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.dmap file format

The on-disk archive is built with rkyv and is memory-mappable with no parsing step. See FORMAT.md in the repo for the authoritative byte-level spec.

A small fixed-size header containing:

  • 4-byte magic (DMAP).
  • Format version (u16).
  • Offset and length of the rkyv-archived Dictionary body.
  • SHA-256 of the body for integrity verification.

Body​

The body is a single Dictionary value containing two Vec<TagEntry> slices:

  • A public slice keyed by (group, element) (sorted lex).
  • A private slice keyed by (group, element_offset, creator_hash) (sorted lex).

Lookup is binary search; no hash table, no allocations.

Tag entry​

Each TagEntry carries: VR, VM, keyword, name, retired flag, private-creator string (for private tags), and an optional description.

Alignment​

rkyv requires 4-byte alignment on the body. The reader maps the file with Mmap::map (which is page-aligned) and slices the body out at the correct offset. For embedded mode the bytes are wrapped in #[repr(C, align(4))] to satisfy the requirement after include_bytes!.

Versioning​

Format version bumps are recorded in CHANGELOG.md. Old readers refuse to open a newer-format archive and vice versa.