0-4-1-understand-commit-object

name	0.4.1-understand-commit_object
description	[0.4.1] Type-3 git object containing tree SHA, parent SHA(s), author, committer, timestamp, and message; forms the his

understand-commit_object

CALL NUMBER: git_internals_and_the_object_model.commit_object : deep_commit_object(8), deep_tree_object(8) DEFINITION: Type-3 git object containing tree SHA, parent SHA(s), author, committer, timestamp, and message; forms the history DAG backbone.

Invoke this skill to understand commit_object down to its primitives. The RELATIVE ROOT below is the least-fixed-point closure of everything it bundles from — the full import cone, grouped by the lib each prim comes from. Projected from a prover-typed KB (MAP/SWI-Prolog consistency gate): every reference below resolves.

THE RELATIVE ROOT (the import cone, by lib)

from deep_commit_object

• commit_message_encoding_encoding_header (d1): Optional header line 'encoding ' in a commit object's raw text; git stores whatever the user wrote verbatim.
• commit_message_encoding_encoding_value (d2): The character encoding name stored in the encoding header (UTF-8, ISO-8859-1, Windows-1252, etc.); controls byte-to-character interpretation of the message bytes.
• commit_message_encoding_message_bytes (d3): Raw byte sequence forming the commit message body; decoded using encoding_value (or assumed UTF-8) to produce display characters.
• commit_message_encoding_original_encoding (d3): Alias for encoding_value; the encoding recorded at commit time distinguishing the message's original byte interpretation from the display encoding.
• commit_message_encoding_transcoding_logic (d3): git's internal conversion from the stored encoding_value to the requested log_encoding_flag encoding; uses iconv-style conversion with loss handling.
• commit_message_encoding_encoding_fallback (d4): Fallback behavior when transcoding_logic cannot map a character; typically replaces with a substitution character or skips the byte.
• commit_message_encoding_transcoded_message (d4): The output of transcoding_logic: message_bytes decoded then re-encoded into log_encoding_flag; git log emits this to the terminal.
• commit_message_encoding_display_encoding (d5): The terminal or output stream encoding git uses when writing the transcoded_message; git assumes UTF-8 output if terminal encoding is unset.

from deep_tree_object

• executable_file_mode (d3): Mode 100755; regular file with execute bits set; Git marks it this way to preserve the executable bit across platforms that handle it differently.
• mode_bit_format (d3): Base-8 positional numeral system used to encode file mode; Git stores exactly 6 octal digits representing 18 bits of mode information.
• mode_permission_bits (d3): The lower 9 bits (three octal digits) encoding Unix owner/group/other read-write-execute permissions; displayed as the last three digits of the 6-digit octal mode.
• mode_type_bits (d3): The upper bits of the octal mode identifying the entry's object category: regular file, directory, symlink, or submodule; encoded in the first two octal digits.
• regular_file_mode (d3): Mode 100644; standard blob-backed file with read permission for all and write for owner; the default for non-executable files committed to Git.
• subtree_mode (d3): Mode 040000; entry points to a nested tree object representing a directory; the tree entry name contains a forward slash but the slash is not stored in the entry itself.
• symlink_mode (d3): Mode 120000; entry points to a blob object containing the target path as raw bytes; the blob content is the symlink destination, not the link data itself.
• mode_zero_padding (d4): The leading zero in modes like 100644 and 040000 ensures consistent 6-digit width; the zero fills the third octal digit reserved for special file type flags.

from git_internals_and_the_object_model

• author_field (d1): Name and email in commit object identifying primary creator; distinct from committer who applied the patch.
• commit_graph (d1): Binary file .git/objects/info/commit-graph storing commit reachability, generation numbers, and bloom filter data off the critical path.
• commit_message_encoding (d1): Commit object may record encoding header; git log --encoding=UTF-8 transcodes as needed; enables non-ASCII commit messages.
• committer_field (d1): Name, email, timestamp, and timezone offset in commit object; records who committed (not necessarily who authored).
• empty_tree_sha (d1): SHA-1 of the empty tree (tree with zero entries): 4b825dc642cb6eb9a060e54bf8d69288fbee4904; used as synthetic parent of root commits.
• head_pointer (d1): Symbolic ref .git/HEAD pointing to current branch name or directly to a SHA-1 in detached mode; moves on checkout.
• note_object (d1): Blob-like object linked via refs/notes/commits/ to commits; provides annotation layer outside normal commit history.
• object_type_inspection (d1): git cat-file -t reveals object type; git cat-file -s shows byte size; git cat-file -p pretty-prints content; git rev-list walks commit graph.
• parent_commit_list (d1): Comma-separated list of parent SHA-1s in commit object; first parent first enables linear history heuristics; octopush uses multi-parent.
• root_commit (d1): Commit with zero parent SHAs; git mktree produces empty tree SHA; git rev-list excludes root commits with --max-parents=0 when negating.
• shallow_clone (d1): Clone with .git/shallow cut-off file listing shallow commit SHAs; prevents fetching ancestors beyond the boundary.
• signed_commit (d1): Commit with GPGSIG header containing cryptographic signature over commit contents; verified by git verify-commit.
• bloom_filter_index (d2): Commit-graph embedded data structure encoding paths modified per commit; powers git log --S --throughput optimization.
• commit_graph_verify (d2): git commit-graph verify reads graph and validates generation numbers, checksum, and fanout structure; reports corruption.
• commit_graph_write (d2): git commit-graph write traverses reachable commits and writes binary graph; --split merges small chains; --append adds commits to existing graph.
• generation_number (d2): Integer in commit-graph measuring topological distance from roots; enables fast ancestor queries without traversing full commit history.

CONSUMERS (what needs this)

cherry_pick_delta, merkle_tree_property, ours_strategy, rebase_replay, revert_delta, sha1_hash

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Projected from the git internals and the object model KB (162 concepts / 159 relations) — consistency-typed by MAP; the facet list after the colon IS the cross-lib dependency web.

(leaf — this is an actual skill.)