2 files changed, 44 insertions, 3 deletions

diff --git a/helix-core/src/lib.rs b/helix-core/src/lib.rs
index 8675fc6f..7a0518d8 100644
--- a/helix-core/src/lib.rs
+++ b/helix-core/src/lib.rs
@@ -5,7 +5,7 @@ mod selection;
 mod state;
 mod transaction;
 
-pub use ropey::Rope;
+pub use ropey::{Rope, RopeSlice};
 pub use tendril::StrTendril as Tendril;
 
 pub use buffer::Buffer;
diff --git a/helix-core/src/state.rs b/helix-core/src/state.rs
index 1ffd708e..ccbfd28a 100644
--- a/helix-core/src/state.rs
+++ b/helix-core/src/state.rs
@@ -1,5 +1,5 @@
-use crate::graphemes::{nth_next_grapheme_boundary, nth_prev_grapheme_boundary};
-use crate::{Buffer, Selection, SelectionRange};
+use crate::graphemes::{nth_next_grapheme_boundary, nth_prev_grapheme_boundary, RopeGraphemes};
+use crate::{Buffer, Rope, RopeSlice, Selection, SelectionRange};
 
 /// A state represents the current editor state of a single buffer.
 pub struct State {
@@ -117,3 +117,44 @@ impl State {
         // TODO: update selection in state via transaction
     }
 }
+
+/// Coordinates are a 0-indexed line and column pair.
+type Coords = (usize, usize); // line, col
+
+/// Convert a character index to (line, column) coordinates.
+pub fn coords_at_pos(text: &RopeSlice, pos: usize) -> Coords {
+    let line = text.char_to_line(pos);
+    let line_start = text.line_to_char(line);
+    let col = RopeGraphemes::new(&text.slice(line_start..pos)).count();
+    (line, col)
+}
+
+/// Convert (line, column) coordinates to a character index.
+pub fn pos_at_coords(text: &RopeSlice, coords: Coords) -> usize {
+    let (line, col) = coords;
+    let line_start = text.line_to_char(line);
+    nth_next_grapheme_boundary(text, line_start, col)
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_coords_at_pos() {
+        let text = Rope::from("ḧëḷḷö\nẅöṛḷḋ");
+        assert_eq!(coords_at_pos(&text.slice(..), 0), (0, 0));
+        assert_eq!(coords_at_pos(&text.slice(..), 5), (0, 5)); // position on \n
+        assert_eq!(coords_at_pos(&text.slice(..), 6), (1, 0)); // position on w
+        assert_eq!(coords_at_pos(&text.slice(..), 11), (1, 5)); // position on d
+    }
+
+    #[test]
+    fn test_pos_at_coords() {
+        let text = Rope::from("ḧëḷḷö\nẅöṛḷḋ");
+        assert_eq!(pos_at_coords(&text.slice(..), (0, 0)), 0);
+        assert_eq!(pos_at_coords(&text.slice(..), (0, 5)), 5); // position on \n
+        assert_eq!(pos_at_coords(&text.slice(..), (1, 0)), 6); // position on w
+        assert_eq!(pos_at_coords(&text.slice(..), (1, 5)), 11); // position on d
+    }
+}