completions: add folder flexible search as an option

Provide an option to change the completion style when selecting a folder
from completing with folders starting with the input string to
completing with folders in which the input string is a substring present
at any point in the folder name.

References: https://lists.sr.ht/~sircmpwn/aerc/%3C20201129181020.186984-1-inwit%40sindominio.net%3E
References: https://lists.sr.ht/~sircmpwn/aerc/%3C20210223202536.199355-1-clayton%40craftyguy.net%3E
Signed-off-by: inwit <inwit@sindominio.net>

commands/commands.go

@@ -121,7 +121,7 @@ func GetFolders(aerc *widgets.Aerc, args []string) []string {
 		return aerc.SelectedAccount().Directories().List()
 	}
 	for _, dir := range aerc.SelectedAccount().Directories().List() {
-		if hasCaseSmartPrefix(dir, args[0]) {
+		if foundInString(dir, args[0], aerc.SelectedAccount().UiConfig().FuzzyFolderComplete) {
 			out = append(out, dir)
 		}
 	}
@@ -177,6 +177,14 @@ func GetLabels(aerc *widgets.Aerc, args []string) []string {
 	return out
 }
 
+func foundInString(s, substring string, fuzzy bool) bool {
+	if fuzzy {
+		return caseInsensitiveContains(s, substring)
+	} else {
+		return hasCaseSmartPrefix(s, substring)
+	}
+}
+
 // hasCaseSmartPrefix checks whether s starts with prefix, using a case
 // sensitive match if and only if prefix contains upper case letters.
 func hasCaseSmartPrefix(s, prefix string) bool {
@@ -186,6 +194,11 @@ func hasCaseSmartPrefix(s, prefix string) bool {
 	return strings.HasPrefix(strings.ToLower(s), strings.ToLower(prefix))
 }
 
+func caseInsensitiveContains(s, substr string) bool {
+	s, substr = strings.ToUpper(s), strings.ToUpper(substr)
+	return strings.Contains(s, substr)
+}
+
 func hasUpper(s string) bool {
 	for _, r := range s {
 		if unicode.IsUpper(r) {

config/aerc.conf.in

@@ -112,6 +112,10 @@ stylesets-dirs=@SHAREDIR@/stylesets/
 # Default: default
 styleset-name=default
 
+# Activates fuzzy search for IMAP folders: the typed string is search in the
+#   folder tree in any position, not necessarily at the beginning.
+#fuzzy-folder-complete=false
+
 #[ui:account=foo]
 #
 # Enable threading in the ui. Only works with notmuch:// and imap:// accounts

config/config.go

@@ -43,6 +43,7 @@ type UIConfig struct {
 	EmptyDirlist        string        `ini:"empty-dirlist"`
 	MouseEnabled        bool          `ini:"mouse-enabled"`
 	ThreadingEnabled    bool          `ini:"threading-enabled"`
+	FuzzyFolderComplete bool          `ini:"fuzzy-folder-complete"`
 	NewMessageBell      bool          `ini:"new-message-bell"`
 	Spinner             string        `ini:"spinner"`
 	SpinnerDelimiter    string        `ini:"spinner-delimiter"`
@@ -563,6 +564,7 @@ func LoadConfigFromFile(root *string, sharedir string, logger *log.Logger) (*Aer
 			EmptyDirlist:        "(no folders)",
 			MouseEnabled:        false,
 			NewMessageBell:      true,
+			FuzzyFolderComplete: false,
 			Spinner:             "[..]    , [..]   ,  [..]  ,   [..] ,    [..],   [..] ,  [..]  , [..]   ",
 			SpinnerDelimiter:    ",",
 			DirListFormat:       "%n %>r",

doc/aerc-config.5.scd

@@ -222,6 +222,12 @@ These options are configured in the *[ui]* section of aerc.conf.
 
 	Have a look at *aerc-stylesets*(7) as to how a styleset looks like.
 
+*fuzzy-folder-complete*
+	When finding a folder with cf or move, for example, the popover will
+	how not only the folders /starting/ with the string input by the user,
+	but it will show coincidences of folders /containing/ the string in any
+	position of their name. This is case-independent.
+
 *threading-enabled*
 	Enable a threaded viewing of messages, works with IMAP (when there's
 	server support) and NotMuch backends.