Add command completion with tab key

In the command prompt, press <tab> to get a list of all available
commands and pick one (using vis-menu). This works also after typing the
first letters of a command (p.e. `:la<tab>`).

lua/plugins/complete-filename.lua

@@ -22,9 +22,19 @@ local complete_filename = function(expand)
 		range.finish = pos
 	end
 
-	local cmdfmt = "vis-complete --file '%s'"
-	if expand then cmdfmt = "vis-open -- '%s'*" end
-	local status, out, err = vis:pipe(cmdfmt:format(prefix:gsub("'", "'\\''")))
+	local status, out, err
+	if prefix:sub(1, 1) == ":" then
+		status, out, err = vis:complete_command(prefix:sub(2))
+		if out then
+			out = out:gsub("\n$", ""):sub(#prefix) .. " "
+		end
+		pos = range.start + #prefix
+		expand = false
+	else
+		local cmdfmt = "vis-complete --file '%s'"
+		if expand then cmdfmt = "vis-open -- '%s'*" end
+		status, out, err = vis:pipe(cmdfmt:format(prefix:gsub("'", "'\\''")))
+	end
 	if status ~= 0 or not out then
 		if err then vis:info(err) end
 		return
@@ -48,4 +58,4 @@ end, "Complete file name")
 -- complete file path at primary selection location using vis-open(1)
 vis:map(vis.modes.INSERT, "<C-x><C-o>", function()
 	complete_filename(true);
-end, "Complete file name (expands path)")
+end, "Complete file name (expands path) or command")

vis-cmds.c

@@ -696,6 +696,19 @@ static bool print_cmd(const char *key, void *value, void *data) {
 	return text_appendf(data, "  %-30s %s\n", usage, help ? help : "");
 }
 
+static bool print_cmd_name(const char *key, void *value, void *data) {
+	CommandDef *cmd = value;
+	size_t len_data = strlen(data);
+	size_t remaining_capacity = VIS_COMMAND_BUFFER_MAX - len_data;
+	if (remaining_capacity >= strlen(cmd->name) + 2)  /* adding 2 for \n and \0 */
+		snprintf(data + len_data, remaining_capacity, "%s\n", cmd->name);
+	return true;
+}
+
+void vis_print_cmds(Vis *vis, char *cmd_list) {
+	map_iterate(vis->cmds, print_cmd_name, cmd_list);
+}
+
 static bool print_option(const char *key, void *value, void *txt) {
 	char desc[256];
 	const OptionDef *opt = value;

vis-lua.c

@@ -1146,6 +1146,44 @@ static int command_register(lua_State *L) {
 	return 1;
 }
 
+/***
+ * Let user pick a command matching the given prefix.
+ *
+ * The editor core will be blocked while the external process is running.
+ *
+ * @function complete_command
+ * @tparam string prefix the prefix of the command to be completed
+ * @treturn int code the exit status of the executed command
+ * @treturn string stdout the data written to stdout
+ * @treturn string stderr the data written to stderr
+ */
+static int complete_command(lua_State *L) {
+	Vis *vis = obj_ref_check(L, 1, "vis");
+	const char *prefix = luaL_checkstring(L, 2);
+	char *out = NULL, *err = NULL;
+	File *file = vis->win ? vis->win->file : NULL;
+	Filerange range = text_range_new(0, 0);
+
+	char cmd_list[VIS_COMMAND_BUFFER_MAX] = {0};
+	vis_print_cmds(vis, cmd_list);
+	char cmd[VIS_COMMAND_BUFFER_MAX + 32] = {0}; /* 32 for echo/grep/vis-menu */
+	sprintf(cmd, "echo '%s' | grep '^%s' | vis-menu -b", cmd_list, prefix);
+
+	int status = vis_pipe_collect(vis, file, &range, (const char*[]){ cmd, NULL }, &out, &err, false);
+	lua_pushinteger(L, status);
+	if (out)
+		lua_pushstring(L, out);
+	else
+		lua_pushnil(L);
+	free(out);
+	if (err)
+		lua_pushstring(L, err);
+	else
+		lua_pushnil(L);
+	free(err);
+	return 3;
+}
+
 /***
  * Push keys to input queue and interpret them.
  *
@@ -1531,6 +1569,7 @@ static const struct luaL_Reg vis_lua[] = {
 	{ "option_register", option_register },
 	{ "option_unregister", option_unregister },
 	{ "command_register", command_register },
+	{ "complete_command", complete_command },
 	{ "feedkeys", feedkeys },
 	{ "insert", insert },
 	{ "replace", replace },

vis.h

@@ -40,6 +40,9 @@ typedef struct Win Win;
 /* maximum bytes needed for string representation of a (pseudo) key */
 #define VIS_KEY_LENGTH_MAX 64
 
+/* maximum bytes used for list of available commands */
+#define VIS_COMMAND_BUFFER_MAX 1000
+
 /** Union used to pass arguments to key action functions. */
 typedef union {
 	bool b;
@@ -838,6 +841,9 @@ bool vis_option_unregister(Vis*, const char *name);
 /** Execute any kind (``:``, ``?``, ``/``) of prompt command */
 bool vis_prompt_cmd(Vis*, const char *cmd);
 
+/** Write newline separated list of available commands to ``cmd_list`` */
+void vis_print_cmds(Vis*, char *cmd_list);
+
 /**
  * Pipe a given file range to an external process.
  *