var b strings.Builder
a := strconv.Itoa(i)
b.Grow(len(dir) + 1 + len(a) + len(cArenaFilePrefix))
b.WriteString(dir)
b.WriteByte('/')
b.WriteString(a)
b.WriteString(cArenaFilePrefix)

func BenchmarkSpeeds(b *testing.B) {
	b.Run("compare speeds", func(b *testing.B) {
		var dir = "/home/user/datadir"
		var p = ""
		b.Run("builder", func(b *testing.B) {
			b.ReportAllocs()
			for i := 0; i < b.N; i++ {
				var b strings.Builder
                                a := strconv.Itoa(i)
				b.Grow(len(dir) + 1 + len(a) + len("_arena.dat"))
				b.WriteString(dir)
				b.WriteByte('/')
				b.WriteString(a)
				b.WriteString("_arena.dat")
				p = b.String()
			}
		})
		b.Run("slice", func(b *testing.B) {
			var filePath []byte
			b.ReportAllocs()
			for i := 0; i < b.N; i++ {
				filePath = append(filePath[:0], dir...)
				filePath = append(filePath, '/')
				filePath = strconv.AppendInt(filePath, int64(i), 10)
				filePath = append(filePath, "_arena.dat"...)
				p = string(filePath)
			}
		})
		b.Run("path", func(b *testing.B) {
			b.ReportAllocs()
			for i := 0; i < b.N; i++ {
				fileName := strconv.Itoa(i) + "_arena.dat"
				p = path.Join(dir, fileName)
			}
		})
		runtime.KeepAlive(p)
	})
}