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