ncdu: add sort by average size in directory

Add keyboard shortcut 'A' which sort by average size in directory.

If files/folders have same avgSize sort by actual size

Fixes: #4699

cmd/ncdu/ncdu.go

@@ -71,7 +71,7 @@ func helpText() (tr []string) {
 		" ←,h to return",
 		" c toggle counts",
 		" g toggle graph",
-		" n,s,C sort by name,size,count",
+		" n,s,C,A sort by name,size,count,average size",
 		" d delete file/directory",
 	}
 	if !clipboard.Unsupported {
@@ -108,6 +108,7 @@ type UI struct {
 	sortByName        int8          // +1 for normal, 0 for off, -1 for reverse
 	sortBySize        int8
 	sortByCount       int8
+	sortByAverageSize int8
 	dirPosMap         map[string]dirPos // store for directory positions
 }
 
@@ -496,9 +497,17 @@ type ncduSort struct {
 
 // Less is part of sort.Interface.
 func (ds *ncduSort) Less(i, j int) bool {
+	var iAvgSize, jAvgSize float64
 	isize, icount, _, _ := ds.d.AttrI(ds.sortPerm[i])
 	jsize, jcount, _, _ := ds.d.AttrI(ds.sortPerm[j])
 	iname, jname := ds.entries[ds.sortPerm[i]].Remote(), ds.entries[ds.sortPerm[j]].Remote()
+	if icount > 0 {
+		iAvgSize = float64(isize / icount)
+	}
+	if jcount > 0 {
+		jAvgSize = float64(jsize / jcount)
+	}
+
 	switch {
 	case ds.u.sortByName < 0:
 		return iname > jname
@@ -520,6 +529,18 @@ func (ds *ncduSort) Less(i, j int) bool {
 		if icount != jcount {
 			return icount > jcount
 		}
+	case ds.u.sortByAverageSize < 0:
+		if iAvgSize != jAvgSize {
+			return iAvgSize < jAvgSize
+		}
+		// if avgSize is equal, sort by size
+		return isize < jsize
+	case ds.u.sortByAverageSize > 0:
+		if iAvgSize != jAvgSize {
+			return iAvgSize > jAvgSize
+		}
+		// if avgSize is equal, sort by size
+		return isize > jsize
 	}
 	// if everything equal, sort by name
 	return iname < jname
@@ -628,6 +649,7 @@ func (u *UI) toggleSort(sortType *int8) {
 	u.sortBySize = 0
 	u.sortByCount = 0
 	u.sortByName = 0
+	u.sortByAverageSize = 0
 	if old == 0 {
 		*sortType = 1
 	} else {
@@ -742,6 +764,8 @@ outer:
 					u.toggleSort(&u.sortBySize)
 				case 'C':
 					u.toggleSort(&u.sortByCount)
+				case 'A':
+					u.toggleSort(&u.sortByAverageSize)
 				case 'y':
 					u.copyPath()
 				case 'Y':