feat: add drop key functionality (#6)

Currently, the `Table.Drop()` function is deprecated because it is not implemented yet. Let's add that functionality.

- Adds true drop functionality to the table, through `Table.Drop()`.
- Adds tests for functionality.
- Rewrites fuzz test using `go_fuzz_utils`, to test arbitrary usage patterns.
- Rewrite `bucket` to allow a capacity of zero.
- Rename `Table.Capacity()` to `Table.TotalCapacity()`, to reflect to different between the capacity of the buckets vs. the whole table.
- Enforce 100% mutation test coverage.

Reviewed-on: #6
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

.gremlins.yaml

@@ -4,4 +4,6 @@ unleash:
   timeout-coefficient: 50
   
   workers: 4
-  dry-run: false
+  dry-run: false
+  threshold:
+    efficacy: 1.0

Makefile

@@ -16,10 +16,12 @@ test-unit: ## Run unit tests with coverage
 test-mutation: ## Run mutation tests with gremlins
 	gremlins unleash
 
+FUZZ_TIME ?= 30
+
 test-fuzz: ## Run all fuzz tests for 30s each
 	@for func in $$(grep -r --include='*_test.go' -oh 'func Fuzz\w*' . | sed 's/func //'); do \
 		echo "Fuzzing $$func..."; \
-		go test ./... -fuzz="^$$func$$" -fuzztime=30s; \
+		go test ./... -fuzz="^$$func$$" -fuzztime=$(FUZZ_TIME)s; \
 	done
 
 test: test-unit test-mutation test-fuzz ## Run all tests

bucket.go

@@ -17,15 +17,37 @@ type bucket[K, V any] struct {
 	compare        EqualFunc[K]
 }
 
+// location determines where in the bucket a certain key would be placed. If the
+// capacity is 0, this will panic.
 func (b bucket[K, V]) location(key K) uint64 {
 	return b.hash(key) % b.capacity
 }
 
 func (b bucket[K, V]) get(key K) (value V, found bool) {
+	if b.capacity == 0 {
+		return
+	}
+
 	slot := b.slots[b.location(key)]
 	return slot.value, slot.occupied && b.compare(slot.key, key)
 }
 
+func (b *bucket[K, V]) drop(key K) (occupied bool) {
+	if b.capacity == 0 {
+		return
+	}
+
+	slot := &b.slots[b.location(key)]
+
+	if slot.occupied && b.compare(slot.key, key) {
+		slot.occupied = false
+		b.size--
+		return true
+	}
+
+	return false
+}
+
 func (b *bucket[K, V]) resize(capacity uint64) {
 	b.slots = make([]slot[K, V], capacity)
 	b.capacity = capacity
@@ -33,6 +55,10 @@ func (b *bucket[K, V]) resize(capacity uint64) {
 }
 
 func (b bucket[K, V]) update(key K, value V) (updated bool) {
+	if b.capacity == 0 {
+		return
+	}
+
 	slot := &b.slots[b.location(key)]
 
 	if slot.occupied && b.compare(slot.key, key) {
@@ -44,6 +70,10 @@ func (b bucket[K, V]) update(key K, value V) (updated bool) {
 }
 
 func (b *bucket[K, V]) evict(insertion entry[K, V]) (evicted entry[K, V], eviction bool) {
+	if b.capacity == 0 {
+		return insertion, true
+	}
+
 	slot := &b.slots[b.location(insertion.key)]
 
 	if !slot.occupied {

cuckoo_fuzz_test.go

@@ -1,11 +1,11 @@
 package cuckoo_test
 
 import (
-	"bytes"
-	"encoding/binary"
+	"maps"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	go_fuzz_utils "github.com/trailofbits/go-fuzz-utils"
 
 	"git.maximhutz.com/tools/go-cuckoo"
 )
@@ -19,31 +19,60 @@ func offsetHash(seed uint32) cuckoo.Hash[uint32] {
 	}
 }
 
+type fuzzStep struct {
+	drop       bool
+	key, value uint32
+}
+
+type fuzzScenario struct {
+	seedA, seedB uint32
+	steps        []fuzzStep
+}
+
 func FuzzInsertLookup(f *testing.F) {
-	f.Fuzz(func(t *testing.T, data []byte, seedA, seedB uint32) {
+	f.Fuzz(func(t *testing.T, data []byte) {
+		var scenario fuzzScenario
 		assert := assert.New(t)
 
-		table := cuckoo.NewCustomTable[uint32, uint32](
-			offsetHash(seedA),
-			offsetHash(seedB),
-			func(a, b uint32) bool { return a == b },
-		)
-
-		if seedA == seedB {
+		if tp, err := go_fuzz_utils.NewTypeProvider(data); err != nil {
+			return
+		} else if err := tp.Fill(&scenario); err != nil {
 			return
 		}
 
-		r := bytes.NewReader(data)
-		var key, value uint32
-		for binary.Read(r, binary.LittleEndian, &key) == nil &&
-			binary.Read(r, binary.LittleEndian, &value) == nil {
+		if scenario.seedA == scenario.seedB {
+			return
+		}
 
-			err := table.Put(key, value)
-			assert.NoError(err)
+		actual := cuckoo.NewCustomTable[uint32, uint32](
+			offsetHash(scenario.seedA),
+			offsetHash(scenario.seedB),
+			func(a, b uint32) bool { return a == b },
+		)
 
-			found, err := table.Get(key)
-			assert.NoError(err)
-			assert.Equal(value, found)
+		expected := map[uint32]uint32{}
+
+		for _, step := range scenario.steps {
+			if step.drop {
+				err := actual.Drop(step.key)
+				assert.NoError(err)
+
+				delete(expected, step.key)
+
+				_, err = actual.Get(step.key)
+				assert.Error(err)
+			} else {
+				err := actual.Put(step.key, step.value)
+				assert.NoError(err)
+
+				expected[step.key] = step.value
+
+				found, err := actual.Get(step.key)
+				assert.NoError(err)
+				assert.Equal(step.value, found)
+			}
+
+			assert.Equal(expected, maps.Collect(actual.Entries()))
 		}
 	})
 }

cuckoo_internal_test.go

@@ -25,6 +25,6 @@ func TestLoad(t *testing.T) {
 	for i := range 16 {
 		err := table.Put(i, true)
 		assert.NoError(err)
-		assert.Equal(float64(table.Size())/float64(table.Capacity()), table.load())
+		assert.Equal(float64(table.Size())/float64(table.TotalCapacity()), table.load())
 	}
 }

cuckoo_test.go

@@ -1,6 +1,7 @@
 package cuckoo_test
 
 import (
+	"errors"
 	"maps"
 	"math/rand/v2"
 	"testing"
@@ -64,7 +65,7 @@ func TestStartingCapacity(t *testing.T) {
 	assert := assert.New(t)
 	table := cuckoo.NewTable[int, bool](cuckoo.Capacity(64))
 
-	assert.Equal(uint64(128), table.Capacity())
+	assert.Equal(uint64(128), table.TotalCapacity())
 }
 
 func TestResizeCapacity(t *testing.T) {
@@ -74,12 +75,12 @@ func TestResizeCapacity(t *testing.T) {
 		cuckoo.GrowthFactor(2),
 	)
 
-	for table.Capacity() == 16 {
+	for table.TotalCapacity() == 16 {
 		err := table.Put(rand.Int(), true)
 		assert.NoError(err)
 	}
 
-	assert.Equal(uint64(32), table.Capacity())
+	assert.Equal(uint64(32), table.TotalCapacity())
 }
 
 func TestPutMany(t *testing.T) {
@@ -128,3 +129,113 @@ func TestRemove(t *testing.T) {
 
 	assert.True(table.Has(0))
 }
+
+func TestDropExistingItem(t *testing.T) {
+	assert := assert.New(t)
+	key, value := 0, true
+	table := cuckoo.NewTable[int, bool]()
+	(table.Put(key, value))
+
+	err := table.Drop(key)
+
+	assert.NoError(err)
+	assert.Equal(0, table.Size())
+	assert.False(table.Has(key))
+}
+
+func TestDropNoItem(t *testing.T) {
+	assert := assert.New(t)
+	key := 0
+	table := cuckoo.NewTable[int, bool]()
+
+	err := table.Drop(key)
+
+	assert.NoError(err)
+	assert.Equal(0, table.Size())
+	assert.False(table.Has(key))
+}
+
+func TestDropItemCapacity(t *testing.T) {
+	assert := assert.New(t)
+	key := 0
+	table := cuckoo.NewTable[int, bool](
+		cuckoo.Capacity(64),
+		cuckoo.GrowthFactor(2),
+	)
+
+	startingCapacity := table.TotalCapacity()
+	err := table.Drop(key)
+	endingCapacity := table.TotalCapacity()
+
+	assert.NoError(err)
+	assert.Equal(0, table.Size())
+	assert.Equal(uint64(128), startingCapacity)
+	assert.Equal(uint64(64), endingCapacity)
+}
+
+func TestPutNoCapacity(t *testing.T) {
+	assert := assert.New(t)
+	key, value := 0, true
+	table := cuckoo.NewTable[int, bool](
+		cuckoo.Capacity(0),
+	)
+
+	err := table.Put(key, value)
+
+	assert.NoError(err)
+	assert.Equal(1, table.Size())
+	assert.True(table.Has(key))
+}
+
+func TestBadHashCapacity(t *testing.T) {
+	assert := assert.New(t)
+	table := cuckoo.NewCustomTable[int, bool](
+		func(int) uint64 { return 0 },
+		func(int) uint64 { return 0 },
+		func(a, b int) bool { return a == b },
+		cuckoo.Capacity(20),
+	)
+
+	err1 := table.Put(0, true)
+	err2 := table.Put(1, true)
+	err3 := table.Put(2, true)
+
+	assert.NoError(err1)
+	assert.NoError(err2)
+	assert.Error(err3)
+
+	assert.Equal(uint64(80), table.TotalCapacity())
+}
+
+func TestDropResizeCapacity(t *testing.T) {
+	assert := assert.New(t)
+	table := cuckoo.NewTable[int, bool](
+		cuckoo.Capacity(10),
+	)
+
+	err1 := table.Put(0, true)
+	err2 := table.Put(1, true)
+	err3 := table.Drop(1)
+
+	assert.NoError(errors.Join(err1, err2, err3))
+	assert.Equal(uint64(20), table.TotalCapacity())
+}
+
+func TestNewTableBy(t *testing.T) {
+	type User struct {
+		_    func()
+		id   string
+		name string
+	}
+
+	assert := assert.New(t)
+	table := cuckoo.NewTableBy[User, bool](
+		func(u User) string { return u.id },
+	)
+
+	err := table.Put(User{nil, "1", "Robert"}, true)
+
+	assert.NoError(err)
+	assert.Equal(1, table.Size())
+	assert.True(table.Has(User{nil, "1", "Robbie"}))
+}

go.mod

@@ -4,8 +4,11 @@ go 1.25.6
 
 require github.com/stretchr/testify v1.11.1
 
+require github.com/kr/pretty v0.3.1 // indirect
+
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/trailofbits/go-fuzz-utils v0.0.0-20260318143407-0907cafe7589
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

go.sum

@@ -1,10 +1,21 @@
+github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
+github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
+github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=
+github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
+github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/rogpeppe/go-internal v1.9.0 h1:73kH8U+JUqXU8lRuOHeVHaa/SZPifC7BkcraZVejAe8=
+github.com/rogpeppe/go-internal v1.9.0/go.mod h1:WtVeX8xhTBvf0smdhujwtBcq4Qrzq/fJaraNFVN+nFs=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+github.com/trailofbits/go-fuzz-utils v0.0.0-20260318143407-0907cafe7589 h1:UmBZCTPdDYore2IEHN+U4eIqEaRq6METh9pKiPumkqc=
+github.com/trailofbits/go-fuzz-utils v0.0.0-20260318143407-0907cafe7589/go.mod h1:zh+T+w9XT/3o4E0WLEGCdmLJ8Yqx/zY3o538tQY3OjY=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

table.go

@@ -16,9 +16,9 @@ type Table[K, V any] struct {
 	minLoadFactor    float64
 }
 
-// Capacity returns the number of slots allocated for the [Table]. To get the
+// TotalCapacity returns the number of slots allocated for the [Table]. To get the
 // number of slots filled, look at [Table.Size].
-func (t Table[K, V]) Capacity() uint64 {
+func (t Table[K, V]) TotalCapacity() uint64 {
 	return t.bucketA.capacity + t.bucketB.capacity
 }
 
@@ -28,25 +28,34 @@ func (t Table[K, V]) Size() int {
 }
 
 func log2(n uint64) (m int) {
-	return bits.Len64(n) - 1
+	return max(0, bits.Len64(n)-1)
 }
 
 func (t Table[K, V]) maxEvictions() int {
-	return 3 * log2(t.Capacity())
+	return 3 * log2(t.TotalCapacity())
 }
 
 func (t Table[K, V]) load() float64 {
-	return float64(t.Size()) / float64(t.Capacity())
+	// When there are no slots in the table, we still treat the load as 100%.
+	// Every slot in the table is full.
+	if t.TotalCapacity() == 0 {
+		return 1.0
+	}
+
+	return float64(t.Size()) / float64(t.TotalCapacity())
 }
 
-func (t *Table[K, V]) resize() error {
+// resize clears all buckets, changes the sizes of them to a specific capacity,
+// and fills them back up again. It is a helper function for [Table.grow] and
+// [Table.shrink]; use them instead.
+func (t *Table[K, V]) resize(capacity uint64) error {
 	entries := make([]entry[K, V], 0, t.Size())
 	for k, v := range t.Entries() {
 		entries = append(entries, entry[K, V]{k, v})
 	}
 
-	t.bucketA.resize(t.growthFactor * t.bucketA.capacity)
-	t.bucketB.resize(t.growthFactor * t.bucketB.capacity)
+	t.bucketA.resize(capacity)
+	t.bucketB.resize(capacity)
 
 	for _, entry := range entries {
 		if err := t.Put(entry.key, entry.value); err != nil {
@@ -57,6 +66,26 @@ func (t *Table[K, V]) resize() error {
 	return nil
 }
 
+// grow increases the table's capacity by the [Table.growthFactor]. If the
+// capacity is 0, it increases it to 1.
+func (t *Table[K, V]) grow() error {
+	var newCapacity uint64
+
+	if t.TotalCapacity() == 0 {
+		newCapacity = 1
+	} else {
+		newCapacity = t.bucketA.capacity * t.growthFactor
+	}
+
+	return t.resize(newCapacity)
+}
+
+// shrink reduces the table's capacity by the [Table.growthFactor]. It may
+// reduce it down to 0.
+func (t *Table[K, V]) shrink() error {
+	return t.resize(t.bucketA.capacity / t.growthFactor)
+}
+
 // Get fetches the value for a key in the [Table]. Returns an error if no value
 // is found.
 func (t Table[K, V]) Get(key K) (value V, err error) {
@@ -99,10 +128,10 @@ func (t *Table[K, V]) Put(key K, value V) (err error) {
 	}
 
 	if t.load() < t.minLoadFactor {
-		return fmt.Errorf("bad hash: resize on load %d/%d = %f", t.Size(), t.Capacity(), t.load())
+		return fmt.Errorf("bad hash: resize on load %d/%d = %f", t.Size(), t.TotalCapacity(), t.load())
 	}
 
-	if err := t.resize(); err != nil {
+	if err := t.grow(); err != nil {
 		return err
 	}
 
@@ -111,10 +140,15 @@ func (t *Table[K, V]) Put(key K, value V) (err error) {
 
 // Drop removes a value for a key in the table. Returns an error if its value
 // cannot be removed.
-//
-// Deprecated: Do not use.
-func (t Table[K, V]) Drop(_ K) {
-	panic("Not implemented")
+func (t *Table[K, V]) Drop(key K) (err error) {
+	t.bucketA.drop(key)
+	t.bucketB.drop(key)
+
+	if t.load() < t.minLoadFactor {
+		return t.shrink()
+	}
+
+	return nil
 }
 
 // Entries returns an unordered sequence of all key-value pairs in the table.