1 files changed, 117 insertions, 0 deletions
diff --git a/src/offset.rs b/src/offset.rs
new file mode 100644
index 0000000..f1f436d
--- /dev/null
+++ b/src/offset.rs
@@ -0,0 +1,117 @@
+//! Source code offsets.
+//!
+//! The [`DefaultEmitter`](crate::DefaultEmitter) is generic over an [`Offset`].
+//! This library comes with two Offset implementations:
+//!
+//! * [`NoopOffset`] for when you don't want to track source offsets
+//! * `usize` for when you do want to track source offsets
+//!
+//! To use the latter your reader however has to implement [`Position<usize>`].
+//! You can easily use any existing reader by wrapping it in the [`PosTrackingReader`] struct
+//! which implements the [`Position<usize>`] trait and takes care of tracking the current position.
+
+use std::fmt::Debug;
+use std::ops::{Add, AddAssign, Sub};
+
+use crate::reader::{IntoReader, Reader};
+
+/// A byte offset in the source.
+pub trait Offset:
+    Default
+    + Copy
+    + Eq
+    + Ord
+    + Add<usize, Output = Self>
+    + Sub<usize, Output = Self>
+    + AddAssign<usize>
+    + Debug
+{
+}
+
+impl Offset for usize {}
+
+impl Offset for NoopOffset {}
+
+/// A zero-sized no-op implementation of [`Offset`] (for when you don't want to track offsets).
+#[derive(Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
+pub struct NoopOffset;
+
+/// A trait to be implemented by readers that track their own position.
+pub trait Position<T> {
+    /// Returns the byte index of the current position.
+    fn position(&self) -> T;
+}
+
+impl<R: Reader> Position<NoopOffset> for R {
+    fn position(&self) -> NoopOffset {
+        NoopOffset
+    }
+}
+
+/// Wraps a [`Reader`] so that it implements [`Position<usize>`].
+pub struct PosTrackingReader<R> {
+    /// The wrapped reader.
+    reader: R,
+    /// The current position.
+    position: usize,
+}
+
+impl<R> PosTrackingReader<R> {
+    /// Wraps the given [`Reader`] so that it implements [`Position<usize>`] with the position starting from 0.
+    pub fn new<'a>(into_reader: impl IntoReader<'a, Reader = R>) -> Self {
+        Self {
+            reader: into_reader.into_reader(),
+            position: 0,
+        }
+    }
+}
+
+impl<R> Position<usize> for PosTrackingReader<R> {
+    fn position(&self) -> usize {
+        self.position - 1
+    }
+}
+
+impl Add<usize> for NoopOffset {
+    type Output = Self;
+
+    fn add(self, _rhs: usize) -> NoopOffset {
+        self
+    }
+}
+
+impl Sub<usize> for NoopOffset {
+    type Output = Self;
+
+    fn sub(self, _rhs: usize) -> NoopOffset {
+        self
+    }
+}
+
+impl AddAssign<usize> for NoopOffset {
+    fn add_assign(&mut self, _rhs: usize) {}
+}
+
+impl<R: Reader> Reader for PosTrackingReader<R> {
+    type Error = R::Error;
+
+    fn read_char(&mut self) -> Result<Option<char>, Self::Error> {
+        match self.reader.read_char()? {
+            Some(char) => {
+                self.position += char.len_utf8();
+                Ok(Some(char))
+            }
+            None => Ok(None),
+        }
+    }
+
+    fn try_read_string(&mut self, s: &str, case_sensitive: bool) -> Result<bool, Self::Error> {
+        match self.reader.try_read_string(s, case_sensitive)? {
+            true => {
+                self.position += s.len();
+                Ok(true)
+            }
+            false => Ok(false),
+        }
+    }
+}