Don't hardcode Sha1 (#413)

Author: Byron

git-hash/src/lib.rs

@@ -144,6 +144,16 @@ impl Kind {
         }
     }
 
+    /// Returns the kind of hash that would fit the given `hex_len`, or `None` if there is no fitting hash.
+    /// Note that 0 as `hex_len` fits always yields Sha1.
+    #[inline]
+    pub const fn from_hex_len(hex_len: usize) -> Option<Self> {
+        Some(match hex_len {
+            0..=40 => Kind::Sha1,
+            _ => return None,
+        })
+    }
+
     /// Converts a size in bytes as obtained by `Kind::len_in_bytes()` into the corresponding hash kind, if possible.
     ///
     /// **Panics** if the hash length doesn't match a known hash.

git-hash/src/owned/prefix.rs

@@ -123,7 +123,7 @@ impl Prefix {
             hex::FromHexError::OddLength | hex::FromHexError::InvalidStringLength => panic!("This is already checked"),
         })?;
 
-        let mut bytes = ObjectId::null(crate::Kind::Sha1);
+        let mut bytes = ObjectId::null(crate::Kind::from_hex_len(value.len()).expect("hex-len is already checked"));
         let dst = bytes.as_mut_slice();
         let copy_len = src.len();
         dst[..copy_len].copy_from_slice(&src);

git-hash/tests/integration.rs

@@ -1 +1,2 @@
+mod kind;
 mod oid;

git-hash/tests/kind/mod.rs

@@ -0,0 +1,16 @@
+mod from_hex_len {
+    use git_hash::Kind;
+
+    #[test]
+    fn some_sha1() {
+        assert_eq!(Kind::from_hex_len(0), Some(Kind::Sha1));
+        assert_eq!(Kind::from_hex_len(10), Some(Kind::Sha1));
+        assert_eq!(Kind::from_hex_len(20), Some(Kind::Sha1));
+        assert_eq!(Kind::from_hex_len(40), Some(Kind::Sha1));
+    }
+
+    #[test]
+    fn none_if_there_is_no_fit() {
+        assert_eq!(Kind::from_hex_len(65), None);
+    }
+}