go-05-lru-cache
0.000
Challenge · difficulty 4/5
# Generic LRU cache
Implement **`solution.go`** in `package challenge` exporting a generic
least-recently-used cache:
```go
type LRUCache[K comparable, V any] struct { /* unexported fields */ }
func NewLRUCache[K comparable, V any](capacity int) *LRUCache[K, V]
func (c *LRUCache[K, V]) Get(key K) (V, bool)
func (c *LRUCache[K, V]) Put(key K, value V)
func (c *LRUCache[K, V]) Len() int
```
Behavior:
- `NewLRUCache(capacity)` creates an empty cache that holds at most `capacity`
entries. If `capacity <= 0`, treat it as `0`: the cache stores nothing and
`Len()` is always `0`.
- `Get(key)` returns the stored value and `true` if `key` is present, or the
zero value of `V` and `false` otherwise. A successful `Get` counts as a
**use**, making `key` the most-recently-used entry.
- `Put(key, value)` inserts or updates `key`. Inserting or updating makes `key`
the most-recently-used entry. If adding a **new** key would exceed `capacity`,
the **least-recently-used** entry is evicted first. Updating the value of an
existing key never evicts anything.
- `Len()` returns the current number of stored entries.
- Works for any `comparable` key type and any value type (e.g. `string`/`int`
keys, struct or pointer values).
Examples:
```go
c := NewLRUCache[string, int](2)
c.Put("a", 1)
c.Put("b", 2)
c.Get("a") // (1, true); now "b" is least-recently-used
c.Put("c", 3) // evicts "b"
c.Get("b") // (0, false)
c.Len() // 2
```
tests/solution_test.go
package challenge
import "testing"
func TestLRUBasicGetPut(t *testing.T) {
c := NewLRUCache[string, int](2)
if _, ok := c.Get("missing"); ok {
t.Fatalf("Get on empty cache returned ok=true")
}
c.Put("a", 1)
c.Put("b", 2)
if v, ok := c.Get("a"); !ok || v != 1 {
t.Fatalf("Get(a) = (%d, %v), want (1, true)", v, ok)
}
if v, ok := c.Get("b"); !ok || v != 2 {
t.Fatalf("Get(b) = (%d, %v), want (2, true)", v, ok)
}
if c.Len() != 2 {
t.Fatalf("Len = %d, want 2", c.Len())
}
}
func TestLRUZeroValueOnMiss(t *testing.T) {
c := NewLRUCache[int, string](2)
if v, ok := c.Get(99); ok || v != "" {
t.Fatalf("Get(miss) = (%q, %v), want (\"\", false)", v, ok)
}
}
func TestLRUEvictsLeastRecentlyUsed(t *testing.T) {
c := NewLRUCache[string, int](2)
c.Put("a", 1)
c.Put("b", 2)
c.Put("c", 3) // capacity 2: "a" is LRU, evicted
if _, ok := c.Get("a"); ok {
t.Fatalf("expected a to be evicted")
}
if v, ok := c.Get("b"); !ok || v != 2 {
t.Fatalf("Get(b) = (%d, %v), want (2, true)", v, ok)
}
if v, ok := c.Get("c"); !ok || v != 3 {
t.Fatalf("Get(c) = (%d, %v), want (3, true)", v, ok)
}
if c.Len() != 2 {
t.Fatalf("Len = %d, want 2", c.Len())
}
}
func TestLRUGetCountsAsUse(t *testing.T) {
c := NewLRUCache[string, int](2)
c.Put("a", 1)
c.Put("b", 2)
if v, ok := c.Get("a"); !ok || v != 1 { // "a" now most-recently-used
t.Fatalf("Get(a) = (%d, %v), want (1, true)", v, ok)
}
c.Put("c", 3) // "b" is LRU now, should be evicted
if _, ok := c.Get("b"); ok {
t.Fatalf("expected b to be evicted (Get should have refreshed a)")
}
if v, ok := c.Get("a"); !ok || v != 1 {
t.Fatalf("Get(a) = (%d, %v), want (1, true)", v, ok)
}
if v, ok := c.Get("c"); !ok || v != 3 {
t.Fatalf("Get(c) = (%d, %v), want (3, true)", v, ok)
}
}
func TestLRUUpdateExistingKey(t *testing.T) {
c := NewLRUCache[string, int](2)
c.Put("a", 1)
c.Put("b", 2)
c.Put("a", 100) // update value, refresh a; no eviction
if c.Len() != 2 {
t.Fatalf("Len = %d, want 2 (update must not evict)", c.Len())
}
if v, ok := c.Get("a"); !ok || v != 100 {
t.Fatalf("Get(a) = (%d, %v), want (100, true)", v, ok)
}
c.Put("c", 3) // "b" is LRU, evicted
if _, ok := c.Get("b"); ok {
t.Fatalf("expected b to be evicted after updating a then inserting c")
}
}
func TestLRUUpdateRefreshesRecency(t *testing.T) {
c := NewLRUCache[string, int](2)
c.Put("a", 1)
c.Put("b", 2)
c.Put("a", 10) // refresh "a" via update => "b" becomes LRU
c.Put("c", 3) // evicts "b"
if _, ok := c.Get("b"); ok {
t.Fatalf("expected b to be evicted; updating a should refresh its recency")
}
if v, ok := c.Get("a"); !ok || v != 10 {
t.Fatalf("Get(a) = (%d, %v), want (10, true)", v, ok)
}
}
func TestLRUCapacityOne(t *testing.T) {
c := NewLRUCache[int, int](1)
c.Put(1, 10)
c.Put(2, 20) // evicts 1
if _, ok := c.Get(1); ok {
t.Fatalf("expected key 1 evicted in capacity-1 cache")
}
if v, ok := c.Get(2); !ok || v != 20 {
t.Fatalf("Get(2) = (%d, %v), want (20, true)", v, ok)
}
if c.Len() != 1 {
t.Fatalf("Len = %d, want 1", c.Len())
}
}
func TestLRUZeroCapacity(t *testing.T) {
c := NewLRUCache[string, int](0)
c.Put("a", 1)
if _, ok := c.Get("a"); ok {
t.Fatalf("zero-capacity cache must store nothing")
}
if c.Len() != 0 {
t.Fatalf("Len = %d, want 0", c.Len())
}
}
func TestLRUStringValues(t *testing.T) {
c := NewLRUCache[int, string](3)
c.Put(1, "one")
c.Put(2, "two")
c.Put(3, "three")
c.Get(1) // refresh 1
c.Put(4, "four") // evicts 2 (LRU)
if _, ok := c.Get(2); ok {
t.Fatalf("expected key 2 evicted")
}
for _, want := range []struct {
k int
v string
}{{1, "one"}, {3, "three"}, {4, "four"}} {
if v, ok := c.Get(want.k); !ok || v != want.v {
t.Errorf("Get(%d) = (%q, %v), want (%q, true)", want.k, v, ok, want.v)
}
}
}
func TestLRUEvictionChain(t *testing.T) {
c := NewLRUCache[int, int](3)
for i := 0; i < 6; i++ {
c.Put(i, i*i)
}
// Only the last 3 inserts survive: 3,4,5.
for _, k := range []int{0, 1, 2} {
if _, ok := c.Get(k); ok {
t.Errorf("expected key %d evicted", k)
}
}
for _, k := range []int{3, 4, 5} {
if v, ok := c.Get(k); !ok || v != k*k {
t.Errorf("Get(%d) = (%d, %v), want (%d, true)", k, v, ok, k*k)
}
}
if c.Len() != 3 {
t.Fatalf("Len = %d, want 3", c.Len())
}
}
Proposed solution
```go
package challenge
type node[K comparable, V any] struct {
key K
value V
prev *node[K, V]
next *node[K, V]
}
type LRUCache[K comparable, V any] struct {
capacity int
items map[K]*node[K, V]
head *node[K, V]
tail *node[K, V]
}
func NewLRUCache[K comparable, V any](capacity int) *LRUCache[K, V] {
if capacity <= 0 {
capacity = 0
}
head := &node[K, V]{}
tail := &node[K, V]{}
head.next = tail
tail.prev = head
return &LRUCache[K, V]{
capacity: capacity,
items: make(map[K]*node[K, V]),
head: head,
tail: tail,
}
}
func (c *LRUCache[K, V]) Get(key K) (V, bool) {
if n, ok := c.items[key]; ok {
c.moveToFront(n)
return n.value, true
}
var zero V
return zero, false
}
func (c *LRUCache[K, V]) Put(key K, value V) {
if n, ok := c.items[key]; ok {
n.value = value
c.moveToFront(n)
return
}
if len(c.items) == c.capacity {
lru := c.tail.prev
if lru != c.head {
c.removeItem(lru)
delete(cErrors (stderr)
no code extracted from response