defmodule Exgit do
  @moduledoc """
  Pure-Elixir git client — clone, fetch, push, plus a path-oriented
  filesystem API for reading blobs by path.

  No `git` binary, no libgit2, no shelling out.

  ## Top-level entry points

    * `init/1` — create an empty repo (memory or on-disk).
    * `open/2` — open an existing on-disk repo.
    * `clone/2` — full, lazy (`lazy: true`), or partial
      (`filter: {:blob, :none}`) clone.
    * `fetch/3` — fetch updates into an existing repo.
    * `push/3` — push objects + ref updates to a remote.

  See individual function docs for options and semantics. See also
  `Exgit.FS`, `Exgit.Walk`, `Exgit.Diff`, `Exgit.Repository`, and
  the `Exgit.Transport` protocol.
  """

  alias Exgit.{Config, ObjectStore, Pack, RefStore, Repository, Transport}

  @typedoc """
  A remote endpoint for `clone/2`, `fetch/3`, and `push/3`.

  Either a URL string (`"https://..."` or `"file://..."`), one of the
  built-in transport structs (`Exgit.Transport.File`,
  `Exgit.Transport.HTTP`), or any struct implementing the
  `Exgit.Transport` protocol.
  """
  @type remote :: String.t() | Transport.File.t() | Transport.HTTP.t() | struct()

  @doc """
  Create an empty bare repository.

      {:ok, repo} = Exgit.init()
      {:ok, repo} = Exgit.init(path: "/tmp/repo.git")

  ## Options

    * `:path` — create the repo on disk at this path, backed by
      `ObjectStore.Disk`/`RefStore.Disk`. Default: in-memory
      (lost on process exit).

  ## Returns

  `{:ok, %Repository{}}` with `HEAD` pointing at `refs/heads/main`.
  When `:path` is set and any directory or file write fails, returns
  `{:error, {:init_failed, reason}}` with the underlying posix reason.
  """
  @spec init(keyword()) :: {:ok, Repository.t()} | {:error, term()}
  def init(opts \\ []) do
    case Keyword.get(opts, :path) do
      nil -> {:ok, Repository.new(ObjectStore.Memory.new(), RefStore.Memory.new())}
      path -> init_disk(path)
    end
  end

  @doc """
  Open an existing on-disk repository.

      {:ok, repo} = Exgit.open("/tmp/repo.git")

  `path` must point at a git directory — a bare repo or a `.git`
  directory — containing a `HEAD` file and an `objects` directory.
  The repo's `config` file is read when present; a missing or
  unparseable config falls back to an empty `Exgit.Config`.

  ## Returns

  `{:ok, %Repository{}}` backed by `ObjectStore.Disk`/`RefStore.Disk`,
  or `{:error, {:not_a_repository, reason}}` when the path doesn't
  look like a git directory.
  """
  @spec open(Path.t(), keyword()) :: {:ok, Repository.t()} | {:error, term()}
  def open(path, _opts \\ []) do
    head_path = Path.join(path, "HEAD")
    objects_path = Path.join(path, "objects")

    cond do
      not File.exists?(head_path) ->
        {:error, {:not_a_repository, "missing HEAD"}}

      not File.dir?(objects_path) ->
        {:error, {:not_a_repository, "missing objects directory"}}

      true ->
        config =
          case Config.read(Path.join(path, "config")) do
            {:ok, c} -> c
            {:error, _} -> Config.new()
          end

        {:ok,
         Repository.new(
           ObjectStore.Disk.new(path),
           RefStore.Disk.new(path),
           config: config,
           path: path
         )}
    end
  end

  @doc """
  Clone a git repository.

      {:ok, repo} = Exgit.clone("https://github.com/user/repo")

  ## Options

    * `:path` — persistent on-disk clone. When set, the clone is
      backed by `ObjectStore.Disk`/`RefStore.Disk` and survives
      process death. Default: in-memory (lost on process exit).

    * `:lazy` — when `true`, defer object fetching. Returns a
      `%Repository{mode: :lazy}` whose object store is an
      `ObjectStore.Promisor` that fetches on demand. The initial
      clone pulls **refs only** — any subsequent object read
      triggers a `want <sha>` fetch against the transport. Great
      for agent loops that read a handful of files from large
      repos. Default: `false` (full clone).

    * `:filter` — partial-clone filter spec. Valid values:
      `{:blob, :none}`, `{:blob, {:limit, bytes}}`,
      `{:tree, depth}`, `{:raw, "filter=..."}`. When set, the
      server ships a pack that omits blobs (or trees) matching
      the filter; omitted objects are fetched on demand. Implies
      `:lazy`-like semantics for omitted objects but keeps the
      default branch's commit+tree history resident.

    * `:if_unsupported` — `:error` (default) or `:ignore`. When
      `:filter` is set but the server doesn't advertise `filter`
      capability, `:error` fails the clone and `:ignore` proceeds
      as a full clone.

    * `:remote` — remote name to record. Default: `"origin"`.

  ## Matrix of modes

  | Options | Clone-time fetch | On read |
  |---|---|---|
  | `clone(url)` | refs + all objects | local always |
  | `clone(url, lazy: true)` | refs only | per-object fetch |
  | `clone(url, filter: {:blob, :none})` | refs + commits + trees | blobs fetched on read |
  | `clone(url, filter: ..., lazy: true)` | refs only | everything on read |

  ## `:path` + `:lazy` / `:filter`

  On-disk partial clones are not yet supported; `:path` combined
  with `:lazy` or `:filter` returns
  `{:error, :disk_partial_clone_unsupported}`.

  ## Returns

  `{:ok, %Repository{}}` with `:mode => :eager` for full clones
  and `:mode => :lazy` for any form of partial/lazy clone. Use
  `Exgit.Repository.materialize/2` to convert `:lazy` → `:eager`
  after reading what you need.
  """
  @spec clone(remote(), keyword()) :: {:ok, Repository.t()} | {:error, term()}
  def clone(source, opts \\ []) do
    cond do
      disk_partial_clone?(opts) ->
        {:error, :disk_partial_clone_unsupported}

      Keyword.get(opts, :lazy, false) or Keyword.has_key?(opts, :filter) ->
        clone_partial(source, opts)

      true ->
        clone_full(source, opts)
    end
  end

  defp disk_partial_clone?(opts) do
    Keyword.has_key?(opts, :path) and
      (Keyword.get(opts, :lazy, false) or Keyword.has_key?(opts, :filter))
  end

  # Full (eager) clone: pull every reachable object up front, populate
  # a Memory or Disk object store, return an `:eager` repo.
  defp clone_full(source, opts) do
    transport = to_transport(source, opts)

    # Asking for `HEAD` as a ref-prefix pulls the server's HEAD line
    # into the ls-refs output, complete with a symref-target attribute —
    # so we pick the server's actual default branch instead of guessing
    # from the advertised refs.
    with {:ok, repo} <- init(opts),
         {:ok, refs, meta} <-
           safe_ls_refs(transport, prefix: ["HEAD", "refs/heads/", "refs/tags/"]),
         {:ok, repo} <- fetch_into(repo, transport, refs, opts) do
      set_head_to_default(repo, refs, meta)
    end
  end

  # Partial / lazy clone: return an `:lazy` repo with a Promisor
  # object store. Triggered by `:lazy` or `:filter` opts.
  defp clone_partial(source, opts) do
    transport = to_transport(source, opts)

    with {:ok, filter_spec} <- resolve_filter(opts),
         :ok <- check_filter_capability(transport, filter_spec, opts),
         {:ok, refs, meta} <-
           safe_ls_refs(transport, prefix: ["HEAD", "refs/heads/", "refs/tags/"]) do
      promisor =
        ObjectStore.Promisor.new(transport,
          default_fetch_opts: promisor_fetch_opts(filter_spec)
        )

      ref_store =
        Enum.reduce(refs, RefStore.Memory.new(), fn {name, sha}, rs ->
          case RefStore.write(rs, name, sha, []) do
            {:ok, rs2} ->
              rs2

            {:error, reason} ->
              # Memory-backed stores shouldn't fail a write, but
              # if they do we at least surface the fact via
              # telemetry so a lazy-cloned repo that's missing
              # refs isn't a silent mystery.
              :telemetry.execute(
                [:exgit, :ref_store, :write_failed],
                %{count: 1},
                %{ref: name, reason: reason, context: :clone_partial}
              )

              rs
          end
        end)

      repo = Repository.new(promisor, ref_store, mode: :lazy)

      with {:ok, repo} <- set_head_to_default(repo, refs, meta) do
        maybe_eager_prefetch(repo, refs, meta, filter_spec)
      end
    end
  end

  # All ref reads from a transport MUST go through this wrapper. Ref
  # names from the wire are hostile input; see Exgit.RefName for the
  # validation rules and `[:exgit, :security, :ref_rejected]` telemetry.
  #
  # Transports are responsible for applying `Exgit.RefName.valid?/1`
  # to their output; this wrapper applies the check again as
  # defense-in-depth for third-party transports that forgot. The
  # return shape mirrors `Transport.ls_refs/2` exactly:
  # `{:ok, refs, meta}`.
  defp safe_ls_refs(transport, opts) do
    case Transport.ls_refs(transport, opts) do
      {:ok, refs, meta} ->
        {:ok, Enum.filter(refs, &keep_ref?(&1, transport)), meta}

      other ->
        other
    end
  end

  defp keep_ref?({ref, _sha}, transport) do
    if Exgit.RefName.valid?(ref) do
      true
    else
      :telemetry.execute(
        [:exgit, :security, :ref_rejected],
        %{count: 1},
        %{source: describe_transport(transport), ref: ref}
      )

      false
    end
  end

  defp describe_transport(%{url: url}), do: url
  defp describe_transport(%{path: path}), do: path
  defp describe_transport(other), do: inspect(other.__struct__)

  # Translate a user-facing filter option into a wire string (or :none).
  defp resolve_filter(opts) do
    case Keyword.get(opts, :filter, :none) do
      :none ->
        {:ok, :none}

      spec ->
        case Exgit.Filter.encode(spec) do
          {:ok, wire} -> {:ok, wire}
          {:error, _} = err -> err
        end
    end
  end

  # Probe the server's advertised fetch capabilities for "filter" support.
  # When the user asked for a filter but the server doesn't support it,
  # error unless `if_unsupported: :ignore` is set.
  defp check_filter_capability(_transport, :none, _opts), do: :ok

  defp check_filter_capability(transport, _wire, opts) do
    case Transport.capabilities(transport) do
      {:ok, caps} ->
        fetch_caps = String.split(Map.get(caps, "fetch", ""), " ", trim: true)

        cond do
          "filter" in fetch_caps ->
            :ok

          Keyword.get(opts, :if_unsupported) == :ignore ->
            :ok

          true ->
            {:error, {:filter_unsupported, fetch_caps}}
        end

      _ ->
        :ok
    end
  end

  # Options the Promisor should send with every on-demand fetch. Blobs
  # that come back because of a follow-up `want <blob_sha>` shouldn't
  # themselves be filtered; leave the filter off the per-object fetch.
  defp promisor_fetch_opts(:none), do: []
  defp promisor_fetch_opts(_wire), do: []

  # When a filter is in effect, pull the commits+trees pack eagerly at
  # clone time. Without a filter, a lazy clone stays fast (refs only).
  defp maybe_eager_prefetch(repo, _refs, _meta, :none), do: {:ok, repo}

  defp maybe_eager_prefetch(repo, refs, meta, wire) do
    case find_default_ref(refs, meta) do
      nil ->
        {:ok, repo}

      {_name, commit_sha} ->
        # Promisor.fetch_with_filter/3 triggers the tree-only fetch and
        # imports everything into the cache.
        case ObjectStore.Promisor.fetch_with_filter(repo.object_store, [commit_sha], filter: wire) do
          {:ok, new_store} ->
            {:ok, %{repo | object_store: new_store}}

          {:error, _} = err ->
            err
        end
    end
  end

  defp set_head_to_default(repo, refs, meta) do
    case find_default_ref(refs, meta) do
      nil ->
        {:ok, repo}

      {ref_name, sha} ->
        with {:ok, ref_store} <-
               RefStore.write(repo.ref_store, "HEAD", {:symbolic, ref_name}, []),
             {:ok, ref_store} <- RefStore.write(ref_store, ref_name, sha, []) do
          {:ok, %{repo | ref_store: ref_store}}
        end
    end
  end

  @doc """
  Fetch updates from a remote into an existing repository.

      {:ok, repo} = Exgit.fetch(repo, "https://github.com/user/repo")

  Lists the remote's refs, fetches any objects reachable from them,
  and records remote-tracking refs under `refs/remotes/<remote>/`
  (for branches) and `refs/tags/` (for tags). Local branches and
  `HEAD` are never moved.

  ## Options

    * `:remote` — remote name to record tracking refs under.
      Default: `"origin"`.

    * `:prefix` — ref prefixes to list on the remote.
      Default: `["refs/heads/", "refs/tags/"]`.

  ## Returns

  `{:ok, %Repository{}}` with the updated object and ref stores, or
  `{:error, reason}` when listing refs or fetching the pack fails.
  """
  @spec fetch(Repository.t(), remote(), keyword()) ::
          {:ok, Repository.t()} | {:error, term()}
  def fetch(%Repository{} = repo, source, opts \\ []) do
    transport = to_transport(source, opts)
    remote_name = Keyword.get(opts, :remote, "origin")
    prefix = Keyword.get(opts, :prefix, ["refs/heads/", "refs/tags/"])

    with {:ok, refs, _meta} <- safe_ls_refs(transport, prefix: prefix) do
      fetch_into(repo, transport, refs, Keyword.put(opts, :remote, remote_name))
    end
  end

  @doc """
  Push local refs to a remote.

  The `:refspecs` keyword takes a list where each entry is one of:

    * `"refs/heads/main"` — a ref name; pushes the local sha for that
      ref to the same name on the remote (creating it if absent).
    * `{:delete, "refs/heads/branch"}` — delete the ref on the remote.

  When `:refspecs` is omitted, the repo's current branch — `HEAD`'s
  symref target — is pushed. If `HEAD` is not a symbolic ref (e.g.
  detached or unset), the push fails with `{:error, :no_refspecs}`.

  ## Errors

    * `{:error, :no_refspecs}` — `:refspecs` omitted and `HEAD`
      doesn't point at a branch.
    * `{:error, {:local_ref_not_found, name}}` — a named refspec
      doesn't resolve in the local ref store. The whole push fails;
      nothing is sent to the remote. (Deleting a ref the remote
      doesn't have remains a silent no-op.)

  Returns `{:ok, %{ref_results: [...]}}` on success.
  """
  @spec push(Repository.t(), remote(), keyword()) ::
          {:ok, map()} | {:error, term()}
  def push(%Repository{} = repo, dest, opts \\ []) do
    transport = to_transport(dest, opts)

    with {:ok, refspecs} <- push_refspecs(repo, opts) do
      ref_names = Enum.map(refspecs, &refspec_ref_name/1)
      remote_refs = load_remote_refs_for_push(transport, ref_names)

      case plan_push_updates(refspecs, repo, remote_refs) do
        {:ok, [], _objects} ->
          {:ok, %{ref_results: []}}

        {:ok, updates, objects} ->
          pack = build_push_pack(updates, objects)
          Transport.push(transport, Enum.reverse(updates), pack, [])

        {:error, _} = err ->
          err
      end
    end
  end

  # When the caller doesn't name refspecs, push the current branch —
  # HEAD's symref target. A detached or unset HEAD leaves nothing
  # sensible to push, so the caller must name refspecs explicitly.
  defp push_refspecs(repo, opts) do
    case Keyword.fetch(opts, :refspecs) do
      {:ok, refspecs} ->
        {:ok, refspecs}

      :error ->
        case RefStore.read(repo.ref_store, "HEAD") do
          {:ok, {:symbolic, ref_name}} -> {:ok, [ref_name]}
          _ -> {:error, :no_refspecs}
        end
    end
  end

  defp refspec_ref_name({:delete, name}), do: name
  defp refspec_ref_name(name) when is_binary(name), do: name

  defp load_remote_refs_for_push(transport, ref_names) do
    case safe_ls_refs(transport, prefix: ref_names) do
      {:ok, refs, _meta} -> Map.new(refs)
      _ -> %{}
    end
  end

  defp plan_push_updates(refspecs, repo, remote_refs) do
    Enum.reduce_while(refspecs, {:ok, [], []}, fn refspec, {:ok, upd, objs} ->
      case plan_push(refspec, repo, remote_refs) do
        {:update, ref_name, old_sha, new_sha, sha_for_objects} ->
          new_objs =
            if sha_for_objects,
              do: collect_push_objects(repo.object_store, sha_for_objects, remote_refs),
              else: []

          {:cont, {:ok, [{ref_name, old_sha, new_sha} | upd], objs ++ new_objs}}

        :skip ->
          {:cont, {:ok, upd, objs}}

        {:error, _} = err ->
          {:halt, err}
      end
    end)
  end

  # A receive-pack request body ends with the pkt-line flush followed
  # by the pack bytes. An empty-objects list is legal for:
  #
  #   (a) pure ref-delete pushes — no new objects needed
  #   (b) pure ref-move pushes where every commit referenced is
  #       already present on the remote (e.g. fast-forward to a
  #       commit the remote learned about via another branch)
  #
  # Most git servers accept an empty body after the flush in case
  # (a), but some reject a completely-missing PACK header in case
  # (b). Emit an empty PACK (header + 0 objects + SHA trailer) when
  # we have updates that aren't all deletes — this is the shape
  # `git send-pack` produces for the same case and is accepted by
  # every modern server we've tested.
  defp build_push_pack(_updates, [_ | _] = objects), do: Pack.Writer.build(objects)

  defp build_push_pack(updates, []) do
    if all_deletes?(updates), do: <<>>, else: Pack.Writer.build([])
  end

  defp all_deletes?(updates), do: Enum.all?(updates, fn {_ref, _old, new} -> new == nil end)

  # Translate one user-supplied refspec entry into a concrete update.
  defp plan_push({:delete, ref_name}, _repo, remote_refs) when is_binary(ref_name) do
    case Map.fetch(remote_refs, ref_name) do
      {:ok, old_sha} -> {:update, ref_name, old_sha, nil, nil}
      # Nothing to delete; silently skip so deleting a missing ref is a
      # no-op rather than an error.
      :error -> :skip
    end
  end

  defp plan_push(ref_name, repo, remote_refs) when is_binary(ref_name) do
    case RefStore.resolve(repo.ref_store, ref_name) do
      {:ok, sha} ->
        old_sha = Map.get(remote_refs, ref_name)
        {:update, ref_name, old_sha, sha, sha}

      _ ->
        {:error, {:local_ref_not_found, ref_name}}
    end
  end

  # --- Internal ---

  # Accepts any struct that implements the Exgit.Transport protocol, or a
  # URL string that we can map to the built-in File/HTTP transports.
  defp to_transport(url, opts) when is_binary(url) do
    if String.starts_with?(url, "file://") do
      path = String.replace_prefix(url, "file://", "")
      Transport.File.new(path)
    else
      Transport.HTTP.new(url, opts)
    end
  end

  defp to_transport(transport, _opts) when is_struct(transport) do
    # Any struct implementing the Transport protocol is fine as-is.
    transport
  end

  defp init_disk(path) do
    with :ok <- File.mkdir_p(path),
         :ok <- File.mkdir_p(Path.join(path, "objects")),
         :ok <- File.mkdir_p(Path.join(path, "objects/info")),
         :ok <- File.mkdir_p(Path.join(path, "objects/pack")),
         :ok <- File.mkdir_p(Path.join(path, "refs")),
         :ok <- File.mkdir_p(Path.join(path, "refs/heads")),
         :ok <- File.mkdir_p(Path.join(path, "refs/tags")),
         :ok <- File.write(Path.join(path, "HEAD"), "ref: refs/heads/main\n"),
         config <- bare_config(),
         :ok <- Config.write(config, Path.join(path, "config")) do
      {:ok,
       Repository.new(
         ObjectStore.Disk.new(path),
         RefStore.Disk.new(path),
         config: config,
         path: path
       )}
    else
      {:error, reason} -> {:error, {:init_failed, reason}}
    end
  end

  defp fetch_into(repo, transport, refs, opts) do
    remote_name = Keyword.get(opts, :remote, "origin")
    wants = for {_ref, sha} <- refs, sha != nil, do: sha
    wants = Enum.uniq(wants)

    if wants == [] do
      {:ok, repo}
    else
      # Pass the object store so HTTP transport can stream pack bytes
      # directly into it via Pack.StreamParser, avoiding the O(pack_size)
      # intermediate binary and the full-pack-in-memory parse.
      fetch_opts = Keyword.put_new(opts, :object_store, repo.object_store)

      case Transport.fetch(transport, wants, fetch_opts) do
        # Streaming path: objects already written; summary carries the
        # updated store (required for value-typed stores like Memory).
        {:ok, <<>>, %{store: updated_store}} ->
          repo = %{repo | object_store: updated_store}
          update_remote_refs(repo, refs, remote_name)

        # Non-streaming path (e.g. file transport, or empty response).
        {:ok, pack_data, _summary} when byte_size(pack_data) > 0 ->
          with {:ok, repo} <- unpack_into(repo, pack_data) do
            update_remote_refs(repo, refs, remote_name)
          end

        {:ok, _, _} ->
          {:ok, repo}

        error ->
          error
      end
    end
  end

  defp unpack_into(repo, pack_data) do
    case Pack.Reader.parse(pack_data) do
      {:ok, objects} ->
        {:ok, object_store} = ObjectStore.import_objects(repo.object_store, objects)
        {:ok, %{repo | object_store: object_store}}

      error ->
        error
    end
  end

  defp update_remote_refs(repo, refs, remote_name) do
    ref_store =
      Enum.reduce(refs, repo.ref_store, fn {ref, sha}, ref_store ->
        if sha == nil do
          ref_store
        else
          remote_ref =
            cond do
              String.starts_with?(ref, "refs/heads/") ->
                branch = String.replace_prefix(ref, "refs/heads/", "")
                "refs/remotes/#{remote_name}/#{branch}"

              String.starts_with?(ref, "refs/tags/") ->
                ref

              true ->
                nil
            end

          if remote_ref do
            case RefStore.write(ref_store, remote_ref, sha, []) do
              {:ok, rs} ->
                rs

              # Preserve previous store on write failure; propagate err via
              # reduce? Keeping the reduce-to-store structure means we
              # silently skip failed writes. That mirrors real-world git
              # which treats remote-ref writes as best-effort — but we log.
              {:error, reason} ->
                :logger.warning(
                  "exgit: failed to write remote ref #{remote_ref}: #{inspect(reason)}"
                )

                ref_store
            end
          else
            ref_store
          end
        end
      end)

    {:ok, %{repo | ref_store: ref_store}}
  end

  # Pick the default ref to point HEAD at. Prefer the server's own
  # HEAD symref target (surfaced via `meta.head` from the
  # protocol-v2 `symrefs` argument). Falls back to
  # `refs/heads/main` / `refs/heads/master` / any `refs/heads/*`
  # only when the server didn't advertise HEAD's target.
  # Previously we always guessed — that meant consecutive clones of
  # a repo whose default was neither `main` nor `master` could land
  # on different branches depending on wire-order.
  defp find_default_ref(refs, meta) do
    case Map.get(meta, :head) do
      nil ->
        fallback_default_ref(refs)

      target ->
        case Enum.find(refs, fn {ref, _} -> ref == target end) do
          nil -> fallback_default_ref(refs)
          entry -> entry
        end
    end
  end

  defp fallback_default_ref(refs) do
    heads =
      Enum.filter(refs, fn {ref, _} -> String.starts_with?(ref, "refs/heads/") end)

    Enum.find(heads, fn {ref, _} -> ref == "refs/heads/main" end) ||
      Enum.find(heads, fn {ref, _} -> ref == "refs/heads/master" end) ||
      List.first(heads)
  end

  defp collect_push_objects(store, sha, remote_refs) do
    remote_shas = Map.new(Map.values(remote_refs), &{&1, true})
    collect_reachable(store, [sha], remote_shas)
  end

  # Iterative (non-recursive) reachability walk. Uses a single shared
  # `seen` accumulator so shared subtrees are visited exactly once, and
  # is bounded by O(heap) rather than O(stack) so it handles deep
  # histories (millions of commits) without stack overflow.
  # `seen` is a plain map used as a membership set (`sha => true`) rather
  # than a `MapSet`. MapSet is an opaque type, and threading it through this
  # self-recursive accumulator — whose base clause binds `_seen` as a
  # wildcard — trips a Dialyzer opacity false-positive. A plain map is
  # semantically identical here (set of seen shas), not opaque, and a touch
  # faster.
  @typep seen_set :: %{optional(binary()) => true}

  @spec collect_reachable(ObjectStore.t(), [binary()], seen_set()) :: [Exgit.Object.t()]
  defp collect_reachable(store, initial_shas, seen) do
    do_collect_reachable(store, initial_shas, seen, [])
  end

  @spec do_collect_reachable(ObjectStore.t(), [binary()], seen_set(), [Exgit.Object.t()]) ::
          [Exgit.Object.t()]
  defp do_collect_reachable(_store, [], _seen, acc), do: Enum.reverse(acc)

  defp do_collect_reachable(store, [sha | rest], seen, acc) do
    if Map.has_key?(seen, sha) do
      do_collect_reachable(store, rest, seen, acc)
    else
      seen = Map.put(seen, sha, true)

      case ObjectStore.get(store, sha) do
        {:ok, obj} ->
          children = object_children(obj)
          do_collect_reachable(store, children ++ rest, seen, [obj | acc])

        _ ->
          do_collect_reachable(store, rest, seen, acc)
      end
    end
  end

  defp object_children(%Exgit.Object.Commit{} = c),
    do: [Exgit.Object.Commit.tree(c) | Exgit.Object.Commit.parents(c)]

  defp object_children(%Exgit.Object.Tree{entries: entries}), do: Enum.map(entries, &elem(&1, 2))
  defp object_children(%Exgit.Object.Tag{object: sha}), do: [sha]
  defp object_children(%Exgit.Object.Blob{}), do: []

  defp bare_config do
    Config.new()
    |> Config.set("core", nil, "repositoryformatversion", "0")
    |> Config.set("core", nil, "filemode", "true")
    |> Config.set("core", nil, "bare", "true")
  end
end