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Copy pathcm4.go
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92 lines (77 loc) · 2.02 KB
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package tinylfu
// cm4 is a small conservative-update count-min sketch implementation with 4-bit counters
type cm4 struct {
s [depth]nvec
mask uint32
}
const depth = 4
func newCM4(w int) *cm4 {
if w < 1 {
panic("cm4: bad width")
}
w32 := nextPowerOfTwo(uint32(w))
c := cm4{
mask: w32 - 1,
}
for i := 0; i < depth; i++ {
c.s[i] = newNvec(int(w32))
}
return &c
}
func (c *cm4) add(keyh uint64) {
// The loop unrolling prevents this function from being inlined, but it still results in a slight overall speedup.
c.s[3].inc(c.counterOffset(keyh, 3))
c.s[2].inc(c.counterOffset(keyh, 2))
c.s[1].inc(c.counterOffset(keyh, 1))
c.s[0].inc(c.counterOffset(keyh, 0))
}
func (c *cm4) counterOffset(keyh uint64, level int) uint32 {
// counterOffset gets inlined and the compiler removes the duplicated computations of h1 and h2, so there is no
// benefit to accepting h1 and h2 as arguments.
h1, h2 := uint32(keyh), uint32(keyh>>32)
return (h1 + uint32(level)*h2) & c.mask
}
func (c *cm4) estimate(keyh uint64) byte {
var min byte = 255
bmin := func(v, min byte) byte {
// bmin gets inlined and the else branch gets optimized away
if v < min {
return v
} else {
return min
}
}
min = bmin(c.s[3].get(c.counterOffset(keyh, 3)), min)
min = bmin(c.s[2].get(c.counterOffset(keyh, 2)), min)
min = bmin(c.s[1].get(c.counterOffset(keyh, 1)), min)
min = bmin(c.s[0].get(c.counterOffset(keyh, 0)), min)
return min
}
func (c *cm4) reset() {
// There is no point in unrolling this loop, the cost is dominated by nvec.reset, which is O(n)
for _, n := range c.s {
n.reset()
}
}
// nybble vector
type nvec []byte
func newNvec(w int) nvec {
return make(nvec, w/2)
}
func (n nvec) get(i uint32) byte {
// Ugly, but as a single expression so the compiler will inline it :/
return byte(n[i/2]>>((i&1)*4)) & 0x0f
}
func (n nvec) inc(i uint32) {
idx := i / 2
shift := (i & 1) * 4
v := (n[idx] >> shift) & 0x0f
if v < 15 {
n[idx] += 1 << shift
}
}
func (n nvec) reset() {
for i := range n {
n[i] = (n[i] >> 1) & 0x77
}
}