From 59666f7e0f3d1c7fa91fa916b6943f908236521f Mon Sep 17 00:00:00 2001
From: Alvin Lindstam <alvin.lindstam@gmail.com>
Date: Sun, 19 Jan 2020 15:48:43 +0100
Subject: [PATCH 1/3] Make string iterator test include non-ascii

---
 tests/snippets/strings.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tests/snippets/strings.py b/tests/snippets/strings.py
index e7b0da07f..1659d3b35 100644
--- a/tests/snippets/strings.py
+++ b/tests/snippets/strings.py
@@ -277,7 +277,7 @@ assert "\u9487" == "钇"
 assert "\U0001F609" == "😉"
 
 # test str iter
-iterable_str = "123456789"
+iterable_str = "12345678😉"
 str_iter = iter(iterable_str)
 
 assert next(str_iter) == "1"
@@ -288,13 +288,13 @@ assert next(str_iter) == "5"
 assert next(str_iter) == "6"
 assert next(str_iter) == "7"
 assert next(str_iter) == "8"
-assert next(str_iter) == "9"
+assert next(str_iter) == "😉"
 assert next(str_iter, None) == None
 assert_raises(StopIteration, next, str_iter)
 
 str_iter_reversed = reversed(iterable_str)
 
-assert next(str_iter_reversed) == "9"
+assert next(str_iter_reversed) == "😉"
 assert next(str_iter_reversed) == "8"
 assert next(str_iter_reversed) == "7"
 assert next(str_iter_reversed) == "6"

From 9203eda6c65e33539e78d23ca1f0d6e8686dfe65 Mon Sep 17 00:00:00 2001
From: Alvin Lindstam <alvin.lindstam@gmail.com>
Date: Sun, 19 Jan 2020 15:49:14 +0100
Subject: [PATCH 2/3] Skip string char traversal on each next in python str
 iterator

Previosly, each call to `__next__` on a str iterator did a `chars().count()`
to find the length of the full string and traversed the chars of the string
until the expected char position from the start.

This approach ignores the char position and only cares about the byte position,
allowing it to only act on the specific part of the string binary it's interested
in.
---
 vm/src/obj/objstr.rs | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/vm/src/obj/objstr.rs b/vm/src/obj/objstr.rs
index 1f299867c..219ce280e 100644
--- a/vm/src/obj/objstr.rs
+++ b/vm/src/obj/objstr.rs
@@ -98,7 +98,7 @@ impl TryIntoRef<PyString> for &str {
 #[derive(Debug)]
 pub struct PyStringIterator {
     pub string: PyStringRef,
-    position: Cell<usize>,
+    byte_position: Cell<usize>,
 }
 
 impl PyValue for PyStringIterator {
@@ -111,15 +111,16 @@ impl PyValue for PyStringIterator {
 impl PyStringIterator {
     #[pymethod(name = "__next__")]
     fn next(&self, vm: &VirtualMachine) -> PyResult {
-        let pos = self.position.get();
+        let pos = self.byte_position.get();
 
-        if pos < self.string.value.chars().count() {
-            self.position.set(self.position.get() + 1);
+        if pos < self.string.value.len() {
+            // We can be sure that chars() has a value, because of the pos check above.
+            let char_ = self.string.value[pos..].chars().next().unwrap();
 
-            #[allow(clippy::range_plus_one)]
-            let value = self.string.value.do_slice(pos..pos + 1);
+            self.byte_position
+                .set(self.byte_position.get() + char_.len_utf8());
 
-            value.into_pyobject(vm)
+            char_.to_string().into_pyobject(vm)
         } else {
             Err(objiter::new_stop_iteration(vm))
         }
@@ -1198,7 +1199,7 @@ impl PyString {
     #[pymethod(name = "__iter__")]
     fn iter(zelf: PyRef<Self>, _vm: &VirtualMachine) -> PyStringIterator {
         PyStringIterator {
-            position: Cell::new(0),
+            byte_position: Cell::new(0),
             string: zelf,
         }
     }

From 98d942ac7f31499789560cf4a0d881d16c1fdf95 Mon Sep 17 00:00:00 2001
From: Alvin Lindstam <alvin.lindstam@gmail.com>
Date: Sun, 19 Jan 2020 16:26:33 +0100
Subject: [PATCH 3/3] Add string iteration benchmark

---
 benchmarks/benchmarks/strings.py | 4 ++++
 benchmarks/test_benchmarks.py    | 1 +
 2 files changed, 5 insertions(+)
 create mode 100644 benchmarks/benchmarks/strings.py

diff --git a/benchmarks/benchmarks/strings.py b/benchmarks/benchmarks/strings.py
new file mode 100644
index 000000000..e2e46e90b
--- /dev/null
+++ b/benchmarks/benchmarks/strings.py
@@ -0,0 +1,4 @@
+long_string = "a" * 50000
+
+for char in long_string:
+    pass
diff --git a/benchmarks/test_benchmarks.py b/benchmarks/test_benchmarks.py
index c1b80582d..2626461aa 100644
--- a/benchmarks/test_benchmarks.py
+++ b/benchmarks/test_benchmarks.py
@@ -18,6 +18,7 @@ pythons = [
 benchmarks = [
     ['benchmarks/nbody.py'],
     ['benchmarks/mandelbrot.py'],
+    ['benchmarks/strings.py'],
 ]
 
 exe_ids = ['cpython', 'rustpython']