feat: provide StringView::char_code_at

Author: Yu-zh

builtin/pkg.generated.mbti

@@ -855,10 +855,12 @@ fn Bytes::of_string(String) -> Bytes
 fn Bytes::to_unchecked_string(Bytes, offset? : Int, length? : Int) -> String
 
 #alias("_[_]")
+#deprecated
 fn StringView::at(Self, Int) -> Int
 fn StringView::char_length(Self) -> Int
 fn StringView::char_length_eq(Self, Int) -> Bool
 fn StringView::char_length_ge(Self, Int) -> Bool
+fn StringView::code_unit_at(Self, Int) -> UInt16
 fn StringView::contains(Self, Self) -> Bool
 fn StringView::contains_any(Self, chars~ : Self) -> Bool
 fn StringView::contains_char(Self, Char) -> Bool

builtin/stringview.mbt

@@ -41,13 +41,25 @@ fn StringView::make_view(str : String, start : Int, end : Int) -> StringView = "
 ///   inspect(view[4], content="55358")
 /// ```
 #alias("_[_]")
+#deprecated("Use `code_unit_at` instead which returns UInt16")
 pub fn StringView::at(self : StringView, index : Int) -> Int {
   guard index >= 0 && index < self.length() else {
     abort("Index out of bounds")
   }
   self.str().unsafe_charcode_at(self.start() + index)
 }
 
+///|
+/// Returns the UTF-16 code unit at the given index.
+/// 
+/// This method has O(1) complexity.
+pub fn StringView::code_unit_at(self : StringView, index : Int) -> UInt16 {
+  guard index >= 0 && index < self.length() else {
+    abort("Index out of bounds")
+  }
+  self.str().code_unit_at(self.start() + index)
+}
+
 ///|
 /// Returns the length of the view.
 /// 

builtin/stringview_test.mbt

@@ -30,3 +30,22 @@ test "StringView::suffixes" {
     .collect()
   @json.inspect(suffixes_no_empty, content=["a🤣b", "🤣b", "b"])
 }
+
+///|
+test "code_unit_at" {
+  let s = "Hello, 世界!😀"
+  assert_true(s[:].code_unit_at(0) is 'H')
+  assert_true(s[:].code_unit_at(7) is '世')
+  assert_true(s[:].code_unit_at(8) is '界')
+  assert_true(s[:].code_unit_at(0) is ('A'..='Z'))
+  inspect(s[:].code_unit_at(0), content="72") // 'H'
+  inspect(s[:].code_unit_at(7), content="19990") // '世'
+  inspect(s[:].code_unit_at(8), content="30028") // '界'
+  inspect(s[:].code_unit_at(10), content="55357") // High surrogate of '😀'
+  inspect(s[:].code_unit_at(11), content="56832") // Low surrogate of '😀'
+  let view = s[7:] // "世界!😀"
+  inspect(view.code_unit_at(0), content="19990") // '世'
+  inspect(view.code_unit_at(1), content="30028") // '界'
+  inspect(view.code_unit_at(3), content="55357") // High surrogate of '😀'
+  inspect(view.code_unit_at(4), content="56832") // Low surrogate of '😀'
+}