text: add text_{v,}printf function

Convenient way to insert formated data into a Text.

text.c

@@ -634,6 +634,24 @@ bool text_insert(Text *txt, size_t pos, const char *data, size_t len) {
 	return true;
 }
 
+bool text_printf(Text *txt, size_t pos, const char *format, ...) {
+	va_list ap;
+	va_start(ap, format);
+	bool ret = text_vprintf(txt, pos, format, ap);
+	va_end(ap);
+	return ret;
+}
+
+bool text_vprintf(Text *txt, size_t pos, const char *format, va_list ap) {
+	int len = vsnprintf(NULL, 0, format, ap);
+	if (len == -1)
+		return false;
+	char *buf = malloc(len+1);
+	bool ret = buf && (vsnprintf(buf, len+1, format, ap) == len) && text_insert(txt, pos, buf, len);
+	free(buf);
+	return ret;
+}
+
 static size_t action_undo(Text *txt, Action *a) {
 	size_t pos = EPOS;
 	for (Change *c = a->change; c; c = c->next) {

text.h

@@ -4,6 +4,7 @@
 #include <stdbool.h>
 #include <time.h>
 #include <unistd.h>
+#include <stdarg.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 
@@ -36,6 +37,8 @@ typedef struct {
 Text *text_load(const char *filename);
 /* file information at time of load or last save */
 struct stat text_stat(Text*);
+bool text_printf(Text*, size_t pos, const char *format, ...);
+bool text_vprintf(Text*, size_t pos, const char *format, va_list ap);
 /* insert `len' bytes starting from `data' at `pos' which has to be
  * in the interval [0, text_size(txt)] */
 bool text_insert(Text*, size_t pos, const char *data, size_t len);