add iter() method for StringView (#1503)

* add iter for stringview

* info

* tweak

string/string.mbti

@@ -16,6 +16,7 @@ impl Show for StringIndex
 
 type StringView
 impl StringView {
+  iter(Self) -> Iter[Char]
   length(Self) -> Int
   length_eq(Self, Int) -> Bool
   length_ge(Self, Int) -> Bool

string/view.mbt

@@ -411,3 +411,25 @@ pub impl Show for StringView with output(self, logger) {
   let substr = self.str.substring(start=self.start, end=self.end)
   String::output(substr, logger)
 }
+
+///|
+/// Returns an iterator over the Unicode characters in the string view.
+pub fn StringView::iter(self : StringView) -> Iter[Char] {
+  Iter::new(fn(yield_) {
+    for index = self.start; index < self.end; index = index + 1 {
+      let c1 = self.str[index]
+      if is_leading_surrogate(c1) && index + 1 < self.end {
+        let c2 = self.str[index + 1]
+        if is_trailing_surrogate(c2) {
+          let c = code_point_of_surrogate_pair(c1, c2)
+          guard let IterContinue = yield_(c) else { x => break x }
+          continue index + 2
+        }
+      }
+      guard let IterContinue = yield_(c1) else { x => break x }
+
+    } else {
+      IterContinue
+    }
+  })
+}

string/view_test.mbt

@@ -444,3 +444,13 @@ test "panic rev_get2" {
   let _ = view.rev_get(-1)
 
 }
+
+test "stringview iter" {
+  let str = "hello🤣🤣🤣"
+  guard let Some(start) = str.index_at(1)
+  guard let Some(end) = str.index_at(6)
+  let view = str[start:end]
+  let iter = view.iter()
+  inspect!(iter.count(), content="5")
+  inspect!(iter, content="['e', 'l', 'l', 'o', '🤣']")
+}