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>

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>

cuckoo_fuzz_test.go

@@ -75,9 +75,9 @@ func FuzzInsertLookup(f *testing.F) {
 				delete(expected, step.key)
 
 				_, ok = actual.Get(step.key)
-				assert.True(ok)
+				assert.False(ok)
 			} else {
-				_, err := actual.Put(step.key, step.value)
+				err := actual.Put(step.key, step.value)
 				assert.NoError(err)
 
 				expected[step.key] = step.value

cuckoo_internal_test.go

@@ -23,7 +23,7 @@ func TestLoad(t *testing.T) {
 	table := New[int, bool](Capacity(8))
 
 	for i := range 16 {
-		_, err := table.Put(i, true)
+		err := table.Put(i, true)
 		assert.NoError(err)
 		assert.Equal(float64(table.Size())/float64(table.TotalCapacity()), table.load())
 	}

cuckoo_test.go

@@ -25,7 +25,7 @@ func TestAddItem(t *testing.T) {
 	key, value := 0, true
 	table := cuckoo.New[int, bool]()
 
-	_, err := table.Put(key, value)
+	err := table.Put(key, value)
 
 	assert.NoError(err)
 	assert.Equal(1, table.Size())
@@ -38,7 +38,7 @@ func TestPutOverwrite(t *testing.T) {
 	table := cuckoo.New[int, int]()
 	(table.Put(key, value))
 
-	_, err := table.Put(key, newValue)
+	err := table.Put(key, newValue)
 
 	assert.NoError(err)
 	assert.Equal(1, table.Size())
@@ -52,9 +52,9 @@ func TestSameHash(t *testing.T) {
 	hash := func(int) uint64 { return 0 }
 	table := cuckoo.NewCustom[int, bool](hash, hash, cuckoo.DefaultEqualFunc[int])
 
-	_, errA := table.Put(0, true)
+	errA := table.Put(0, true)
-	_, errB := table.Put(1, true)
+	errB := table.Put(1, true)
-	_, errC := table.Put(2, true)
+	errC := table.Put(2, true)
 
 	assert.NoError(errA)
 	assert.NoError(errB)
@@ -76,7 +76,7 @@ func TestResizeCapacity(t *testing.T) {
 	)
 
 	for table.TotalCapacity() == 16 {
-		_, err := table.Put(rand.Int(), true)
+		err := table.Put(rand.Int(), true)
 		assert.NoError(err)
 	}
 
@@ -89,7 +89,7 @@ func TestPutMany(t *testing.T) {
 
 	for i := range 1_000 {
 		expected[i] = true
-		_, err := actual.Put(i, true)
+		err := actual.Put(i, true)
 
 		assert.NoError(err)
 	}
@@ -103,7 +103,7 @@ func TestGetMany(t *testing.T) {
 	table := cuckoo.New[int, bool]()
 
 	for i := range 1_000 {
-		_, err := table.Put(i, true)
+		err := table.Put(i, true)
 		assert.NoError(err)
 	}
 
@@ -167,7 +167,7 @@ func TestPutNoCapacity(t *testing.T) {
 		cuckoo.Capacity(0),
 	)
 
-	_, err := table.Put(key, value)
+	err := table.Put(key, value)
 
 	assert.NoError(err)
 	assert.Equal(1, table.Size())
@@ -183,9 +183,9 @@ func TestBadHashCapacity(t *testing.T) {
 		cuckoo.Capacity(20),
 	)
 
-	_, err1 := table.Put(0, true)
+	err1 := table.Put(0, true)
-	_, err2 := table.Put(1, true)
+	err2 := table.Put(1, true)
-	_, err3 := table.Put(2, true)
+	err3 := table.Put(2, true)
 
 	assert.NoError(err1)
 	assert.NoError(err2)
@@ -200,8 +200,8 @@ func TestDropResizeCapacity(t *testing.T) {
 		cuckoo.Capacity(10),
 	)
 
-	_, err1 := table.Put(0, true)
+	err1 := table.Put(0, true)
-	_, err2 := table.Put(1, true)
+	err2 := table.Put(1, true)
 	table.Drop(1)
 
 	assert.NoError(errors.Join(err1, err2))
@@ -218,7 +218,7 @@ func TestNewTableBy(t *testing.T) {
 	assert := assert.New(t)
 	table := cuckoo.NewBy[User, bool](func(u User) string { return u.id })
 
-	_, err := table.Put(User{nil, "1", "Robert"}, true)
+	err := table.Put(User{nil, "1", "Robert"}, true)
 
 	assert.NoError(err)
 	assert.Equal(1, table.Size())

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

@@ -10,7 +10,7 @@ import (
 func Example_basic() {
 	table := cuckoo.New[int, string]()
 
-	if _, err := table.Put(1, "Hello, World!"); err != nil {
+	if err := table.Put(1, "Hello, World!"); err != nil {
 		fmt.Println("Put error:", err)
 	}
 

subtable.go

@@ -1,13 +1,13 @@
 package cuckoo
 
-// An Entry is a key-value pair.
+// An entry is a key-value pair.
-type Entry[K, V any] struct {
+type entry[K, V any] struct {
-	Key   K
+	key   K
-	Value V
+	value V
 }
 
 type slot[K, V any] struct {
-	Entry[K, V]
+	entry[K, V]
 	occupied bool
 }
 
@@ -18,81 +18,81 @@ type subtable[K, V any] struct {
 	compare        EqualFunc[K]
 }
 
-// location determines where in the bucket a certain key would be placed. If the
+// location determines where in the subtable a certain key would be placed. If
-// capacity is 0, this will panic.
+// the capacity is 0, this will panic.
-func (b *subtable[K, V]) location(key K) uint64 {
+func (t *subtable[K, V]) location(key K) uint64 {
-	return b.hash(key) % b.capacity
+	return t.hash(key) % t.capacity
 }
 
-func (b *subtable[K, V]) get(key K) (value V, found bool) {
+func (t *subtable[K, V]) get(key K) (value V, found bool) {
-	if b.capacity == 0 {
+	if t.capacity == 0 {
 		return
 	}
 
-	slot := b.slots[b.location(key)]
+	slot := t.slots[t.location(key)]
-	return slot.Value, slot.occupied && b.compare(slot.Key, key)
+	return slot.value, slot.occupied && t.compare(slot.key, key)
 }
 
-func (b *subtable[K, V]) drop(key K) (occupied bool) {
+func (t *subtable[K, V]) drop(key K) (occupied bool) {
-	if b.capacity == 0 {
+	if t.capacity == 0 {
 		return
 	}
 
-	slot := &b.slots[b.location(key)]
+	slot := &t.slots[t.location(key)]
 
-	if slot.occupied && b.compare(slot.Key, key) {
+	if slot.occupied && t.compare(slot.key, key) {
 		slot.occupied = false
-		b.size--
+		t.size--
 		return true
 	}
 
 	return false
 }
 
-func (b *subtable[K, V]) resized(capacity uint64) *subtable[K, V] {
+func (t *subtable[K, V]) resized(capacity uint64) *subtable[K, V] {
 	return &subtable[K, V]{
 		slots:    make([]slot[K, V], capacity),
 		capacity: capacity,
-		hash:     b.hash,
+		hash:     t.hash,
-		compare:  b.compare,
+		compare:  t.compare,
 	}
 }
 
-func (b *subtable[K, V]) update(key K, value V) (updated bool) {
+func (t *subtable[K, V]) update(key K, value V) (updated bool) {
-	if b.capacity == 0 {
+	if t.capacity == 0 {
 		return
 	}
 
-	slot := &b.slots[b.location(key)]
+	slot := &t.slots[t.location(key)]
 
-	if slot.occupied && b.compare(slot.Key, key) {
+	if slot.occupied && t.compare(slot.key, key) {
-		slot.Value = value
+		slot.value = value
 		return true
 	}
 
 	return false
 }
 
-func (b *subtable[K, V]) insert(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 b.capacity == 0 {
+	if t.capacity == 0 {
 		return insertion, true
 	}
 
-	slot := &b.slots[b.location(insertion.Key)]
+	slot := &t.slots[t.location(insertion.key)]
 
 	if !slot.occupied {
-		slot.Entry = insertion
+		slot.entry = insertion
 		slot.occupied = true
-		b.size++
+		t.size++
 		return
 	}
 
-	if b.compare(slot.Key, insertion.Key) {
+	if t.compare(slot.key, insertion.key) {
-		slot.Value = insertion.Value
+		slot.value = insertion.value
 		return
 	}
 
-	insertion, slot.Entry = slot.Entry, insertion
+	insertion, slot.entry = slot.entry, insertion
 	return insertion, true
 }
 

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.
@@ -57,12 +57,12 @@ func (t *Table[K, V]) load() float64 {
 // 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) {
+func (t *Table[K, V]) insert(entry entry[K, V]) (displaced entry[K, V], homeless bool) {
-	if t.tableA.update(entry.Key, entry.Value) {
+	if t.tableA.update(entry.key, entry.value) {
 		return
 	}
 
-	if t.tableB.update(entry.Key, entry.Value) {
+	if t.tableB.update(entry.key, entry.value) {
 		return
 	}
 
@@ -97,7 +97,7 @@ 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 {
+		if _, failed := updated.insert(entry[K, V]{k, v}); failed {
 			return false
 		}
 	}
@@ -153,14 +153,10 @@ func (t *Table[K, V]) Has(key K) (exists bool) {
 	return
 }
 
-// Put sets the value for a key. If it cannot be set, an error is returned,
+// Put sets the value for a key. If it cannot be set, an error is returned.
-// along with the last displaced entry.
+func (t *Table[K, V]) Put(key K, value V) (err error) {
-//
-// On failure, the returned entry and the current table contents together
-// preserve all previously inserted entries and the attempted entry.
-func (t *Table[K, V]) Put(key K, value V) (displaced Entry[K, V], err error) {
 	var (
-		entry    = Entry[K, V]{key, value}
+		entry    = entry[K, V]{key, value}
 		homeless bool
 	)
 
@@ -173,18 +169,18 @@ func (t *Table[K, V]) Put(key K, value V) (displaced Entry[K, V], err error) {
 		// early when the table is sparse, while the latter catches cases where
 		// growing never helps.
 		if t.load() < t.minLoadFactor {
-			return entry, fmt.Errorf("bad hash: resize on load %d/%d", t.Size(), t.TotalCapacity())
+			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 entry, fmt.Errorf("bad hash: could not redistribute entries into larger table")
+			return fmt.Errorf("could not redistribute entries into larger table: %w", ErrBadHash)
 		}
 	}
 
-	return entry, fmt.Errorf("bad hash: could not place entry after %d resizes", defaultGrowthLimit)
+	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
@@ -206,7 +202,7 @@ 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) {
+				if !yield(slot.key, slot.value) {
 					return
 				}
 			}
@@ -214,7 +210,7 @@ func (t *Table[K, V]) Entries() iter.Seq2[K, V] {
 
 		for _, slot := range t.tableB.slots {
 			if slot.occupied {
-				if !yield(slot.Key, slot.Value) {
+				if !yield(slot.key, slot.value) {
 					return
 				}
 			}