1 files changed, 111 insertions, 0 deletions
diff --git a/src/util/location.rs b/src/util/location.rs
new file mode 100644
index 0000000..0c9c426
--- /dev/null
+++ b/src/util/location.rs
@@ -0,0 +1,111 @@
+//! Deal with positions in a file.
+//!
+//! * Convert between byte indices and unist points.
+//! * Convert between byte indices into a string which is built up of several
+//!   slices in a whole document, and byte indices into that whole document.
+
+use crate::unist::Point;
+use alloc::{vec, vec::Vec};
+
+/// Each stop represents a new slice, which contains the byte index into the
+/// corresponding string where the slice starts (`0`), and the byte index into
+/// the whole document where that slice starts (`1`).
+pub type Stop = (usize, usize);
+
+#[derive(Debug)]
+pub struct Location {
+    /// List, where each index is a line number (0-based), and each value is
+    /// the byte index *after* where the line ends.
+    indices: Vec<usize>,
+}
+
+impl Location {
+    /// Get an index for the given `bytes`.
+    ///
+    /// Port of <https://github.com/vfile/vfile-location/blob/main/index.js>
+    #[must_use]
+    pub fn new(bytes: &[u8]) -> Self {
+        let mut index = 0;
+        let mut location_index = Self { indices: vec![] };
+
+        while index < bytes.len() {
+            if bytes[index] == b'\r' {
+                if index + 1 < bytes.len() && bytes[index + 1] == b'\n' {
+                    location_index.indices.push(index + 2);
+                } else {
+                    location_index.indices.push(index + 1);
+                }
+            } else if bytes[index] == b'\n' {
+                location_index.indices.push(index + 1);
+            }
+
+            index += 1;
+        }
+
+        location_index.indices.push(index + 1);
+        location_index
+    }
+
+    /// Get the line and column-based `point` for `offset` in the bound indices.
+    ///
+    /// Returns `None` when given out of bounds input.
+    ///
+    /// Port of <https://github.com/vfile/vfile-location/blob/main/index.js>
+    #[must_use]
+    pub fn to_point(&self, offset: usize) -> Option<Point> {
+        let mut index = 0;
+
+        if let Some(end) = self.indices.last() {
+            if offset < *end {
+                while index < self.indices.len() {
+                    if self.indices[index] > offset {
+                        break;
+                    }
+
+                    index += 1;
+                }
+
+                let previous = if index > 0 {
+                    self.indices[index - 1]
+                } else {
+                    0
+                };
+                return Some(Point {
+                    line: index + 1,
+                    column: offset + 1 - previous,
+                    offset,
+                });
+            }
+        }
+
+        None
+    }
+
+    /// Like `to_point`, but takes a relative offset from a certain string
+    /// instead of an absolute offset into the whole document.
+    ///
+    /// The relative offset is made absolute based on `stops`, which represent
+    /// where that certain string is in the whole document.
+    #[must_use]
+    pub fn relative_to_point(&self, stops: &[Stop], relative: usize) -> Option<Point> {
+        Location::relative_to_absolute(stops, relative).and_then(|absolute| self.to_point(absolute))
+    }
+
+    /// Turn a relative offset into an absolute offset.
+    #[must_use]
+    pub fn relative_to_absolute(stops: &[Stop], relative: usize) -> Option<usize> {
+        let mut index = 0;
+
+        while index < stops.len() && stops[index].0 <= relative {
+            index += 1;
+        }
+
+        // There are no points: that only occurs if there was an empty string.
+        if index == 0 {
+            None
+        } else {
+            let (stop_relative, stop_absolute) = &stops[index - 1];
+            Some(stop_absolute + (relative - stop_relative))
+        }
+    }
+}