user can configure visible columns in track-list

backend/tracklist/config.go

@@ -0,0 +1,104 @@
+// Package tracklist manages track-list display configuration.
+package tracklist
+
+import (
+	"errors"
+	"fmt"
+	"slices"
+)
+
+var (
+	errUnknownColumnID = errors.New("unknown track-list column ID")
+	errDuplicateColumn = errors.New("duplicate column ID")
+)
+
+// ColumnID identifies a displayable column in the track list.
+type ColumnID string
+
+// Valid column identifiers.
+const (
+	ColTrackName   ColumnID = "trackName"
+	ColArtistName  ColumnID = "artistName"
+	ColTrackLength ColumnID = "trackLength"
+	ColAlbum       ColumnID = "album"
+	ColGenre       ColumnID = "genre"
+	ColYear        ColumnID = "year"
+	ColComposer    ColumnID = "composer"
+	ColTrackNumber ColumnID = "trackNumber"
+	ColDiscNumber  ColumnID = "discNumber"
+	ColFilePath    ColumnID = "filePath"
+	ColFileType    ColumnID = "fileType"
+)
+
+// AllColumnIDs lists every recognised column in default display
+// order.
+var AllColumnIDs = []ColumnID{
+	ColTrackName,
+	ColArtistName,
+	ColTrackLength,
+	ColAlbum,
+	ColGenre,
+	ColYear,
+	ColComposer,
+	ColTrackNumber,
+	ColDiscNumber,
+	ColFilePath,
+	ColFileType,
+}
+
+// DefaultColumns is the initial column configuration matching the
+// original hardcoded layout.
+var DefaultColumns = []Column{
+	{ID: ColTrackName},
+	{ID: ColArtistName},
+	{ID: ColTrackLength},
+}
+
+// Column represents a visible column in the track list.
+type Column struct {
+	ID ColumnID `json:"id" toml:"ID"`
+}
+
+// Config holds track-list display preferences.
+type Config struct {
+	Columns []Column `json:"columns" toml:"Columns"`
+}
+
+// ApplyDefaults fills zero-value fields with sensible defaults.
+func (c *Config) ApplyDefaults() {
+	if len(c.Columns) == 0 {
+		c.Columns = make([]Column, len(DefaultColumns))
+		copy(c.Columns, DefaultColumns)
+	}
+}
+
+// Validate checks that every column ID is recognised and that
+// there are no duplicates.
+func (c *Config) Validate() error {
+	c.ApplyDefaults()
+
+	seen := make(map[ColumnID]bool, len(c.Columns))
+
+	for _, col := range c.Columns {
+		if !isValidColumnID(col.ID) {
+			return fmt.Errorf(
+				"%w: %q", errUnknownColumnID, col.ID,
+			)
+		}
+
+		if seen[col.ID] {
+			return fmt.Errorf(
+				"%w: %q", errDuplicateColumn, col.ID,
+			)
+		}
+
+		seen[col.ID] = true
+	}
+
+	return nil
+}
+
+// isValidColumnID returns true when id matches a known column.
+func isValidColumnID(id ColumnID) bool {
+	return slices.Contains(AllColumnIDs, id)
+}