radicle-surf: refactor design: part 4

- Update design doc.
- Refactor History API.
- Use FromRevision and IntoCommit traits.
- Add more test cases.

Signed-off-by: Han Xu <keepsimple@gmail.com>

radicle-surf/docs/refactor-design.md

@@ -9,29 +9,27 @@ can use it to create a GitHub-like UI for a git repo:
 
 1. Code browsing: given a specific commit/ref, browse files and directories.
 2. Diff between two revisions that resolve into two commits.
-3. Retrieve the history of the commits.
-4. Retrieve a specific object: all its metadata.
-5. Retrieve the refs: Branches, Tags, Remotes, Notes and user-defined
-"categories", where a category is: refs/<category>/<...>.
+3. Retrieve the history of commits with a given head, and optionally a file.
+4. List refs and retrieve their metadata: Branches, Tags, Remotes,
+Notes and user-defined "categories", where a category is: refs/<category>/<...>.
 
 ## Motivation
 
 The `radicle-surf` crate aims to provide a safe and easy-to-use API that
 supports the features listed in [Introduction]. Based on the existing API,
 the main goals of the refactoring are:
 
-- API review: make API simpler whenever possible.
-- Address open issues in the original `radicle-surf` repo as much as possible.
-- Not to shy away from being `git` specific. (i.e. not to consider supporting
-other VCS systems)
-- Hide away `git2` from the API. The use of `git2` should be an implementation
-detail.
+- API review: identify the issues with the current API.
+- New API: propose a new API that could reuse parts of the existing API.
+- Address open issues in the original `radicle-surf` repo.
+- Be `git` specific. (i.e. no need to support other VCS systems)
+- Remove `git2` from the public API. The use of `git2` should be an
+implementation detail.
 
 ## API review
 
-The current API has quite a bit accidental complexity that is not inherent with
-the requirements, especially when we can be git specific and don't care about
-other VCS systems.
+In this section, we review some core types in the current API and propose
+changes to them. The main theme is to make the API simpler and easier to use.
 
 ### Remove the `Browser`
 
@@ -80,39 +78,121 @@ We no longer need another layer of indirection defined by `Vcs` trait.
 With the changes proposed in the previous section, we describe what the new API
 would look like and how they meet the requirements.
 
-### Common principles
+### Basic types
 
-#### How to identify things that resolve into a commit
+#### Revision and Commit
 
-In our API, it will be good to have a single way to identify all refs and
-objects that resolve into commits. In other words, we try to avoid using
-different ways at different places. Currently there are multiple types in the
-API for this purpose:
+In Git, `Revision` commonly resolves into a `Commit` but could refers to other
+objects for example a `Blob`. Hence we need to keep both concepts in the API.
+Currently we have multiple types to identify a `Commit` or `Revision`.
 
 - Commit
 - Oid
 - Rev
 
-Because `Rev` is the most high level among those and supports refs already,
-I think we should use `Rev` in our API as much as possible.
+The relations between them are: all `Rev` and `Commit` can resolve into `Oid`,
+and most `Rev`s can resolve into `Commit`.
 
-#### How to identify History
+On one hand, `Oid` is the ultimate unique identifer but it is more machine-
+friendly than human-friendly. On the other hand, `Revision` is most human-
+friendly and better suited in the API interface. A conversion from `Revision`
+to `Oid` will be useful.
 
-TBD
+For the places where `Commit` is required, we should explicitly ask for
+`Commit` instead of `Revision`.
+
+In conclusion, we define two new traits to support the use of `Revision` and
+`Commit`:
+
+```Rust
+pub trait Revision {
+    /// Resolves a revision into an object id in `repo`.
+    fn object_id(&self, repo: &RepositoryRef) -> Result<Oid, Error>;
+}
+
+pub trait ToCommit {
+    /// Converts to a commit in `repo`.
+    fn to_commit(self, repo: &RepositoryRef) -> Result<Commit, Error>;
+}
+```
+
+These two traits will be implemented for most common representations of
+`Revision` and `Commit`, for example `&str`, refs like `Branch`, `Tag`, etc.
+Our API will use these traits where we expect a `Revision` or a `Commit`.
+
+#### History
+
+The current `History` is generic over VCS types and also retrieves the full list
+of commits when the history is created. The VCS part can be removed and the
+history can lazy-load the list of commits by implmenting `Iterator` to support
+ potentially very long histories.
+
+We can also store the head commit with the history so that it's easy to get
+the start point and it helps to identify the history.
+
+To support getting the history of a file, we provide methods to modify a
+`History` to filter by a file path.
+
+The new `History` type would look like this:
+
+```Rust
+pub struct History<'a> {
+    repo: RepositoryRef<'a>,
+    head: Commit,
+    revwalk: git2::Revwalk<'a>,
+    filter_by: Option<FilterBy>,
+}
+
+enum FilterBy {
+    File { path: file_system::Path },
+}
+```
+
+For the methods provided by `History`, please see section [Retrieve the history]
+(#retrieve-the-history) below.
+
+#### Commit
+
+`Commit` is a central concept in Git. In `radicle-surf` we define `Commit` type
+to represent its metadata:
+
+```Rust
+pub struct Commit {
+    /// Object Id
+    pub id: Oid,
+    /// The author of the commit.
+    pub author: Author,
+    /// The actor who committed this commit.
+    pub committer: Author,
+    /// The long form message of the commit.
+    pub message: String,
+    /// The summary message of the commit.
+    pub summary: String,
+    /// The parents of this commit.
+    pub parents: Vec<Oid>,
+}
+```
+
+To get the content (i.e. the tree object) of the commit, the user should use
+`snapshot` method described in [Code browsing](#code-browsing) section.
+
+To get the diff of the commit, the user should use `diff_from_parent` method
+described in [Diffs](#diffs) section. Note that we might move that method to
+`Commit` type itself.
 
 ### Code browsing
 
 The user should be able to browse the files and directories for any given
-commits or references. The core API is:
+commit. The core API is:
 
 - Create a root Directory:
 ```Rust
-imp RepositoryRef {
-    pub fn snapshot(&self, rev: &Rev) -> Result<Directory, Error>;
+impl RepositoryRef {
+    pub fn snapshot<C: ToCommit>(&self, commit: C) -> Result<Directory, Error>;
 }
 ```
 
-- Browse a Directory:
+- Browse a Directory's contents:
 ```Rust
 impl Directory {
     pub fn contents(&self) -> impl Iterator<Item = &DirectoryContents>;
@@ -135,26 +215,142 @@ pub enum DirectoryContents {
 
 ### Diffs
 
-The user would be able to create a diff between any two revisions that resolve
-into two commits.
+The user would be able to create a diff between any two revisions. In the first
+implementation, these revisions have to resolve into commits. But in future,
+the revisions could refer to other objects, e.g. files (blobs).
 
-The main change is to use `Rev` instead of `Oid` to identify `from` and `to`.
 The core API is:
 
 ```Rust
-imp RepositoryRef {
-    pub fn diff(&self, from: &Rev, to: &Rev) -> Result<Diff, Error>;
+impl RepositoryRef {
+    /// Returns the diff between two revisions.
+    pub fn diff<R: Revision>(&self, from: R, to: R) -> Result<Diff, Error>;
 }
 ```
 
-To help convert from `Oid` to `Rev`, we provide a helper method:
+We used to have the following method:
+```Rust
+    /// Returns the diff between a revision and the initial state of the repo.
+    pub fn initial_diff<R: Revision>(&self, rev: R) -> Result<Diff, Error>;
+```
+
+However, it is not comparing with any other commit so the output is basically
+the snapshot of `R`. I am not sure if it is necessary. My take is that we can
+remove this method.
+
+We also have the following method:
 ```Rust
-imp RepositoryRef {
-    /// Returns the Oid of `rev`.
-    pub fn rev_oid(&self, rev: &Rev) -> Result<Oid, Error>;
+    /// Returns the diff of a specific commit.
+    pub fn diff_from_parent<C: ToCommit>(&self, commit: C) -> Result<Diff, Error>;
+```
+
+I think it is probably better to instead define the above method as
+`Commit::diff()` as its output is associated with a `Commit`.
+
+### Retrieve the history
+
+The user would be able to get the list of previous commits reachable from a
+particular commit.
+
+To create a `History` from a repo with a given head:
+```Rust
+impl RepositoryRef {
+    pub fn history<C: ToCommit>(&self, head: C) -> Result<History, Error>;
 }
 ```
 
+`History` implements `Iterator` that produces `Result<Commit, Error>`, and
+also provides these methods:
+
+```Rust
+impl<'a> History<'a> {
+    pub fn new<C: ToCommit>(repo: RepositoryRef<'a>, head: C) -> Result<Self, Error>;
+
+    pub fn head(&self) -> &Commit;
+
+    // Modifies a history with a filter by `path`.
+    // This is to support getting the history of a file.
+    pub fn by_path(mut self, path: file_system::Path) -> Self;
+```
+
+- Alternative design:
+
+One potential downside of define `History` as an iterator is that:
+`history.next()` takes a mutable history object. A different design is to use
+`History` as immutable object that produces an iterator on-demand:
+
+```Rust
+pub struct History<'a> {
+    repo: RepositoryRef<'a>,
+    head: Commit,
+}
+
+impl<'a> History<'a> {
+    /// This method creats a new `RevWalk` internally and return an
+    /// iterator for all commits in a history.
+    pub fn iter(&self) -> impl Iterator<Item = Commit>;
+}
+```
+
+In this design, `History` does not keep `RevWalk` in its state. It will create
+a new one when `iter()` is called. I like the immutable interface of this design
+but did not implement it in the current code mainly because the libgit2 doc says
+[creating a new `RevWalk` is relatively expensive](https://libgit2.org/libgit2/#HEAD/group/revwalk/git_revwalk_new).
+
+### List refs and retrieve their metadata
+
+Git refs are simple names that point to objects using object IDs. `radicle-surf`
+support refs by its `Ref` type.
+
+```Rust
+pub enum Ref {
+    /// A git tag, which can be found under `.git/refs/tags/`.
+    Tag {
+        /// The name of the tag, e.g. `v1.0.0`.
+        name: TagName,
+    },
+    /// A git branch, which can be found under `.git/refs/heads/`.
+    LocalBranch {
+        /// The name of the branch, e.g. `master`.
+        name: BranchName,
+    },
+    /// A git branch, which can be found under `.git/refs/remotes/`.
+    RemoteBranch {
+        /// The remote name, e.g. `origin`.
+        remote: String,
+        /// The name of the branch, e.g. `master`.
+        name: BranchName,
+    },
+    /// A git namespace, which can be found under `.git/refs/namespaces/`.
+    ///
+    /// Note that namespaces can be nested.
+    Namespace {
+        /// The name value of the namespace.
+        namespace: String,
+        /// The reference under that namespace, e.g. The
+        /// `refs/remotes/origin/master/ portion of `refs/namespaces/
+        /// moi/refs/remotes/origin/master`.
+        reference: Box<Ref>,
+    },
+    /// A git notes, which can be found under `.git/refs/notes`
+    Notes {
+        /// The default name is "commits".
+        name: String,
+    }
+}
+```
+
+### Git Objects
+
+Git has four kinds of objects: Blob, Tree, Commit and Tag. We have already
+discussed `Commit` (a struct) and `Tag` (`Ref::Tag`) types. For `blob`, we
+use `File` type to represent, and for `tree`, we use `Directory` type to
+represent. The motivation is to let the user "surf" a repo as a file system
+as much as possible, and to avoid Git internal concepts in our API if possible.
+
+Open question: there could be some cases where the names `blob` and `tree`
+shall be used. We need to define such cases clearly if they exist.
+
 ## Error handling
 
 TBD

radicle-surf/examples/diff.rs

@@ -32,10 +32,7 @@ fn main() {
     let base_oid = Oid::from_str(&options.base_revision).unwrap();
     let now = Instant::now();
     let elapsed_nanos = now.elapsed().as_nanos();
-    let diff = repo
-        .as_ref()
-        .diff(&base_oid.into(), &head_oid.into())
-        .unwrap();
+    let diff = repo.as_ref().diff(base_oid, head_oid).unwrap();
     print_diff_summary(&diff, elapsed_nanos);
 }