1 files changed, 120 insertions, 0 deletions
diff --git a/components/layout/flow/inline/line_breaker.rs b/components/layout/flow/inline/line_breaker.rs
new file mode 100644
index 00000000000..28301fdadf8
--- /dev/null
+++ b/components/layout/flow/inline/line_breaker.rs
@@ -0,0 +1,120 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use std::ops::Range;
+
+use icu_segmenter::LineSegmenter;
+
+pub(crate) struct LineBreaker {
+    linebreaks: Vec<usize>,
+    current_offset: usize,
+}
+
+impl LineBreaker {
+    pub(crate) fn new(string: &str) -> Self {
+        let line_segmenter = LineSegmenter::new_auto();
+        Self {
+            // From https://docs.rs/icu_segmenter/1.5.0/icu_segmenter/struct.LineSegmenter.html
+            // > For consistency with the grapheme, word, and sentence segmenters, there is always a
+            // > breakpoint returned at index 0, but this breakpoint is not a meaningful line break
+            // > opportunity.
+            //
+            // Skip this first line break opportunity, as it isn't interesting to us.
+            linebreaks: line_segmenter.segment_str(string).skip(1).collect(),
+            current_offset: 0,
+        }
+    }
+
+    pub(crate) fn advance_to_linebreaks_in_range(&mut self, text_range: Range<usize>) -> &[usize] {
+        let linebreaks_in_range = self.linebreaks_in_range_after_current_offset(text_range);
+        self.current_offset = linebreaks_in_range.end;
+        &self.linebreaks[linebreaks_in_range]
+    }
+
+    fn linebreaks_in_range_after_current_offset(&self, text_range: Range<usize>) -> Range<usize> {
+        assert!(text_range.start <= text_range.end);
+
+        let mut linebreaks_range = self.current_offset..self.linebreaks.len();
+
+        while self.linebreaks[linebreaks_range.start] < text_range.start &&
+            linebreaks_range.len() > 1
+        {
+            linebreaks_range.start += 1;
+        }
+
+        let mut ending_linebreak_index = linebreaks_range.start;
+        while self.linebreaks[ending_linebreak_index] < text_range.end &&
+            ending_linebreak_index < self.linebreaks.len() - 1
+        {
+            ending_linebreak_index += 1;
+        }
+        linebreaks_range.end = ending_linebreak_index;
+        linebreaks_range
+    }
+}
+
+#[test]
+fn test_linebreaker_ranges() {
+    let linebreaker = LineBreaker::new("abc def");
+    assert_eq!(linebreaker.linebreaks, [4, 7]);
+    assert_eq!(
+        linebreaker.linebreaks_in_range_after_current_offset(0..5),
+        0..1
+    );
+    // The last linebreak should not be included for the text range we are interested in.
+    assert_eq!(
+        linebreaker.linebreaks_in_range_after_current_offset(0..7),
+        0..1
+    );
+
+    let linebreaker = LineBreaker::new("abc d def");
+    assert_eq!(linebreaker.linebreaks, [4, 6, 9]);
+    assert_eq!(
+        linebreaker.linebreaks_in_range_after_current_offset(0..5),
+        0..1
+    );
+    assert_eq!(
+        linebreaker.linebreaks_in_range_after_current_offset(0..7),
+        0..2
+    );
+    assert_eq!(
+        linebreaker.linebreaks_in_range_after_current_offset(0..9),
+        0..2
+    );
+
+    assert_eq!(
+        linebreaker.linebreaks_in_range_after_current_offset(4..9),
+        0..2
+    );
+
+    std::panic::catch_unwind(|| {
+        let linebreaker = LineBreaker::new("abc def");
+        linebreaker.linebreaks_in_range_after_current_offset(5..2);
+    })
+    .expect_err("Reversed range should cause an assertion failure.");
+}
+
+#[test]
+fn test_linebreaker_stateful_advance() {
+    let mut linebreaker = LineBreaker::new("abc d def");
+    assert_eq!(linebreaker.linebreaks, [4, 6, 9]);
+    assert!(linebreaker.advance_to_linebreaks_in_range(0..7) == &[4, 6]);
+    assert!(linebreaker.advance_to_linebreaks_in_range(8..9).is_empty());
+
+    // We've already advanced, so a range from the beginning shouldn't affect things.
+    assert!(linebreaker.advance_to_linebreaks_in_range(0..9).is_empty());
+
+    linebreaker.current_offset = 0;
+
+    // Sending a value out of range shoudn't break things.
+    assert!(linebreaker.advance_to_linebreaks_in_range(0..999) == &[4, 6]);
+
+    linebreaker.current_offset = 0;
+
+    std::panic::catch_unwind(|| {
+        let mut linebreaker = LineBreaker::new("abc d def");
+        linebreaker.advance_to_linebreaks_in_range(2..0);
+    })
+    .expect_err("Reversed range should cause an assertion failure.");
+}