ref-filter: implement %(if), %(then), and %(else) atoms

Implement %(if), %(then) and %(else) atoms. Used as
%(if)..%(then)..%(end) or %(if)..%(then)..%(else)..%(end). If there is
an atom with value or string literal after the %(if) then everything
after the %(then) is printed, else if the %(else) atom is used, then
everything after %(else) is printed. If the string contains only
whitespaces, then it is not considered. Nesting of this construct is
possible.

This is in preperation for porting over `git branch -l` to use
ref-filter APIs for printing.

Add Documentation and tests regarding the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 177 insertions, 10 deletions

diff --git a/Documentation/git-for-each-ref.txt b/Documentation/git-for-each-ref.txt
index 16b4ac5614..768a5128e4 100644
--- a/Documentation/git-for-each-ref.txt
+++ b/Documentation/git-for-each-ref.txt
@@ -134,9 +134,14 @@ align::
 	`<position>` is either left, right or middle, default being
 	left and `<width>` is the total length of the content with
 	alignment. If the contents length is more than the width then
-	no alignment is performed. If used with '--quote' everything
-	in between %(align:...) and %(end) is quoted, but if nested
-	then only the topmost level performs quoting.
+	no alignment is performed.
+
+if::
+	Used as %(if)..%(then)..(%end) or %(if)..%(then)..%(else)..%(end).
+	If there is an atom with value or string literal after the
+	%(if) then everything after the %(then) is printed, else if
+	the %(else) atom is used, then everything after %(else) is
+	printed.
 
 In addition to the above, for commit and tag objects, the header
 field names (`tree`, `parent`, `object`, `type`, and `tag`) can
@@ -169,6 +174,18 @@ the date by adding one of `:default`, `:relative`, `:short`, `:local`,
 `:iso8601`, `:rfc2822` or `:raw` to the end of the fieldname; e.g.
 `%(taggerdate:relative)`.
 
+Some atoms like %(align) and %(if) always require a matching %(end).
+We call them "opening atoms" and sometimes denote them as %($open).
+
+When a scripting language specific quoting is in effect, except for
+opening atoms, replacement from every %(atom) is quoted when and only
+when it appears at the top-level (that is, when it appears outside
+%($open)...%(end)).
+
+When a scripting language specific quoting is in effect, everything
+between a top-level opening atom and its matching %(end) is evaluated
+according to the semantics of the opening atom and its result is
+quoted.
 
 EXAMPLES
 --------
diff --git a/ref-filter.c b/ref-filter.c
index dbd8fcec24..93b07f5b85 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -55,6 +55,9 @@ static struct {
 	{ "color" },
 	{ "align" },
 	{ "end" },
+	{ "if" },
+	{ "then" },
+	{ "else" },
 };
 
 #define REF_FORMATTING_STATE_INIT  { 0, NULL }
@@ -69,10 +72,17 @@ struct contents {
 	struct object_id oid;
 };
 
+struct if_then_else {
+	unsigned int if_atom : 1,
+		then_atom : 1,
+		else_atom : 1,
+		condition_satisfied : 1;
+};
+
 struct ref_formatting_stack {
 	struct ref_formatting_stack *prev;
 	struct strbuf output;
-	void (*at_end)(struct ref_formatting_stack *stack);
+	void (*at_end)(struct ref_formatting_stack **stack);
 	void *at_end_data;
 };
 
@@ -216,13 +226,14 @@ static void pop_stack_element(struct ref_formatting_stack **stack)
 	*stack = prev;
 }
 
-static void end_align_handler(struct ref_formatting_stack *stack)
+static void end_align_handler(struct ref_formatting_stack **stack)
 {
-	struct align *align = (struct align *)stack->at_end_data;
+	struct ref_formatting_stack *cur = *stack;
+	struct align *align = (struct align *)cur->at_end_data;
 	struct strbuf s = STRBUF_INIT;
 
-	strbuf_utf8_align(&s, align->position, align->width, stack->output.buf);
-	strbuf_swap(&stack->output, &s);
+	strbuf_utf8_align(&s, align->position, align->width, cur->output.buf);
+	strbuf_swap(&cur->output, &s);
 	strbuf_release(&s);
 }
 
@@ -236,6 +247,85 @@ static void align_atom_handler(struct atom_value *atomv, struct ref_formatting_s
 	new->at_end_data = &atomv->u.align;
 }
 
+static void if_then_else_handler(struct ref_formatting_stack **stack)
+{
+	struct ref_formatting_stack *cur = *stack;
+	struct ref_formatting_stack *prev = cur->prev;
+	struct if_then_else *if_then_else = (struct if_then_else *)cur->at_end_data;
+
+	/*
+	 * If the 'if' condition is satisfied, then if there exists an
+	 * 'else' atom drop the 'else' atom's state. Else we swap the
+	 * buffer of the 'else' atom with the previous state and drop
+	 * that state. If the condition is satisfied and no 'else' atom
+	 * exists, then just clear the buffer.
+	 */
+	if (if_then_else->condition_satisfied && if_then_else->else_atom) {
+		strbuf_reset(&cur->output);
+		pop_stack_element(&cur);
+	} else if (if_then_else->else_atom) {
+		strbuf_swap(&cur->output, &prev->output);
+		strbuf_reset(&cur->output);
+		pop_stack_element(&cur);
+	} else if (!if_then_else->condition_satisfied)
+		strbuf_reset(&cur->output);
+	*stack = cur;
+	free(if_then_else);
+}
+
+static void if_atom_handler(struct atom_value *atomv, struct ref_formatting_state *state)
+{
+	struct ref_formatting_stack *new;
+	struct if_then_else *if_then_else = xcalloc(sizeof(struct if_then_else), 1);
+
+	if_then_else->if_atom = 1;
+	push_stack_element(&state->stack);
+	new = state->stack;
+	new->at_end = if_then_else_handler;
+	new->at_end_data = if_then_else;
+}
+
+static int is_empty(const char * s){
+	while (*s != '\0') {
+		if (!isspace(*s))
+			return 0;
+		s++;
+	}
+	return 1;
+}
+
+static void then_atom_handler(struct atom_value *atomv, struct ref_formatting_state *state)
+{
+	struct ref_formatting_stack *cur = state->stack;
+	struct if_then_else *if_then_else = (struct if_then_else *)cur->at_end_data;
+
+	if (!if_then_else)
+		die(_("format: %%(then) atom used without an %%(if) atom"));
+	if_then_else->then_atom = 1;
+	/*
+	 * If there exists non-empty string between the 'if' and
+	 * 'then' atom then the 'if' condition is satisfied.
+	 */
+	if (cur->output.len && !is_empty(cur->output.buf))
+		if_then_else->condition_satisfied = 1;
+	strbuf_reset(&cur->output);
+}
+
+static void else_atom_handler(struct atom_value *atomv, struct ref_formatting_state *state)
+{
+	struct ref_formatting_stack *prev = state->stack;
+	struct if_then_else *if_then_else = (struct if_then_else *)state->stack->at_end_data;
+
+	if (!if_then_else)
+		die(_("format: %%(else) atom used without an %%(if) atom"));
+	if (!if_then_else->then_atom)
+		die(_("format: %%(else) atom used without a %%(then) atom"));
+	if_then_else->else_atom = 1;
+	push_stack_element(&state->stack);
+	state->stack->at_end_data = prev->at_end_data;
+	state->stack->at_end = prev->at_end;
+}
+
 static void end_atom_handler(struct atom_value *atomv, struct ref_formatting_state *state)
 {
 	struct ref_formatting_stack *current = state->stack;
@@ -243,14 +333,17 @@ static void end_atom_handler(struct atom_value *atomv, struct ref_formatting_sta
 
 	if (!current->at_end)
 		die(_("format: %%(end) atom used without corresponding atom"));
-	current->at_end(current);
+	current->at_end(&state->stack);
+
+	/*  Stack may have been popped, hence reset the current pointer */
+	current = state->stack;
 
 	/*
 	 * Perform quote formatting when the stack element is that of
 	 * a supporting atom. If nested then perform quote formatting
 	 * only on the topmost supporting atom.
 	 */
-	if (!state->stack->prev->prev) {
+	if (!current->prev->prev) {
 		quote_formatting(&s, current->output.buf, state->quote_style);
 		strbuf_swap(&current->output, &s);
 	}
@@ -920,6 +1013,15 @@ static void populate_value(struct ref_array_item *ref)
 		} else if (!strcmp(name, "end")) {
 			v->handler = end_atom_handler;
 			continue;
+		} else if (!strcmp(name, "if")) {
+			v->handler = if_atom_handler;
+			continue;
+		} else if (!strcmp(name, "then")) {
+			v->handler = then_atom_handler;
+			continue;
+		} else if (!strcmp(name, "else")) {
+			v->handler = else_atom_handler;
+			continue;
 		} else
 			continue;
 
diff --git a/t/t6302-for-each-ref-filter.sh b/t/t6302-for-each-ref-filter.sh
index fe4796cc9c..ec13675ba5 100755
--- a/t/t6302-for-each-ref-filter.sh
+++ b/t/t6302-for-each-ref-filter.sh
@@ -255,4 +255,52 @@ test_expect_success 'reverse version sort' '
 	test_cmp expect actual
 '
 
+test_expect_success 'improper usage of %(if), %(then), %(else) and %(end) atoms' '
+	test_must_fail git for-each-ref --format="%(if)" &&
+	test_must_fail git for-each-ref --format="%(then)" &&
+	test_must_fail git for-each-ref --format="%(else)" &&
+	test_must_fail git for-each-ref --format="%(if) %(else)" &&
+	test_must_fail git for-each-ref --format="%(then) %(else)" &&
+	test_must_fail git for-each-ref --format="%(if) %(else)" &&
+	test_must_fail git for-each-ref --format="%(if) %(then) %(else)"
+'
+
+test_expect_success 'check %(if)...%(then)...%(end) atoms' '
+	git for-each-ref --format="%(if)%(authorname)%(then)%(authorname): %(refname)%(end)" >actual &&
+	cat >expect <<-\EOF &&
+	A U Thor: refs/heads/master
+	A U Thor: refs/heads/side
+	A U Thor: refs/odd/spot
+
+	A U Thor: refs/tags/foo1.10
+	A U Thor: refs/tags/foo1.3
+	A U Thor: refs/tags/foo1.6
+	A U Thor: refs/tags/four
+	A U Thor: refs/tags/one
+
+	A U Thor: refs/tags/three
+	A U Thor: refs/tags/two
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'check %(if)...%(then)...%(else)...%(end) atoms' '
+	git for-each-ref --format="%(if)%(authorname)%(then)%(authorname)%(else)No author%(end): %(refname)" >actual &&
+	cat >expect <<-\EOF &&
+	A U Thor: refs/heads/master
+	A U Thor: refs/heads/side
+	A U Thor: refs/odd/spot
+	No author: refs/tags/double-tag
+	A U Thor: refs/tags/foo1.10
+	A U Thor: refs/tags/foo1.3
+	A U Thor: refs/tags/foo1.6
+	A U Thor: refs/tags/four
+	A U Thor: refs/tags/one
+	No author: refs/tags/signed-tag
+	A U Thor: refs/tags/three
+	A U Thor: refs/tags/two
+	EOF
+	test_cmp expect actual
+'
+
 test_done