From eb263b7666cacfc99db4d14c1f4f97c4efddc812 Mon Sep 17 00:00:00 2001
From: Seebs <seebs@seebs.net>
Date: Wed, 4 Mar 2020 09:55:01 -0600
Subject: [PATCH] don't fill up empty space with non-functional ops logs

Two changes:

1. Don't write batch/roaring adds or removes when N is 0, because
a write of no bits is not a meaningful write.
2. When unmarshalling roaring things, if a roaring bitmap didn't
change many bits, treat it as having changed at least 1 bit per 8 bytes,
so an 8KB hunk of roaring data counts as 1K changes, which will
nudge us towards snapshotting. This should keep us from having
Large Files show up so much.

This was particularly noticeable on the existence field, which
tends to a steady state of "completely full" very quickly in a lot
of cases.
---
 roaring/roaring.go | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/roaring/roaring.go b/roaring/roaring.go
index 99a87a0d9..ffc26e369 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -297,7 +297,7 @@ func (b *Bitmap) AddN(a ...uint64) (changed int, err error) {
 
 	changed = b.DirectAddN(a...) // modifies a in-place
 
-	if b.OpWriter != nil {
+	if b.OpWriter != nil && changed > 0 {
 		op := &op{
 			typ:    opTypeAddBatch,
 			values: a[:changed],
@@ -404,7 +404,7 @@ func (b *Bitmap) RemoveN(a ...uint64) (changed int, err error) {
 
 	changed = b.DirectRemoveN(a...) // modifies a in-place
 
-	if b.OpWriter != nil {
+	if b.OpWriter != nil && changed > 0 {
 		op := &op{
 			typ:    opTypeRemoveBatch,
 			values: a[:changed],
@@ -1727,7 +1727,7 @@ func (b *Bitmap) ImportRoaringBits(data []byte, clear bool, log bool, rowSize ui
 		return changed, rowSet, itrErr
 	}
 	err = nil
-	if log {
+	if log && changed > 0 {
 		op := op{opN: changed, roaring: data}
 		if clear {
 			op.typ = opTypeRemoveRoaring
@@ -4820,6 +4820,11 @@ func (op *op) UnmarshalBinary(data []byte) error {
 			return fmt.Errorf("op data truncated - expected %d, got %d", 13+op.value, len(data))
 		}
 		op.opN = int(binary.LittleEndian.Uint32(data[13:17]))
+		// gratuitous hack: treat any roaring write as having at least 1/8 of
+		// its length in bits, even if it didn't actually change things.
+		if op.opN < int(op.value/8) {
+			op.opN = int(op.value / 8)
+		}
 		op.roaring = data[17 : 17+op.value]
 		_, _ = h.Write(data[13 : 17+op.value])
 		// op.value = 0