From a2d66433f605fa25be92d716c7d9849cfdbe0ae9 Mon Sep 17 00:00:00 2001
From: maypok86 <alex.mayshev.86@gmail.com>
Date: Mon, 11 Dec 2023 21:15:45 +0300
Subject: [PATCH] [#17] Improve S3-FIFO

---
 internal/s3fifo/main.go   | 12 ++++++++++
 internal/s3fifo/policy.go | 26 +++++++++-------------
 internal/s3fifo/small.go  | 46 +++++++++++++++++++--------------------
 3 files changed, 44 insertions(+), 40 deletions(-)

diff --git a/internal/s3fifo/main.go b/internal/s3fifo/main.go
index ca5a94d..50484a6 100644
--- a/internal/s3fifo/main.go
+++ b/internal/s3fifo/main.go
@@ -6,6 +6,8 @@ import (
 	"github.com/maypok86/otter/internal/node"
 )
 
+const maxReinsertions = 20
+
 type main[K comparable, V any] struct {
 	q       *deque.Deque[*node.Node[K, V]]
 	cost    uint32
@@ -26,6 +28,7 @@ func (m *main[K, V]) insert(n *node.Node[K, V]) {
 }
 
 func (m *main[K, V]) evict(deleted []*node.Node[K, V]) []*node.Node[K, V] {
+	reinsertions := 0
 	for m.cost > 0 {
 		n := m.q.PopFront()
 		if n.Meta.IsDeleted() {
@@ -41,6 +44,15 @@ func (m *main[K, V]) evict(deleted []*node.Node[K, V]) []*node.Node[K, V] {
 			return append(deleted, n)
 		}
 
+		// to avoid the worst case O(n), we remove the 20th reinserted consecutive element.
+		reinsertions++
+		if reinsertions >= maxReinsertions {
+			n.Meta = n.Meta.UnmarkMain().MarkDeleted()
+			m.cost -= n.Cost()
+			// can remove
+			return append(deleted, n)
+		}
+
 		m.q.PushBack(n)
 		n.Meta = n.Meta.DecrementFrequency()
 	}
diff --git a/internal/s3fifo/policy.go b/internal/s3fifo/policy.go
index e991876..74530e6 100644
--- a/internal/s3fifo/policy.go
+++ b/internal/s3fifo/policy.go
@@ -52,44 +52,38 @@ func (p *Policy[K, V]) read(deleted []*node.Node[K, V], n *node.Node[K, V]) []*n
 }
 
 func (p *Policy[K, V]) insert(deleted []*node.Node[K, V], n *node.Node[K, V]) []*node.Node[K, V] {
-	for p.isFull() {
-		deleted = p.evict(deleted)
-	}
-
 	if n.Meta.IsDeleted() {
 		return deleted
 	}
 
 	if p.ghost.isGhost(n) {
-		if !n.Meta.IsMain() {
-			p.main.insert(n)
-		}
-		return deleted
+		p.main.insert(n)
+	} else {
+		p.small.insert(n)
 	}
 
-	if !n.Meta.IsSmall() {
-		p.small.insert(n)
+	for p.isFull() {
+		deleted = p.evict(deleted)
 	}
 
 	return deleted
 }
 
 func (p *Policy[K, V]) update(deleted []*node.Node[K, V], n *node.Node[K, V], costDiff uint32) []*node.Node[K, V] {
-	for p.isFull() {
-		deleted = p.evict(deleted)
-	}
-
 	if n.Meta.IsDeleted() {
 		return deleted
 	}
 
 	if n.Meta.IsSmall() {
 		p.small.cost += costDiff
-	}
-	if n.Meta.IsMain() {
+	} else if n.Meta.IsMain() {
 		p.main.cost += costDiff
 	}
 
+	for p.isFull() {
+		deleted = p.evict(deleted)
+	}
+
 	return deleted
 }
 
diff --git a/internal/s3fifo/small.go b/internal/s3fifo/small.go
index 2625aa2..91578dd 100644
--- a/internal/s3fifo/small.go
+++ b/internal/s3fifo/small.go
@@ -34,34 +34,32 @@ func (s *small[K, V]) insert(n *node.Node[K, V]) {
 }
 
 func (s *small[K, V]) evict(deleted []*node.Node[K, V]) []*node.Node[K, V] {
-	for s.cost > 0 {
-		n := s.q.PopFront()
-		s.cost -= n.Cost()
-		n.Meta = n.Meta.UnmarkSmall()
-		if n.Meta.IsDeleted() {
-			return deleted
-		}
-		if n.IsExpired() {
-			n.Meta = n.Meta.MarkDeleted()
-			// can remove
-			return append(deleted, n)
-		}
+	if s.cost == 0 {
+		return deleted
+	}
 
-		if n.Meta.GetFrequency() > 1 {
-			if !n.Meta.IsMain() {
-				s.main.insert(n)
-				for s.main.isFull() {
-					deleted = s.main.evict(deleted)
-				}
-				n.Meta = n.Meta.ResetFrequency()
-				continue
-			}
-		}
+	n := s.q.PopFront()
+	s.cost -= n.Cost()
+	n.Meta = n.Meta.UnmarkSmall()
+	if n.Meta.IsDeleted() {
+		return deleted
+	}
+	if n.IsExpired() {
+		n.Meta = n.Meta.MarkDeleted()
+		// can remove
+		return append(deleted, n)
+	}
 
-		return s.ghost.insert(deleted, n)
+	if n.Meta.GetFrequency() > 1 {
+		s.main.insert(n)
+		for s.main.isFull() {
+			deleted = s.main.evict(deleted)
+		}
+		n.Meta = n.Meta.ResetFrequency()
+		return deleted
 	}
 
-	return deleted
+	return s.ghost.insert(deleted, n)
 }
 
 func (s *small[K, V]) length() int {