1 files changed, 80 insertions, 0 deletions
diff --git a/helix-vcs/src/git.rs b/helix-vcs/src/git.rs
new file mode 100644
index 00000000..82b2b558
--- /dev/null
+++ b/helix-vcs/src/git.rs
@@ -0,0 +1,80 @@
+use std::path::Path;
+
+use git::objs::tree::EntryMode;
+use git::sec::trust::DefaultForLevel;
+use git::{Commit, ObjectId, Repository, ThreadSafeRepository};
+use git_repository as git;
+
+use crate::DiffProvider;
+
+#[cfg(test)]
+mod test;
+
+pub struct Git;
+
+impl Git {
+    fn open_repo(path: &Path, ceiling_dir: Option<&Path>) -> Option<ThreadSafeRepository> {
+        // custom open options
+        let mut git_open_opts_map = git::sec::trust::Mapping::<git::open::Options>::default();
+
+        // don't use the global git configs (not needed)
+        let config = git::permissions::Config {
+            system: false,
+            git: false,
+            user: false,
+            env: true,
+            includes: true,
+            git_binary: false,
+        };
+        // change options for config permissions without touching anything else
+        git_open_opts_map.reduced = git_open_opts_map.reduced.permissions(git::Permissions {
+            config,
+            ..git::Permissions::default_for_level(git::sec::Trust::Reduced)
+        });
+        git_open_opts_map.full = git_open_opts_map.full.permissions(git::Permissions {
+            config,
+            ..git::Permissions::default_for_level(git::sec::Trust::Full)
+        });
+
+        let mut open_options = git::discover::upwards::Options::default();
+        if let Some(ceiling_dir) = ceiling_dir {
+            open_options.ceiling_dirs = vec![ceiling_dir.to_owned()];
+        }
+
+        ThreadSafeRepository::discover_with_environment_overrides_opts(
+            path,
+            open_options,
+            git_open_opts_map,
+        )
+        .ok()
+    }
+}
+
+impl DiffProvider for Git {
+    fn get_diff_base(&self, file: &Path) -> Option<Vec<u8>> {
+        debug_assert!(!file.exists() || file.is_file());
+        debug_assert!(file.is_absolute());
+
+        // TODO cache repository lookup
+        let repo = Git::open_repo(file.parent()?, None)?.to_thread_local();
+        let head = repo.head_commit().ok()?;
+        let file_oid = find_file_in_commit(&repo, &head, file)?;
+
+        let file_object = repo.find_object(file_oid).ok()?;
+        Some(file_object.detach().data)
+    }
+}
+
+/// Finds the object that contains the contents of a file at a specific commit.
+fn find_file_in_commit(repo: &Repository, commit: &Commit, file: &Path) -> Option<ObjectId> {
+    let repo_dir = repo.work_dir()?;
+    let rel_path = file.strip_prefix(repo_dir).ok()?;
+    let tree = commit.tree().ok()?;
+    let tree_entry = tree.lookup_entry_by_path(rel_path).ok()??;
+    match tree_entry.mode() {
+        // not a file, everything is new, do not show diff
+        EntryMode::Tree | EntryMode::Commit | EntryMode::Link => None,
+        // found a file
+        EntryMode::Blob | EntryMode::BlobExecutable => Some(tree_entry.object_id()),
+    }
+}