docs: congruency target

## 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>

.golangci.yml

@@ -114,6 +114,9 @@ linters:
     # Reports uses of functions with replacement inside the testing package.
     - usetesting
 
+    # Reports mixed receiver types in structs/interfaces.
+    - recvcheck
+
   settings:
     revive:
       rules:

adr/001_interface_design.md

@@ -0,0 +1,542 @@
+# Designing an Idiomatic API Interface
+
+We (the maintainers) built `go-cuckoo`'s API interface without design intent.
+Up until now, we paid more attention implementing the underlying functionality of the cuckoo hashing.
+
+With the fundamentals of the algorithm built, we should revisit the interface.
+It should align closer to the following principles:
+
+- **Congruency**
+  A `go-cuckoo` table should have the same core functionality as Go's built-in map.
+
+- **Familiarity**
+  A `go-cuckoo` table should behave similarly to Go's standard map, so users will intuitively know how to use it.
+  In effect, its users will carry less cognitive load.
+
+## Current State
+
+### Interface of the built-in Map
+
+Listed below is every interface provided by Go to the built-in map object.
+Also included, are the functions from the package `maps` in the standard library.
+
+<details>
+<summary>Interfaces</summary>
+
+| #   | built-in Interface                 | Description                                                                                                                                           |
+| --- | ---------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------- |
+| 1   | `m := make(map[K]V)`               | Returns an empty map using the built-in `make()` function.                                                                                            |
+| 2   | `m := make(map[K]V, hint)`         | Returns an empty map using `make()`, with a capacity 'hint'. This hint is how many items the map expects to hold, _not_ a measure of how large it is. |
+| 3   | `m := map[K]V{...}`                | Returns a map, which may be filled with entries in the ellipsis (optional).                                                                           |
+| 4   | `var m map[K]V`                    | Defines an empty _variable_ that holds a map. This differs from #1 because `m` is uninitialized (nil) here.                                           |
+| 5   | `m[k] := v`                        | Assigns the value of `k` to `v`.                                                                                                                      |
+| 6   | `v := m[k]`                        | Returns the value of `k` if it exists. Otherwise, `v` is uninitialized.                                                                               |
+| 7   | `v, ok := m[k]`                    | Similar to #6, except `ok` is equal to whether `v` is initialized. This is comma-ok notation.                                                         |
+| 8   | `for k, v := range m`              | Iterates over every key-value pair in `m`. The order is random.                                                                                       |
+| 9   | `delete(m, k)`                     | Unassigns the value `k`. Returns no value.                                                                                                            |
+| 10  | `clear(m)`                         | Unassigns all keys in `m`. Returns no value.                                                                                                          |
+| 11  | `n := len(m)`                      | Returns the number of entries in `m`. If nil, `m` returns 0.                                                                                          |
+| 12  | `m2 := maps.Clone(m)`              | Returns a copy of `m`.                                                                                                                                |
+| 13  | `maps.Copy(dst, src)`              | Assigns every entry of `src` in `dst`.                                                                                                                |
+| 14  | `ok := maps.Equal(m1, m2)`         | Returns true iff `m1` and `m2` the same entries.                                                                                                      |
+| 15  | `ok := maps.EqualFunc(m1, m2, fn)` | Like #14, but with a custom comparator for non-comparable values.                                                                                     |
+| 16  | `maps.DeleteFunc(m, fn)`           | Removes every entry in `m` which satisfies `fn`. Returns no value.                                                                                    |
+| 17  | `it2 := maps.All(m)`               | Returns an 2D iterator over every key-value pair.                                                                                                     |
+| 18  | `it := maps.Keys(m)`               | Returns an iterator over every key.                                                                                                                   |
+| 19  | `it := maps.Values(m)`             | Returns an iterator over every value. There can be duplicates.                                                                                        |
+| 20  | `m := maps.Collect(seq)`           | Returns a map, with every entry defined in a 2D iterator over key-value pairs.                                                                        |
+| 21  | `maps.Insert(m, seq)`              | Assigns to `m` all key-value pairs in 2D iterator `seq`. Returns no value.                                                                            |
+
+</details>
+
+### Interface of `go-cuckoo`
+
+On the other hand, here is the current contract for `go-cuckoo`.
+
+<details>
+<summary>Interfaces</summary>
+
+| #   | built-in Interface                                 | Description                                                                                                                  |
+| --- | -------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------- |
+| 1   | `m := New(opts...)`                                | Creates a table using the default hash and equal function. The options configure its behavior. Confined to comparable keys.  |
+| 2   | `m := NewBy(keyFunc, opts...)`                     | Like #1, but allows any key type. A `keyFunc` is used to derive a comparable key.                                            |
+| 3   | `m := NewCustom(hashA, hashB, equalFunc, opts...)` | Like #1, but allows control over the hashes used to allow any key type. An `equalFunc` determines key equality.              |
+| 4   | `seq := m.Entries()`                               | Returns an unordered 2D iterator of all key-value pairs in the table.                                                        |
+| 5   | `v := m.Find(k)`                                   | Removes the value for `k`. Returns true if `k` existed.                                                                      |
+| 6   | `v, ok := m.Get(k)`                                | Returns the value for `k` in the table. Also, returns true if the `k` exists, otherwise false. When false, `v` is undefined. |
+| 7   | `ok := m.Has(k)`                                   | Returns true if `k` is in the table.                                                                                         |
+| 8   | `err := m.Put(k, v)`                               | Sets value `v` for key `k`. Otherwise, returns error.                                                                        |
+| 9   | `n := m.Size()`                                    | Returns the number of items in `m`.                                                                                          |
+| 10  | `str := m.String()`                                | Returns `m` as a string in the format "table[k1:v1 k2:v2 ...]".                                                              |
+| 11  | `cap := m.TotalCapacity()`                         | Returns how many slots `m` has allocated.                                                                                    |
+| 12  | `ok := m.Drop(k)`                                  | Removes `k` from the table. Returns whether the key had existed.                                                             |
+
+</details>
+
+### Determining Congruency
+
+So, how does the core functionality compare?
+Listed below is an analysis of every interface in Go's standard map.
+Each is compared against what `go-cuckoo` offers, and categorized into the following groups:
+
+- ✅ Covered: an analog exists.
+- ⚠️ Partial: workaround available.
+- ❌ Gap: no analog yet; addressed in [Target State](#solving-congruency).
+
+Specifically, here we are checking for functionality.
+Is there functionality that this offers which `go-cuckoo` does not?
+We are checking accessibility, but not discoverability.
+The latter will be considered later.
+
+<details>
+<summary>✅ <code>m := make(map[K]V)</code></summary>
+
+The analog is `m := New()`.
+
+</details>
+
+<details>
+<summary>⚠️ <code>m := make(map[K]V, hint)</code></summary>
+
+This has no simple analog.
+
+It is close to `m := New(Capacity(hint))`, but it assigns starting capacity, not expected size.
+For the built-in map, these are two separate things.
+
+- Capacity is an internal measure, used to optimize space/speed.
+  It is hidden from the user because it depends on the underlying implementation, which may change.
+- Expected size requires the map must hold a number of items before resizing.
+  This is tangeable and agnostic to implementation, hence why it is given to the user.
+
+In short, this interface defines expected size, but `Capacity()` defines capacity.
+
+</details>
+
+<details>
+<summary>❌ <code>m := map[K]V{...}</code></summary>
+
+This has no simple analog, the closest being:
+
+```go
+m := New[K, V]()
+for k, v := range startingEntries {
+  m.Put(k, v)
+}
+```
+
+It is idiomatic, but far less ergonomic.
+
+</details>
+
+<details>
+<summary>✅ <code>var m map[K]V</code></summary>
+
+The analog is `var m Table[K, V]`.
+
+</details>
+
+<details>
+<summary>✅ <code>m[k] := v</code></summary>
+
+The analog is `err := m.Put(k, v)`.
+
+</details>
+
+<details>
+<summary>✅ <code>v := m[k]</code></summary>
+
+The analog is `v := m.Find(k)`.
+
+</details>
+
+<details>
+<summary>✅ <code>v, ok := m[k]</code></summary>
+
+The analog is `v, ok := m.Get(k)`.
+
+</details>
+
+<details>
+<summary>✅ <code>for k, v := range m</code></summary>
+
+The analog is `for k, v := range m.Entries()`.
+
+</details>
+
+<details>
+<summary>✅ <code>delete(m, k)</code></summary>
+
+The analog is `ok := m.Drop(k)`.
+
+</details>
+
+<details>
+<summary>❌ <code>clear(m)</code></summary>
+
+There is no analog.
+
+The easiest may to do this is to delete all items individually:
+
+```go
+for k := range m.Entries() {
+  m.Drop(k)
+}
+```
+
+</details>
+
+<details>
+<summary>✅ <code>n := len(m)</code></summary>
+
+The analog is `n := m.Size()`.
+
+</details>
+
+<details>
+<summary>❌ <code>m2 := maps.Clone(m)</code></summary>
+
+There is no analog.
+
+The easiest way to do this currently is to make a new map, and manually add the items.
+
+```go
+m2 := cuckoo.Table[K, V]()
+
+for k, v := range m.Entries() {
+  m2.Put(k, v)
+}
+```
+
+This gets complicated by the various options available to the user.
+Furthermore, any custom `EqualFunc`, `keyFunc` or `Hash` is not transferred.
+
+</details>
+
+<details>
+<summary>❌ <code>maps.Copy(dst, src)</code></summary>
+
+There is no analog.
+
+The simplest way to do this is with a for-loop.
+
+```go
+for k, v := range src.Entries() {
+  dst.Put(k, v)
+}
+```
+
+</details>
+
+<details>
+<summary>❌ <code>ok := maps.Equal(m1, m2)</code></summary>
+
+There is no analog.
+
+Users have to manually check the key-value pairs to determine equality.
+
+</details>
+
+<details>
+<summary>❌ <code>ok := maps.EqualFunc(m1, m2, fn)</code></summary>
+
+There is no analog.
+
+Users have to manually check the key-value pairs to determine equality.
+
+</details>
+
+<details>
+<summary>❌ <code>maps.DeleteFunc(m, fn)</code></summary>
+
+There is no analog.
+
+Users have to manually delete keys.
+
+</details>
+
+<details>
+<summary>✅ <code>it2 := maps.All(m)</code></summary>
+
+The analog is `it2 := m.Entries()`.
+
+</details>
+
+<details>
+<summary>⚠️ <code>it := maps.Keys(m)</code></summary>
+
+There is no simple analog.
+
+A close neighbor is `it2 := m.Entries()`.
+Users can use this in a for-loop, and pick out just the keys:
+
+```go
+for k := range m.Entries() {
+  // ...
+}
+```
+
+</details>
+
+<details>
+<summary>⚠️ <code>it := maps.Values(m)</code></summary>
+
+There is no simple analog.
+
+A close neighbor is `it2 := m.Entries()`.
+Users can use this in a for-loop, and pick out just the values:
+
+```go
+for _, v := range m.Entries() {
+  // ...
+}
+```
+
+</details>
+
+<details>
+<summary>❌ <code>m := maps.Collect(seq)</code></summary>
+
+There is no analog.
+
+</details>
+
+<details>
+<summary>❌ <code>maps.Insert(m, seq)</code></summary>
+
+There is no analog.
+
+</details>
+
+## Target State
+
+### Solving Congruency
+
+We should make the following changes to accomodate for congruency:
+
+<details>
+<summary><code>ok := maps.EqualFunc(m1, m2, fn)</code></summary>
+
+We should implement a new function:
+
+```go
+func EqualFunc[K, V1, V2 any](t1 *Table[K, V1], t2 *Table[K, V2], eq func(V1, V2) bool) bool
+```
+
+This function is free, and not bound as a receiver function.
+(It is called `cuckoo.Equal(t1, t2)`, not `t1.Equals(t2)`.)
+The latter implies `t1` has authority, when in fact neither do.
+
+We define equality as:
+
+1. Neither table has a key the other doesn't.
+2. Each key has the same value in each table.
+   Parameter `eq` determines this equality.
+
+Custom `EqualFunc`'s complicate this, as they modulate key identity in tables.
+If two tables may differ on whether two keys are different, this function might break.
+So, we must assume that:
+
+- Both tables have `EqualFunc`'s which 'agree' on the identity of the keys present in the tables.
+  Agreement is defined as: if two keys are distinct in one table, they are distinct in the other.
+
+The name `EqualFunc` is already taken by `EqualFunc[K, V]`: an alias for `func(a, b K) bool`.
+Inlining `EqualFunc[K, V]` would solve this problem.
+We will move the documentation attached to it to `DefaultEqualFunc`.
+
+</details>
+
+<details>
+<summary><code>ok := maps.Equal(m1, m2)</code></summary>
+
+We should implement a new function, to conform with the standard library:
+
+```go
+func Equal[K any, V comparable](t1, t2 *Table[K, V]) bool
+```
+
+It uses the same equality check as in `EqualFunc`.
+Once again, the function is free because it is symmetric.
+
+</details>
+
+<details>
+<summary><code>maps.Insert(m, seq)</code></summary>
+
+We should implement a new receiver for the table:
+
+```go
+func (t *Table[K, V]) Insert(seq iter.Seq2[K, V]) error
+```
+
+A receiver fits better even though `maps.Insert` is a free function, because copying it is asymmetric.
+Map `dst` receives entries from map `src`.
+It's only free because Go's standard map is built into the language, and so cannot have receivers.
+
+In terms of naming, `t.Extend` is more accurate, and has precedent in [Python](docs.python.org/3/tutorial/datastructures.html#more-on-lists) and [Rust](https://doc.rust-lang.org/std/iter/trait.Extend.html).
+When [adding iterator function](https://github.com/golang/go/issues/61900) to the `maps` package, the Go team chose to frame it as 'sources' and 'sinks'.
+With this model, `maps.Insert` made more sense than `maps.Extend`.
+Ultimately, `t.Insert()` is a better choice to be consistent with `maps`.
+
+</details>
+
+<details>
+<summary><code>maps.Copy(dst, src)</code></summary>
+
+We should implement a new receiver for the table:
+
+```go
+func (t *Table[K, V]) Copy(src *Table[K, V]) error
+```
+
+It's functionality should match that of `t.Insert()`.
+
+A receiver fits better even though `maps.Copy` is a free function, 'copying' it is asymmetric: `dst` is writen into by `src`.
+It is only free because Go's standard map is built into the language, and so cannot have receivers.
+
+The name `t.Merge()` might be more accurate, but it does work because:
+
+- `t.Copy()` matches Go's built-in `copy()`, and `io.Copy()`. The Go team used [the same logic](https://github.com/golang/go/discussions/47330#discussioncomment-1167799) to name `maps.Copy()`.
+  In this case, `t.Merge()` would be an outlier.
+- `t.Merge()` implies some sort of conflict-resolution, when there is not.
+  It simply overwrites the values.
+
+</details>
+
+<details>
+<summary><code>maps.DeleteFunc(m, fn)</code></summary>
+
+We should implement a new receiver for the table:
+
+```go
+func (t *Table[K, V]) DeleteFunc(del func(K, V) bool)
+```
+
+It would have the same functionality as `maps.DeleteFunc`.
+
+A free function could work here, but `t` has clear authority over `del`.
+Other than being consistent with the `maps` package, `t.DeleteFunc` follows the Go convention of appending `Func` to higher-order equivalents of functions.
+This trumps names like `t.DeleteIf`, which lend more to [Java](https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html#removeIf-java.util.function.Predicate-) or [C++](https://en.cppreference.com/cpp/algorithm/remove).
+The word `Delete` is also convention, tying back to the built-in `delete()`.
+
+</details>
+
+<details>
+<summary><code>m := maps.Collect(seq)</code></summary>
+
+We should implement a new constructor.
+
+```go
+func Collect[K comparable, V any](seq iter.Seq2[K, V]) (*Table[K, V], error)
+```
+
+It would create a `New()` table, and insert all entries in `seq`.
+
+This reveicer only supports the standard table constructor, with comparable keys.
+It is tempting to add `CollectBy` or `CollectCustom` to support all table types, but doing so would pollute the public interface.
+
+It would be just one more line to initialize the table and then call `t.Insert` directly:
+
+```go
+t := // ...
+err := t.Insert(seq)
+```
+
+</details>
+
+<details>
+<summary><code>m := map[K]V{...}</code></summary>
+
+We should make a new constructor, because entries are generic.
+So, creating an option with inialized entries doesn't work.
+
+With the previous additions, users have a few options.
+If they want to use a `New()` table, `t.Collect` matches well:
+
+```go
+t, err := cuckoo.Collect(func(yield func(K, V) bool) {
+    yield(key1, val1)
+    yield(key2, val2)
+})
+```
+
+For `NewCustom()` or `NewBy()` tables, users can call `t.Insert` after initialization:
+
+```go
+t := // ...
+err := t.Insert(func(yield func(K, V) bool) {
+    yield(key1, val1)
+    yield(key2, val2)
+})
+```
+
+It is one more line.
+But, the alternative is polluting the public interface with corresponding `*WithEntries` constuctors.
+
+</details>
+
+<details>
+<summary><code>m := make(map[K]V, hint)</code></summary>
+
+We should add a new option:
+
+```go
+func ExpectedSize(n int) Option
+```
+
+When fed to a table, it will allocate enough space to hold `n` entries without a resize.
+
+</details>
+
+<details>
+<summary><code>clear(m)</code></summary>
+
+We should implement a new receiver:
+
+```go
+func (t *Table[K, V]) Clear()
+```
+
+It will remove all entries from the table.
+
+</details>
+
+<details>
+<summary><code>m2 := maps.Clone(m)</code></summary>
+
+We should implement a matching function:
+
+```go
+func (t *Table[K, V]) Clone() *Table[K, V]
+```
+
+Also, it will copy the hash, equality function, and options used in the table.
+
+</details>
+
+<details>
+<summary><code>it := maps.Keys(m)</code></summary>
+
+We should implement a matching function:
+
+```go
+func (t *Table[K, V]) Keys() iter.Seq[K]
+```
+
+It is tempting to just have `All()`, but it returns a `Seq2`, not a `Seq`.
+There is no iterator adaptor between `Seq` and `Seq2`, and will not be for the foreseeable future.
+This function, while it feels superfluous, is required.
+
+</details>
+
+<details>
+<summary><code>it := maps.Values(m)</code></summary>
+
+We should implement a matching function:
+
+```go
+func (t *Table[K, V]) Values() iter.Seq[V]
+```
+
+For the same reason we need `Keys()`, we also need `Values()`.
+
+</details>

compare.go

@@ -1,11 +1,11 @@
 package cuckoo
 
 // An EqualFunc determines whethers two keys are 'equal'. Keys that are 'equal'
-// are teated as the same by the [HashTable]. A good EqualFunc is pure,
+// are teated as the same by the [Table]. A good EqualFunc is pure,
 // deterministic, and fast. By default, [New] uses [DefaultEqualFunc].
 //
 // This function MUST NOT return true if the [Hash] digest of two keys
-// are different: the [HashTable] will not work.
+// are different: the [Table] will not work.
 type EqualFunc[K any] = func(a, b K) bool
 
 // DefaultEqualFunc compares two keys by strict equality. Returns true if the

cuckoo_fuzz_test.go

@@ -68,21 +68,22 @@ 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)
 
-				_, err = actual.Get(step.key)
+				_, ok = actual.Get(step.key)
-				assert.Error(err)
+				assert.False(ok)
 			} else {
 				err := actual.Put(step.key, step.value)
 				assert.NoError(err)
 
 				expected[step.key] = step.value
 
-				found, err := actual.Get(step.key)
+				found, ok := actual.Get(step.key)
-				assert.NoError(err)
+				assert.True(ok)
 				assert.Equal(step.value, found)
 			}
 

cuckoo_test.go

@@ -108,12 +108,12 @@ func TestGetMany(t *testing.T) {
 	}
 
 	for i := range 2_000 {
-		value, err := table.Get(i)
+		value, ok := table.Get(i)
 		if i < 1_000 {
-			assert.NoError(err)
+			assert.True(ok)
 			assert.Equal(value, true)
 		} else {
-			assert.Error(err)
+			assert.False(ok)
 		}
 	}
 }
@@ -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

doc_example_test.go

@@ -14,19 +14,19 @@ func Example_basic() {
 		fmt.Println("Put error:", err)
 	}
 
-	if item, err := table.Get(1); err != nil {
+	if item, ok := table.Get(1); !ok {
-		fmt.Println("Error:", err)
+		fmt.Println("Not Found 1!")
 	} else {
 		fmt.Println("Found 1:", item)
 	}
 
-	if item, err := table.Get(0); err != nil {
+	if item, ok := table.Get(0); !ok {
-		fmt.Println("Error:", err)
+		fmt.Println("Not Found 0!")
 	} else {
 		fmt.Println("Found 0:", item)
 	}
 
 	// Output:
 	// Found 1: Hello, World!
-	// Error: key '0' not found
+	// Not Found 0!
 }

hash.go

@@ -7,9 +7,9 @@ import (
 // A Hash function maps any data to a fixed-length value (in this case, a
 // [uint64]).
 //
-// It is used by the [HashTable] to evenly distribute values
+// It is used by the [Table] to evenly distribute values
 // amongst its slots. A good hash function is uniform, [chaotic], and
-// deterministic. [HashTable] uses [NewDefaultHash] by default, which is built on
+// deterministic. [Table] uses [NewDefaultHash] by default, which is built on
 // [maphash.Comparable].
 //
 // [chaotic]: https://en.wikipedia.org/wiki/Avalanche_effect

hash_table.go

@@ -1,246 +0,0 @@
-package cuckoo
-
-import (
-	"errors"
-	"fmt"
-	"iter"
-	"math/bits"
-	"strings"
-)
-
-// ErrBadHash occurs when the hashes given to a [Table] cause too many key
-// collisions. Try rebuilding the table using:
-//
-//  1. Different hash seeds. Equal seeds produce equal hash functions, which
-//     always cycle.
-//  2. A different [Hash] algorithm.
-var ErrBadHash = errors.New("bad hash")
-
-// A HashTable which uses cuckoo hashing to resolve collision. Create
-// one with [New]. Or if you want more granularity, use [NewBy] or
-// [NewCustom].
-type HashTable[K, V any] struct {
-	tableA, tableB table[K, V]
-	growthFactor   uint64
-	minLoadFactor  float64
-}
-
-// TotalCapacity returns the number of slots allocated for the [HashTable]. To get the
-// number of slots filled, look at [HashTable.Size].
-func (t *HashTable[K, V]) TotalCapacity() uint64 {
-	return t.tableA.capacity + t.tableB.capacity
-}
-
-// Size returns how many slots are filled in the [HashTable].
-func (t *HashTable[K, V]) Size() int {
-	return int(t.tableA.size + t.tableB.size)
-}
-
-func log2(n uint64) (m int) {
-	return max(0, bits.Len64(n)-1)
-}
-
-func (t *HashTable[K, V]) maxEvictions() int {
-	return 3 * log2(t.TotalCapacity())
-}
-
-func (t *HashTable[K, V]) load() float64 {
-	// 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())
-}
-
-// 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 [HashTable.grow] and
-// [HashTable.shrink]; use them instead.
-func (t *HashTable[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.tableA.resize(capacity)
-	t.tableB.resize(capacity)
-
-	for _, entry := range entries {
-		if err := t.Put(entry.key, entry.value); err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
-// grow increases the table's capacity by the growth factor. If the
-// capacity is 0, it increases it to 1.
-func (t *HashTable[K, V]) grow() error {
-	var newCapacity uint64
-
-	if t.TotalCapacity() == 0 {
-		newCapacity = 1
-	} else {
-		newCapacity = t.tableA.capacity * t.growthFactor
-	}
-
-	return t.resize(newCapacity)
-}
-
-// shrink reduces the table's capacity by the growth factor. It may
-// reduce it down to 0.
-func (t *HashTable[K, V]) shrink() error {
-	return t.resize(t.tableA.capacity / t.growthFactor)
-}
-
-// Get fetches the value for a key in the [HashTable]. Returns an error if no value
-// is found.
-func (t *HashTable[K, V]) Get(key K) (value V, err error) {
-	if item, ok := t.tableA.get(key); ok {
-		return item, nil
-	}
-
-	if item, ok := t.tableB.get(key); ok {
-		return item, nil
-	}
-
-	return value, fmt.Errorf("key '%v' not found", key)
-}
-
-// Has returns true if a key has a value in the table.
-func (t *HashTable[K, V]) Has(key K) (exists bool) {
-	_, err := t.Get(key)
-	return err == nil
-}
-
-// Put sets the value for a key. Returns error if its value cannot be set.
-func (t *HashTable[K, V]) Put(key K, value V) (err error) {
-	if t.tableA.update(key, value) {
-		return nil
-	}
-
-	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
-		}
-
-		if entry, eviction = t.tableB.evict(entry); !eviction {
-			return nil
-		}
-	}
-
-	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)
-}
-
-// Drop removes a value for a key in the table. Returns an error if its value
-// cannot be removed.
-func (t *HashTable[K, V]) Drop(key K) (err error) {
-	t.tableA.drop(key)
-	t.tableB.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.
-func (t *HashTable[K, V]) Entries() iter.Seq2[K, V] {
-	return func(yield func(K, V) bool) {
-		for _, slot := range t.tableA.slots {
-			if slot.occupied {
-				if !yield(slot.key, slot.value) {
-					return
-				}
-			}
-		}
-
-		for _, slot := range t.tableB.slots {
-			if slot.occupied {
-				if !yield(slot.key, slot.value) {
-					return
-				}
-			}
-		}
-	}
-}
-
-// String returns the entries of the table as a string in the format:
-// "table[k1:v1 k2:v2 ...]".
-func (t *HashTable[K, V]) String() string {
-	var sb strings.Builder
-	sb.WriteString("table[")
-
-	first := true
-	for k, v := range t.Entries() {
-		if !first {
-			sb.WriteString(" ")
-		}
-
-		fmt.Fprintf(&sb, "%v:%v", k, v)
-		first = false
-	}
-
-	sb.WriteString("]")
-	return sb.String()
-}
-
-// NewCustom creates a [HashTable] with custom [Hash] and [EqualFunc]
-// functions, along with any [Option] the user provides.
-func NewCustom[K, V any](hashA, hashB Hash[K], compare EqualFunc[K], options ...Option) *HashTable[K, V] {
-	settings := &settings{
-		growthFactor:  DefaultGrowthFactor,
-		bucketSize:    DefaultCapacity,
-		minLoadFactor: defaultMinimumLoad,
-	}
-
-	for _, option := range options {
-		option(settings)
-	}
-
-	return &HashTable[K, V]{
-		growthFactor:  settings.growthFactor,
-		minLoadFactor: settings.minLoadFactor,
-		tableA:        newTable[K, V](settings.bucketSize, hashA, compare),
-		tableB:        newTable[K, V](settings.bucketSize, hashB, compare),
-	}
-}
-
-func pipe[X, Y, Z any](a func(X) Y, b func(Y) Z) func(X) Z {
-	return func(x X) Z { return b(a(x)) }
-}
-
-// NewBy creates a [HashTable] for any key type by using keyFunc to derive a
-// comparable key. Two keys with the same derived key are treated as equal.
-func NewBy[K, V any, C comparable](keyFunc func(K) C, options ...Option) *HashTable[K, V] {
-	return NewCustom[K, V](
-		pipe(keyFunc, NewDefaultHash[C]()),
-		pipe(keyFunc, NewDefaultHash[C]()),
-		func(a, b K) bool { return keyFunc(a) == keyFunc(b) },
-		options...,
-	)
-}
-
-// New creates a [HashTable] using the default [Hash] and [EqualFunc]. Use
-// the [Option] functions to configure its behavior. Note that this constructor
-// is only provided for comparable keys. For arbitrary keys, consider
-// [NewBy] or [NewCustom].
-func New[K comparable, V any](options ...Option) *HashTable[K, V] {
-	return NewCustom[K, V](NewDefaultHash[K](), NewDefaultHash[K](), DefaultEqualFunc[K], options...)
-}

settings.go

@@ -2,34 +2,39 @@ package cuckoo
 
 import "fmt"
 
-// DefaultCapacity is the initial capacity of a [HashTable]. It is inspired from
+// DefaultCapacity is the initial capacity of a [Table]. It is inspired from
 // Java's [HashMap] implementation, which also uses 16.
 //
 // [HashMap]: https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#HashMap--
 const DefaultCapacity uint64 = 16
 
-// DefaultGrowthFactor is the standard resize multiplier for a [HashTable]. Most
+// DefaultGrowthFactor is the standard resize multiplier for a [Table]. Most
 // implementations use 2.
 const DefaultGrowthFactor uint64 = 2
 
-// defaultMinimumLoad is the default lowest acceptable occupancy of a [HashTable].
+// defaultMinimumLoad is the default lowest acceptable occupancy of a [Table].
-// The higher the minimum load, the more likely that a [HashTable.Put] will not
+// The higher the minimum load, the more likely that a [Table.Put] will not
 // succeed. The value of 5% is taken from [libcuckoo].
 //
 // [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
 	bucketSize    uint64
 }
 
-// An Option modifies the settings of a [HashTable]. It is used in its constructors
+// An Option modifies the settings of a [Table]. It is used in its constructors
 // like [New], for example.
 type Option func(*settings)
 
-// Capacity modifies the starting capacity of each table of the [HashTable]. The
+// Capacity modifies the starting capacity of each subtable of the [Table]. The
 // value must be non-negative.
 func Capacity(value int) Option {
 	if value < 0 {
@@ -39,7 +44,7 @@ func Capacity(value int) Option {
 	return func(s *settings) { s.bucketSize = uint64(value) }
 }
 
-// GrowthFactor controls how much the capacity of the [HashTable] multiplies when
+// GrowthFactor controls how much the capacity of the [Table] multiplies when
 // it must resize. The value must be greater than 1.
 func GrowthFactor(value int) Option {
 	if value < 2 {

subtable.go

@@ -0,0 +1,107 @@
+package cuckoo
+
+// An entry is a key-value pair.
+type entry[K, V any] struct {
+	key   K
+	value V
+}
+
+type slot[K, V any] struct {
+	entry[K, V]
+	occupied bool
+}
+
+type subtable[K, V any] struct {
+	hash           Hash[K]
+	slots          []slot[K, V]
+	capacity, size uint64
+	compare        EqualFunc[K]
+}
+
+// location determines where in the subtable a certain key would be placed. If
+// the capacity is 0, this will panic.
+func (t *subtable[K, V]) location(key K) uint64 {
+	return t.hash(key) % t.capacity
+}
+
+func (t *subtable[K, V]) get(key K) (value V, found bool) {
+	if t.capacity == 0 {
+		return
+	}
+
+	slot := t.slots[t.location(key)]
+	return slot.value, slot.occupied && t.compare(slot.key, key)
+}
+
+func (t *subtable[K, V]) drop(key K) (occupied bool) {
+	if t.capacity == 0 {
+		return
+	}
+
+	slot := &t.slots[t.location(key)]
+
+	if slot.occupied && t.compare(slot.key, key) {
+		slot.occupied = false
+		t.size--
+		return true
+	}
+
+	return false
+}
+
+func (t *subtable[K, V]) resized(capacity uint64) *subtable[K, V] {
+	return &subtable[K, V]{
+		slots:    make([]slot[K, V], capacity),
+		capacity: capacity,
+		hash:     t.hash,
+		compare:  t.compare,
+	}
+}
+
+func (t *subtable[K, V]) update(key K, value V) (updated bool) {
+	if t.capacity == 0 {
+		return
+	}
+
+	slot := &t.slots[t.location(key)]
+
+	if slot.occupied && t.compare(slot.key, key) {
+		slot.value = value
+		return true
+	}
+
+	return false
+}
+
+func (t *subtable[K, V]) insert(insertion entry[K, V]) (evicted entry[K, V], eviction bool) {
+	if t.capacity == 0 {
+		return insertion, true
+	}
+
+	slot := &t.slots[t.location(insertion.key)]
+
+	if !slot.occupied {
+		slot.entry = insertion
+		slot.occupied = true
+		t.size++
+		return
+	}
+
+	if t.compare(slot.key, insertion.key) {
+		slot.value = insertion.value
+		return
+	}
+
+	insertion, slot.entry = slot.entry, insertion
+	return insertion, true
+}
+
+func newSubtable[K, V any](capacity uint64, hash Hash[K], compare EqualFunc[K]) *subtable[K, V] {
+	return &subtable[K, V]{
+		hash:     hash,
+		capacity: capacity,
+		compare:  compare,
+		size:     0,
+		slots:    make([]slot[K, V], capacity),
+	}
+}

table.go

@@ -1,103 +1,283 @@
 package cuckoo
 
-type entry[K, V any] struct {
+import (
-	key   K
+	"errors"
-	value V
+	"fmt"
+	"iter"
+	"math/bits"
+	"strings"
+)
+
+// ErrBadHash occurs when the hashes given to a [Table] cause too many key
+// 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.
+//  2. A different [Hash] algorithm.
+var ErrBadHash = errors.New("bad hash")
+
+// A Table which uses cuckoo hashing to resolve collision. Create
+// one with [New]. Or if you want more granularity, use [NewBy] or
+// [NewCustom].
+type Table[K, V any] struct {
+	tableA, tableB *subtable[K, V]
+	growthFactor   uint64
+	minLoadFactor  float64
 }
 
-type slot[K, V any] struct {
+// TotalCapacity returns the number of slots allocated for the [Table]. To get the
-	entry[K, V]
+// number of slots filled, look at [Table.Size].
-	occupied bool
+func (t *Table[K, V]) TotalCapacity() uint64 {
+	return t.tableA.capacity + t.tableB.capacity
 }
 
-type table[K, V any] struct {
+// Size returns how many slots are filled in the [Table].
-	hash           Hash[K]
+func (t *Table[K, V]) Size() int {
-	slots          []slot[K, V]
+	return int(t.tableA.size + t.tableB.size)
-	capacity, size uint64
-	compare        EqualFunc[K]
 }
 
-// location determines where in the table a certain key would be placed. If the
+func log2(n uint64) (m int) {
-// capacity is 0, this will panic.
+	return max(0, bits.Len64(n)-1)
-func (t table[K, V]) location(key K) uint64 {
-	return t.hash(key) % t.capacity
 }
 
-func (t table[K, V]) get(key K) (value V, found bool) {
+func (t *Table[K, V]) maxEvictions() int {
-	if t.capacity == 0 {
+	return 3 * log2(t.TotalCapacity())
+}
+
+func (t *Table[K, V]) load() float64 {
+	// 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())
+}
+
+// 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
 	}
 
-	slot := t.slots[t.location(key)]
+	if t.tableB.update(entry.key, entry.value) {
-	return slot.value, slot.occupied && t.compare(slot.key, key)
-}
-
-func (t *table[K, V]) drop(key K) (occupied bool) {
-	if t.capacity == 0 {
 		return
 	}
 
-	slot := &t.slots[t.location(key)]
+	for range t.maxEvictions() {
-
+		if entry, homeless = t.tableA.insert(entry); !homeless {
-	if slot.occupied && t.compare(slot.key, key) {
+			return
-		slot.occupied = false
-		t.size--
-		return true
 		}
 
+		if entry, homeless = t.tableB.insert(entry); !homeless {
+			return
+		}
+	}
+
+	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
-}
+		}
-
-func (t *table[K, V]) resize(capacity uint64) {
-	t.slots = make([]slot[K, V], capacity)
-	t.capacity = capacity
-	t.size = 0
-}
-
-func (t table[K, V]) update(key K, value V) (updated bool) {
-	if t.capacity == 0 {
-		return
 	}
 
-	slot := &t.slots[t.location(key)]
+	*t = *updated
-
-	if slot.occupied && t.compare(slot.key, key) {
-		slot.value = value
 	return true
-	}
-
-	return false
 }
 
-func (t *table[K, V]) evict(insertion entry[K, V]) (evicted entry[K, V], eviction bool) {
+// grow increases the table's capacity by the growth factor. If the
-	if t.capacity == 0 {
+// capacity is 0, it increases it to 1.
-		return insertion, true
+func (t *Table[K, V]) grow() bool {
+	var newCapacity uint64
+
+	if t.TotalCapacity() == 0 {
+		newCapacity = 1
+	} else {
+		newCapacity = t.tableA.capacity * t.growthFactor
 	}
 
-	slot := &t.slots[t.location(insertion.key)]
+	return t.resize(newCapacity)
+}
 
-	if !slot.occupied {
+// shrink reduces the table's capacity by the growth factor. It may
-		slot.entry = insertion
+// reduce it down to 0.
-		slot.occupied = true
+func (t *Table[K, V]) shrink() bool {
-		t.size++
+	return t.resize(t.tableA.capacity / t.growthFactor)
+}
+
+// Get fetches the value for a key in the [Table]. Matches the comma-ok pattern
+// of a builtin map; see [Table.Find] for plain indexing.
+func (t *Table[K, V]) Get(key K) (value V, ok bool) {
+	if item, ok := t.tableA.get(key); ok {
+		return item, true
+	}
+
+	if item, ok := t.tableB.get(key); ok {
+		return item, true
+	}
+
+	return
+}
+
+// Find fetches the value of a key. Matches direct indexing of a builtin map;
+// see [Table.Get] for a comma-ok pattern.
+func (t *Table[K, V]) Find(key K) (value V) {
+	value, _ = t.Get(key)
+	return
+}
+
+// Has returns true if a key has a value in the table.
+func (t *Table[K, V]) Has(key K) (exists bool) {
+	_, exists = t.Get(key)
+	return
+}
+
+// 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) {
+	var (
+		entry    = entry[K, V]{key, value}
+		homeless bool
+	)
+
+	for range defaultGrowthLimit {
+		if entry, homeless = t.insert(entry); !homeless {
 			return
 		}
 
-	if t.compare(slot.key, insertion.key) {
+		// Both this and the growth limit are necessary: this catches bad hashes
-		slot.value = insertion.value
+		// 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)
+		}
+	}
+
+	return fmt.Errorf("could not place entry after %d resizes: %w", defaultGrowthLimit, ErrBadHash)
+}
+
+// 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 {
+		// The error is not handled here, because table-shrinking is an internal
+		// optimization.
+		t.shrink()
+	}
+
+	return occupied
+}
+
+// Entries returns an unordered sequence of all key-value pairs in the table.
+func (t *Table[K, V]) Entries() iter.Seq2[K, V] {
+	return func(yield func(K, V) bool) {
+		for _, slot := range t.tableA.slots {
+			if slot.occupied {
+				if !yield(slot.key, slot.value) {
 					return
 				}
+			}
+		}
 
-	insertion, slot.entry = slot.entry, insertion
+		for _, slot := range t.tableB.slots {
-	return insertion, true
+			if slot.occupied {
-}
+				if !yield(slot.key, slot.value) {
-
+					return
-func newTable[K, V any](capacity uint64, hash Hash[K], compare EqualFunc[K]) table[K, V] {
+				}
-	return table[K, V]{
+			}
-		hash:     hash,
+		}
-		capacity: capacity,
-		compare:  compare,
-		size:     0,
-		slots:    make([]slot[K, V], capacity),
 	}
 }
+
+// String returns the entries of the table as a string in the format:
+// "table[k1:v1 k2:v2 ...]".
+func (t *Table[K, V]) String() string {
+	var sb strings.Builder
+	sb.WriteString("table[")
+
+	first := true
+	for k, v := range t.Entries() {
+		if !first {
+			sb.WriteString(" ")
+		}
+
+		fmt.Fprintf(&sb, "%v:%v", k, v)
+		first = false
+	}
+
+	sb.WriteString("]")
+	return sb.String()
+}
+
+// NewCustom creates a [Table] with custom [Hash] and [EqualFunc]
+// functions, along with any [Option] the user provides.
+func NewCustom[K, V any](hashA, hashB Hash[K], compare EqualFunc[K], options ...Option) *Table[K, V] {
+	settings := &settings{
+		growthFactor:  DefaultGrowthFactor,
+		bucketSize:    DefaultCapacity,
+		minLoadFactor: defaultMinimumLoad,
+	}
+
+	for _, option := range options {
+		option(settings)
+	}
+
+	return &Table[K, V]{
+		growthFactor:  settings.growthFactor,
+		minLoadFactor: settings.minLoadFactor,
+		tableA:        newSubtable[K, V](settings.bucketSize, hashA, compare),
+		tableB:        newSubtable[K, V](settings.bucketSize, hashB, compare),
+	}
+}
+
+func pipe[X, Y, Z any](a func(X) Y, b func(Y) Z) func(X) Z {
+	return func(x X) Z { return b(a(x)) }
+}
+
+// NewBy creates a [Table] for any key type by using keyFunc to derive a
+// comparable key. Two keys with the same derived key are treated as equal.
+func NewBy[K, V any, C comparable](keyFunc func(K) C, options ...Option) *Table[K, V] {
+	return NewCustom[K, V](
+		pipe(keyFunc, NewDefaultHash[C]()),
+		pipe(keyFunc, NewDefaultHash[C]()),
+		func(a, b K) bool { return keyFunc(a) == keyFunc(b) },
+		options...,
+	)
+}
+
+// New creates a [Table] using the default [Hash] and [EqualFunc]. Use
+// the [Option] functions to configure its behavior. Note that this constructor
+// is only provided for comparable keys. For arbitrary keys, consider
+// [NewBy] or [NewCustom].
+func New[K comparable, V any](options ...Option) *Table[K, V] {
+	return NewCustom[K, V](NewDefaultHash[K](), NewDefaultHash[K](), DefaultEqualFunc[K], options...)
+}