From: William Chargin <wchargin@gmail.com>
Date: Sun, 23 Dec 2018 22:41:20 GMT
Subject: Parsing trailers
Message-ID: <CAFW+GMDazFSDzBrvzMqaPGwew=+CP7tw7G5FfDqcAUYd3qjPuQ@mail.gmail.com>

I'm interested in parsing the output of `git-interpret-trailers` in a
script. I had hoped that the `--parse` option would make this easy, but
it seems that the `trailer.separators` configuration option is used to
specify both the input format (which separators may indicate a trailer)
and the output format of `git interpret-trailers --parse`. Given that
`trailer.separators` may contain any (non-NUL) characters, including
whitespace, parsing the output is not straightforward.

Here's what I've come up with. The output format is "<tok><sep> <val>",
where "<tok>" and "<val>" have been trimmed and so have no leading or
trailing whitespace, but "<val>" may have internal whitespace while
"<tok>" may not. Thus, the first space character in the output may
correspond to either "<sep>" or the fixed space, but we should be able
to determine which is the case: the first space is immediately followed
by a second space if and only if the first space corresponds to "<sep>".

Assuming that the above analysis is correct, the following procedure
should suffice to safely parse the output:

  - Let `i` be the index of the first space in `s`.
  - If `s[i+1]` is a space, let `sep_pos` be `i`. Otherwise, let
    `sep_pos` be `i - 1`.
  - The substring `s[:sep_pos]` is the token.
  - The character at index `sep_pos` is the separator.
  - The character at index `sep_pos + 1` is the fixed space.
  - The substring `s[sep_pos+2:nl]` is the value, where `nl` is the
    index of the first newline in `s` after `sep_pos`.

(It seems unfortunately complicated when all we want to do is parse the
output of `--parse`, but I don't see a better approach!)

My questions:

  - Is this accurate?
  - Is this algorithm guaranteed to remain correct in future versions of
    Git?
  - Is there a simpler way to extract the token-value pairs from a
    commit message string?

Would appreciate any advice.

Thanks!
WC