Update

vendor/github.com/klauspost/compress/huff0/build_table.go

@@ -0,0 +1,168 @@
+package huff0
+
+import "errors"
+
+// BuildCTable builds a Huffman compression table from a precomputed symbol
+// histogram and installs it as the previous (reuse) table on s.
+//
+// After this call:
+//   - EstimateSize/CanUseTable can probe the table against other histograms.
+//   - Compress1X/Compress4X with Reuse = ReusePolicyMust will encode without
+//     emitting a new table header.
+//   - TransferCTable can hand the table to a sibling Scratch.
+//
+// count[i] is the number of occurrences of symbol i. The histogram must have
+// at least 2 distinct non-zero symbols; ErrUseRLE is returned for a single
+// symbol and an error is returned for an empty histogram.
+func (s *Scratch) BuildCTable(count *[256]uint32) error {
+	if s == nil {
+		return errors.New("huff0: BuildCTable on nil Scratch")
+	}
+	if count == nil {
+		return errors.New("huff0: nil count passed to BuildCTable")
+	}
+	var err error
+	s, err = s.prepare(nil)
+	if err != nil {
+		return err
+	}
+	s.count = *count
+	var total, maxCount int
+	var symLen uint16
+	for i, v := range s.count {
+		total += int(v)
+		if int(v) > maxCount {
+			maxCount = int(v)
+		}
+		if v != 0 {
+			symLen = uint16(i) + 1
+		}
+	}
+	if total == 0 {
+		return errors.New("huff0: empty histogram")
+	}
+	if symLen < 2 || maxCount == total {
+		return ErrUseRLE
+	}
+	// huff0's internal rank table assumes total ≤ BlockSizeMax (it uses
+	// highBit32(count+1) + 1 as a rank index into a fixed-size array).
+	// Histograms summed across multiple blocks can exceed that; scale the
+	// counts down preserving the distribution. Non-zero entries round up so
+	// rare symbols stay representable.
+	if total > BlockSizeMax {
+		shift := uint(0)
+		for total>>shift > BlockSizeMax {
+			shift++
+		}
+		round := uint32(1<<shift) - 1
+		var newTotal, newMax int
+		for i, v := range s.count {
+			if v == 0 {
+				continue
+			}
+			scaled := (v + round) >> shift
+			if scaled == 0 {
+				scaled = 1
+			}
+			s.count[i] = scaled
+			newTotal += int(scaled)
+			if int(scaled) > newMax {
+				newMax = int(scaled)
+			}
+		}
+		total = newTotal
+		maxCount = newMax
+		if maxCount == total {
+			return ErrUseRLE
+		}
+	}
+	s.symbolLen = symLen
+	s.maxCount = maxCount
+	s.srcLen = total
+	if err := s.buildCTable(); err != nil {
+		return err
+	}
+	if cap(s.prevTable) < len(s.cTable) {
+		s.prevTable = make(cTable, 0, maxSymbolValue+1)
+	}
+	s.prevTable = s.prevTable[:len(s.cTable)]
+	copy(s.prevTable, s.cTable)
+	s.prevTableLog = s.actualTableLog
+	// Force the next Compress* to recount from real input.
+	s.clearCount = true
+	s.maxCount = 0
+	return nil
+}
+
+// EstimateSize returns an estimated compressed payload size in bytes for the
+// supplied histogram using the table currently stored in prevTable. It returns
+// -1 when the table cannot encode every non-zero symbol of hist (i.e. when
+// CanUseTable would return false). The estimate excludes the table header.
+func (s *Scratch) EstimateSize(hist *[256]uint32) int {
+	if s == nil || hist == nil || len(s.prevTable) == 0 {
+		return -1
+	}
+	pt := s.prevTable
+	nbBits := uint32(7)
+	for i, v := range hist {
+		if v == 0 {
+			continue
+		}
+		if i >= len(pt) || pt[i].nBits == 0 {
+			return -1
+		}
+		nbBits += uint32(pt[i].nBits) * v
+	}
+	return int(nbBits >> 3)
+}
+
+// CanUseTable reports whether the table in prevTable can encode every
+// non-zero symbol present in hist.
+func (s *Scratch) CanUseTable(hist *[256]uint32) bool {
+	if s == nil || hist == nil || len(s.prevTable) == 0 {
+		return false
+	}
+	pt := s.prevTable
+	for i, v := range hist {
+		if v == 0 {
+			continue
+		}
+		if i >= len(pt) || pt[i].nBits == 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// AppendTable serializes the table currently stored in prevTable (e.g. as
+// installed by BuildCTable or carried over from a previous Compress call)
+// into a self-delimiting zstd-style header and appends it to dst. The
+// returned slice can be parsed back by ReadTable.
+func (s *Scratch) AppendTable(dst []byte) ([]byte, error) {
+	if s == nil || len(s.prevTable) == 0 {
+		return dst, errors.New("huff0: AppendTable with empty table")
+	}
+	// cTable.write reads s.actualTableLog, s.symbolLen, s.huffWeight, s.fse
+	// and writes into s.Out. Save/restore Out so we don't disturb in-flight
+	// compression buffers.
+	saveOut := s.Out
+	saveTL := s.actualTableLog
+	saveSL := s.symbolLen
+	if s.fse == nil {
+		// Lazily init in case AppendTable is called on a fresh Scratch.
+		if _, err := s.prepare(nil); err != nil {
+			return dst, err
+		}
+		saveOut = s.Out
+	}
+	s.Out = s.Out[:0]
+	s.actualTableLog = s.prevTableLog
+	s.symbolLen = uint16(len(s.prevTable))
+	if err := s.prevTable.write(s); err != nil {
+		s.Out, s.actualTableLog, s.symbolLen = saveOut, saveTL, saveSL
+		return dst, err
+	}
+	dst = append(dst, s.Out...)
+	s.Out, s.actualTableLog, s.symbolLen = saveOut, saveTL, saveSL
+	return dst, nil
+}