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slidingwindow.go
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slidingwindow.go
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package ss2022
import "math/bits"
const swfBlockBits = bits.UintSize
// SlidingWindowFilter maintains a sliding window of uint64 counters.
type SlidingWindowFilter struct {
size uint64
last uint64
ring []uint
ringBlockIndexMask uint64
}
// NewSlidingWindowFilter returns a new sliding window filter with the given size.
func NewSlidingWindowFilter(size uint64) *SlidingWindowFilter {
ringBits := uint64(1 << bits.Len64(size+swfBlockBits-1))
ringBlocks := ringBits / swfBlockBits
return &SlidingWindowFilter{
size: size,
ring: make([]uint, ringBlocks),
ringBlockIndexMask: ringBlocks - 1,
}
}
// Size returns the size of the sliding window.
func (f *SlidingWindowFilter) Size() uint64 {
return f.size
}
// Reset resets the filter to its initial state.
func (f *SlidingWindowFilter) Reset() {
f.last = 0
f.ring[0] = 0
}
func (*SlidingWindowFilter) unmaskedBlockIndex(counter uint64) uint64 {
return counter / swfBlockBits
}
func (f *SlidingWindowFilter) blockIndex(counter uint64) uint64 {
return counter / swfBlockBits & f.ringBlockIndexMask
}
func (*SlidingWindowFilter) bitIndex(counter uint64) uint64 {
return counter % swfBlockBits
}
// IsOk checks whether counter can be accepted by the sliding window filter.
func (f *SlidingWindowFilter) IsOk(counter uint64) bool {
// Accept counter if it is ahead of window.
if counter > f.last {
return true
}
// Reject counter if it is behind window.
if f.last-counter >= f.size {
return false
}
// Within window, accept if not seen by window.
return f.ring[f.blockIndex(counter)]&(1<<f.bitIndex(counter)) == 0
}
// MustAdd adds counter to the sliding window without checking if the counter is valid.
// Call IsOk beforehand to make sure the counter is valid.
func (f *SlidingWindowFilter) MustAdd(counter uint64) {
blockIndex := f.unmaskedBlockIndex(counter)
// When counter is ahead of window, clear blocks ahead.
if counter > f.last {
lastBlockIndex := f.unmaskedBlockIndex(f.last)
clearBlockCount := min(int(blockIndex-lastBlockIndex), len(f.ring))
// Clear blocks ahead.
for range clearBlockCount {
lastBlockIndex = (lastBlockIndex + 1) & f.ringBlockIndexMask
f.ring[lastBlockIndex] = 0
}
f.last = counter
}
blockIndex &= f.ringBlockIndexMask
f.ring[blockIndex] |= 1 << f.bitIndex(counter)
}
// Add attempts to add counter to the sliding window and returns
// whether the counter is successfully added to the sliding window.
func (f *SlidingWindowFilter) Add(counter uint64) bool {
unmaskedBlockIndex := f.unmaskedBlockIndex(counter)
blockIndex := unmaskedBlockIndex & f.ringBlockIndexMask
bitIndex := f.bitIndex(counter)
switch {
case counter > f.last: // Ahead of window, clear blocks ahead.
lastBlockIndex := f.unmaskedBlockIndex(f.last)
clearBlockCount := min(int(unmaskedBlockIndex-lastBlockIndex), len(f.ring))
// Clear blocks ahead.
for range clearBlockCount {
lastBlockIndex = (lastBlockIndex + 1) & f.ringBlockIndexMask
f.ring[lastBlockIndex] = 0
}
f.last = counter
case f.last-counter >= f.size: // Behind window.
return false
case f.ring[blockIndex]&(1<<bitIndex) != 0: // Within window, already seen.
return false
}
f.ring[blockIndex] |= 1 << bitIndex
return true
}