core: Add string_utils char boundary functions

core/src/display_object/edit_text.rs

@@ -10,6 +10,7 @@ use crate::font::{round_down_to_pixel, Glyph};
 use crate::html::{BoxBounds, FormatSpans, LayoutBox, LayoutContent, TextFormat};
 use crate::prelude::*;
 use crate::shape_utils::DrawCommand;
+use crate::string_utils;
 use crate::tag_utils::SwfMovie;
 use crate::transform::Transform;
 use crate::types::{Degrees, Percent};
@@ -1023,11 +1024,7 @@ impl<'gc> EditText<'gc> {
                             && local_position.1 <= params.height()
                         {
                             if local_position.0 >= x + (advance / 2) {
-                                let mut idx = pos + 1;
-                                while !text.is_char_boundary(idx) {
-                                    idx += 1;
-                                }
-                                result = Some(idx);
+                                result = Some(string_utils::next_char_boundary(text, pos));
                             } else {
                                 result = Some(pos);
                             }
@@ -1065,10 +1062,7 @@ impl<'gc> EditText<'gc> {
                     if selection.start() > 0 {
                         // Delete previous character
                         let text = self.text();
-                        let mut start = selection.start() - 1;
-                        while !text.is_char_boundary(start) {
-                            start -= 1;
-                        }
+                        let start = string_utils::prev_char_boundary(&text, selection.start());
                         self.replace_text(start, selection.start(), "", context);
                         self.set_selection(
                             Some(TextSelection::for_position(start)),
@@ -1082,10 +1076,7 @@ impl<'gc> EditText<'gc> {
                     if selection.end() < self.text_length() {
                         // Delete next character
                         let text = self.text();
-                        let mut end = selection.start() + 1;
-                        while !text.is_char_boundary(end) {
-                            end += 1;
-                        }
+                        let end = string_utils::next_char_boundary(&text, selection.start());
                         self.replace_text(selection.start(), end, "", context);
                         // No need to change selection
                         changed = true;
@@ -1450,19 +1441,13 @@ impl<'gc> TDisplayObject<'gc> for EditText<'gc> {
                     let length = text.len();
                     match key_code {
                         ButtonKeyCode::Left if selection.to > 0 => {
-                            selection.to -= 1;
-                            while !text.is_char_boundary(selection.to) {
-                                selection.to -= 1;
-                            }
+                            selection.to = string_utils::prev_char_boundary(text, selection.to);
                             if !context.input.is_key_down(KeyCode::Shift) {
                                 selection.from = selection.to;
                             }
                         }
                         ButtonKeyCode::Right if selection.to < length => {
-                            selection.to += 1;
-                            while !text.is_char_boundary(selection.to) {
-                                selection.to += 1;
-                            }
+                            selection.to = string_utils::next_char_boundary(text, selection.to);
                             if !context.input.is_key_down(KeyCode::Shift) {
                                 selection.from = selection.to;
                             }

core/src/html/layout.rs

@@ -7,6 +7,7 @@ use crate::font::{EvalParameters, Font};
 use crate::html::dimensions::{BoxBounds, Position, Size};
 use crate::html::text_format::{FormatSpans, TextFormat, TextSpan};
 use crate::shape_utils::DrawCommand;
+use crate::string_utils;
 use crate::tag_utils::SwfMovie;
 use gc_arena::{Collect, GcCell, MutationContext};
 use std::cmp::{max, min};
@@ -720,16 +721,10 @@ impl<'gc> LayoutBox<'gc> {
 
                             // This ensures that the space causing the line break
                             // is included in the line it broke.
-                            let next_breakpoint = if last_breakpoint + breakpoint + 1 >= text.len()
-                            {
-                                text.len()
-                            } else {
-                                let mut nb = last_breakpoint + breakpoint + 1;
-                                while !text.is_char_boundary(nb) {
-                                    nb += 1;
-                                }
-                                nb
-                            };
+                            let next_breakpoint = string_utils::next_char_boundary(
+                                text,
+                                last_breakpoint + breakpoint,
+                            );
 
                             layout_context.append_text(
                                 &text[last_breakpoint..next_breakpoint],

core/src/string_utils.rs

@@ -1,5 +1,29 @@
 ///! Utilities for operating on strings in SWF files.
 
+/// Gets the position of the previous char
+/// `pos` must already lie on a char boundary
+pub fn prev_char_boundary(slice: &str, pos: usize) -> usize {
+    if pos == 0 {
+        return pos;
+    }
+
+    let mut idx = pos - 1;
+    while !slice.is_char_boundary(idx) {
+        idx -= 1;
+    }
+    idx
+}
+
+/// Gets the byte position of the next char
+/// `pos` must already lie on a char boundary
+pub fn next_char_boundary(slice: &str, pos: usize) -> usize {
+    if let Some(c) = slice[pos..].chars().next() {
+        pos + c.len_utf8()
+    } else {
+        slice.len()
+    }
+}
+
 /// Creates a `String` from an iterator of UTF-16 code units.
 /// TODO: Unpaired surrogates will get replaced with the Unicode replacement character.
 pub fn utf16_iter_to_string<I: Iterator<Item = u16>>(it: I) -> String {