Felipe Contreras wrote:
> [...]

Okay, you used nth_prior in this one.

> There is no need to call this function recursively with the branch of
> @{-N} substituted because dwim_{ref,log} already replaces it.

I figured that the recursion is because dwim_{ref,log} didn't exist
when this was written.

> However, there's one corner-case where @{-N} resolves to a detached
> HEAD, in which case we wouldn't get any ref back.
>
> So we parse the nth-prior manually, and deal with it depending on
> weather it's a SHA-1, or a ref.

Right.  _This_ is the special case, which the old logic didn't quite
convey.  The end-user version of this is: 'git checkout -' won't bring
you back to the branch if you said git checkout HEAD~1 earlier.

> diff --git a/sha1_name.c b/sha1_name.c
> index 3820f28..6428001 100644
> --- a/sha1_name.c
> +++ b/sha1_name.c
> @@ -431,13 +431,14 @@ static inline int upstream_mark(const char *string, int 
> len)
>  }
>
>  static int get_sha1_1(const char *name, int len, unsigned char *sha1, 
> unsigned lookup_flags);
> +static int interpret_nth_prior_checkout(const char *name, struct strbuf 
> *buf);

It didn't strike me to use interpret_nth_prior_checkout() directly.  I
was still stuck at interpret_branch_name() returning a positive value.

>  static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
>  {
>         static const char *warn_msg = "refname '%.*s' is ambiguous.";
>         char *real_ref = NULL;
>         int refs_found = 0;
> -       int at, reflog_len;
> +       int at, reflog_len, nth_prior = 0;
>
>         if (len == 40 && !get_sha1_hex(str, sha1))
>                 return 0;
> @@ -447,6 +448,10 @@ static int get_sha1_basic(const char *str, int len, 
> unsigned char *sha1)
>         if (len && str[len-1] == '}') {
>                 for (at = len-2; at >= 0; at--) {
>                         if (str[at] == '@' && str[at+1] == '{') {
> +                               if (at == 0 && str[2] == '-') {
> +                                       nth_prior = 1;
> +                                       continue;
> +                               }

Looking at this closely once again.
You've already hit the beginning.  What are you continuing?  Take the
example of a compound expression with @{-

@{-1}@{0}
     ^ at is here
     "@{-" is not matched

@{-1}@{0}
^ at is here
"@{-" is matched
What's to continue? at is already at 0

On another note, I think you've fixed a bug: @{-1}{0} was parsing to
the same value as @{-1}@{0} before your patch.

> @@ -460,20 +465,22 @@ static int get_sha1_basic(const char *str, int len, 
> unsigned char *sha1)
>         if (len && ambiguous_path(str, len))
>                 return -1;
>
> -       if (!len && reflog_len) {
> +       if (nth_prior) {

nth_prior makes this much cleaner overall.

>                 struct strbuf buf = STRBUF_INIT;
> -               int ret;
> -               /* try the @{-N} syntax for n-th checkout */
> -               ret = interpret_branch_name(str+at, &buf);
> -               if (ret > 0) {
> -                       /* substitute this branch name and restart */
> -                       return get_sha1_1(buf.buf, buf.len, sha1, 0);
> -               } else if (ret == 0) {
> -                       return -1;
> +               int detached;
> +
> +               if (interpret_nth_prior_checkout(str, &buf) > 0) {
> +                       detached = (buf.len == 40 && !get_sha1_hex(buf.buf, 
> sha1));
> +                       strbuf_release(&buf);
> +                       if (detached)
> +                               return 0;

Neat.  I'd set reflog_len to zero and made sure that the last part of
the function wouldn't be executed.  How did you get away without
setting refs_found to 1 though?

>                 }
> +       }
> +
> +       if (!len && reflog_len)
>                 /* allow "@{...}" to mean the current branch reflog */
>                 refs_found = dwim_ref("HEAD", 4, sha1, &real_ref);

I got this part wrong too: I said dwim_log() instead of dwim_ref().

> -       } else if (reflog_len)
> +       else if (reflog_len)
>                 refs_found = dwim_log(str, len, sha1, &real_ref);
>         else
>                 refs_found = dwim_ref(str, len, sha1, &real_ref);
> --
> 1.8.3.rc0.401.g45bba44
>
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