1 files changed, 60 insertions, 0 deletions

diff --git a/src/utils/cursor.rs b/src/utils/cursor.rs
new file mode 100644
index 0000000..c41270a
--- /dev/null
+++ b/src/utils/cursor.rs
@@ -0,0 +1,60 @@
+use std::io::{Error, ErrorKind, Read, Result, Seek, SeekFrom, Write};
+
+pub struct IoCursor<Io> {
+    io: Io,
+    cursor: u64,
+    bounds: (u64, u64),
+}
+
+impl<Io: Seek> IoCursor<Io> {
+    pub fn new(mut io: Io, start: u64, end: u64) -> Result<Self> {
+        let cursor = io.seek(SeekFrom::Start(start))?;
+        Ok(Self {
+            io,
+            cursor,
+            bounds: (cursor, end),
+        })
+    }
+}
+
+impl<Io: Read> Read for IoCursor<Io> {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        let upper = buf.len().min((self.bounds.1 - self.cursor) as usize);
+        let bytes = self.io.read(&mut buf[..upper])?;
+        self.cursor += bytes as u64;
+        Ok(bytes)
+    }
+}
+
+impl<Io: Write> Write for IoCursor<Io> {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        let upper = buf.len().min((self.bounds.1 - self.cursor) as usize);
+        let bytes = self.io.write(&buf[..upper])?;
+        self.cursor += bytes as u64;
+        Ok(bytes)
+    }
+
+    #[inline]
+    fn flush(&mut self) -> Result<()> {
+        self.io.flush()
+    }
+}
+
+impl<Io: Seek> Seek for IoCursor<Io> {
+    fn seek(&mut self, pos: SeekFrom) -> Result<u64> {
+        self.cursor = match pos {
+            SeekFrom::Start(0) => return Ok(self.cursor - self.bounds.0),
+            SeekFrom::Start(offset) => self.bounds.0.checked_add(offset),
+            SeekFrom::End(offset) => self.bounds.1.checked_add_signed(offset),
+            SeekFrom::Current(offset) => self.cursor.checked_add_signed(offset),
+        }
+        .filter(|v| *v >= self.bounds.0)
+        .ok_or(Error::new(
+            ErrorKind::InvalidInput,
+            "Invalid seek to a negative or overflowing position",
+        ))?
+        .min(self.bounds.1);
+
+        Ok(self.io.seek(SeekFrom::Start(self.cursor))? - self.bounds.0)
+    }
+}