commit-graph: Support lookups of entries in a commit-graph

This change introduces `git_commit_graph_entry_find()` and
`git_commit_graph_entry_parent()`. These two functions allow a much
faster lookup of commits by ID, since the ODB does not need to be
consulted, the commit object does not need to be inflated, and the
contents of the commit object do not need to be parsed.

Part of: #5757

1 files changed, 41 insertions, 0 deletions

diff --git a/src/commit_graph.h b/src/commit_graph.h
index 01512d76f..f3a431705 100644
--- a/src/commit_graph.h
+++ b/src/commit_graph.h
@@ -57,7 +57,48 @@ typedef struct git_commit_graph_file {
 	git_buf filename;
 } git_commit_graph_file;
 
+/**
+ * An entry in the commit-graph file. Provides a subset of the information that
+ * can be obtained from the commit header.
+ */
+typedef struct git_commit_graph_entry {
+	/* The generation number of the commit within the graph */
+	size_t generation;
+
+	/* Time in seconds from UNIX epoch. */
+	git_time_t commit_time;
+
+	/* The number of parents of the commit. */
+	size_t parent_count;
+
+	/*
+	 * The indices of the parent commits within the Commit Data table. The value
+	 * of `GIT_COMMIT_GRAPH_MISSING_PARENT` indicates that no parent is in that
+	 * position.
+	 */
+	size_t parent_indices[2];
+
+	/* The index within the Extra Edge List of any parent after the first two. */
+	size_t extra_parents_index;
+
+	/* The SHA-1 hash of the root tree of the commit. */
+	git_oid tree_oid;
+
+	/* The SHA-1 hash of the requested commit. */
+	git_oid sha1;
+} git_commit_graph_entry;
+
 int git_commit_graph_open(git_commit_graph_file **cgraph_out, const char *path);
+int git_commit_graph_entry_find(
+		git_commit_graph_entry *e,
+		const git_commit_graph_file *cgraph,
+		const git_oid *short_oid,
+		size_t len);
+int git_commit_graph_entry_parent(
+		git_commit_graph_entry *parent,
+		const git_commit_graph_file *cgraph,
+		const git_commit_graph_entry *entry,
+		size_t n);
 int git_commit_graph_close(git_commit_graph_file *cgraph);
 void git_commit_graph_free(git_commit_graph_file *cgraph);