// Encode encodes the distance using the parameter l. Dist can have values from
// the full range of uint32 values. To get the distance offset the actual match
// distance has to be decreased by 1. A distance offset of 0xffffffff (eos)
// indicates the end of the stream.
func (dc *distCodec) Encode(e *rangeEncoder, dist uint32, l uint32) (err error) {
// Compute the posSlot using nlz32
var posSlot uint32
var bits uint32
if dist < startPosModel {
posSlot = dist
} else {
bits = uint32(30 - u32.NLZ(dist))
posSlot = startPosModel - 2 + (bits << 1)
posSlot += (dist >> uint(bits)) & 1
}
if err = dc.posSlotCodecs[lenState(l)].Encode(e, posSlot); err != nil {
return
}
switch {
case posSlot < startPosModel:
return nil
case posSlot < endPosModel:
tc := &dc.posModel[posSlot-startPosModel]
return tc.Encode(dist, e)
}
dic := directCodec(bits - alignBits)
if err = dic.Encode(e, dist>>alignBits); err != nil {
return
}
return dc.alignCodec.Encode(dist, e)
}
// hashTableExponent derives the hash table exponent from the history length.
func hashTableExponent(n uint32) int {
e := 30 - u32.NLZ(n)
switch {
case e < minTableExponent:
e = minTableExponent
case e > maxTableExponent:
e = maxTableExponent
}
return e
}
// distBits returns the number of bits required to encode dist.
func distBits(dist uint32) int {
if dist < startPosModel {
return 6
}
// slot s > 3, dist d
// s = 2(bits(d)-1) + bit(d, bits(d)-2)
// s>>1 = bits(d)-1
// bits(d) = 32-nlz32(d)
// s>>1=31-nlz32(d)
// n = 5 + (s>>1) = 36 - nlz32(d)
return 36 - u32.NLZ(dist)
}
