ref-filter: implement an align atom

Implement an `align` atom which left-, middle-, or right-aligns the
content between %(align:...) and %(end).

The "align:" is followed by `<width>` and `<position>` in any order
separated by a comma, where the `<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.  e.g. to align a refname atom to the
middle with a total width of 40 we can do:
--format="%(align:middle,40)%(refname)%(end)".

We introduce an `at_end` function for each element of the stack which
is to be called when the `end` atom is encountered. Using this we
implement end_align_handler() for the `align` atom, this aligns the
final strbuf by calling `strbuf_utf8_align()` from utf8.c.

Ensure that quote formatting is performed on the whole of
%(align:...)...%(end) rather than individual atoms inside. We skip
quote formatting for individual atoms when the current stack element
is handling an %(align:...) atom and perform quote formatting at the
end when we encounter the %(end) atom of the second element of then
stack.

Add documentation and tests for the same.

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

ref-filter.c

@@ -10,6 +10,7 @@
 #include "quote.h"
 #include "ref-filter.h"
 #include "revision.h"
+#include "utf8.h"
 
 typedef enum { FIELD_STR, FIELD_ULONG, FIELD_TIME } cmp_type;
 
@@ -53,13 +54,22 @@ static struct {
 	{ "flag" },
 	{ "HEAD" },
 	{ "color" },
+	{ "align" },
+	{ "end" },
 };
 
 #define REF_FORMATTING_STATE_INIT  { 0, NULL }
 
+struct align {
+	align_type position;
+	unsigned int width;
+};
+
 struct ref_formatting_stack {
 	struct ref_formatting_stack *prev;
 	struct strbuf output;
+	void (*at_end)(struct ref_formatting_stack *stack);
+	void *at_end_data;
 };
 
 struct ref_formatting_state {
@@ -69,6 +79,9 @@ struct ref_formatting_state {
 
 struct atom_value {
 	const char *s;
+	union {
+		struct align align;
+	} u;
 	void (*handler)(struct atom_value *atomv, struct ref_formatting_state *state);
 	unsigned long ul; /* used for sorting when not FIELD_STR */
 };
@@ -165,7 +178,16 @@ static void quote_formatting(struct strbuf *s, const char *str, int quote_style)
 
 static void append_atom(struct atom_value *v, struct ref_formatting_state *state)
 {
-	quote_formatting(&state->stack->output, v->s, state->quote_style);
+	/*
+	 * Quote formatting is only done when the stack has a single
+	 * element. Otherwise quote formatting is done on the
+	 * element's entire output strbuf when the %(end) atom is
+	 * encountered.
+	 */
+	if (!state->stack->prev)
+		quote_formatting(&state->stack->output, v->s, state->quote_style);
+	else
+		strbuf_addstr(&state->stack->output, v->s);
 }
 
 static void push_stack_element(struct ref_formatting_stack **stack)
@@ -189,6 +211,48 @@ static void pop_stack_element(struct ref_formatting_stack **stack)
 	*stack = prev;
 }
 
+static void end_align_handler(struct ref_formatting_stack *stack)
+{
+	struct align *align = (struct align *)stack->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_release(&s);
+}
+
+static void align_atom_handler(struct atom_value *atomv, struct ref_formatting_state *state)
+{
+	struct ref_formatting_stack *new;
+
+	push_stack_element(&state->stack);
+	new = state->stack;
+	new->at_end = end_align_handler;
+	new->at_end_data = &atomv->u.align;
+}
+
+static void end_atom_handler(struct atom_value *atomv, struct ref_formatting_state *state)
+{
+	struct ref_formatting_stack *current = state->stack;
+	struct strbuf s = STRBUF_INIT;
+
+	if (!current->at_end)
+		die(_("format: %%(end) atom used without corresponding atom"));
+	current->at_end(current);
+
+	/*
+	 * 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) {
+		quote_formatting(&s, current->output.buf, state->quote_style);
+		strbuf_swap(&current->output, &s);
+	}
+	strbuf_release(&s);
+	pop_stack_element(&state->stack);
+}
+
 static int match_atom_name(const char *name, const char *atom_name, const char **val)
 {
 	const char *body;
@@ -773,6 +837,48 @@ static void populate_value(struct ref_array_item *ref)
 			else
 				v->s = " ";
 			continue;
+		} else if (match_atom_name(name, "align", &valp)) {
+			struct align *align = &v->u.align;
+			struct strbuf **s, **to_free;
+			int width = -1;
+
+			if (!valp)
+				die(_("expected format: %%(align:<width>,<position>)"));
+
+			/*
+			 * TODO: Implement a function similar to strbuf_split_str()
+			 * which would omit the separator from the end of each value.
+			 */
+			s = to_free = strbuf_split_str(valp, ',', 0);
+
+			align->position = ALIGN_LEFT;
+
+			while (*s) {
+				/*  Strip trailing comma */
+				if (s[1])
+					strbuf_setlen(s[0], s[0]->len - 1);
+				if (!strtoul_ui(s[0]->buf, 10, (unsigned int *)&width))
+					;
+				else if (!strcmp(s[0]->buf, "left"))
+					align->position = ALIGN_LEFT;
+				else if (!strcmp(s[0]->buf, "right"))
+					align->position = ALIGN_RIGHT;
+				else if (!strcmp(s[0]->buf, "middle"))
+					align->position = ALIGN_MIDDLE;
+				else
+					die(_("improper format entered align:%s"), s[0]->buf);
+				s++;
+			}
+
+			if (width < 0)
+				die(_("positive width expected with the %%(align) atom"));
+			align->width = width;
+			strbuf_list_free(to_free);
+			v->handler = align_atom_handler;
+			continue;
+		} else if (!strcmp(name, "end")) {
+			v->handler = end_atom_handler;
+			continue;
 		} else
 			continue;
 
@@ -1347,6 +1453,8 @@ void show_ref_array_item(struct ref_array_item *info, const char *format, int qu
 		resetv.s = color;
 		append_atom(&resetv, &state);
 	}
+	if (state.stack->prev)
+		die(_("format: %%(end) atom missing"));
 	final_buf = &state.stack->output;
 	fwrite(final_buf->buf, 1, final_buf->len, stdout);
 	pop_stack_element(&state.stack);