Function shuffleQueriersForUser

pkg/scheduler/queue/user_queues.go:367–387

shuffleQueriersForUser returns nil if queriersToSelect is 0 or there are not enough queriers to select from. In that case *all* queriers should be used. Scratchpad is used for shuffling, to avoid new allocations. If nil, new slice is allocated.

(userSeed int64, queriersToSelect int, allSortedQueriers []string, scratchpad []string)

// In that case *all* queriers should be used.
// Scratchpad is used for shuffling, to avoid new allocations. If nil, new slice is allocated.
func shuffleQueriersForUser(userSeed int64, queriersToSelect int, allSortedQueriers []string, scratchpad []string) map[string]struct{} {
	if queriersToSelect == 0 || len(allSortedQueriers) <= queriersToSelect {
		return nil
	}

	queriers := make(map[string]struct{}, queriersToSelect)
	rnd := rand.New(rand.NewSource(userSeed))

	scratchpad = scratchpad[:0]
	scratchpad = append(scratchpad, allSortedQueriers...)

	last := len(scratchpad) - 1
	for range queriersToSelect {
		r := rnd.Intn(last + 1)
		queriers[scratchpad[r]] = struct{}{}
		scratchpad[r], scratchpad[last] = scratchpad[last], scratchpad[r]
		last--
	}

	return queriers
}

// getPriorityList returns a list of priorities, each priority repeated as much as number of reserved queriers.
// This is used when creating map of reserved queriers.