Add checksum to pack (in preparation for validation)

Author: Byron

README.md

@@ -164,7 +164,7 @@ trailed by a newline.
 echo c56a8e7aa92c86c41a923bc760d2dc39e8a31cf7  | git cat-file --batch | tail +2 > fixture
 ```
 
-Thus one has to post-process the file by reducing its size by one using `truncate -s -1 fixture`, removing the newline byte.
+Thus one has to post-process the file by reducing its size by one using `truncate -s -1 fixture`, **removing the newline byte**.
 
 
 ## Shortcomings

gitoxide-odb/src/pack/file/mod.rs

@@ -1,6 +1,6 @@
 use byteorder::{BigEndian, ByteOrder};
 use filebuffer::FileBuffer;
-use git_object::SHA1_SIZE;
+use git_object::{Id, SHA1_SIZE};
 use quick_error::quick_error;
 use std::{convert::TryFrom, convert::TryInto, mem::size_of, path::Path};
 
@@ -58,6 +58,11 @@ impl File {
     pub fn num_objects(&self) -> u32 {
         self.num_objects
     }
+    pub fn checksum(&self) -> Id {
+        let mut v = [0u8; 20];
+        v.copy_from_slice(&self.data[self.data.len() - SHA1_SIZE..]);
+        v
+    }
 
     fn assure_v2(&self) {
         assert!(

gitoxide-odb/tests/pack/file.rs

@@ -2,13 +2,27 @@ use crate::fixture_path;
 use git_odb::pack;
 use std::convert::TryFrom;
 
-fn new_pack(at: &str) -> pack::File {
+fn pack_at(at: &str) -> pack::File {
     pack::File::try_from(fixture_path(at).as_path()).unwrap()
 }
 
+mod method {
+    use crate::pack::file::pack_at;
+    use crate::pack::SMALL_PACK;
+
+    #[test]
+    fn checksum() {
+        let p = pack_at(SMALL_PACK);
+        assert_eq!(
+            hex::encode(p.checksum()),
+            "0f3ea84cd1bba10c2a03d736a460635082833e59"
+        );
+    }
+}
+
 /// All hardcoded offsets are obtained via `git verify-pack --verbose  tests/fixtures/packs/pack-a2bf8e71d8c18879e499335762dd95119d93d9f1.idx`
 mod decode_entry {
-    use crate::{pack::file::new_pack, pack::SMALL_PACK};
+    use crate::{pack::file::pack_at, pack::SMALL_PACK};
     use bstr::ByteSlice;
     use git_odb::pack::ResolvedBase;
 
@@ -44,7 +58,7 @@ mod decode_entry {
             panic!("should not want to resolve an id here")
         }
 
-        let p = new_pack(SMALL_PACK);
+        let p = pack_at(SMALL_PACK);
         let entry = p.entry(offset);
         let mut buf = Vec::new();
         p.decode_entry(entry, &mut buf, resolve_with_panic).unwrap();
@@ -53,7 +67,7 @@ mod decode_entry {
 }
 
 mod decompress_entry {
-    use crate::{pack::file::new_pack, pack::SMALL_PACK};
+    use crate::{pack::file::pack_at, pack::SMALL_PACK};
     use bstr::ByteSlice;
 
     #[test]
@@ -93,7 +107,7 @@ mod decompress_entry {
     }
 
     fn decompress_entry_at_offset(offset: u64) -> Vec<u8> {
-        let p = new_pack(SMALL_PACK);
+        let p = pack_at(SMALL_PACK);
         let entry = p.entry(offset);
 
         let size = entry.size as usize;