sequencer: introduce todo_list_write_to_file()

This introduces a new function to recreate the text of a todo list from
its commands and write it to a file.  This will be useful as the next
few commits will change the use of the buffer in struct todo_list so it
will no longer be a mirror of the file on disk.

This functionality already exists in todo_list_transform(), but this
function was made to replace the buffer of a todo list, which is not
what we want here.  Thus, the part of todo_list_transform() that
replaces the buffer is dropped, and the function is renamed
todo_list_to_strbuf().  It is called by todo_list_write_to_file() to
fill the buffer to write to the disk.

todo_list_write_to_file() can also take care of appending the help text
to the buffer before writing it to the disk, or to write only the first
n items of the list.  This feature will be used by
skip_unnecessary_picks(), which has to write done commands in a file.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -4562,26 +4562,28 @@ int sequencer_add_exec_commands(struct repository *r,
 	return i;
 }
 
-void todo_list_transform(struct repository *r, struct todo_list *todo_list,
-			 unsigned flags)
+static void todo_list_to_strbuf(struct repository *r, struct todo_list *todo_list,
+				struct strbuf *buf, int num, unsigned flags)
 {
-	struct strbuf buf = STRBUF_INIT;
 	struct todo_item *item;
-	int i;
+	int i, max = todo_list->nr;
 
-	for (item = todo_list->items, i = 0; i < todo_list->nr; i++, item++) {
+	if (num > 0 && num < max)
+		max = num;
+
+	for (item = todo_list->items, i = 0; i < max; i++, item++) {
 		/* if the item is not a command write it and continue */
 		if (item->command >= TODO_COMMENT) {
-			strbuf_addf(&buf, "%.*s\n", item->arg_len,
+			strbuf_addf(buf, "%.*s\n", item->arg_len,
 				    todo_item_get_arg(todo_list, item));
 			continue;
 		}
 
 		/* add command to the buffer */
 		if (flags & TODO_LIST_ABBREVIATE_CMDS)
-			strbuf_addch(&buf, command_to_char(item->command));
+			strbuf_addch(buf, command_to_char(item->command));
 		else
-			strbuf_addstr(&buf, command_to_string(item->command));
+			strbuf_addstr(buf, command_to_string(item->command));
 
 		/* add commit id */
 		if (item->commit) {
@@ -4591,28 +4593,48 @@ void todo_list_transform(struct repository *r, struct todo_list *todo_list,
 
 			if (item->command == TODO_MERGE) {
 				if (item->flags & TODO_EDIT_MERGE_MSG)
-					strbuf_addstr(&buf, " -c");
+					strbuf_addstr(buf, " -c");
 				else
-					strbuf_addstr(&buf, " -C");
+					strbuf_addstr(buf, " -C");
 			}
 
-			strbuf_addf(&buf, " %s", oid);
+			strbuf_addf(buf, " %s", oid);
 		}
 
 		/* add all the rest */
 		if (!item->arg_len)
-			strbuf_addch(&buf, '\n');
+			strbuf_addch(buf, '\n');
 		else
-			strbuf_addf(&buf, " %.*s\n", item->arg_len,
+			strbuf_addf(buf, " %.*s\n", item->arg_len,
 				    todo_item_get_arg(todo_list, item));
 	}
+}
 
-	strbuf_reset(&todo_list->buf);
-	strbuf_add(&todo_list->buf, buf.buf, buf.len);
+int todo_list_write_to_file(struct repository *r, struct todo_list *todo_list,
+			    const char *file, const char *shortrevisions,
+			    const char *shortonto, int num, unsigned flags)
+{
+	int edit_todo = !(shortrevisions && shortonto), res;
+	struct strbuf buf = STRBUF_INIT;
+
+	todo_list_to_strbuf(r, todo_list, &buf, num, flags);
+
+	if (flags & TODO_LIST_APPEND_TODO_HELP) {
+		int command_count = count_commands(todo_list);
+		if (!edit_todo) {
+			strbuf_addch(&buf, '\n');
+			strbuf_commented_addf(&buf, Q_("Rebase %s onto %s (%d command)",
+						       "Rebase %s onto %s (%d commands)",
+						       command_count),
+					      shortrevisions, shortonto, command_count);
+		}
+		append_todo_help(edit_todo, flags & TODO_LIST_KEEP_EMPTY, &buf);
+	}
+
+	res = write_message(buf.buf, buf.len, file, 0);
 	strbuf_release(&buf);
 
-	if (todo_list_parse_insn_buffer(r, todo_list->buf.buf, todo_list))
-		BUG("unusable todo list");
+	return res;
 }
 
 int transform_todo_file(struct repository *r, unsigned flags)
@@ -4629,9 +4651,8 @@ int transform_todo_file(struct repository *r, unsigned flags)
 		return error(_("unusable todo list: '%s'"), todo_file);
 	}
 
-	todo_list_transform(r, &todo_list, flags);
-
-	res = write_message(todo_list.buf.buf, todo_list.buf.len, todo_file, 0);
+	res = todo_list_write_to_file(r, &todo_list, todo_file,
+				      NULL, NULL, -1, flags);
 	todo_list_release(&todo_list);
 
 	if (res)

sequencer.h

@@ -121,8 +121,9 @@ struct todo_list {
 
 int todo_list_parse_insn_buffer(struct repository *r, char *buf,
 				struct todo_list *todo_list);
-void todo_list_transform(struct repository *r, struct todo_list *todo_list,
-			 unsigned flags);
+int todo_list_write_to_file(struct repository *r, struct todo_list *todo_list,
+			    const char *file, const char *shortrevisions,
+			    const char *shortonto, int num, unsigned flags);
 void todo_list_release(struct todo_list *todo_list);
 const char *todo_item_get_arg(struct todo_list *todo_list,
 			      struct todo_item *item);
@@ -145,8 +146,10 @@ int sequencer_remove_state(struct replay_opts *opts);
  * commits should be rebased onto the new base, this flag needs to be passed.
  */
 #define TODO_LIST_REBASE_COUSINS (1U << 4)
-int sequencer_make_script(struct repository *repo, FILE *out,
-			  int argc, const char **argv,
+#define TODO_LIST_APPEND_TODO_HELP (1U << 5)
+
+int sequencer_make_script(struct repository *r, FILE *out, int argc,
+			  const char **argv,
 			  unsigned flags);
 
 int sequencer_add_exec_commands(struct repository *r, const char *command);