#tac

Version 0.1.4 behavior is documented here. The support record separates local verification from published MoonX behavior; moonx examples use registry releases.

Write records in reverse order:

printf 'one\ntwo\n' | moonx cli/tac
printf 'a--b--' | moonx cli/tac -b -s --

-s/--separator selects a fixed separator and -b/--before places separators before records. Empty fixed separators mean NUL. Records retain their original adjoining separator, including a missing final separator. Ordered file operands insert no extra bytes; - consumes stdin. Read/open failures retain completed output and later operands continue.

-r/--regex uses the frozen GNU Emacs byte syntax: bare +/?, escaped groups/alternation, literal braces, line anchors and buffer assertions. Dot excludes LF, and POSIX bracket-class syntax is disabled in this dialect. Search selects the rightmost start and longest end inside the preceding logical prefix. Nullable patterns make backward progress; an empty regex rejects before input is opened. The matcher borrows prefixes without copying the complete remainder.

#Large inputs and temporary storage

Fixed separators on regular files are searched backwards in 64 KiB blocks, including separator matches that cross block boundaries. Selected records are copied in bounded blocks, so a very long record does not require a record-sized buffer. Memory is O(block size + separator length); file offsets are 64-bit. The file is opened once, but concurrent modification is not a snapshot guarantee. Regular pseudo files whose zero or nominal size disagrees with a positional end probe are consumed through the stream spool before any output is written.

Standard input and nonregular files are first copied into an invocation-owned temporary file. Regex mode also uses this spool so matching and output use the same bytes. Spools require temporary-directory write access and use exclusive creation with owner-only permissions where supported. The input spool can grow to the complete input size. No output is written before spooling succeeds; normal errors and observable cancellation close and remove owned temporary storage. Process termination that cannot unwind, including SIGKILL, does not guarantee cleanup.

Regex mode preserves arbitrary-length separator matches. Its current matcher requires a contiguous byte-view string with 32-bit string indices, retaining O(input) memory in addition to the disk spool; reversed output is streamed without a second input-sized output buffer. Bounded-memory arbitrary-span regex matching requires a shared matcher API over a paged or seekable byte source, with anchor, overlap and nullable-separator regressions on both backends. An output error may retain an already written prefix; later operands continue after an input error, as before.

#Standalone metadata

Standalone --version prints exactly tac 0.1.4 followed by one newline. Standalone --help starts with Usage: tac, lists both metadata options, and ends with a newline. Both write to stdout, leave stdin unread, and exit with status 0. Exact-version publication and verification remain recorded separately in the support record.

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