plumbing: commitgraph, fix types and handle commit-graph-chains

Unfortunately the original variant makes some incorrect typing
assumptions about commit-graphs which make handling graph chains
difficult to do correctly. This creates a new subpackage and deprecates
the old one. It then adds support commit graph chains.

Signed-off-by: Andrew Thornton <art27@cantab.net>

1 files changed, 48 insertions, 0 deletions

diff --git a/plumbing/format/commitgraph/v2/chunk.go b/plumbing/format/commitgraph/v2/chunk.go
new file mode 100644
index 0000000..ab24320
--- /dev/null
+++ b/plumbing/format/commitgraph/v2/chunk.go
@@ -0,0 +1,48 @@
+package v2
+
+import "bytes"
+
+const (
+	chunkSigLen    = 4 // Length of a chunk signature
+	chunkSigOffset = 4 // Offset of each chunk signature in chunkSignatures
+)
+
+// chunkSignatures contains the coalesced byte signatures for each chunk type.
+// The order of the signatures must match the order of the ChunkType constants.
+// (When adding new chunk types you must avoid introducing ambiguity, and you may need to add padding separators to this list or reorder these signatures.)
+// (i.e. it would not be possible to add a new chunk type with the signature "IDFO" without some reordering or the addition of separators.)
+var chunkSignatures = []byte("OIDFOIDLCDATGDA2GDO2EDGEBIDXBDATBASE\000\000\000\000")
+
+// ChunkType represents the type of a chunk in the commit graph file.
+type ChunkType int
+
+const (
+	OIDFanoutChunk              ChunkType = iota // "OIDF"
+	OIDLookupChunk                               // "OIDL"
+	CommitDataChunk                              // "CDAT"
+	GenerationDataChunk                          // "GDA2"
+	GenerationDataOverflowChunk                  // "GDO2"
+	ExtraEdgeListChunk                           // "EDGE"
+	BloomFilterIndexChunk                        // "BIDX"
+	BloomFilterDataChunk                         // "BDAT"
+	BaseGraphsListChunk                          // "BASE"
+	ZeroChunk                                    // "\000\000\000\000"
+)
+
+// Signature returns the byte signature for the chunk type.
+func (ct ChunkType) Signature() []byte {
+	if ct >= BaseGraphsListChunk || ct < 0 { // not a valid chunk type just return ZeroChunk
+		return chunkSignatures[ZeroChunk*chunkSigOffset : ZeroChunk*chunkSigOffset+chunkSigLen]
+	}
+
+	return chunkSignatures[ct*chunkSigOffset : ct*chunkSigOffset+chunkSigLen]
+}
+
+// ChunkTypeFromBytes returns the chunk type for the given byte signature.
+func ChunkTypeFromBytes(b []byte) (ChunkType, bool) {
+	idx := bytes.Index(chunkSignatures, b)
+	if idx == -1 || idx%chunkSigOffset != 0 { // not found, or not aligned at chunkSigOffset
+		return -1, false
+	}
+	return ChunkType(idx / chunkSigOffset), true
+}