add strbuf_expand_dict_cb(), a helper for simple cases

The new callback function strbuf_expand_dict_cb() can be used together
with strbuf_expand() if there is only a small number of placeholders
for static replacement texts.  It expects its dictionary as an array of
placeholder+value pairs as context parameter, terminated by an entry
with the placeholder member set to NULL.

The new helper is intended to aid converting the remaining calls of
interpolate().  strbuf_expand() is smaller, more flexible and can be
used to go faster than interpolate(), so it should replace the latter.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-strbuf.txt

@@ -205,6 +205,13 @@ In order to facilitate caching and to make it possible to give
 parameters to the callback, `strbuf_expand()` passes a context pointer,
 which can be used by the programmer of the callback as she sees fit.
 
+`strbuf_expand_dict_cb`::
+
+	Used as callback for `strbuf_expand()`, expects an array of
+	struct strbuf_expand_dict_entry as context, i.e. pairs of
+	placeholder and replacement string.  The array needs to be
+	terminated by an entry with placeholder set to NULL.
+
 `strbuf_addf`::
 
 	Add a formatted string to the buffer.

strbuf.c

@@ -237,6 +237,22 @@ void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn,
 	}
 }
 
+size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder,
+		void *context)
+{
+	struct strbuf_expand_dict_entry *e = context;
+	size_t len;
+
+	for (; e->placeholder && (len = strlen(e->placeholder)); e++) {
+		if (!strncmp(placeholder, e->placeholder, len)) {
+			if (e->value)
+				strbuf_addstr(sb, e->value);
+			return len;
+		}
+	}
+	return 0;
+}
+
 size_t strbuf_fread(struct strbuf *sb, size_t size, FILE *f)
 {
 	size_t res;

strbuf.h

@@ -111,6 +111,11 @@ extern void strbuf_adddup(struct strbuf *sb, size_t pos, size_t len);
 
 typedef size_t (*expand_fn_t) (struct strbuf *sb, const char *placeholder, void *context);
 extern void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn, void *context);
+struct strbuf_expand_dict_entry {
+	const char *placeholder;
+	const char *value;
+};
+extern size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder, void *context);
 
 __attribute__((format(printf,2,3)))
 extern void strbuf_addf(struct strbuf *sb, const char *fmt, ...);