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
package challenge

import "container/list"

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

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

func (c *LRUCache[K, V]) Get(key K) (V, bool) {
	if c.capacity == 0 {
		var zero V
		return zero, false
	}
	if elem, ok := c.items[key]; ok {
		c.list.MoveToFront(elem)
		return elem.Value.(V), true
	}
	var zero V
	return zero, false
}

func (c *LRUCache[K, V]) Put(key K, value V) {
	if c.capacity == 0 {
		return
	}
	if elem, ok := c.items[key]; ok {
		elem.Value = value
		c.list.MoveToFront(elem)
		return
	}
	if c.list.Len() == c.capacity {
		tail := c.list.Back()
		if tail != nil {
			delete(c.items, tail.Value.(K))
			c.list.Remove(tail)
		}
	}
	elem := c.list.PushFront(value)
	c.items[key] = elem
}

func (c *LRUCache[K, V]) Len() int {
	return c.list.Len()
}
```

[
  {
    "total": 3,
    "answer": "```go\npackage challenge\n\nimport \"container/list\"\n\ntype LRUCache[K comparable, V any] struct {\n\tcapacity int\n\titems    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\tcapacity = 0\n\t}\n\treturn &LRUCache[K, V]{\n\t\tcapacity: capacity,\n\t\titems:    make(map[K]*list.Element),\n\t\tlist:     list.New(),\n\t}\n}\n\nfunc (c *LRUCache[K, V]) Get(key K) (V, bool) {\n\tif c.capacity == 0 {\n\t\tvar zero V\n\t\treturn zero, false\n\t}\n\tif elem, ok := c.items[key]; ok {\n\t\tc.list.MoveToFront(elem)\n\t\treturn elem.Value.(V), true\n\t}\n\tvar zero V\n\treturn zero, false\n}\n\nfunc (c *LRUCache[K, V]) Put(key K, value V) {\n\tif c.capacity == 0 {\n\t\treturn\n\t}\n\tif elem, ok := c.items[key]; ok {\n\t\telem.Value = value\n\t\tc.list.MoveToFront(elem)\n\t\treturn\n\t}\n\tif c.list.Len() == c.capacity {\n\t\ttail := c.list.Back()\n\t\tif tail != nil {\n\t\t\tdelete(c.items, tail.Value.(K))\n\t\t\tc.list.Remove(tail)\n\t\t}\n\t}\n\telem := c.list.PushFront(value)\n\tc.items[key] = elem\n}\n\nfunc (c *LRUCache[K, V]) Len() int {\n\treturn c.list.Len()\n}\n```",
    "passed": 2,
    "test_error": "e\":\"2026-07-01T23:36:11.187370337+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"\\t/home/psc/miniconda3/go/src/testing/testing.go:1977 +0x349\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187371367+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"panic({0x55f4a0?, 0xc3ed1e633b0?})\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187372237+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"\\t/home/psc/miniconda3/go/src/runtime/panic.go:860 +0x13a\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187374677+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"challenge.(*LRUCache[...]).Put(0x59dc20, {0x588ad5, 0x1}, 0x3)\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187375897+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"\\t/tmp/llmlab-go-05-lru-cache-1rc_hifa/solution.go:47 +0x3f1\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187376787+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"challenge.TestLRUEvictsLeastRecentlyUsed(0xc3ed1f066c8)\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187377677+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"\\t/tmp/llmlab-go-05-lru-cache-1rc_hifa/solution_test.go:34 +0xd5\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187378857+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"testing.tRunner(0xc3ed1f066c8, 0x597078)\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187379807+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"\\t/home/psc/miniconda3/go/src/testing/testing.go:2036 +0xea\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187380637+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"created by testing.(*T).Run in goroutine 1\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187381817+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Output\":\"\\t/home/psc/miniconda3/go/src/testing/testing.go:2101 +0x4c5\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187494868+02:00\",\"Action\":\"fail\",\"Package\":\"challenge\",\"Test\":\"TestLRUEvictsLeastRecentlyUsed\",\"Elapsed\":0}\n{\"Time\":\"2026-07-01T23:36:11.187503938+02:00\",\"Action\":\"output\",\"Package\":\"challenge\",\"Output\":\"FAIL\\tchallenge\\t0.003s\\n\"}\n{\"Time\":\"2026-07-01T23:36:11.187508778+02:00\",\"Action\":\"fail\",\"Package\":\"challenge\",\"Elapsed\":0.003}"
  },
  {
    "total": 10,
    "answer": "The panic occurs in the `Put` method when trying to access the key from the list element's value. The issue is that the list stores values of type `V`, but we're trying to treat them as keys (`K`) when evicting. We need to store both the key and value in the list elements to properly handle eviction.\n\nHere's the corrected solution:\n\n```go\npackage challenge\n\nimport \"container/list\"\n\ntype entry[K comparable, V any] struct {\n\tkey   K\n\tvalue V\n}\n\ntype LRUCache[K comparable, V any] struct {\n\tcapacity int\n\titems    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\tcapacity = 0\n\t}\n\treturn &LRUCache[K, V]{\n\t\tcapacity: capacity,\n\t\titems:    make(map[K]*list.Element),\n\t\tlist:     list.New(),\n\t}\n}\n\nfunc (c *LRUCache[K, V]) Get(key K) (V, bool) {\n\tif c.capacity == 0 {\n\t\tvar zero V\n\t\treturn zero, false\n\t}\n\tif elem, ok := c.items[key]; ok {\n\t\tc.list.MoveToFront(elem)\n\t\treturn elem.Value.(entry[K, V]).value, true\n\t}\n\tvar zero V\n\treturn zero, false\n}\n\nfunc (c *LRUCache[K, V]) Put(key K, value V) {\n\tif c.capacity == 0 {\n\t\treturn\n\t}\n\tif elem, ok := c.items[key]; ok {\n\t\telem.Value = entry[K, V]{key, value}\n\t\tc.list.MoveToFront(elem)\n\t\treturn\n\t}\n\tif c.list.Len() == c.capacity {\n\t\ttail := c.list.Back()\n\t\tif tail != nil {\n\t\t\tdelete(c.items, tail.Value.(entry[K, V]).key)\n\t\t\tc.list.Remove(tail)\n\t\t}\n\t}\n\telem := c.list.PushFront(entry[K, V]{key, value})\n\tc.items[key] = elem\n}\n\nfunc (c *LRUCache[K, V]) Len() int {\n\treturn c.list.Len()\n}\n```\n\nKey changes made:\n1. Created an `entry` struct to store both key and value together in the list\n2. Updated the list to store `entry[K, V]` instead of just `V`\n3. Modified `Get` to extract the value from the entry\n4. Modified `Put` to:\n   - Store entries with both key and value\n   - Properly delete the key when evicting\n   - Update existing entries correctly\n\nThis solution should now pass all tests, including the eviction test that was failing before.",
    "passed": 10,
    "test_error": ""
  }
]