Only run the merge tool on files with differences

If the backup file and the data file are equal, there is no need to run
the merge tool.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.c

@@ -290,13 +290,21 @@ static inline void key_generic_reload(void)
 			io_save_todo(path_todo_backup);
 			io_save_mutex_unlock();
 
-			const char *arg_apts[] = { conf.mergetool, path_apts,
-						   path_apts_backup, NULL };
-			wins_launch_external(arg_apts);
+			if (!io_files_equal(path_apts, path_apts_backup)) {
+				const char *arg_apts[] = { conf.mergetool,
+							   path_apts,
+							   path_apts_backup,
+							   NULL };
+				wins_launch_external(arg_apts);
+			}
 
-			const char *arg_todo[] = { conf.mergetool, path_todo,
-						   path_todo_backup, NULL };
-			wins_launch_external(arg_todo);
+			if (!io_files_equal(path_todo, path_todo_backup)) {
+				const char *arg_todo[] = { conf.mergetool,
+							   path_todo,
+							   path_todo_backup,
+							   NULL };
+				wins_launch_external(arg_todo);
+			}
 
 			xfree(path_apts_backup);
 			xfree(path_todo_backup);

src/calcurse.h

@@ -781,6 +781,7 @@ void io_stop_psave_thread(void);
 void io_set_lock(void);
 unsigned io_dump_pid(char *);
 unsigned io_get_pid(char *);
+int io_files_equal(const char *, const char *);
 int io_file_is_empty(char *);
 int io_file_cp(const char *, const char *);
 void io_unset_modified(void);

src/io.c

@@ -1332,6 +1332,32 @@ int io_file_is_empty(char *file)
 	return -1;
 }
 
+/*
+ * Check whether two files are equal.
+ */
+int io_files_equal(const char *file1, const char *file2)
+{
+	FILE *fp1, *fp2;
+	int ret = 0;
+
+	if (!file1 || !file2)
+		return 0;
+
+	fp1 = fopen(file1, "rb");
+	fp2 = fopen(file2, "rb");
+
+	while (!feof(fp1) && !feof(fp2)) {
+		if (fgetc(fp1) != fgetc(fp2))
+			goto cleanup;
+	}
+
+	ret = 1;
+cleanup:
+	fclose(fp1);
+	fclose(fp2);
+	return ret;
+}
+
 /*
  * Copy an existing file to a new location.
  */