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>

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,
-// 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})
+// insert attempts to put/update an entry in the table, without modifying the
+// size of the table. Returns a displaced entry and 'homeless = true' if an
+// entry could not be placed after exhausting evictions.
+func (t *Table[K, V]) insert(entry entry[K, V]) (displaced entry[K, V], homeless bool) {
+	if t.tableA.update(entry.key, entry.value) {
+		return
 	}
 
-	t.tableA.resize(capacity)
-	t.tableB.resize(capacity)
+	if t.tableB.update(entry.key, entry.value) {
+		return
+	}
 
-	for _, entry := range entries {
-		if err := t.Put(entry.key, entry.value); err != nil {
-			return err
+	for range t.maxEvictions() {
+		if entry, homeless = t.tableA.insert(entry); !homeless {
+			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) {
-		return nil
-	}
+	var (
+		entry    = entry[K, V]{key, value}
+		homeless bool
+	)
 
-	if t.tableB.update(key, value) {
-		return nil
-	}
-
-	entry, eviction := entry[K, V]{key, value}, false
-	for range t.maxEvictions() {
-		if entry, eviction = t.tableA.evict(entry); !eviction {
-			return nil
+	for range defaultGrowthLimit {
+		if entry, homeless = t.insert(entry); !homeless {
+			return
 		}
 
-		if entry, eviction = t.tableB.evict(entry); !eviction {
-			return nil
+		// Both this and the growth limit are necessary: this catches bad hashes
+		// 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("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)
+	return fmt.Errorf("could not place entry after %d resizes: %w", defaultGrowthLimit, ErrBadHash)
 }
 
-// Drop removes a value for a key in the table. Returns an error if its value
-// cannot be removed.
-func (t *Table[K, V]) Drop(key K) (err error) {
-	t.tableA.drop(key)
-	t.tableB.drop(key)
+// Drop removes a value for a key in the table. Returns whether the key had
+// existed.
+func (t *Table[K, V]) Drop(key K) bool {
+	occupied := t.tableA.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.