interpret_nth_last_branch(): avoid traversing the reflog twice

You can have quite a many reflog entries, but you typically won't recall
which branch you were on after switching branches for more than several
times.

Instead of reading the reflog twice, this reads the branch switching event
and keeps as many entries as the user asked from the latest such entries,
which is the minimum required to be able to switch back to the branch we
were recently on.

[jc: improvements from Dscho squashed in]

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

diff --git a/sha1_name.c b/sha1_name.c
index 9e1538e3d2fbbabf3c39a1592c02339fe6f58038..d6972f2d6a2fd5af38b9babbacd97c9388962bb4 100644 (file)
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -685,8 +685,7 @@ static int get_sha1_oneline(const char *prefix, unsigned char *sha1)
 }
 
 struct grab_nth_branch_switch_cbdata {
-       int counting;
-       int nth;
+       long cnt, alloc;
        struct strbuf *buf;
 };
 
@@ -697,6 +696,7 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
        struct grab_nth_branch_switch_cbdata *cb = cb_data;
        const char *match = NULL, *target = NULL;
        size_t len;
+       int nth;
 
        if (!prefixcmp(message, "checkout: moving from ")) {
                match = message + strlen("checkout: moving from ");
@@ -711,16 +711,9 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
        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;
-       }
+       nth = cb->cnt++ % cb->alloc;
+       strbuf_reset(&cb->buf[nth]);
+       strbuf_add(&cb->buf[nth], match, len);
        return 0;
 }
 
@@ -737,7 +730,8 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
  */
 int interpret_nth_last_branch(const char *name, struct strbuf *buf)
 {
-       int nth;
+       long nth;
+       int i;
        struct grab_nth_branch_switch_cbdata cb;
        const char *brace;
        char *num_end;
@@ -750,19 +744,22 @@ int interpret_nth_last_branch(const char *name, struct strbuf *buf)
        nth = strtol(name+3, &num_end, 10);
        if (num_end != brace)
                return -1;
-
-       cb.counting = 1;
-       cb.nth = 0;
-       cb.buf = buf;
+       if (nth <= 0)
+               return -1;
+       cb.alloc = nth;
+       cb.buf = xmalloc(nth * sizeof(struct strbuf));
+       for (i = 0; i < nth; i++)
+               strbuf_init(&cb.buf[i], 20);
+       cb.cnt = 0;
        for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
-
-       if (cb.nth < nth)
+       if (cb.cnt < nth)
                return 0;
-
-       cb.counting = 0;
-       cb.nth -= nth;
-       cb.buf = buf;
-       for_each_reflog_ent("HEAD", grab_nth_branch_switch, &cb);
+       i = cb.cnt % nth;
+       strbuf_reset(buf);
+       strbuf_add(buf, cb.buf[i].buf, cb.buf[i].len);
+       for (i = 0; i < nth; i++)
+               strbuf_release(&cb.buf[i]);
+       free(cb.buf);
 
        return brace-name+1;
 }

diff --git a/t/t2012-checkout-last.sh b/t/t2012-checkout-last.sh
index 320f6eb2be6c1e3ee0a0fbf4b7e478b626415387..87b30a268c66848726baec5676e73b8db686806d 100755 (executable)
--- a/t/t2012-checkout-last.sh
+++ b/t/t2012-checkout-last.sh
@@ -47,4 +47,48 @@ test_expect_success '"checkout -" detaches again' '
        test_must_fail git symbolic-ref HEAD
 '
 
+test_expect_success 'more switches' '
+       for i in 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
+       do
+               git checkout -b branch$i
+       done
+'
+
+more_switches () {
+       for i in 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
+       do
+               git checkout branch$i
+       done
+}
+
+test_expect_success 'switch to the last' '
+       more_switches &&
+       git checkout @{-1} &&
+       test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch2"
+'
+
+test_expect_success 'switch to second from the last' '
+       more_switches &&
+       git checkout @{-2} &&
+       test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch3"
+'
+
+test_expect_success 'switch to third from the last' '
+       more_switches &&
+       git checkout @{-3} &&
+       test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch4"
+'
+
+test_expect_success 'switch to fourth from the last' '
+       more_switches &&
+       git checkout @{-4} &&
+       test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch5"
+'
+
+test_expect_success 'switch to twelfth from the last' '
+       more_switches &&
+       git checkout @{-12} &&
+       test "z$(git symbolic-ref HEAD)" = "zrefs/heads/branch13"
+'
+
 test_done