| package compress |
| |
| import "math" |
| |
| // Estimate returns a normalized compressibility estimate of block b. |
| // Values close to zero are likely uncompressible. |
| // Values above 0.1 are likely to be compressible. |
| // Values above 0.5 are very compressible. |
| // Very small lengths will return 0. |
| func Estimate(b []byte) float64 { |
| if len(b) < 16 { |
| return 0 |
| } |
| |
| // Correctly predicted order 1 |
| hits := 0 |
| lastMatch := false |
| var o1 [256]byte |
| var hist [256]int |
| c1 := byte(0) |
| for _, c := range b { |
| if c == o1[c1] { |
| // We only count a hit if there was two correct predictions in a row. |
| if lastMatch { |
| hits++ |
| } |
| lastMatch = true |
| } else { |
| lastMatch = false |
| } |
| o1[c1] = c |
| c1 = c |
| hist[c]++ |
| } |
| |
| // Use x^0.6 to give better spread |
| prediction := math.Pow(float64(hits)/float64(len(b)), 0.6) |
| |
| // Calculate histogram distribution |
| variance := float64(0) |
| avg := float64(len(b)) / 256 |
| |
| for _, v := range hist { |
| Δ := float64(v) - avg |
| variance += Δ * Δ |
| } |
| |
| stddev := math.Sqrt(float64(variance)) / float64(len(b)) |
| exp := math.Sqrt(1 / float64(len(b))) |
| |
| // Subtract expected stddev |
| stddev -= exp |
| if stddev < 0 { |
| stddev = 0 |
| } |
| stddev *= 1 + exp |
| |
| // Use x^0.4 to give better spread |
| entropy := math.Pow(stddev, 0.4) |
| |
| // 50/50 weight between prediction and histogram distribution |
| return math.Pow((prediction+entropy)/2, 0.9) |
| } |
| |
| // ShannonEntropyBits returns the number of bits minimum required to represent |
| // an entropy encoding of the input bytes. |
| // https://en.wiktionary.org/wiki/Shannon_entropy |
| func ShannonEntropyBits(b []byte) int { |
| if len(b) == 0 { |
| return 0 |
| } |
| var hist [256]int |
| for _, c := range b { |
| hist[c]++ |
| } |
| shannon := float64(0) |
| invTotal := 1.0 / float64(len(b)) |
| for _, v := range hist[:] { |
| if v > 0 { |
| n := float64(v) |
| shannon += math.Ceil(-math.Log2(n*invTotal) * n) |
| } |
| } |
| return int(math.Ceil(shannon)) |
| } |