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gitea/vendor/github.com/dsnet/compress/internal/prefix/range.go

94 lines
2.5 KiB

// Copyright 2015, Joe Tsai. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE.md file.
package prefix
type RangeCode struct {
Base uint32 // Starting base offset of the range
Len uint32 // Bit-length of a subsequent integer to add to base offset
}
type RangeCodes []RangeCode
type RangeEncoder struct {
rcs RangeCodes
lut [1024]uint32
minBase uint
}
// End reports the non-inclusive ending range.
func (rc RangeCode) End() uint32 { return rc.Base + (1 << rc.Len) }
// MakeRangeCodes creates a RangeCodes, where each region is assumed to be
// contiguously stacked, without any gaps, with bit-lengths taken from bits.
func MakeRangeCodes(minBase uint, bits []uint) (rc RangeCodes) {
for _, nb := range bits {
rc = append(rc, RangeCode{Base: uint32(minBase), Len: uint32(nb)})
minBase += 1 << nb
}
return rc
}
// Base reports the inclusive starting range for all ranges.
func (rcs RangeCodes) Base() uint32 { return rcs[0].Base }
// End reports the non-inclusive ending range for all ranges.
func (rcs RangeCodes) End() uint32 { return rcs[len(rcs)-1].End() }
// checkValid reports whether the RangeCodes is valid. In order to be valid,
// the following must hold true:
// rcs[i-1].Base <= rcs[i].Base
// rcs[i-1].End <= rcs[i].End
// rcs[i-1].End >= rcs[i].Base
//
// Practically speaking, each range must be increasing and must not have any
// gaps in between. It is okay for ranges to overlap.
func (rcs RangeCodes) checkValid() bool {
if len(rcs) == 0 {
return false
}
pre := rcs[0]
for _, cur := range rcs[1:] {
preBase, preEnd := pre.Base, pre.End()
curBase, curEnd := cur.Base, cur.End()
if preBase > curBase || preEnd > curEnd || preEnd < curBase {
return false
}
pre = cur
}
return true
}
func (re *RangeEncoder) Init(rcs RangeCodes) {
if !rcs.checkValid() {
panic("invalid range codes")
}
*re = RangeEncoder{rcs: rcs, minBase: uint(rcs.Base())}
for sym, rc := range rcs {
base := int(rc.Base) - int(re.minBase)
end := int(rc.End()) - int(re.minBase)
if base >= len(re.lut) {
break
}
if end > len(re.lut) {
end = len(re.lut)
}
for i := base; i < end; i++ {
re.lut[i] = uint32(sym)
}
}
}
func (re *RangeEncoder) Encode(offset uint) (sym uint) {
if idx := int(offset - re.minBase); idx < len(re.lut) {
return uint(re.lut[idx])
}
sym = uint(re.lut[len(re.lut)-1])
retry:
if int(sym) >= len(re.rcs) || re.rcs[sym].Base > uint32(offset) {
return sym - 1
}
sym++
goto retry // Avoid for-loop so that this function can be inlined
}