checkout: implement "@{-N}" shortcut name for N-th last branch

Implement a shortcut @{-N} for the N-th last branch checked out, that
works by parsing the reflog for the message added by previous
git-checkout invocations.  We expand the @{-N} to the branch name, so
that you end up on an attached HEAD on that branch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin-checkout.c b/builtin-checkout.c
index b5dd9c07b42e0130384259715730f52bc8c4e6c3..a3b69d6b94dce47b3341f5eb77ae4438bae20cc6 100644 (file)
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -361,8 +361,14 @@ struct branch_info {
 static void setup_branch_path(struct branch_info *branch)
 {
        struct strbuf buf = STRBUF_INIT;
-       strbuf_addstr(&buf, "refs/heads/");
-       strbuf_addstr(&buf, branch->name);
+
+       if (!interpret_nth_last_branch(branch->name, &buf)) {
+               branch->name = xstrdup(buf.buf);
+               strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
+       } else {
+               strbuf_addstr(&buf, "refs/heads/");
+               strbuf_addstr(&buf, branch->name);
+       }
        branch->path = strbuf_detach(&buf, NULL);
 }
 

diff --git a/cache.h b/cache.h
index 8e1af2669bd2e9af03a73b7058bec014d4d3a3aa..0dd9168be58be2147edeb5ca81dca6dd7abd8f05 100644 (file)
--- a/cache.h
+++ b/cache.h
@@ -663,6 +663,7 @@ extern int read_ref(const char *filename, unsigned char *sha1);
 extern const char *resolve_ref(const char *path, unsigned char *sha1, int, int *);
 extern int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
 extern int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
+extern int interpret_nth_last_branch(const char *str, struct strbuf *);
 
 extern int refname_match(const char *abbrev_name, const char *full_name, const char **rules);
 extern const char *ref_rev_parse_rules[];

diff --git a/sha1_name.c b/sha1_name.c
index 159c2ab84fa2cdde0e540024a1ca22e0bbb43af8..6377264300c134db82354d11a5dd62fd7b783a59 100644 (file)
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -674,6 +674,84 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
        return retval;
 }
 
+struct grab_nth_branch_switch_cbdata {
+       int counting;
+       int nth;
+       struct strbuf *buf;
+};
+
+static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
+                                 const char *email, unsigned long timestamp, int tz,
+                                 const char *message, void *cb_data)
+{
+       struct grab_nth_branch_switch_cbdata *cb = cb_data;
+       const char *match = NULL;
+
+       if (!prefixcmp(message, "checkout: moving to "))
+               match = message + strlen("checkout: moving to ");
+       else if (!prefixcmp(message, "checkout: moving from ")) {
+               const char *cp = message + strlen("checkout: moving from ");
+               if ((cp = strstr(cp, " to ")) != NULL) {
+                       match = cp + 4;
+               }
+       }
+
+       if (!match)
+               return 0;
+
+       if (cb->counting) {
+               cb->nth++;
+               return 0;
+       }
+
+       if (--cb->nth <= 0) {
+               size_t len = strlen(match);
+               while (match[len-1] == '\n')
+                       len--;
+               strbuf_reset(cb->buf);
+               strbuf_add(cb->buf, match, len);
+               return 1;
+       }
+       return 0;
+}
+
+/*
+ * This reads "@{-N}" syntax, finds the name of the Nth previous
+ * branch we were on, and places the name of the branch in the given
+ * buf and returns 0 if successful.
+ *
+ * If the input is not of the accepted format, it returns a negative
+ * number to signal an error.
+ */
+int interpret_nth_last_branch(const char *name, struct strbuf *buf)
+{
+       int nth, i;
+       struct grab_nth_branch_switch_cbdata cb;
+
+       if (name[0] != '@' || name[1] != '{' || name[2] != '-')
+               return -1;
+       for (i = 3, nth = 0; name[i] && name[i] != '}'; i++) {
+               char ch = name[i];
+               if ('0' <= ch && ch <= '9')
+                       nth = nth * 10 + ch - '0';
+               else
+                       return -1;
+       }
+       if (nth < 0 || 10 <= nth)
+               return -1;
+
+       cb.counting = 1;
+       cb.nth = 0;
+       cb.buf = buf;
+       for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+
+       cb.counting = 0;
+       cb.nth -= nth;
+       cb.buf = buf;
+       for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+       return 0;
+}
+
 /*
  * This is like "get_sha1_basic()", except it allows "sha1 expressions",
  * notably "xyz^" for "parent of xyz"