text: // Copyright 2019+ Klaus Post. All rights reserved.
  // License information can be found in the LICENSE file.
  // Based on work by Yann Collet, released under BSD License.
  
  package zstd
  
  import "math/bits"
  
  type seqCoders struct {
  	llEnc, ofEnc, mlEnc    *fseEncoder
  	llPrev, ofPrev, mlPrev *fseEncoder
  }
  
  // swap coders with another (block).
  func (s *seqCoders) swap(other *seqCoders) {
  	*s, *other = *other, *s
  }
  
  // setPrev will update the previous encoders to the actually used ones
  // and make sure a fresh one is in the main slot.
  func (s *seqCoders) setPrev(ll, ml, of *fseEncoder) {
  	compareSwap := func(used *fseEncoder, current, prev **fseEncoder) {
  		// We used the new one, more current to history and reuse the previous history
  		if *current == used {
  			*prev, *current = *current, *prev
  			c := *current
  			p := *prev
  			c.reUsed = false
  			p.reUsed = true
  			return
  		}
  		if used == *prev {
  			return
  		}
  		// Ensure we cannot reuse by accident
  		prevEnc := *prev
  		prevEnc.symbolLen = 0
  		return
  	}
  	compareSwap(ll, &s.llEnc, &s.llPrev)
  	compareSwap(ml, &s.mlEnc, &s.mlPrev)
  	compareSwap(of, &s.ofEnc, &s.ofPrev)
  }
  
  func highBit(val uint32) (n uint32) {
  	return uint32(bits.Len32(val) - 1)
  }
  
  var llCodeTable = [64]byte{0, 1, 2, 3, 4, 5, 6, 7,
  	8, 9, 10, 11, 12, 13, 14, 15,
  	16, 16, 17, 17, 18, 18, 19, 19,
  	20, 20, 20, 20, 21, 21, 21, 21,
  	22, 22, 22, 22, 22, 22, 22, 22,
  	23, 23, 23, 23, 23, 23, 23, 23,
  	24, 24, 24, 24, 24, 24, 24, 24,
  	24, 24, 24, 24, 24, 24, 24, 24}
  
  // Up to 6 bits
  const maxLLCode = 35
  
  // llBitsTable translates from ll code to number of bits.
  var llBitsTable = [maxLLCode + 1]byte{
  	0, 0, 0, 0, 0, 0, 0, 0,
  	0, 0, 0, 0, 0, 0, 0, 0,
  	1, 1, 1, 1, 2, 2, 3, 3,
  	4, 6, 7, 8, 9, 10, 11, 12,
  	13, 14, 15, 16}
  
  // llCode returns the code that represents the literal length requested.
  func llCode(litLength uint32) uint8 {
  	const llDeltaCode = 19
  	if litLength <= 63 {
  		// Compiler insists on bounds check (Go 1.12)
  		return llCodeTable[litLength&63]
  	}
  	return uint8(highBit(litLength)) + llDeltaCode
  }
  
  var mlCodeTable = [128]byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
  	16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
  	32, 32, 33, 33, 34, 34, 35, 35, 36, 36, 36, 36, 37, 37, 37, 37,
  	38, 38, 38, 38, 38, 38, 38, 38, 39, 39, 39, 39, 39, 39, 39, 39,
  	40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40,
  	41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41,
  	42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42,
  	42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42}
  
  // Up to 6 bits
  const maxMLCode = 52
  
  // mlBitsTable translates from ml code to number of bits.
  var mlBitsTable = [maxMLCode + 1]byte{
  	0, 0, 0, 0, 0, 0, 0, 0,
  	0, 0, 0, 0, 0, 0, 0, 0,
  	0, 0, 0, 0, 0, 0, 0, 0,
  	0, 0, 0, 0, 0, 0, 0, 0,
  	1, 1, 1, 1, 2, 2, 3, 3,
  	4, 4, 5, 7, 8, 9, 10, 11,
  	12, 13, 14, 15, 16}
  
  // note : mlBase = matchLength - MINMATCH;
  // because it's the format it's stored in seqStore->sequences
  func mlCode(mlBase uint32) uint8 {
  	const mlDeltaCode = 36
  	if mlBase <= 127 {
  		// Compiler insists on bounds check (Go 1.12)
  		return mlCodeTable[mlBase&127]
  	}
  	return uint8(highBit(mlBase)) + mlDeltaCode
  }
  
  func ofCode(offset uint32) uint8 {
  	// A valid offset will always be > 0.
  	return uint8(bits.Len32(offset) - 1)
  }
  