75 lines
2.1 KiB
Go
75 lines
2.1 KiB
Go
package subsync
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import "math/bits"
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// signal is "somebody is speaking" as one bit per time bin.
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//
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// A bitset rather than a []bool because the search is the whole cost of this feature: a
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// two-hour film at 50ms is around 144,000 bins, and every one of ~2,400 shifts has to
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// compare all of them against every stretch candidate. Packed into words, one comparison
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// is an AND and a population count over 2,250 words instead of 144,000 byte reads, which
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// is the difference between a button that answers while somebody is still looking at the
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// menu and one they wait on. math/bits.OnesCount64 compiles to a single instruction on
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// every architecture this runs on.
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type signal struct {
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words []uint64
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bins int
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on int
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}
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func newSignal(bins int) *signal {
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if bins < 1 {
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bins = 1
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}
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return &signal{words: make([]uint64, (bins+63)/64), bins: bins}
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}
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// set marks the half-open bin range [from, to) as speech.
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func (s *signal) set(from, to int) {
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if from < 0 {
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from = 0
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}
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if to > s.bins {
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to = s.bins
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}
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for i := from; i < to; i++ {
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word, bit := i/64, uint(i%64)
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if s.words[word]&(1<<bit) == 0 {
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s.words[word] |= 1 << bit
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s.on++
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}
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}
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}
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// overlap counts the bins where both signals are speaking, with other displaced by shift
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// bins. A positive shift means other is read later — the value to add to this signal's
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// times to bring them onto other's.
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//
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// Negative shifts are answered by swapping the two, which is the same count from the other
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// side and saves writing the word arithmetic twice in mirror image.
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func (s *signal) overlap(other *signal, shift int) int {
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if shift < 0 {
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return other.overlap(s, -shift)
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}
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wordShift, bitShift := shift/64, uint(shift%64)
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total := 0
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for i := range s.words {
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mine := s.words[i]
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if mine == 0 {
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continue
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}
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j := i + wordShift
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if j >= len(other.words) {
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break
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}
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theirs := other.words[j] >> bitShift
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// Go defines a shift of 64 or more as zero, so this term vanishes when bitShift
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// is zero rather than needing a branch of its own.
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if j+1 < len(other.words) {
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theirs |= other.words[j+1] << (64 - bitShift)
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}
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total += bits.OnesCount64(mine & theirs)
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}
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return total
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}
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