0-4-5-understand-sha1-hash

name	0.4.5-understand-sha1_hash
description	[0.4.5] 160-bit cryptographic hash computed over object content; serves as object's unique addressable name in git's c

understand-sha1_hash

CALL NUMBER: git_internals_and_the_object_model.sha1_hash : deep_commit_object(8), deep_tree_object(8) DEFINITION: 160-bit cryptographic hash computed over object content; serves as object's unique addressable name in git's content-addressable store.

Invoke this skill to understand sha1_hash 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 (d2): Optional header line 'encoding ' in a commit object's raw text; git stores whatever the user wrote verbatim.
• commit_message_encoding_encoding_value (d3): 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 (d4): 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 (d4): 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 (d4): 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 (d5): Fallback behavior when transcoding_logic cannot map a character; typically replaces with a substitution character or skips the byte.
• commit_message_encoding_transcoded_message (d5): 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 (d6): 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

• alternates_object_db (d1): File .git/objects/info/alternates listing paths to shared object stores; enables disk-space savings across clones.
• blob_object (d1): Type-1 git object storing raw file contents verbatim; identified by SHA-1 of content; no filename or metadata retained.
• commit_object (d1): Type-3 git object containing tree SHA, parent SHA(s), author, committer, timestamp, and message; forms the history DAG backbone.
• content_addressable_store (d1): Property that object identity derives solely from its content hash; same content always yields identical identifier regardless of path or history.
• replace_ref (d1): Reference under refs/replace/ mapping an object SHA to a substitute SHA; transparent to most commands; allows history rewriting without modifying history.
• tag_object (d1): Type-4 git object wrapping another object with a tag name, tagger info, and message; typically used for annotated release markers.
• tree_object (d1): Type-2 git object listing directory entries as (mode, name, sha1) tuples; represents a single directory snapshot at a point in time.
• object_database (d2): Directory-backed key-value store under .git/objects/; loose objects stored as zlib-deflated files keyed by SHA-1; packed objects aggregated into packfiles.
• object_type_inspection (d2): 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.
• partial_clone (d2): Clone with --filter omitting blob objects from transfer; missing blobs retrieved on demand via fetch-object; enables huge-repository workflows.
• author_field (d2): Name and email in commit object identifying primary creator; distinct from committer who applied the patch.
• commit_graph (d2): Binary file .git/objects/info/commit-graph storing commit reachability, generation numbers, and bloom filter data off the critical path.
• commit_message_encoding (d2): Commit object may record encoding header; git log --encoding=UTF-8 transcodes as needed; enables non-ASCII commit messages.
• committer_field (d2): Name, email, timestamp, and timezone offset in commit object; records who committed (not necessarily who authored).
• empty_tree_sha (d2): SHA-1 of the empty tree (tree with zero entries): 4b825dc642cb6eb9a060e54bf8d69288fbee4904; used as synthetic parent of root commits.
• head_pointer (d2): Symbolic ref .git/HEAD pointing to current branch name or directly to a SHA-1 in detached mode; moves on checkout.

CONSUMERS (what needs this)

sha1_collision_resistance

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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.)