Git Commit-Graph Format: Additional Chunks and Historical Notes

with corrected commit date offsets that cannot be stored within 31 bits. * Generation Data Overflow chunk is present only when Generation Data chunk is present and at least one corrected commit date offset cannot be stored within 31 bits. ==== 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 support value 2 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". Version 1 Bloom filters have a bug that appears when char is signed and the repository has path names that have characters >= 0x80; Git supports reading and writing them, but this ability will be removed in a future version of Git. - 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 one, with either all bits set to zero or one respectively. * 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. == Historical Notes: The Generation Data (GDA2) and Generation Data Overflow (GDO2) chunks have the number '2' in their chunk IDs because a previous version of Git wrote possibly erroneous data in these chunks with the IDs "GDAT" and "GDOV". By changing the IDs, newer versions of Git will silently ignore those older chunks and write the new information without trusting the incorrect data. GIT --- Part of the linkgit:git[1] suite

The Git commit-graph format includes additional chunks such as Extra Edge List, Bloom Filter Index, Bloom Filter Data, and Base Graphs List, which store information about commit parents, Bloom filters, and commit-graph chains, and also notes the historical context of certain chunk IDs to ensure compatibility and data integrity across different Git versions.