diff --git a/envelopes.go b/envelopes.go
index 8f401a4..5e6bc75 100644
--- a/envelopes.go
+++ b/envelopes.go
@@ -195,7 +195,7 @@ func (v CountEnvelope) MarshalJSON() ([]byte, error) {
 		w.RawString(strconv.FormatInt(*v.Count, 10))
 		if v.HyperLogLog != nil {
 			w.RawString(`,"hll":"`)
-			hllHex := make([]byte, 0, 512)
+			hllHex := make([]byte, 512)
 			hex.Encode(hllHex, v.HyperLogLog)
 			w.Buffer.AppendBytes(hllHex)
 			w.RawString(`"`)
diff --git a/nip45/hll_event.go b/nip45/hll_event.go
new file mode 100644
index 0000000..dc4d01f
--- /dev/null
+++ b/nip45/hll_event.go
@@ -0,0 +1,42 @@
+package nip45
+
+import (
+	"iter"
+	"strconv"
+
+	"github.com/nbd-wtf/go-nostr"
+)
+
+func HyperLogLogEventPubkeyOffsetsAndReferencesForEvent(evt *nostr.Event) iter.Seq2[string, int] {
+	return func(yield func(string, int) bool) {
+		switch evt.Kind {
+		case 3:
+			//
+			// follower counts
+			for _, tag := range evt.Tags {
+				if len(tag) >= 2 && tag[0] == "p" && nostr.IsValid32ByteHex(tag[1]) {
+					// 32th nibble of each "p" tag
+					p, _ := strconv.ParseInt(tag[1][32:33], 16, 64)
+					if !yield(tag[1], int(p+8)) {
+						return
+					}
+				}
+			}
+		case 7:
+			//
+			// reaction counts:
+			// (only the last "e" tag counts)
+			lastE := evt.Tags.GetLast([]string{"e", ""})
+			if lastE != nil {
+				v := (*lastE)[1]
+				if nostr.IsValid32ByteHex(v) {
+					// 32th nibble of "e" tag
+					p, _ := strconv.ParseInt(v[32:33], 16, 64)
+					if !yield(v, int(p+8)) {
+						return
+					}
+				}
+			}
+		}
+	}
+}
diff --git a/nip45/hll_filter.go b/nip45/hll_filter.go
new file mode 100644
index 0000000..c075b38
--- /dev/null
+++ b/nip45/hll_filter.go
@@ -0,0 +1,49 @@
+package nip45
+
+import (
+	"strconv"
+
+	"github.com/nbd-wtf/go-nostr"
+)
+
+// HyperLogLogEventPubkeyOffsetForFilter returns the deterministic pubkey offset that will be used
+// when computing hyperloglogs in the context of a specific filter.
+//
+// It returns -1 when the filter is not eligible for hyperloglog calculation.
+func HyperLogLogEventPubkeyOffsetForFilter(filter nostr.Filter) int {
+	if filter.IDs != nil || filter.Since != nil || filter.Until != nil || filter.Authors != nil ||
+		len(filter.Kinds) != 1 || filter.Search != "" || len(filter.Tags) != 1 {
+		// obvious cases in which we won't bother to do hyperloglog stuff
+		return -1
+	}
+
+	// only serve the cases explicitly defined by the NIP:
+	if pTags, ok := filter.Tags["p"]; ok {
+		//
+		// follower counts:
+		if filter.Kinds[0] == 3 && len(pTags) == 1 {
+			// 32th nibble of "p" tag
+			p, err := strconv.ParseInt(pTags[0][32:33], 16, 64)
+			if err != nil {
+				return -1
+			}
+			return int(p + 8)
+		}
+	} else if eTags, ok := filter.Tags["e"]; ok {
+		if len(eTags) == 1 {
+			//
+			// reaction counts:
+			if filter.Kinds[0] == 7 {
+				// 32th nibble of "e" tag
+				p, err := strconv.ParseInt(eTags[0][32:33], 16, 64)
+				if err != nil {
+					return -1
+				}
+				return int(p + 8)
+			}
+		}
+	}
+
+	// everything else is false at least for now
+	return -1
+}
diff --git a/nip45/hyperloglog/hll.go b/nip45/hyperloglog/hll.go
index 16f98ed..e546b67 100644
--- a/nip45/hyperloglog/hll.go
+++ b/nip45/hyperloglog/hll.go
@@ -3,21 +3,37 @@ package hyperloglog
 import (
 	"encoding/binary"
 	"encoding/hex"
+	"fmt"
 )
 
 // Everything is hardcoded to use precision 8, i.e. 256 registers.
 type HyperLogLog struct {
+	offset    int
 	registers []uint8
 }
 
-func New() *HyperLogLog {
+func New(offset int) *HyperLogLog {
+	if offset < 0 || offset > 32-8 {
+		panic(fmt.Errorf("invalid offset %d", offset))
+	}
+
 	// precision is always 8
 	// the number of registers is always 256 (1<<8)
-	hll := &HyperLogLog{}
+	hll := &HyperLogLog{offset: offset}
 	hll.registers = make([]uint8, 256)
 	return hll
 }
 
+func NewWithRegisters(registers []byte, offset int) *HyperLogLog {
+	if offset < 0 || offset > 32-8 {
+		panic(fmt.Errorf("invalid offset %d", offset))
+	}
+	if len(registers) != 256 {
+		panic(fmt.Errorf("invalid number of registers %d", len(registers)))
+	}
+	return &HyperLogLog{registers: registers, offset: offset}
+}
+
 func (hll *HyperLogLog) GetRegisters() []byte    { return hll.registers }
 func (hll *HyperLogLog) SetRegisters(enc []byte) { hll.registers = enc }
 func (hll *HyperLogLog) MergeRegisters(other []byte) {
@@ -34,8 +50,22 @@ func (hll *HyperLogLog) Clear() {
 	}
 }
 
-func (hll *HyperLogLog) Add(id string) {
-	x, _ := hex.DecodeString(id[32 : 32+8*2])
+// Add takes a Nostr event pubkey which will be used as the item "key" (that combined with the offset)
+func (hll *HyperLogLog) Add(pubkey string) {
+	x, _ := hex.DecodeString(pubkey[hll.offset*2 : hll.offset*2+8*2])
+	j := x[0] // register address (first 8 bits, i.e. first byte)
+
+	w := binary.BigEndian.Uint64(x) // number that we will use
+	zeroBits := clz56(w) + 1        // count zeroes (skip the first byte, so only use 56 bits)
+
+	if zeroBits > hll.registers[j] {
+		hll.registers[j] = zeroBits
+	}
+}
+
+// AddBytes is like Add, but takes pubkey as bytes instead of as string
+func (hll *HyperLogLog) AddBytes(pubkey []byte) {
+	x := pubkey[hll.offset : hll.offset+8]
 	j := x[0] // register address (first 8 bits, i.e. first byte)
 
 	w := binary.BigEndian.Uint64(x) // number that we will use
diff --git a/pool.go b/pool.go
index 6222a16..65e70c8 100644
--- a/pool.go
+++ b/pool.go
@@ -476,7 +476,7 @@ func (pool *SimplePool) CountMany(
 	filter Filter,
 	opts []SubscriptionOption,
 ) int {
-	hll := hyperloglog.New()
+	hll := hyperloglog.New(0) // offset is irrelevant here, so we just pass 0
 
 	wg := sync.WaitGroup{}
 	wg.Add(len(urls))