diff --git a/Cargo.toml b/Cargo.toml
index 5ee344f..a8eae12 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,7 +7,7 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+anyhow = "1"
 
 [dev-dependencies]
-anyhow = "1"
 pretty_assertions = "0.7"
diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..35f2467
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,2 @@
+pub type Error = anyhow::Error;
+pub type Result<T> = std::result::Result<T, Error>;
diff --git a/src/lib.rs b/src/lib.rs
index 2d06385..f862ab1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,6 @@
 #![allow(dead_code)]
 
+mod error;
+mod reader;
 mod scanner;
 mod token;
diff --git a/src/reader.rs b/src/reader.rs
new file mode 100644
index 0000000..3176e27
--- /dev/null
+++ b/src/reader.rs
@@ -0,0 +1,205 @@
+use std::collections::VecDeque;
+
+use anyhow::bail;
+
+use crate::error::Result;
+
+#[derive(Debug)]
+struct Reader<I> {
+    src: I,
+    buffer: VecDeque<char>,
+    column: usize,
+    mark: usize,
+
+    reset_column: bool,
+}
+
+impl<I> Reader<I>
+where
+    I: Iterator<Item = Result<char>>,
+{
+    pub fn new(src: I) -> Self {
+        Self {
+            src,
+            buffer: VecDeque::new(),
+            column: 0,
+            mark: 0,
+
+            reset_column: false,
+        }
+    }
+
+    /// Number of chars seen so far
+    pub fn mark(&self) -> usize {
+        self.mark
+    }
+
+    /// How deep into a line we currently are
+    pub fn column(&self) -> usize {
+        self.column
+    }
+
+    /// Reserve amt chars, returning an error if it could not reserve the
+    /// requested amount
+    pub fn reserve_exact(&mut self, amt: usize) -> Result<()> {
+        let reserved = self.reserve(amt)?;
+
+        if reserved != amt {
+            bail!("only reserved {}/{} chars @{}", reserved, amt, self.mark());
+        }
+
+        Ok(())
+    }
+
+    /// Attempt to reserve up to amt chars, returning the actual number added
+    pub fn reserve(&mut self, amt: usize) -> Result<usize> {
+        let mut done = 0;
+
+        while done < amt {
+            match self.src.next() {
+                Some(Ok(c)) => self.buffer.push_front(c),
+                Some(Err(e)) => return Err(e),
+                None => return Ok(done),
+            }
+
+            done += 1;
+        }
+
+        Ok(done)
+    }
+
+    fn next_char(&mut self) -> Result<Option<char>> {
+        match self.read_one()? {
+            Some(c) => {
+                // We hit a line end previously, reset column size
+                if self.reset_column {
+                    self.reset_column = false;
+                    self.column = 0;
+                }
+
+                // We just hit a line end, arm column reset
+                if c == '\n' {
+                    self.reset_column = true;
+                }
+
+                self.column += 1;
+                self.mark += 1;
+
+                Ok(Some(c))
+            }
+            None => Ok(None),
+        }
+    }
+
+    fn read_one(&mut self) -> Result<Option<char>> {
+        match self.buffer.is_empty() {
+            true => self.src.next().transpose(),
+            false => Ok(self.buffer.pop_back()),
+        }
+    }
+}
+
+impl<I> Iterator for Reader<I>
+where
+    I: Iterator<Item = Result<char>>,
+{
+    type Item = Result<char>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.next_char().transpose()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::{assert_eq, assert_ne};
+
+    macro_rules! data {
+        ($data:expr) => {
+            $data.chars().map(Ok)
+        };
+        () => {
+            "1234567890".chars().map(Ok)
+        };
+    }
+
+    #[test]
+    fn amount() {
+        let data = data!();
+        let r = Reader::new(data);
+
+        assert_eq!(r.into_iter().count(), 10);
+    }
+
+    #[test]
+    fn reserve() {
+        let data = data!();
+        let mut r = Reader::new(data);
+
+        let amount = r.reserve(20).expect("impossible to error");
+
+        assert_ne!(amount, 20);
+        assert_eq!(amount, 10);
+    }
+
+    #[test]
+    fn reserve_partial() {
+        let data = data!();
+        let mut r = Reader::new(data);
+
+        let amount = r.reserve(5).expect("impossible to error");
+
+        assert_eq!(amount, 5);
+    }
+
+    #[test]
+    fn reserve_one() {
+        let data = data!();
+        let mut r = Reader::new(data);
+
+        let amount = r.reserve(1).expect("impossible to error");
+
+        assert_eq!(amount, 1);
+    }
+
+    #[test]
+    fn reserve_zero() {
+        let data = data!();
+        let mut r = Reader::new(data);
+
+        let amount = r.reserve(0).expect("impossible to error");
+
+        assert_eq!(amount, 0);
+    }
+
+    #[test]
+    fn column() {
+        let data = data!("abc\nefg\nhijkl");
+        let expected = vec![1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 5];
+        let mut r = Reader::new(data);
+
+        assert_eq!(r.column(), 0);
+
+        for indent in expected {
+            let c = r.next().unwrap().expect("impossible to error");
+
+            assert_eq!(r.column(), indent, "@ char: {}, mark: {}", c, r.mark());
+        }
+    }
+
+    #[test]
+    fn mark() {
+        let data = data!();
+        let expected = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+        let mut r = Reader::new(data);
+
+        assert_eq!(r.mark(), 0);
+
+        for mark in expected {
+            let c = r.next().unwrap().expect("impossible to error");
+
+            assert_eq!(r.mark(), mark, "@ char: {}, mark: {}", c, r.mark());
+        }
+    }
+}