Improve codec to be generic over its inner region

Signed-off-by: Moritz Hoffmann <[email protected]>

src/codec.rs

@@ -1,4 +1,4 @@
-//! TODO
+//! A region that encodes its contents.
 
 use crate::{CopyOnto, CopyRegion, Region};
 
@@ -72,18 +72,22 @@ pub fn consolidate_slice<T: Ord>(slice: &mut [(T, usize)]) -> usize {
 
 /// A region that encodes its data in a codec `C`.
 #[derive(Default, Debug)]
-pub struct CodecRegion<C: Codec> {
-    inner: CopyRegion<u8>,
+pub struct CodecRegion<C: Codec, R = CopyRegion<u8>> {
+    inner: R,
     codec: C,
     staging: Vec<u8>,
 }
 
-impl<C: Codec> Region for CodecRegion<C> {
+impl<C: Codec, R> Region for CodecRegion<C, R>
+where
+    for<'a> R: Region<ReadItem<'a> = &'a [u8]> + 'a,
+    for<'a> &'a [u8]: CopyOnto<R>,
+{
     type ReadItem<'a> = &'a [u8]
     where
         Self: 'a;
 
-    type Index = (usize, usize);
+    type Index = R::Index;
 
     /// Construct a region that can absorb the contents of `regions` in the future.
     fn merge_regions<'a>(regions: impl Iterator<Item = &'a Self> + Clone) -> Self
@@ -92,7 +96,7 @@ impl<C: Codec> Region for CodecRegion<C> {
     {
         let codec = C::new_from(regions.clone().map(|r| &r.codec));
         Self {
-            inner: CopyRegion::merge_regions(regions.map(|r| &r.inner)),
+            inner: R::merge_regions(regions.map(|r| &r.inner)),
             codec,
             staging: vec![],
         }
@@ -116,15 +120,19 @@ impl<C: Codec> Region for CodecRegion<C> {
     }
 }
 
-impl<C: Codec> CopyOnto<CodecRegion<C>> for &[u8] {
-    fn copy_onto(self, target: &mut CodecRegion<C>) -> <CodecRegion<C> as Region>::Index {
+impl<C: Codec, R> CopyOnto<CodecRegion<C, R>> for &[u8]
+where
+    for<'a> R: Region<ReadItem<'a> = &'a [u8]> + 'a,
+    for<'a> &'a [u8]: CopyOnto<R>,
+{
+    fn copy_onto(self, target: &mut CodecRegion<C, R>) -> <CodecRegion<C, R> as Region>::Index {
         target.staging.clear();
         target.codec.encode(self, &mut target.staging);
         target.staging.as_slice().copy_onto(&mut target.inner)
     }
 }
 
-/// TODO
+/// Encode and decode byte strings.
 pub trait Codec: Default + 'static {
     /// Decodes an input byte slice into a sequence of byte slices.
     fn decode<'a>(&'a self, bytes: &'a [u8]) -> &'a [u8];
@@ -189,10 +197,7 @@ mod dictionary {
         fn new_from<'a, I: Iterator<Item = &'a Self> + Clone>(stats: I) -> Self {
             // Collect most popular bytes from combined containers.
             let mut mg = MisraGries::default();
-            for (thing, count) in stats
-                .clone()
-                .flat_map(|stats| stats.stats.0.clone().done())
-            {
+            for (thing, count) in stats.clone().flat_map(|stats| stats.stats.0.clone().done()) {
                 mg.update(thing, count);
             }
             let mut mg = mg.done().into_iter();