Merge branch 'fc/at-head'

Instead of typing four capital letters "HEAD", you can say "@"
instead.

* fc/at-head:
  sha1_name: compare variable with constant, not constant with variable
  Add new @ shortcut for HEAD
  sha1_name: refactor reinterpret()
  sha1_name: check @{-N} errors sooner
  sha1_name: reorganize get_sha1_basic()
  sha1_name: don't waste cycles in the @-parsing loop
  sha1_name: remove unnecessary braces
  sha1_name: remove no-op
  tests: at-combinations: @{N} versus HEAD@{N}
  tests: at-combinations: increase coverage
  tests: at-combinations: improve nonsense()
  tests: at-combinations: check ref names directly
  tests: at-combinations: simplify setup

5 files changed, 123 insertions, 57 deletions

diff --git a/Documentation/git-check-ref-format.txt b/Documentation/git-check-ref-format.txt
index a49be1bab..fc02959ba 100644
--- a/Documentation/git-check-ref-format.txt
+++ b/Documentation/git-check-ref-format.txt
@@ -54,6 +54,8 @@ Git imposes the following rules on how references are named:
 
 . They cannot contain a sequence `@{`.
 
+. They cannot be the single character `@`.
+
 . They cannot contain a `\`.
 
 These rules make it easy for shell script based tools to parse
diff --git a/Documentation/revisions.txt b/Documentation/revisions.txt
index d477b3f6b..09896a37b 100644
--- a/Documentation/revisions.txt
+++ b/Documentation/revisions.txt
@@ -58,6 +58,9 @@ the '$GIT_DIR/refs' directory or from the '$GIT_DIR/packed-refs' file.
 While the ref name encoding is unspecified, UTF-8 is preferred as
 some output processing may assume ref names in UTF-8.
 
+'@'::
+  '@' alone is a shortcut for 'HEAD'.
+
 '<refname>@\{<date>\}', e.g. 'master@\{yesterday\}', 'HEAD@\{5 minutes ago\}'::
   A ref followed by the suffix '@' with a date specification
   enclosed in a brace
diff --git a/refs.c b/refs.c
index d17931a8b..42a7e17f6 100644
--- a/refs.c
+++ b/refs.c
@@ -72,6 +72,10 @@ int check_refname_format(const char *refname, int flags)
 {
 	int component_len, component_count = 0;
 
+	if (!strcmp(refname, "@"))
+		/* Refname is a single character '@'. */
+		return -1;
+
 	while (1) {
 		/* We are at the start of a path component. */
 		component_len = check_refname_component(refname, flags);
diff --git a/sha1_name.c b/sha1_name.c
index a695d16d8..90419efe1 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -431,6 +431,7 @@ 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);
 
 static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 {
@@ -448,7 +449,7 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	unsigned char tmp_sha1[20];
 	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)) {
 		refs_found = dwim_ref(str, len, tmp_sha1, &real_ref);
@@ -464,8 +465,15 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 	/* basic@{time or number or -number} format to query ref-log */
 	reflog_len = at = 0;
 	if (len && str[len-1] == '}') {
-		for (at = len-2; at >= 0; at--) {
+		for (at = len-4; at >= 0; at--) {
 			if (str[at] == '@' && str[at+1] == '{') {
+				if (str[at+2] == '-') {
+					if (at != 0)
+						/* @{-N} not at start */
+						return -1;
+					nth_prior = 1;
+					continue;
+				}
 				if (!upstream_mark(str + at, len - at)) {
 					reflog_len = (len-1) - (at+2);
 					len = at;
@@ -479,20 +487,22 @@ 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) {
 		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;
 		}
+	}
+
+	if (!len && reflog_len)
 		/* allow "@{...}" to mean the current branch reflog */
 		refs_found = dwim_ref("HEAD", 4, sha1, &real_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);
@@ -511,10 +521,6 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 		unsigned long co_time;
 		int co_tz, co_cnt;
 
-		/* a @{-N} placed anywhere except the start is an error */
-		if (str[at+2] == '-')
-			return -1;
-
 		/* Is it asking for N-th entry, or approxidate? */
 		for (i = nth = 0; 0 <= nth && i < reflog_len; i++) {
 			char ch = str[at+2+i];
@@ -996,6 +1002,38 @@ int get_sha1_mb(const char *name, unsigned char *sha1)
 	return st;
 }
 
+/* parse @something syntax, when 'something' is not {.*} */
+static int interpret_empty_at(const char *name, int namelen, int len, struct strbuf *buf)
+{
+	if (len || name[1] == '{')
+		return -1;
+
+	strbuf_reset(buf);
+	strbuf_add(buf, "HEAD", 4);
+	return 1;
+}
+
+static int reinterpret(const char *name, int namelen, int len, struct strbuf *buf)
+{
+	/* we have extra data, which might need further processing */
+	struct strbuf tmp = STRBUF_INIT;
+	int used = buf->len;
+	int ret;
+
+	strbuf_add(buf, name + len, namelen - len);
+	ret = interpret_branch_name(buf->buf, &tmp);
+	/* that data was not interpreted, remove our cruft */
+	if (ret < 0) {
+		strbuf_setlen(buf, used);
+		return len;
+	}
+	strbuf_reset(buf);
+	strbuf_addbuf(buf, &tmp);
+	strbuf_release(&tmp);
+	/* tweak for size of {-N} versus expanded ref name */
+	return ret - used + len;
+}
+
 /*
  * This reads short-hand syntax that not only evaluates to a commit
  * object name, but also can act as if the end user spelled the name
@@ -1025,36 +1063,27 @@ int interpret_branch_name(const char *name, struct strbuf *buf)
 	int len = interpret_nth_prior_checkout(name, buf);
 	int tmp_len;
 
-	if (!len)
+	if (!len) {
 		return len; /* syntax Ok, not enough switches */
-	if (0 < len && len == namelen)
-		return len; /* consumed all */
-	else if (0 < len) {
-		/* we have extra data, which might need further processing */
-		struct strbuf tmp = STRBUF_INIT;
-		int used = buf->len;
-		int ret;
-
-		strbuf_add(buf, name + len, namelen - len);
-		ret = interpret_branch_name(buf->buf, &tmp);
-		/* that data was not interpreted, remove our cruft */
-		if (ret < 0) {
-			strbuf_setlen(buf, used);
-			return len;
-		}
-		strbuf_reset(buf);
-		strbuf_addbuf(buf, &tmp);
-		strbuf_release(&tmp);
-		/* tweak for size of {-N} versus expanded ref name */
-		return ret - used + len;
+	} else if (len > 0) {
+		if (len == namelen)
+			return len; /* consumed all */
+		else
+			return reinterpret(name, namelen, len, buf);
 	}
 
 	cp = strchr(name, '@');
 	if (!cp)
 		return -1;
+
+	len = interpret_empty_at(name, namelen, cp - name, buf);
+	if (len > 0)
+		return reinterpret(name, namelen, len, buf);
+
 	tmp_len = upstream_mark(cp, namelen - (cp - name));
 	if (!tmp_len)
 		return -1;
+
 	len = cp + tmp_len - name;
 	cp = xstrndup(name, cp - name);
 	upstream = branch_get(*cp ? cp : NULL);
diff --git a/t/t1508-at-combinations.sh b/t/t1508-at-combinations.sh
index d5d624417..4db1613f8 100755
--- a/t/t1508-at-combinations.sh
+++ b/t/t1508-at-combinations.sh
@@ -4,17 +4,24 @@ test_description='test various @{X} syntax combinations together'
 . ./test-lib.sh
 
 check() {
-test_expect_${3:-success} "$1 = $2" "
-	echo '$2' >expect &&
-	git log -1 --format=%s '$1' >actual &&
-	test_cmp expect actual
-"
+	test_expect_${4:-success} "$1 = $3" "
+		echo '$3' >expect &&
+		if test '$2' = 'commit'
+		then
+			git log -1 --format=%s '$1' >actual
+		else
+			git rev-parse --symbolic-full-name '$1' >actual
+		fi &&
+		test_cmp expect actual
+	"
 }
+
 nonsense() {
-test_expect_${2:-success} "$1 is nonsensical" "
-	test_must_fail git log -1 '$1'
-"
+	test_expect_${2:-success} "$1 is nonsensical" "
+		test_must_fail git rev-parse --verify '$1'
+	"
 }
+
 fail() {
 	"$@" failure
 }
@@ -31,21 +38,42 @@ test_expect_success 'setup' '
 	git checkout -b new-branch &&
 	test_commit new-one &&
 	test_commit new-two &&
-	git config branch.old-branch.remote . &&
-	git config branch.old-branch.merge refs/heads/master &&
-	git config branch.new-branch.remote . &&
-	git config branch.new-branch.merge refs/heads/upstream-branch
+	git branch -u master old-branch &&
+	git branch -u upstream-branch new-branch
 '
 
-check HEAD new-two
-check "@{1}" new-one
-check "@{-1}" old-two
-check "@{-1}@{1}" old-one
-check "@{u}" upstream-two
-check "@{u}@{1}" upstream-one
-check "@{-1}@{u}" master-two
-check "@{-1}@{u}@{1}" master-one
+check HEAD ref refs/heads/new-branch
+check "@{1}" commit new-one
+check "HEAD@{1}" commit new-one
+check "@{now}" commit new-two
+check "HEAD@{now}" commit new-two
+check "@{-1}" ref refs/heads/old-branch
+check "@{-1}@{0}" commit old-two
+check "@{-1}@{1}" commit old-one
+check "@{u}" ref refs/heads/upstream-branch
+check "HEAD@{u}" ref refs/heads/upstream-branch
+check "@{u}@{1}" commit upstream-one
+check "@{-1}@{u}" ref refs/heads/master
+check "@{-1}@{u}@{1}" commit master-one
+check "@" commit new-two
+check "@@{u}" ref refs/heads/upstream-branch
 nonsense "@{u}@{-1}"
+nonsense "@{0}@{0}"
 nonsense "@{1}@{u}"
+nonsense "HEAD@{-1}"
+nonsense "@{-1}@{-1}"
+
+# @{N} versus HEAD@{N}
+
+check "HEAD@{3}" commit old-two
+nonsense "@{3}"
+
+test_expect_success 'switch to old-branch' '
+	git checkout old-branch
+'
+
+check HEAD ref refs/heads/old-branch
+check "HEAD@{1}" commit new-two
+check "@{1}" commit old-one
 
 test_done