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Fix random distribution in bitArray.PickRandom (#2534)

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* Fix random distribution in bitArray.PickRandom

Previously it was very biased. 63 "_" followed by a single "x" had
much greater odds of being chosen. Additionally, the last element was
skewed. This fixes that by first preproccessing the set of all true
indices, and then randomly selecting a single element from there.

This commit also makes the code here significantly simpler, and
improves test cases.

* unlock mtx right after we select true indices
This commit is contained in:
Dev Ojha
2018-10-05 00:00:50 -07:00
committed by Anton Kaliaev
parent 5b120d788a
commit c648c93807
3 changed files with 60 additions and 42 deletions
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68
libs/common/bit_array.go
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@ -234,49 +234,53 @@ func (bA *BitArray) IsFull() bool {
return (lastElem+1)&((uint64(1)<<uint(lastElemBits))-1) == 0
}
// PickRandom returns a random index in the bit array, and its value.
// PickRandom returns a random index for a set bit in the bit array.
// If there is no such value, it returns 0, false.
// It uses the global randomness in `random.go` to get this index.
func (bA *BitArray) PickRandom() (int, bool) {
if bA == nil {
return 0, false
}
bA.mtx.Lock()
defer bA.mtx.Unlock()
length := len(bA.Elems)
if length == 0 {
bA.mtx.Lock()
trueIndices := bA.getTrueIndices()
bA.mtx.Unlock()
if len(trueIndices) == 0 { // no bits set to true
return 0, false
}
randElemStart := RandIntn(length)
for i := 0; i < length; i++ {
elemIdx := ((i + randElemStart) % length)
if elemIdx < length-1 {
if bA.Elems[elemIdx] > 0 {
randBitStart := RandIntn(64)
for j := 0; j < 64; j++ {
bitIdx := ((j + randBitStart) % 64)
if (bA.Elems[elemIdx] & (uint64(1) << uint(bitIdx))) > 0 {
return 64*elemIdx + bitIdx, true
}
}
PanicSanity("should not happen")
}
} else {
// Special case for last elem, to ignore straggler bits
elemBits := bA.Bits % 64
if elemBits == 0 {
elemBits = 64
}
randBitStart := RandIntn(elemBits)
for j := 0; j < elemBits; j++ {
bitIdx := ((j + randBitStart) % elemBits)
if (bA.Elems[elemIdx] & (uint64(1) << uint(bitIdx))) > 0 {
return 64*elemIdx + bitIdx, true
}
return trueIndices[RandIntn(len(trueIndices))], true
}
func (bA *BitArray) getTrueIndices() []int {
trueIndices := make([]int, 0, bA.Bits)
curBit := 0
numElems := len(bA.Elems)
// set all true indices
for i := 0; i < numElems-1; i++ {
elem := bA.Elems[i]
if elem == 0 {
curBit += 64
continue
}
for j := 0; j < 64; j++ {
if (elem & (uint64(1) << uint64(j))) > 0 {
trueIndices = append(trueIndices, curBit)
}
curBit++
}
}
return 0, false
// handle last element
lastElem := bA.Elems[numElems-1]
numFinalBits := bA.Bits - curBit
for i := 0; i < numFinalBits; i++ {
if (lastElem & (uint64(1) << uint64(i))) > 0 {
trueIndices = append(trueIndices, curBit)
}
curBit++
}
return trueIndices
}
// String returns a string representation of BitArray: BA{<bit-string>},