feat!: Drop returns bool, Put doesn't stack-overflow (#21)

## Description

Closes #11.

## Changes

### Design Decisions

## Checklist

- [ ] Tests pass
- [ ] Docs updated

Reviewed-on: #21
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cuckoo_fuzz_test.go

@@ -68,12 +68,13 @@ func FuzzInsertLookup(f *testing.F) {
 
 		for _, step := range scenario.steps {
 			if step.drop {
-				err := actual.Drop(step.key)
+				ok := actual.Drop(step.key)
-				assert.NoError(err)
+				_, has := expected[step.key]
+				assert.Equal(ok, has)
 
 				delete(expected, step.key)
 
-				_, ok := actual.Get(step.key)
+				_, ok = actual.Get(step.key)
 				assert.False(ok)
 			} else {
 				err := actual.Put(step.key, step.value)

cuckoo_test.go

@@ -124,9 +124,9 @@ func TestDropExistingItem(t *testing.T) {
 	table := cuckoo.New[int, bool]()
 	(table.Put(key, value))
 
-	err := table.Drop(key)
+	had := table.Drop(key)
 
-	assert.NoError(err)
+	assert.True(had)
 	assert.Equal(0, table.Size())
 	assert.False(table.Has(key))
 }
@@ -136,9 +136,9 @@ func TestDropNoItem(t *testing.T) {
 	key := 0
 	table := cuckoo.New[int, bool]()
 
-	err := table.Drop(key)
+	had := table.Drop(key)
 
-	assert.NoError(err)
+	assert.False(had)
 	assert.Equal(0, table.Size())
 	assert.False(table.Has(key))
 }
@@ -152,10 +152,9 @@ func TestDropItemCapacity(t *testing.T) {
 	)
 
 	startingCapacity := table.TotalCapacity()
-	err := table.Drop(key)
+	table.Drop(key)
 	endingCapacity := table.TotalCapacity()
 
-	assert.NoError(err)
 	assert.Equal(0, table.Size())
 	assert.Equal(uint64(128), startingCapacity)
 	assert.Equal(uint64(64), endingCapacity)
@@ -203,9 +202,9 @@ func TestDropResizeCapacity(t *testing.T) {
 
 	err1 := table.Put(0, true)
 	err2 := table.Put(1, true)
-	err3 := table.Drop(1)
+	table.Drop(1)
 
-	assert.NoError(errors.Join(err1, err2, err3))
+	assert.NoError(errors.Join(err1, err2))
 	assert.Equal(uint64(20), table.TotalCapacity())
 }
 

doc.go

@@ -5,5 +5,8 @@
 // a table with any key type using [NewCustom]. Custom [Hash] functions and
 // key comparison are also supported.
 //
+// NOTE: The [Table] is a look-up structure, and not a source of truth. If
+// [ErrBadHash] occurs, the data cannot be restored.
+//
 // See more: https://en.wikipedia.org/wiki/Cuckoo_hashing
 package cuckoo

settings.go

@@ -19,6 +19,11 @@ const DefaultGrowthFactor uint64 = 2
 // [libcuckoo]: https://github.com/efficient/libcuckoo/blob/656714705a055df2b7a605eb3c71586d9da1e119/libcuckoo/cuckoohash_config.hh#L21
 const defaultMinimumLoad float64 = 0.05
 
+// defaultGrowthLimit is the maximum number of times a [Table] can grow in a
+// single [Table.Put], before the library infers it will lead to a stack
+// overflow. The value of '64' was chosen arbirarily.
+const defaultGrowthLimit uint64 = 64
+
 type settings struct {
 	growthFactor  uint64
 	minLoadFactor float64

subtable.go

@@ -1,5 +1,6 @@
 package cuckoo
 
+// An entry is a key-value pair.
 type entry[K, V any] struct {
 	key   K
 	value V
@@ -48,10 +49,13 @@ func (t *subtable[K, V]) drop(key K) (occupied bool) {
 	return false
 }
 
-func (t *subtable[K, V]) resize(capacity uint64) {
+func (t *subtable[K, V]) resized(capacity uint64) *subtable[K, V] {
-	t.slots = make([]slot[K, V], capacity)
+	return &subtable[K, V]{
-	t.capacity = capacity
+		slots:    make([]slot[K, V], capacity),
-	t.size = 0
+		capacity: capacity,
+		hash:     t.hash,
+		compare:  t.compare,
+	}
 }
 
 func (t *subtable[K, V]) update(key K, value V) (updated bool) {
@@ -69,7 +73,7 @@ func (t *subtable[K, V]) update(key K, value V) (updated bool) {
 	return false
 }
 
-func (t *subtable[K, V]) evict(insertion entry[K, V]) (evicted entry[K, V], eviction bool) {
+func (t *subtable[K, V]) insert(insertion entry[K, V]) (evicted entry[K, V], eviction bool) {
 	if t.capacity == 0 {
 		return insertion, true
 	}
@@ -92,8 +96,8 @@ func (t *subtable[K, V]) evict(insertion entry[K, V]) (evicted entry[K, V], evic
 	return insertion, true
 }
 
-func newSubtable[K, V any](capacity uint64, hash Hash[K], compare EqualFunc[K]) subtable[K, V] {
+func newSubtable[K, V any](capacity uint64, hash Hash[K], compare EqualFunc[K]) *subtable[K, V] {
-	return subtable[K, V]{
+	return &subtable[K, V]{
 		hash:     hash,
 		capacity: capacity,
 		compare:  compare,

table.go

@@ -9,7 +9,7 @@ import (
 )
 
 // ErrBadHash occurs when the hashes given to a [Table] cause too many key
-// collisions. Try rebuilding the table using:
+// collisions. Discard the old table, rebuild it from your source data, and try:
 //
 //  1. Different hash seeds. Equal seeds produce equal hash functions, which
 //     always cycle.
@@ -20,7 +20,7 @@ var ErrBadHash = errors.New("bad hash")
 // one with [New]. Or if you want more granularity, use [NewBy] or
 // [NewCustom].
 type Table[K, V any] struct {
-	tableA, tableB subtable[K, V]
+	tableA, tableB *subtable[K, V]
 	growthFactor   uint64
 	minLoadFactor  float64
 }
@@ -54,30 +54,61 @@ func (t *Table[K, V]) load() float64 {
 	return float64(t.Size()) / float64(t.TotalCapacity())
 }
 
-// resize clears all tables, changes the sizes of them to a specific capacity,
+// insert attempts to put/update an entry in the table, without modifying the
-// and fills them back up again. It is a helper function for [Table.grow] and
+// size of the table. Returns a displaced entry and 'homeless = true' if an
-// [Table.shrink]; use them instead.
+// entry could not be placed after exhausting evictions.
-func (t *Table[K, V]) resize(capacity uint64) error {
+func (t *Table[K, V]) insert(entry entry[K, V]) (displaced entry[K, V], homeless bool) {
-	entries := make([]entry[K, V], 0, t.Size())
+	if t.tableA.update(entry.key, entry.value) {
-	for k, v := range t.Entries() {
+		return
-		entries = append(entries, entry[K, V]{k, v})
 	}
 
-	t.tableA.resize(capacity)
+	if t.tableB.update(entry.key, entry.value) {
-	t.tableB.resize(capacity)
+		return
+	}
 
-	for _, entry := range entries {
+	for range t.maxEvictions() {
-		if err := t.Put(entry.key, entry.value); err != nil {
+		if entry, homeless = t.tableA.insert(entry); !homeless {
-			return err
+			return
+		}
+
+		if entry, homeless = t.tableB.insert(entry); !homeless {
+			return
 		}
 	}
 
-	return nil
+	return entry, true
+}
+
+// resized creates an empty copy of the table, with a new capacity for each
+// bucket.
+func (t *Table[K, V]) resized(capacity uint64) *Table[K, V] {
+	return &Table[K, V]{
+		growthFactor:  t.growthFactor,
+		minLoadFactor: t.minLoadFactor,
+		tableA:        t.tableA.resized(capacity),
+		tableB:        t.tableB.resized(capacity),
+	}
+}
+
+// resize creates a new [Table.resized] with 'capacity', inserts all items into
+// the array, and replaces the current table. It is a helper function for
+// [Table.grow] and [Table.shrink]; use them instead.
+func (t *Table[K, V]) resize(capacity uint64) bool {
+	updated := t.resized(capacity)
+
+	for k, v := range t.Entries() {
+		if _, failed := updated.insert(entry[K, V]{k, v}); failed {
+			return false
+		}
+	}
+
+	*t = *updated
+	return true
 }
 
 // grow increases the table's capacity by the growth factor. If the
 // capacity is 0, it increases it to 1.
-func (t *Table[K, V]) grow() error {
+func (t *Table[K, V]) grow() bool {
 	var newCapacity uint64
 
 	if t.TotalCapacity() == 0 {
@@ -91,7 +122,7 @@ func (t *Table[K, V]) grow() error {
 
 // shrink reduces the table's capacity by the growth factor. It may
 // reduce it down to 0.
-func (t *Table[K, V]) shrink() error {
+func (t *Table[K, V]) shrink() bool {
 	return t.resize(t.tableA.capacity / t.growthFactor)
 }
 
@@ -122,49 +153,48 @@ func (t *Table[K, V]) Has(key K) (exists bool) {
 	return
 }
 
-// Put sets the value for a key. Returns error if its value cannot be set.
+// Put sets the value for a key. If it cannot be set, an error is returned.
 func (t *Table[K, V]) Put(key K, value V) (err error) {
-	if t.tableA.update(key, value) {
+	var (
-		return nil
+		entry    = entry[K, V]{key, value}
-	}
+		homeless bool
+	)
 
-	if t.tableB.update(key, value) {
+	for range defaultGrowthLimit {
-		return nil
+		if entry, homeless = t.insert(entry); !homeless {
-	}
+			return
-
-	entry, eviction := entry[K, V]{key, value}, false
-	for range t.maxEvictions() {
-		if entry, eviction = t.tableA.evict(entry); !eviction {
-			return nil
 		}
 
-		if entry, eviction = t.tableB.evict(entry); !eviction {
+		// Both this and the growth limit are necessary: this catches bad hashes
-			return nil
+		// early when the table is sparse, while the latter catches cases where
+		// growing never helps.
+		if t.load() < t.minLoadFactor {
+			return fmt.Errorf("hash functions produced a cycle at load %d/%d: %w", t.Size(), t.TotalCapacity(), ErrBadHash)
+		}
+
+		// It is theoretically possible to have a table with a larger capacity
+		// that is valid. But this chance is astronomically small, so we ignore
+		// it in this implementation.
+		if grew := t.grow(); !grew {
+			return fmt.Errorf("could not redistribute entries into larger table: %w", ErrBadHash)
 		}
 	}
 
-	if t.load() < t.minLoadFactor {
+	return fmt.Errorf("could not place entry after %d resizes: %w", defaultGrowthLimit, ErrBadHash)
-		return fmt.Errorf("hash functions produced a cycle at load %d/%d: %w", t.Size(), t.TotalCapacity(), ErrBadHash)
-	}
-
-	if err := t.grow(); err != nil {
-		return err
-	}
-
-	return t.Put(entry.key, entry.value)
 }
 
-// Drop removes a value for a key in the table. Returns an error if its value
+// Drop removes a value for a key in the table. Returns whether the key had
-// cannot be removed.
+// existed.
-func (t *Table[K, V]) Drop(key K) (err error) {
+func (t *Table[K, V]) Drop(key K) bool {
-	t.tableA.drop(key)
+	occupied := t.tableA.drop(key) || t.tableB.drop(key)
-	t.tableB.drop(key)
 
 	if t.load() < t.minLoadFactor {
-		return t.shrink()
+		// The error is not handled here, because table-shrinking is an internal
+		// optimization.
+		t.shrink()
 	}
 
-	return nil
+	return occupied
 }
 
 // Entries returns an unordered sequence of all key-value pairs in the table.