commands: implement prompt completion

This patch implements :prompt completion.
The completion mechanism only provides completions when there is at least
one argument specified (prompt text).
The mechanism is based on other commands' completions and works as follows:

   1. Attempts to look up a command by the name specified in args[1].

   2.a On success it uses command.Complete.

   2.b Otherwise, if total arguments count is lesser or equals than 2
       (i.e. no command arguments specified), it attempts to complete
       the command's name.

Additional effort is made to preserve prompt text, which often contains
spaces and formatting.

Signed-off-by: Sergey Smirnykh <sergey.smirnykh@siborgium.xyz>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/commands.go

@@ -37,6 +37,13 @@ func (cmds *Commands) Names() []string {
 	return names
 }
 
+func (cmds *Commands) ByName(name string) Command {
+	if cmd, ok := cmds.dict()[name]; ok {
+		return cmd
+	}
+	return nil
+}
+
 func (cmds *Commands) Register(cmd Command) {
 	// TODO enforce unique aliases, until then, duplicate each
 	if len(cmd.Aliases()) < 1 {

commands/prompt.go

@@ -2,8 +2,8 @@ package commands
 
 import (
 	"fmt"
-
 	"git.sr.ht/~rjarry/aerc/widgets"
+	"strings"
 )
 
 type Prompt struct{}
@@ -17,7 +17,55 @@ func (Prompt) Aliases() []string {
 }
 
 func (Prompt) Complete(aerc *widgets.Aerc, args []string) []string {
-	return nil // TODO: add completions
+	argc := len(args)
+	if argc == 0 {
+		return nil
+	}
+	hascommand := argc > 2
+	if argc == 1 {
+		args = append(args, "")
+	}
+
+	cmd := GlobalCommands.ByName(args[1])
+	var cs []string
+	if cmd != nil {
+		cs = cmd.Complete(aerc, args[2:])
+		hascommand = true
+	} else {
+		if hascommand {
+			return nil
+		}
+		cs = GlobalCommands.GetCompletions(aerc, args[1])
+	}
+	if cs == nil {
+		return nil
+	}
+
+	var b strings.Builder
+	// it seems '' quoting is enough
+	// to keep quoted arguments in one piece
+	b.WriteRune('\'')
+	b.WriteString(args[0])
+	b.WriteRune('\'')
+	b.WriteRune(' ')
+	if hascommand {
+		b.WriteString(args[1])
+		b.WriteRune(' ')
+	}
+
+	src := b.String()
+	b.Reset()
+
+	rs := make([]string, 0, len(cs))
+	for _, c := range cs {
+		b.WriteString(src)
+		b.WriteString(c)
+
+		rs = append(rs, b.String())
+		b.Reset()
+	}
+
+	return rs
 }
 
 func (Prompt) Execute(aerc *widgets.Aerc, args []string) error {