Generalize HEAD to refs

how_to/Change_21.md

@@ -0,0 +1,23 @@
+- tag: Generalize HEAD to refs
+
+As part of implementing `tag`, we'll generalize the way we handle HEAD. If you
+think about it, HEAD and tags are similar. They are both ways for ugit to attach
+a name to an OID. In case of HEAD, the name is hardcoded by ugit; in case of
+tags, the name will be provided by the user. It makes sense to handle them
+similarly in *data.py*.
+
+In *data.py*, let's extend the function `set_HEAD` and `get_HEAD` to
+`update_ref` and `get_ref`. "Ref" is a short for reference, and that's the name
+Git uses. The function will now accept the name of the ref and write/read it as
+a file under *.ugit* directory. Logically, a ref is a named pointer to an object.
+
+The important change is in *data.py*. The rest of the changes just rename some
+functions:
+
+```
+- get_HEAD()    ->   get_ref('HEAD')
+- set_HEAD(oid) ->   update_ref('HEAD', oid)
+```
+
+Note that we didn't change any behaviour of ugit here, this is purely
+refactoring.

ugit/base.py

@@ -83,7 +83,7 @@ def read_tree(tree_oid):
 def commit(message):
     commit = f"tree {write_tree()}\n"
 
-    HEAD = data.get_HEAD()
+    HEAD = data.get_ref("HEAD")
     if HEAD:
         commit += f"parent {HEAD}\n"
 
@@ -92,7 +92,7 @@ def commit(message):
 
     oid = data.hash_object(commit.encode(), "commit")
 
-    data.set_HEAD(oid)
+    data.update_ref("HEAD", oid)
 
     return oid
 
@@ -100,7 +100,7 @@ def commit(message):
 def checkout(oid):
     commit = get_commit(oid)
     read_tree(commit.tree)
-    data.set_HEAD(oid)
+    data.update_ref("HEAD", oid)
 
 
 Commit = namedtuple("Commit", ["tree", "parent", "message"])

ugit/cli.py

@@ -85,7 +85,7 @@ def commit(args):
 
 
 def log(args):
-    oid = args.oid or data.get_HEAD()
+    oid = args.oid or data.get_ref("HEAD")
     while oid:
         commit = base.get_commit(oid)
 
@@ -101,5 +101,5 @@ def checkout(args):
 
 
 def tag(args):
-    oid = args.oid or data.get_HEAD()
+    oid = args.oid or data.get_ref("HEAD")
     base.create_tag(args.name, oid)

ugit/data.py

@@ -10,13 +10,13 @@ def init():
     Path.mkdir(f"{GIT_DIR}/objects")
 
 
-def set_HEAD(oid):
-    with open(f"{GIT_DIR}/HEAD", "w") as f:
+def update_ref(ref, oid):
+    with open(f"{GIT_DIR}/{ref}", "w") as f:
         f.write(oid)
 
 
-def get_HEAD():
-    if Path.is_file(f"{GIT_DIR}/HEAD"):
+def get_ref(ref):
+    if Path.is_file(f"{GIT_DIR}/{ref}"):
         with open(f"{GIT_DIR}/HEAD") as f:
             return f.read().strip()