1 files changed, 114 insertions, 0 deletions
diff --git a/src/syntax/lines.rs b/src/syntax/lines.rs
new file mode 100644
index 00000000..86fc461b
--- /dev/null
+++ b/src/syntax/lines.rs
@@ -0,0 +1,114 @@
+//! Conversion of byte positions to line/column locations.
+
+use std::fmt::{self, Debug, Display, Formatter};
+
+use super::Pos;
+use crate::parse::is_newline_char;
+
+/// Enables conversion of byte position to locations.
+pub struct LineMap<'s> {
+    src: &'s str,
+    line_starts: Vec<Pos>,
+}
+
+impl<'s> LineMap<'s> {
+    /// Create a new line map for a source string.
+    pub fn new(src: &'s str) -> Self {
+        let mut line_starts = vec![Pos::ZERO];
+        let mut iter = src.char_indices().peekable();
+
+        while let Some((mut i, c)) = iter.next() {
+            if is_newline_char(c) {
+                i += c.len_utf8();
+                if c == '\r' && matches!(iter.peek(), Some((_, '\n'))) {
+                    i += '\n'.len_utf8();
+                    iter.next();
+                }
+
+                line_starts.push(Pos(i as u32));
+            }
+        }
+
+        Self { src, line_starts }
+    }
+
+    /// Convert a byte position to a location.
+    ///
+    /// # Panics
+    /// This panics if the position is out of bounds.
+    pub fn location(&self, pos: Pos) -> Location {
+        let line_index = match self.line_starts.binary_search(&pos) {
+            Ok(i) => i,
+            Err(i) => i - 1,
+        };
+
+        let line_start = self.line_starts[line_index];
+        let head = &self.src[line_start.to_usize() .. pos.to_usize()];
+        let column_index = head.chars().count();
+
+        Location {
+            line: 1 + line_index as u32,
+            column: 1 + column_index as u32,
+        }
+    }
+}
+
+/// One-indexed line-column position in source code.
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[cfg_attr(feature = "serialize", derive(serde::Serialize))]
+pub struct Location {
+    /// The one-indexed line.
+    pub line: u32,
+    /// The one-indexed column.
+    pub column: u32,
+}
+
+impl Location {
+    /// Create a new location from line and column.
+    pub fn new(line: u32, column: u32) -> Self {
+        Self { line, column }
+    }
+}
+
+impl Debug for Location {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        Display::fmt(self, f)
+    }
+}
+
+impl Display for Location {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{}:{}", self.line, self.column)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST: &str = "äbcde\nf💛g\r\nhi\rjkl";
+
+    #[test]
+    fn test_line_map_new() {
+        let map = LineMap::new(TEST);
+        assert_eq!(map.line_starts, vec![Pos(0), Pos(7), Pos(15), Pos(18)]);
+    }
+
+    #[test]
+    fn test_line_map_location() {
+        let map = LineMap::new(TEST);
+        assert_eq!(map.location(Pos(0)), Location::new(1, 1));
+        assert_eq!(map.location(Pos(2)), Location::new(1, 2));
+        assert_eq!(map.location(Pos(6)), Location::new(1, 6));
+        assert_eq!(map.location(Pos(7)), Location::new(2, 1));
+        assert_eq!(map.location(Pos(8)), Location::new(2, 2));
+        assert_eq!(map.location(Pos(12)), Location::new(2, 3));
+        assert_eq!(map.location(Pos(21)), Location::new(4, 4));
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_line_map_panics_out_of_bounds() {
+        LineMap::new(TEST).location(Pos(22));
+    }
+}