2 files changed, 58 insertions, 0 deletions

diff --git a/libeditor/src/lib.rs b/libeditor/src/lib.rs
index 4e9631a8b..9da71743f 100644
--- a/libeditor/src/lib.rs
+++ b/libeditor/src/lib.rs
@@ -1,6 +1,8 @@
 extern crate libsyntax2;
+extern crate superslice;
 
 mod extend_selection;
+mod line_index;
 
 use libsyntax2::{
     SyntaxNodeRef, AstNode,
diff --git a/libeditor/src/line_index.rs b/libeditor/src/line_index.rs
new file mode 100644
index 000000000..feb482b32
--- /dev/null
+++ b/libeditor/src/line_index.rs
@@ -0,0 +1,56 @@
+use superslice::Ext;
+use ::{TextUnit};
+
+pub struct LineIndex {
+    newlines: Vec<TextUnit>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct LineCol {
+    pub line: u32,
+    pub col: TextUnit,
+}
+
+impl LineIndex {
+    pub fn new(text: &str) -> LineIndex {
+        let mut newlines = vec![0.into()];
+        let mut curr = 0.into();
+        for c in text.chars() {
+            curr += TextUnit::of_char(c);
+            if c == '\n' {
+                newlines.push(curr);
+            }
+        }
+        LineIndex { newlines }
+    }
+
+    pub fn translate(&self, offset: TextUnit) -> LineCol {
+        let line = self.newlines.upper_bound(&offset) - 1;
+        let line_start_offset = self.newlines[line];
+        let col = offset - line_start_offset;
+        return LineCol { line: line as u32, col }
+    }
+}
+
+#[test]
+fn test_line_index() {
+    let text = "hello\nworld";
+    let index = LineIndex::new(text);
+    assert_eq!(index.translate(0.into()), LineCol { line: 0, col: 0.into()});
+    assert_eq!(index.translate(1.into()), LineCol { line: 0, col: 1.into()});
+    assert_eq!(index.translate(5.into()), LineCol { line: 0, col: 5.into()});
+    assert_eq!(index.translate(6.into()), LineCol { line: 1, col: 0.into()});
+    assert_eq!(index.translate(7.into()), LineCol { line: 1, col: 1.into()});
+    assert_eq!(index.translate(8.into()), LineCol { line: 1, col: 2.into()});
+    assert_eq!(index.translate(10.into()), LineCol { line: 1, col: 4.into()});
+    assert_eq!(index.translate(11.into()), LineCol { line: 1, col: 5.into()});
+    assert_eq!(index.translate(12.into()), LineCol { line: 1, col: 6.into()});
+
+    let text = "\nhello\nworld";
+    let index = LineIndex::new(text);
+    assert_eq!(index.translate(0.into()), LineCol { line: 0, col: 0.into()});
+    assert_eq!(index.translate(1.into()), LineCol { line: 1, col: 0.into()});
+    assert_eq!(index.translate(2.into()), LineCol { line: 1, col: 1.into()});
+    assert_eq!(index.translate(6.into()), LineCol { line: 1, col: 5.into()});
+    assert_eq!(index.translate(7.into()), LineCol { line: 2, col: 0.into()});
+}