refactor: unify string helpers and pattern2 derived slot

src/boxes/string_box.rs

@@ -13,6 +13,7 @@
  * - `toLowerCase()` - 小文字変換
  * - `trim()` - 前後の空白除去
  * - `indexOf(search)` - 文字列検索
+ * - `indexOf(search, fromIndex)` - 指定位置から検索
  * - `replace(from, to)` - 文字列置換
  * - `charAt(index)` - 指定位置の文字取得
  *
@@ -71,18 +72,18 @@ impl StringBox {
     /// Env gate: NYASH_STR_CP=1 → return codepoint index; default is byte index
     pub fn find(&self, search: &str) -> Box<dyn NyashBox> {
         use crate::boxes::integer_box::IntegerBox;
-        match self.value.find(search) {
-            Some(byte_pos) => {
-                let use_cp = std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1");
-                let idx = if use_cp {
-                    self.value[..byte_pos].chars().count() as i64
-                } else {
-                    byte_pos as i64
-                };
-                Box::new(IntegerBox::new(idx))
-            }
-            None => Box::new(IntegerBox::new(-1)),
-        }
+        let mode = crate::boxes::string_ops::index_mode_from_env();
+        let idx = crate::boxes::string_ops::index_of(&self.value, search, None, mode);
+        Box::new(IntegerBox::new(idx))
+    }
+
+    /// Find substring starting from a given index (or -1 if not found)
+    /// Env gate: NYASH_STR_CP=1 → indices are codepoint-based; default is byte index
+    pub fn find_from(&self, search: &str, start: i64) -> Box<dyn NyashBox> {
+        use crate::boxes::integer_box::IntegerBox;
+        let mode = crate::boxes::string_ops::index_mode_from_env();
+        let idx = crate::boxes::string_ops::index_of(&self.value, search, Some(start), mode);
+        Box::new(IntegerBox::new(idx))
     }
 
     /// Replace all occurrences of old with new
@@ -94,18 +95,9 @@ impl StringBox {
     /// Env gate: NYASH_STR_CP=1 → return codepoint index; default is byte index.
     pub fn lastIndexOf(&self, search: &str) -> Box<dyn NyashBox> {
         use crate::boxes::integer_box::IntegerBox;
-        match self.value.rfind(search) {
-            Some(byte_pos) => {
-                let use_cp = std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1");
-                let idx = if use_cp {
-                    self.value[..byte_pos].chars().count() as i64
-                } else {
-                    byte_pos as i64
-                };
-                Box::new(IntegerBox::new(idx))
-            }
-            None => Box::new(IntegerBox::new(-1)),
-        }
+        let mode = crate::boxes::string_ops::index_mode_from_env();
+        let idx = crate::boxes::string_ops::last_index_of(&self.value, search, mode);
+        Box::new(IntegerBox::new(idx))
     }
 
     /// Trim whitespace from both ends