go-05-lru-cache

# 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
```

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())
	}
}

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.

```go
// solution.go
package challenge

import (
	"container/list"
)

type LRUCache[K comparable, V any] struct {
	mu sync.Mutex
	capacity int
	cache map[K]*list.Element
	list  *list.List
}

func NewLRUCache[K comparable, V any](capacity int) *LRUCache[K, V] {
	if capacity <= 0 {
		return &LRUCache[K, V]{capacity: 0, cache: make(map[K]*list.Element), list: list.New()}
	}
	return &LRUCache[K, V]{capacity: capacity, cache: make(map[K]*list.Element), list: list.New()}
}

func (c *LRUCache[K, V]) Get(key K) (V, bool) {
	c.mu.Lock()
	defer c.mu.Unlock()

	if element, found := c.cache[key]; found {
		c.list.MoveToFront(element)
		return element.Value.(*entry).value, true
	}
	return V{}, false
}

func (c *LRUCache[K, V]) Put(key K, value V) {
	c.mu.Lock()
	defer c.mu.Unlock()

	if element, found := c.cache[key]; found {
		c.list.MoveToFront(element)
		element.Value.(*entry).value = value
		return
	}

	if c.Len() == c.capacity {
		oldest := c.list.Back()
		c.list.Remove(oldest)
		delete(c.cache, oldest.Value.(*entry).key)
	}

	newElement := c.list.PushFront(&entry{key: key, value: value})
	c.cache[key] = newElement
}

func (c *LRUCache[K, V]) Len() int {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.list.Len()
}

type entry struct {
	key   K
	value V
}

func (e *entry) key() K { return e.key }
func (e *entry) value() V { return e.value }
```

[
  {
    "total": 1,
    "answer": "```go\n// solution.go\npackage challenge\n\nimport (\n\t\"container/list\"\n)\n\ntype LRUCache[K comparable, V any] struct {\n\tmu sync.Mutex\n\tcapacity int\n\tcache map[K]*list.Element\n\tlist  *list.List\n}\n\nfunc NewLRUCache[K comparable, V any](capacity int) *LRUCache[K, V] {\n\tif capacity <= 0 {\n\t\treturn &LRUCache[K, V]{capacity: 0, cache: make(map[K]*list.Element), list: list.New()}\n\t}\n\treturn &LRUCache[K, V]{capacity: capacity, cache: make(map[K]*list.Element), list: list.New()}\n}\n\nfunc (c *LRUCache[K, V]) Get(key K) (V, bool) {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\n\tif element, found := c.cache[key]; found {\n\t\tc.list.MoveToFront(element)\n\t\treturn element.Value.(*entry).value, true\n\t}\n\treturn V{}, false\n}\n\nfunc (c *LRUCache[K, V]) Put(key K, value V) {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\n\tif element, found := c.cache[key]; found {\n\t\tc.list.MoveToFront(element)\n\t\telement.Value.(*entry).value = value\n\t\treturn\n\t}\n\n\tif c.Len() == c.capacity {\n\t\toldest := c.list.Back()\n\t\tc.list.Remove(oldest)\n\t\tdelete(c.cache, oldest.Value.(*entry).key)\n\t}\n\n\tnewElement := c.list.PushFront(&entry{key: key, value: value})\n\tc.cache[key] = newElement\n}\n\nfunc (c *LRUCache[K, V]) Len() int {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\treturn c.list.Len()\n}\n\ntype entry struct {\n\tkey   K\n\tvalue V\n}\n\nfunc (e *entry) key() K { return e.key }\nfunc (e *entry) value() V { return e.value }\n```",
    "passed": 0,
    "test_error": "{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"# challenge [challenge.test]\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:9:5: undefined: sync\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:30:9: invalid composite literal type V (no common underlying type)\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:39:3: cannot assign to element.Value.(*entry).value (neither addressable nor a map index expression)\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:60:8: undefined: K\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:61:8: undefined: V\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:64:17: field and method with the same name key\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"\\t./solution.go:60:2: other declaration of key\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:64:23: undefined: K\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:65:17: field and method with the same name value\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"\\t./solution.go:61:2: other declaration of value\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:65:25: undefined: V\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-fail\"}\n{\"Time\":\"2026-07-03T03:10:04.726886227+02:00\",\"Action\":\"start\",\"Package\":\"challenge\"}\n{\"Time\":\"2026-07-03T03:10:04.726936107+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Output\":\"FAIL\\tchallenge [build failed]\\n\"}\n{\"Time\":\"2026-07-03T03:10:04.726940357+02:00\",\"Action\":\"fail\",\"Package\":\"challenge\",\"Elapsed\":0,\"FailedBuild\":\"challenge [challenge.test]\"}"
  },
  {
    "total": 1,
    "answer": "```go\n// solution.go\npackage challenge\n\nimport (\n\t\"sync\"\n)\n\ntype LRUCache[K comparable, V any] struct {\n\tmu sync.Mutex\n\tcapacity int\n\tcache map[K]*list.Element\n\tlist  *list.List\n}\n\nfunc NewLRUCache[K comparable, V any](capacity int) *LRUCache[K, V] {\n\tif capacity <= 0 {\n\t\treturn &LRUCache[K, V]{capacity: 0, cache: make(map[K]*list.Element), list: list.New()}\n\t}\n\treturn &LRUCache[K, V]{capacity: capacity, cache: make(map[K]*list.Element), list: list.New()}\n}\n\nfunc (c *LRUCache[K, V]) Get(key K) (V, bool) {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\n\tif element, found := c.cache[key]; found {\n\t\tc.list.MoveToFront(element)\n\t\treturn element.Value.(V), true\n\t}\n\treturn V{}, false\n}\n\nfunc (c *LRUCache[K, V]) Put(key K, value V) {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\n\tif element, found := c.cache[key]; found {\n\t\tc.list.MoveToFront(element)\n\t\telement.Value.(V) = value\n\t\treturn\n\t}\n\n\tif c.Len() == c.capacity {\n\t\toldest := c.list.Back()\n\t\tc.list.Remove(oldest)\n\t\tdelete(c.cache, oldest.Value.(*entry).key)\n\t}\n\n\tnewElement := c.list.PushFront(&entry{key: key, value: value})\n\tc.cache[key] = newElement\n}\n\nfunc (c *LRUCache[K, V]) Len() int {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\treturn c.list.Len()\n}\n\ntype entry struct {\n\tkey   K\n\tvalue V\n}\n```",
    "passed": 0,
    "test_error": "{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"# challenge [challenge.test]\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:11:15: undefined: list\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:12:9: undefined: list\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:17:58: undefined: list\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:19:64: undefined: list\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:30:9: invalid composite literal type V (no common underlying type)\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:60:8: undefined: K\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-output\",\"Output\":\"./solution.go:61:8: undefined: V\\n\"}\n{\"ImportPath\":\"challenge [challenge.test]\",\"Action\":\"build-fail\"}\n{\"Time\":\"2026-07-03T03:10:08.29115852+02:00\",\"Action\":\"start\",\"Package\":\"challenge\"}\n{\"Time\":\"2026-07-03T03:10:08.291227911+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Output\":\"FAIL\\tchallenge [build failed]\\n\"}\n{\"Time\":\"2026-07-03T03:10:08.291232301+02:00\",\"Action\":\"fail\",\"Package\":\"challenge\",\"Elapsed\":0,\"FailedBuild\":\"challenge [challenge.test]\"}"
  },
  {
    "total": 1,
    "answer": "```go\n// solution.go\npackage challenge\n\nimport (\n\t\"sync\"\n)\n\ntype LRUCache[K comparable, V any] struct {\n\tmu sync.Mutex\n\tcapacity int\n\tcache map[K]*list.Element\n\tlist  *list.List\n}\n\nfunc NewLRUCache[K comparable, V any](capacity int) *LRUCache[K, V] {\n\tif capacity <= 0 {\n\t\treturn &LRUCache[K, V]{capacity: 0, cache: make(map[K]*list.Element), list: list.New()}\n\t}\n\treturn &LRUCache[K, V]{capacity: capacity, cache: make(map[K]*list.Element), list: list.New()}\n}\n\nfunc (c *LRUCache[K, V]) Get(key K) (V, bool) {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\n\tif element, found := c.cache[key]; found {\n\t\tc.list.MoveToFront(element)\n\t\treturn element.Value.(V), true\n\t}\n\treturn V{}, false\n}\n\nfunc (c *LRUCache[K, V]) Put(key K, value V) {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\n\tif element, found := c.cache[key]; found {\n\t\tc.list.MoveToFront(element)\n\t\telement.Value.(V) = value\n\t\treturn\n\t}\n\n\tif c.Len() == c.capacity {\n\t\toldest := c.list.Back()\n\t\tc.list.Remove(oldest)\n\t\tdelete(c.cache, oldest.Value.(*entry).key)\n\t}\n\n\tnewElement := c.list.PushFront(&entry{key: key, value: value})\n\tc.cache[key] = newElement\n}\n\nfunc (c *LRUCache[K, V]) Len() int {\n\tc.mu.Lock()\n\tdefer c.mu.Unlock()\n\treturn c.list.Len()\n}\n\ntype entry struct {\n\tkey   K\n\tvalue V\n}\n```",
    "passed": 0,
    "test_error": ""
  }
]