2 files changed, 83 insertions, 0 deletions
diff --git a/src/jpeg/mod.rs b/src/jpeg/mod.rs
new file mode 100644
index 0000000..3d6bdc3
--- /dev/null
+++ b/src/jpeg/mod.rs
@@ -0,0 +1,2 @@
+mod segment;
+pub use segment::*;
diff --git a/src/jpeg/segment.rs b/src/jpeg/segment.rs
new file mode 100644
index 0000000..b20ba3b
--- /dev/null
+++ b/src/jpeg/segment.rs
@@ -0,0 +1,81 @@
+use std::{mem::size_of, usize};
+
+use img_parts::jpeg::{markers, Jpeg, JpegSegment};
+use thiserror::Error;
+
+use crate::codec::Codec;
+
+/// Codec for storing payload data in JPEG comment (COM) segments. Can store an arbitrary amount of
+/// data, as long as the number of comment segments does not exceed u64::MAX.
+#[derive(Debug, PartialEq, Eq)]
+pub struct JpegSegmentCodec {
+    /// Index of segment to insert comments at.
+    pub start_index: usize,
+}
+
+impl Codec for JpegSegmentCodec {
+    type Carrier = Vec<u8>;
+    type Payload = Vec<u8>;
+    type Output = Self::Carrier;
+    type Error = JpegSegmentError;
+
+    fn encode<C, P>(&self, carrier: C, payload: P) -> Result<Self::Output, Self::Error>
+    where
+        C: Into<Self::Carrier>,
+        P: Into<Self::Payload>,
+    {
+        let mut jpeg = match Jpeg::from_bytes(carrier.into().into()) {
+            Ok(image) => image,
+            Err(err) => return Err(JpegSegmentError::ParseFailed { inner: err })
+        };
+        let mut payload_bytes: Self::Carrier = payload.into();
+        let segment_count = ((payload_bytes.len() + size_of::<u64>()) as u64).div_ceil((u16::MAX as usize - size_of::<u16>()) as u64);
+        payload_bytes.splice(0..0, segment_count.to_le_bytes());
+        for (index, payload_chunk) in payload_bytes.chunks(u16::MAX as usize - size_of::<u16>()).enumerate() {
+            let segment = JpegSegment::new_with_contents(markers::COM, payload_chunk.to_vec().into());
+            jpeg.segments_mut().insert(self.start_index + index, segment);
+        }
+        Ok(jpeg.encoder().bytes().to_vec())
+    }
+
+    fn decode<E>(&self, encoded: E) -> Result<(Self::Carrier, Self::Payload), Self::Error>
+    where
+        E: Into<Self::Output>,
+    {
+        let mut jpeg = match Jpeg::from_bytes(encoded.into().into()) {
+            Ok(image) => image,
+            Err(err) => return Err(JpegSegmentError::ParseFailed { inner: err })
+        };
+        let segment = jpeg.segments_mut().remove(self.start_index);
+        let segment_bytes = segment.contents();
+        let segment_count = u64::from_le_bytes(segment_bytes[0..size_of::<u64>()].try_into().unwrap()) as usize;
+        let mut payload_vec: Vec<u8> = Vec::with_capacity((u16::MAX as usize - size_of::<u16>()) * segment_count);
+        payload_vec.extend(segment_bytes[size_of::<u64>()..].to_vec());
+
+        for _ in 0..segment_count-1 {
+            let segment = jpeg.segments_mut().remove(self.start_index);
+            payload_vec.extend(segment.contents());
+        }
+
+        Ok((jpeg.encoder().bytes().to_vec(), payload_vec))
+    }
+}
+
+impl Default for JpegSegmentCodec {
+    fn default() -> Self {
+        Self {
+            start_index: 3,
+        }
+    }
+}
+
+/// Errors thrown by the JPEG segment codec.
+#[derive(Error, Debug)]
+pub enum JpegSegmentError {
+    /// Parsing JPEG data failed.
+    #[error("Failed to parse JPEG data: {inner:?}")]
+    ParseFailed { 
+        /// Error thrown by parser.
+        inner: img_parts::Error,
+    }
+}