go-05-lru-cache
1.000
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())
}
}
System prompt
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.
Proposed solution
```go
package challenge
import "container/list"
// LRUCache implements a generic LRU cache
type LRUCache[K comparable, V any] struct {
capacity int
cache map[K]*list.Element
lruList *list.List
}
// entry represents a key-value pair stored in the LRU cache
type entry[K comparable, V any] struct {
key K
value V
}
// NewLRUCache creates a new LRU cache with the given capacity
func NewLRUCache[K comparable, V any](capacity int) *LRUCache[K, V] {
if capacity <= 0 {
capacity = 0
}
return &LRUCache[K, V]{
capacity: capacity,
cache: make(map[K]*list.Element),
lruList: list.New(),
}
}
// Get returns the value for the given key and true if found, or zero value and false otherwise
func (c *LRUCache[K, V]) Get(key K) (V, bool) {
if c.capacity == 0 {
var zero V
return zero, false
}
elem, found := c.cache[key]
if !found {
var zero V
return zero, false
}
// Move this element to the back (most recently used)
c.lruList.MoveToBack(elem)
// Return the value
return elem.Value.(*entry[K, V]).value, true
}
// Put inserts or updates a key-value pair in the cache
func (c *LRUCache[K, V]) Put(key K, value V) {
if c.capacity == 0 {
return
}
// Check if key already exists
if elem, found := c.cache[key]; found {
// Update existing entry
elem.Value.(*entry[K, V]).value = value
// Move to back (most recently used)
c.lruList.MoveToBack(elem)
return
}
// If cache is at capacity, evict the least recently used item
if len(c.cache) == c.capacity {
// Remove from front (least recently used)
front := c.lruList.Front()
if front != nil {
removedEntry := front.Value.(*entry[K, V])
delete(c.cache, removedEntry.key)
c.lruList.Remove(front)
}
}
// Create new entry and add to back (most recently used)
newEntry := &entry[K, V]{key: key, value: value}
elem := c.lruList.PushBack(newEntry)
c.cache[key] = elem
}
// Len returns the current number of entries in the cache
func (c *LRUCache[K, V]) Len() int {
return len(c.cache)
}
```Test output (stdout)
enge","Test":"TestLRUGetCountsAsUse","Output":"--- PASS: TestLRUGetCountsAsUse (0.00s)\n"}
{"Time":"2026-07-01T22:48:55.46980888+02:00","Action":"pass","Package":"challenge","Test":"TestLRUGetCountsAsUse","Elapsed":0}
{"Time":"2026-07-01T22:48:55.46981233+02:00","Action":"run","Package":"challenge","Test":"TestLRUUpdateExistingKey"}
{"Time":"2026-07-01T22:48:55.46981446+02:00","Action":"output","Package":"challenge","Test":"TestLRUUpdateExistingKey","Output":"=== RUN TestLRUUpdateExistingKey\n"}
{"Time":"2026-07-01T22:48:55.4698169+02:00","Action":"output","Package":"challenge","Test":"TestLRUUpdateExistingKey","Output":"--- PASS: TestLRUUpdateExistingKey (0.00s)\n"}
{"Time":"2026-07-01T22:48:55.46981919+02:00","Action":"pass","Package":"challenge","Test":"TestLRUUpdateExistingKey","Elapsed":0}
{"Time":"2026-07-01T22:48:55.469821+02:00","Action":"run","Package":"challenge","Test":"TestLRUUpdateRefreshesRecency"}
{"Time":"2026-07-01T22:48:55.46982266+02:00","Action":"output","Package":"challenge","Test":"TestLRUUpdateRefreshesRecency","Output":"=== RUN TestLRUUpdateRefreshesRecency\n"}
{"Time":"2026-07-01T22:48:55.46982505+02:00","Action":"output","Package":"challenge","Test":"TestLRUUpdateRefreshesRecency","Output":"--- PASS: TestLRUUpdateRefreshesRecency (0.00s)\n"}
{"Time":"2026-07-01T22:48:55.46982718+02:00","Action":"pass","Package":"challenge","Test":"TestLRUUpdateRefreshesRecency","Elapsed":0}
{"Time":"2026-07-01T22:48:55.46982926+02:00","Action":"run","Package":"challenge","Test":"TestLRUCapacityOne"}
{"Time":"2026-07-01T22:48:55.46983128+02:00","Action":"output","Package":"challenge","Test":"TestLRUCapacityOne","Output":"=== RUN TestLRUCapacityOne\n"}
{"Time":"2026-07-01T22:48:55.46983348+02:00","Action":"output","Package":"challenge","Test":"TestLRUCapacityOne","Output":"--- PASS: TestLRUCapacityOne (0.00s)\n"}
{"Time":"2026-07-01T22:48:55.46983547+02:00","Action":"pass","Package":"challenge","Test":"TestLRUCapacityOne","Elapsed":0}
{"Time":"2026-07-01T22:48:55.46983719+02:00","Action":"run","Package":"challenge","Test":"TestLRUZeroCapacity"}
{"Time":"2026-07-01T22:48:55.46983884+02:00","Action":"output","Package":"challenge","Test":"TestLRUZeroCapacity","Output":"=== RUN TestLRUZeroCapacity\n"}
{"Time":"2026-07-01T22:48:55.46984103+02:00","Action":"output","Package":"challenge","Test":"TestLRUZeroCapacity","Output":"--- PASS: TestLRUZeroCapacity (0.00s)\n"}
{"Time":"2026-07-01T22:48:55.46984298+02:00","Action":"pass","Package":"challenge","Test":"TestLRUZeroCapacity","Elapsed":0}
{"Time":"2026-07-01T22:48:55.46984551+02:00","Action":"run","Package":"challenge","Test":"TestLRUStringValues"}
{"Time":"2026-07-01T22:48:55.46984764+02:00","Action":"output","Package":"challenge","Test":"TestLRUStringValues","Output":"=== RUN TestLRUStringValues\n"}
{"Time":"2026-07-01T22:48:55.46985042+02:00","Action":"output","Package":"challenge","Test":"TestLRUStringValues","Output":"--- PASS: TestLRUStringValues (0.00s)\n"}
{"Time":"2026-07-01T22:48:55.46985218+02:00","Action":"pass","Package":"challenge","Test":"TestLRUStringValues","Elapsed":0}
{"Time":"2026-07-01T22:48:55.46985429+02:00","Action":"run","Package":"challenge","Test":"TestLRUEvictionChain"}
{"Time":"2026-07-01T22:48:55.46985883+02:00","Action":"output","Package":"challenge","Test":"TestLRUEvictionChain","Output":"=== RUN TestLRUEvictionChain\n"}
{"Time":"2026-07-01T22:48:55.46986131+02:00","Action":"output","Package":"challenge","Test":"TestLRUEvictionChain","Output":"--- PASS: TestLRUEvictionChain (0.00s)\n"}
{"Time":"2026-07-01T22:48:55.46986345+02:00","Action":"pass","Package":"challenge","Test":"TestLRUEvictionChain","Elapsed":0}
{"Time":"2026-07-01T22:48:55.46986525+02:00","Action":"output","Package":"challenge","Output":"PASS\n"}
{"Time":"2026-07-01T22:48:55.469967481+02:00","Action":"output","Package":"challenge","Output":"ok \tchallenge\t0.001s\n"}
{"Time":"2026-07-01T22:48:55.469981541+02:00","Action":"pass","Package":"challenge","Elapsed":0.001}