1 files changed, 70 insertions, 0 deletions
diff --git a/ext/js/data/string-util.js b/ext/js/data/string-util.js
new file mode 100644
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--- /dev/null
+++ b/ext/js/data/string-util.js
@@ -0,0 +1,70 @@
+/*
+ * Copyright (C) 2023-2024  Yomitan Authors
+ * 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/>.
+ */
+
+/**
+ * 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.
+ */
+export function readCodePointsForward(text, position, count) {
+    const textLength = text.length;
+    let result = '';
+    for (; count > 0; --count) {
+        const char = text[position];
+        result += char;
+        if (++position >= textLength) { break; }
+        const charCode = char.charCodeAt(0);
+        if (charCode >= 0xd800 && charCode < 0xdc00) { // charCode is a high surrogate code
+            const char2 = text[position];
+            const charCode2 = char2.charCodeAt(0);
+            if (charCode2 >= 0xdc00 && charCode2 < 0xe000) { // charCode2 is a low surrogate code
+                result += char2;
+                if (++position >= textLength) { break; }
+            }
+        }
+    }
+    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.
+ */
+export function readCodePointsBackward(text, position, count) {
+    let result = '';
+    for (; count > 0; --count) {
+        const char = text[position];
+        result = char + result;
+        if (--position < 0) { break; }
+        const charCode = char.charCodeAt(0);
+        if (charCode >= 0xdc00 && charCode < 0xe000) { // charCode is a low surrogate code
+            const char2 = text[position];
+            const charCode2 = char2.charCodeAt(0);
+            if (charCode2 >= 0xd800 && charCode2 < 0xdc00) { // charCode2 is a high surrogate code
+                result = char2 + result;
+                if (--position < 0) { break; }
+            }
+        }
+    }
+    return result;
+}