {string} text The text to read the code points from.
+     * @param {number} position The index of the first character to read.
+     * @param {number} count The number of code points to read.
+     * @returns {string} The code points from the string.
+     */
+    static readCodePointsForward(text, position, count) {
+        let result = '';
+        for (; count > 0; --count) {
+            const char = text[position];
+            const charCode = char.charCodeAt(0);
+            result += char;
+            if (charCode >= 0xd800 && charCode < 0xdc00 && ++position < text.length) {
+                const char2 = text[position];
+                const charCode2 = char2.charCodeAt(0);
+                if (charCode2 >= 0xdc00 && charCode2 < 0xe000) {
+                    result += char2;
+                }
+            }
+        }
+        return result;
+    }
+
+    /**
+     * Reads code points from a string in the backward direction.
+     * @param {string} text The text to read the code points from.
+     * @param {number} position The index of the first character to read.
+     * @param {number} count The number of code points to read.
+     * @returns {string} The code points from the string.
+     */
+    static readCodePointsBackward(text, position, count) {
+        let result = '';
+        for (; count > 0; --count) {
+            const char = text[position];
+            const charCode = char.charCodeAt(0);
+            result = char + result;
+            if (charCode >= 0xdc00 && charCode < 0xe000 && position > 0) {
+                const char2 = text[position - 1];
+                const charCode2 = char2.charCodeAt(0);
+                if (charCode2 >= 0xd800 && charCode2 < 0xdc00) {
+                    result = char2 + result;
+                }
+            }
+        }
+        return result;
+    }
+}