Git commit graph format======================= The Git commit graph stores a list of commit OIDs and some associatedmetadata, 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. - The Bloom filter of the commit carrying the paths that were changed between  the commit and its first parent, if requested. These positional references are stored as unsigned 32-bit integerscorresponding to the array position within the list of commit OIDs. Dueto 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 organizethe body into "chunks" and provide a binary lookup table at the beginningof the body. The header includes certain values, such as number of chunksand 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 number (B) of base commit-graphs      We infer the length (H*B) of the Base Graphs chunk      from 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.   Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]    * The ith entry, BIDX[i], stores the number of bytes in all Bloom filters      from commit 0 to commit i (inclusive) in lexicographic order. The Bloom      filter for the i-th commit spans from BIDX[i-1] to BIDX[i] (plus header      length), where BIDX[-1] is 0.    * The BIDX chunk is ignored if the BDAT chunk is not present.   Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]    * It starts with header consisting of three unsigned 32-bit integers:      - Version of the hash algorithm being used. We currently only support	value 1 which corresponds to the 32-bit version of the murmur3 hash	implemented exactly as described in	https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double	hashing technique using seed values 0x293ae76f and 0x7e646e2 as	described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters	in Probabilistic Verification"      - The number of times a path is hashed and hence the number of bit positions	      that cumulatively determine whether a file is present in the commit.      - The minimum number of bits 'b' per entry in the Bloom filter. If the filter	      contains 'n' entries, then the filter size is the minimum number of 64-bit	      words that contain n*b bits.    * The rest of the chunk is the concatenation of all the computed Bloom      filters for the commits in lexicographic order.    * Note: Commits with no changes or more than 512 changes have Bloom filters      of length zero.    * The BDAT chunk is present if and only if BIDX is present.   Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional]      This list of H-byte hashes describe a set of B commit-graph files that      form a commit-graph chain. The graph position for the ith commit in this      file's OID Lookup chunk is equal to i plus the number of commits in all      base graphs.  If B is non-zero, this chunk must exist. TRAILER: 	H-byte HASH-checksum of all of the above.