sha1_name: support @{-N} syntax in get_sha1()

diff --git a/sha1_name.c b/sha1_name.c
index 159c2ab84fa2cdde0e540024a1ca22e0bbb43af8..9e1538e3d2fbbabf3c39a1592c02339fe6f58038 100644 (file)
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -297,6 +297,8 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
        return logs_found;
 }
 
+static int get_sha1_1(const char *name, int len, unsigned char *sha1);
+
 static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 {
        static const char *warning = "warning: refname '%.*s' is ambiguous.\n";
@@ -307,7 +309,7 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
        if (len == 40 && !get_sha1_hex(str, sha1))
                return 0;
 
-       /* basic@{time or number} format to query ref-log */
+       /* basic@{time or number or -number} format to query ref-log */
        reflog_len = at = 0;
        if (str[len-1] == '}') {
                for (at = 0; at < len - 1; at++) {
@@ -324,6 +326,16 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
                return -1;
 
        if (!len && reflog_len) {
+               struct strbuf buf = STRBUF_INIT;
+               int ret;
+               /* try the @{-N} syntax for n-th checkout */
+               ret = interpret_nth_last_branch(str+at, &buf);
+               if (ret > 0) {
+                       /* substitute this branch name and restart */
+                       return get_sha1_1(buf.buf, buf.len, sha1);
+               } else if (ret == 0) {
+                       return -1;
+               }
                /* allow "@{...}" to mean the current branch reflog */
                refs_found = dwim_ref("HEAD", 4, sha1, &real_ref);
        } else if (reflog_len)
@@ -379,8 +391,6 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
        return 0;
 }
 
-static int get_sha1_1(const char *name, int len, unsigned char *sha1);
-
 static int get_parent(const char *name, int len,
                      unsigned char *result, int idx)
 {
@@ -674,6 +684,89 @@ 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, *target = NULL;
+       size_t len;
+
+       if (!prefixcmp(message, "checkout: moving from ")) {
+               match = message + strlen("checkout: moving from ");
+               if ((target = strstr(match, " to ")) != NULL)
+                       target += 4;
+       }
+
+       if (!match)
+               return 0;
+
+       len = target - match - 4;
+       if (target[len] == '\n' && !strncmp(match, target, len))
+               return 0;
+
+       if (cb->counting) {
+               cb->nth++;
+               return 0;
+       }
+
+       if (cb->nth-- <= 0) {
+               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 the number of characters parsed if successful.
+ *
+ * If the input is not of the accepted format, it returns a negative
+ * number to signal an error.
+ *
+ * If the input was ok but there are not N branch switches in the
+ * reflog, it returns 0.
+ */
+int interpret_nth_last_branch(const char *name, struct strbuf *buf)
+{
+       int nth;
+       struct grab_nth_branch_switch_cbdata cb;
+       const char *brace;
+       char *num_end;
+
+       if (name[0] != '@' || name[1] != '{' || name[2] != '-')
+               return -1;
+       brace = strchr(name, '}');
+       if (!brace)
+               return -1;
+       nth = strtol(name+3, &num_end, 10);
+       if (num_end != brace)
+               return -1;
+
+       cb.counting = 1;
+       cb.nth = 0;
+       cb.buf = buf;
+       for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+
+       if (cb.nth < nth)
+               return 0;
+
+       cb.counting = 0;
+       cb.nth -= nth;
+       cb.buf = buf;
+       for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+
+       return brace-name+1;
+}
+
 /*
  * This is like "get_sha1_basic()", except it allows "sha1 expressions",
  * notably "xyz^" for "parent of xyz"