ref-filter: introduce handler function for each atom

Introduce a handler function for each atom, which is called when the
atom is processed in show_ref_array_item().

In this context make append_atom() as the default handler function and
extract quote_formatting() out of append_atom(). Bump this to the top.

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>

ref-filter.c

@@ -69,6 +69,7 @@ struct ref_formatting_state {
 
 struct atom_value {
 	const char *s;
+	void (*handler)(struct atom_value *atomv, struct ref_formatting_state *state);
 	unsigned long ul; /* used for sorting when not FIELD_STR */
 };
 
@@ -141,6 +142,32 @@ int parse_ref_filter_atom(const char *atom, const char *ep)
 	return at;
 }
 
+static void quote_formatting(struct strbuf *s, const char *str, int quote_style)
+{
+	switch (quote_style) {
+	case QUOTE_NONE:
+		strbuf_addstr(s, str);
+		break;
+	case QUOTE_SHELL:
+		sq_quote_buf(s, str);
+		break;
+	case QUOTE_PERL:
+		perl_quote_buf(s, str);
+		break;
+	case QUOTE_PYTHON:
+		python_quote_buf(s, str);
+		break;
+	case QUOTE_TCL:
+		tcl_quote_buf(s, str);
+		break;
+	}
+}
+
+static void append_atom(struct atom_value *v, struct ref_formatting_state *state)
+{
+	quote_formatting(&state->stack->output, v->s, state->quote_style);
+}
+
 static void push_stack_element(struct ref_formatting_stack **stack)
 {
 	struct ref_formatting_stack *s = xcalloc(1, sizeof(struct ref_formatting_stack));
@@ -662,6 +689,8 @@ static void populate_value(struct ref_array_item *ref)
 		const char *formatp;
 		struct branch *branch = NULL;
 
+		v->handler = append_atom;
+
 		if (*name == '*') {
 			deref = 1;
 			name++;
@@ -1228,29 +1257,6 @@ void ref_array_sort(struct ref_sorting *sorting, struct ref_array *array)
 	qsort(array->items, array->nr, sizeof(struct ref_array_item *), compare_refs);
 }
 
-static void append_atom(struct atom_value *v, struct ref_formatting_state *state)
-{
-	struct strbuf *s = &state->stack->output;
-
-	switch (state->quote_style) {
-	case QUOTE_NONE:
-		strbuf_addstr(s, v->s);
-		break;
-	case QUOTE_SHELL:
-		sq_quote_buf(s, v->s);
-		break;
-	case QUOTE_PERL:
-		perl_quote_buf(s, v->s);
-		break;
-	case QUOTE_PYTHON:
-		python_quote_buf(s, v->s);
-		break;
-	case QUOTE_TCL:
-		tcl_quote_buf(s, v->s);
-		break;
-	}
-}
-
 static int hex1(char ch)
 {
 	if ('0' <= ch && ch <= '9')
@@ -1307,7 +1313,7 @@ void show_ref_array_item(struct ref_array_item *info, const char *format, int qu
 		if (cp < sp)
 			append_literal(cp, sp, &state);
 		get_ref_atom_value(info, parse_ref_filter_atom(sp + 2, ep), &atomv);
-		append_atom(atomv, &state);
+		atomv->handler(atomv, &state);
 	}
 	if (*cp) {
 		sp = cp + strlen(cp);