commands: expand and complete template code

Expand and complete template code. The exline does understand template
code and can evaluate it. With this patch, the completion systems
supports writing the templates by showing the available template data
fields and template functions. Pressing <Tab> after }} will show the
evaluated template in the completion menu. Complex template code, such
as if-else statements work as well.

Examples:

:filter -f {{<Tab>
will show the possible template data fields and functions

:filter -f {{index (.To|email) 0}}<Tab>
will show the value of the template expression in the completion list

Pressing <Tab> twice after a completed template expression will
substitute the expression with its value.

:{{if match .Folder "INBOX"}}check-mail{{else}}cf INBOX{{end}}

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 52 insertions, 0 deletions

diff --git a/commands/commands.go b/commands/commands.go
index f50a1e8a..c6f95aaa 100644
--- a/commands/commands.go
+++ b/commands/commands.go
@@ -141,6 +141,58 @@ func (cmds *Commands) ExecuteCommand(
 	return NoSuchCommand(args[0])
 }
 
+func GetTemplateCompletion(
+	aerc *widgets.Aerc, cmd string,
+) ([]string, string, bool) {
+	args, err := splitCmd(cmd)
+	if err != nil || len(args) == 0 {
+		return nil, "", false
+	}
+
+	countLeft := strings.Count(cmd, "{{")
+	if countLeft == 0 {
+		return nil, "", false
+	}
+	countRight := strings.Count(cmd, "}}")
+
+	switch {
+	case countLeft > countRight:
+		// complete template terms
+		var i int
+		for i = len(cmd) - 1; i >= 0; i-- {
+			if strings.ContainsRune("{()| ", rune(cmd[i])) {
+				break
+			}
+		}
+		search, prefix := cmd[i+1:], cmd[:i+1]
+		padding := strings.Repeat(" ",
+			len(search)-len(strings.TrimLeft(search, " ")))
+		options := FilterList(
+			templates.Terms(),
+			strings.TrimSpace(search),
+			"",
+			aerc.SelectedAccountUiConfig().FuzzyComplete,
+		)
+		return options, prefix + padding, true
+	case countLeft == countRight:
+		// expand template
+		data := templateData(aerc, nil, nil)
+		t, err := templates.ParseTemplate("", cmd)
+		if err != nil {
+			log.Warnf("template parsing failed: %v", err)
+			return nil, "", false
+		}
+		var sb strings.Builder
+		if err = templates.Render(t, &sb, data); err != nil {
+			log.Warnf("template rendering failed: %v", err)
+			return nil, "", false
+		}
+		return []string{sb.String()}, "", true
+	}
+
+	return nil, "", false
+}
+
 // GetCompletions returns the completion options and the command prefix
 func (cmds *Commands) GetCompletions(
 	aerc *widgets.Aerc, cmd string,