4 files changed, 110 insertions, 6 deletions
diff --git a/plumbing/format/commitgraph/commitgraph_test.go b/plumbing/format/commitgraph/commitgraph_test.go
index 0e38707..0214f49 100644
--- a/plumbing/format/commitgraph/commitgraph_test.go
+++ b/plumbing/format/commitgraph/commitgraph_test.go
@@ -6,10 +6,8 @@ import (
 	"path"
 	"testing"
 
-	"golang.org/x/exp/mmap"
-
 	. "gopkg.in/check.v1"
-	"gopkg.in/src-d/go-git-fixtures.v3"
+	fixtures "gopkg.in/src-d/go-git-fixtures.v3"
 	"gopkg.in/src-d/go-git.v4/plumbing"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/commitgraph"
 )
@@ -23,7 +21,7 @@ type CommitgraphSuite struct {
 var _ = Suite(&CommitgraphSuite{})
 
 func testDecodeHelper(c *C, path string) {
-	reader, err := mmap.Open(path)
+	reader, err := os.Open(path)
 	c.Assert(err, IsNil)
 	defer reader.Close()
 	index, err := commitgraph.OpenFileIndex(reader)
@@ -85,7 +83,7 @@ func (s *CommitgraphSuite) TestReencode(c *C) {
 	fixtures.ByTag("commit-graph").Test(c, func(f *fixtures.Fixture) {
 		dotgit := f.DotGit()
 
-		reader, err := mmap.Open(path.Join(dotgit.Root(), "objects", "info", "commit-graph"))
+		reader, err := os.Open(path.Join(dotgit.Root(), "objects", "info", "commit-graph"))
 		c.Assert(err, IsNil)
 		defer reader.Close()
 		index, err := commitgraph.OpenFileIndex(reader)
@@ -108,7 +106,7 @@ func (s *CommitgraphSuite) TestReencodeInMemory(c *C) {
 	fixtures.ByTag("commit-graph").Test(c, func(f *fixtures.Fixture) {
 		dotgit := f.DotGit()
 
-		reader, err := mmap.Open(path.Join(dotgit.Root(), "objects", "info", "commit-graph"))
+		reader, err := os.Open(path.Join(dotgit.Root(), "objects", "info", "commit-graph"))
 		c.Assert(err, IsNil)
 		index, err := commitgraph.OpenFileIndex(reader)
 		c.Assert(err, IsNil)
diff --git a/plumbing/format/commitgraph/doc.go b/plumbing/format/commitgraph/doc.go
new file mode 100644
index 0000000..41cd8b1
--- /dev/null
+++ b/plumbing/format/commitgraph/doc.go
@@ -0,0 +1,103 @@
+// Package commitgraph implements encoding and decoding of commit-graph files.
+//
+// Git commit graph format
+// =======================
+//
+// The Git commit graph stores a list of commit OIDs and some associated
+// metadata, including:
+//
+// - The generation number of the commit. Commits with no parents have
+//   generation number 1; commits with parents have generation number
+//   one more than the maximum generation number of its parents. We
+//   reserve zero as special, and can be used to mark a generation
+//   number invalid or as "not computed".
+//
+// - The root tree OID.
+//
+// - The commit date.
+//
+// - The parents of the commit, stored using positional references within
+//   the graph file.
+//
+// These positional references are stored as unsigned 32-bit integers
+// corresponding to the array position within the list of commit OIDs. Due
+// to some special constants we use to track parents, we can store at most
+// (1 << 30) + (1 << 29) + (1 << 28) - 1 (around 1.8 billion) commits.
+//
+// == Commit graph files have the following format:
+//
+// In order to allow extensions that add extra data to the graph, we organize
+// the body into "chunks" and provide a binary lookup table at the beginning
+// of the body. The header includes certain values, such as number of chunks
+// and hash type.
+//
+// All 4-byte numbers are in network order.
+//
+// HEADER:
+//
+//   4-byte signature:
+//       The signature is: {'C', 'G', 'P', 'H'}
+//
+//   1-byte version number:
+//       Currently, the only valid version is 1.
+//
+//   1-byte Hash Version (1 = SHA-1)
+//       We infer the hash length (H) from this value.
+//
+//   1-byte number (C) of "chunks"
+//
+//   1-byte (reserved for later use)
+//      Current clients should ignore this value.
+//
+// CHUNK LOOKUP:
+//
+//   (C + 1) * 12 bytes listing the table of contents for the chunks:
+//       First 4 bytes describe the chunk id. Value 0 is a terminating label.
+//       Other 8 bytes provide the byte-offset in current file for chunk to
+//       start. (Chunks are ordered contiguously in the file, so you can infer
+//       the length using the next chunk position if necessary.) Each chunk
+//       ID appears at most once.
+//
+//   The remaining data in the body is described one chunk at a time, and
+//   these chunks may be given in any order. Chunks are required unless
+//   otherwise specified.
+//
+// CHUNK DATA:
+//
+//   OID Fanout (ID: {'O', 'I', 'D', 'F'}) (256 * 4 bytes)
+//       The ith entry, F[i], stores the number of OIDs with first
+//       byte at most i. Thus F[255] stores the total
+//       number of commits (N).
+//
+//   OID Lookup (ID: {'O', 'I', 'D', 'L'}) (N * H bytes)
+//       The OIDs for all commits in the graph, sorted in ascending order.
+//
+//   Commit Data (ID: {'C', 'D', 'A', 'T' }) (N * (H + 16) bytes)
+//     * The first H bytes are for the OID of the root tree.
+//     * The next 8 bytes are for the positions of the first two parents
+//       of the ith commit. Stores value 0x7000000 if no parent in that
+//       position. If there are more than two parents, the second value
+//       has its most-significant bit on and the other bits store an array
+//       position into the Extra Edge List chunk.
+//     * The next 8 bytes store the generation number of the commit and
+//       the commit time in seconds since EPOCH. The generation number
+//       uses the higher 30 bits of the first 4 bytes, while the commit
+//       time uses the 32 bits of the second 4 bytes, along with the lowest
+//       2 bits of the lowest byte, storing the 33rd and 34th bit of the
+//       commit time.
+//
+//   Extra Edge List (ID: {'E', 'D', 'G', 'E'}) [Optional]
+//       This list of 4-byte values store the second through nth parents for
+//       all octopus merges. The second parent value in the commit data stores
+//       an array position within this list along with the most-significant bit
+//       on. Starting at that array position, iterate through this list of commit
+//       positions for the parents until reaching a value with the most-significant
+//       bit on. The other bits correspond to the position of the last parent.
+//
+// TRAILER:
+//
+// 	H-byte HASH-checksum of all of the above.
+//
+// Source:
+// https://raw.githubusercontent.com/git/git/master/Documentation/technical/commit-graph-format.txt
+package commitgraph
diff --git a/plumbing/format/commitgraph/encoder.go b/plumbing/format/commitgraph/encoder.go
index 648153f..a06871c 100644
--- a/plumbing/format/commitgraph/encoder.go
+++ b/plumbing/format/commitgraph/encoder.go
@@ -22,6 +22,7 @@ func NewEncoder(w io.Writer) *Encoder {
 	return &Encoder{mw, h}
 }
 
+// Encode writes an index into the commit-graph file
 func (e *Encoder) Encode(idx Index) error {
 	var err error
 
diff --git a/plumbing/format/commitgraph/memory.go b/plumbing/format/commitgraph/memory.go
index f084b85..a4a96e9 100644
--- a/plumbing/format/commitgraph/memory.go
+++ b/plumbing/format/commitgraph/memory.go
@@ -4,6 +4,8 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing"
 )
 
+// MemoryIndex provides a way to build the commit-graph in memory
+// for later encoding to file.
 type MemoryIndex struct {
 	commitData []*CommitData
 	indexMap   map[plumbing.Hash]int