cache: add EngineCache with tier-aware Get/Set

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/cache/engine_cache_test.go

@@ -0,0 +1,95 @@
+package cache
+
+import (
+	"context"
+	"log/slog"
+	"testing"
+	"time"
+
+	"github.com/metamorphosis-dev/samsa/internal/contracts"
+)
+
+func TestEngineCacheGetSet(t *testing.T) {
+	// Create a disabled cache for unit testing (nil client)
+	c := &Cache{logger: slog.Default()}
+	ec := NewEngineCache(c, nil)
+
+	ctx := context.Background()
+	cached, ok := ec.Get(ctx, "wikipedia", "abc123")
+	if ok {
+		t.Errorf("Get on disabled cache: expected false, got %v", ok)
+	}
+	_ = cached // unused when ok=false
+}
+
+func TestEngineCacheKeyFormat(t *testing.T) {
+	key := engineCacheKey("wikipedia", "abc123")
+	if key != "samsa:resp:wikipedia:abc123" {
+		t.Errorf("engineCacheKey: expected samsa:resp:wikipedia:abc123, got %s", key)
+	}
+}
+
+func TestEngineCacheIsStale(t *testing.T) {
+	c := &Cache{logger: slog.Default()}
+	ec := NewEngineCache(c, nil)
+
+	// Fresh response (stored 1 minute ago, wikipedia has 24h TTL)
+	fresh := CachedEngineResponse{
+		Engine:   "wikipedia",
+		Response: []byte(`{}`),
+		StoredAt: time.Now().Add(-1 * time.Minute),
+	}
+	if ec.IsStale(fresh, "wikipedia") {
+		t.Errorf("IsStale: 1-minute-old wikipedia should NOT be stale")
+	}
+
+	// Stale response (stored 25 hours ago)
+	stale := CachedEngineResponse{
+		Engine:   "wikipedia",
+		Response: []byte(`{}`),
+		StoredAt: time.Now().Add(-25 * time.Hour),
+	}
+	if !ec.IsStale(stale, "wikipedia") {
+		t.Errorf("IsStale: 25-hour-old wikipedia SHOULD be stale (24h TTL)")
+	}
+
+	// Override: 30 minute TTL for reddit
+	overrides := map[string]time.Duration{"reddit": 30 * time.Minute}
+	ec2 := NewEngineCache(c, overrides)
+
+	// 20 minutes old with 30m override should NOT be stale
+	redditFresh := CachedEngineResponse{
+		Engine:   "reddit",
+		Response: []byte(`{}`),
+		StoredAt: time.Now().Add(-20 * time.Minute),
+	}
+	if ec2.IsStale(redditFresh, "reddit") {
+		t.Errorf("IsStale: 20-min reddit with 30m override should NOT be stale")
+	}
+
+	// 45 minutes old with 30m override SHOULD be stale
+	redditStale := CachedEngineResponse{
+		Engine:   "reddit",
+		Response: []byte(`{}`),
+		StoredAt: time.Now().Add(-45 * time.Minute),
+	}
+	if !ec2.IsStale(redditStale, "reddit") {
+		t.Errorf("IsStale: 45-min reddit with 30m override SHOULD be stale")
+	}
+}
+
+func TestEngineCacheSetResponseType(t *testing.T) {
+	c := &Cache{logger: slog.Default()}
+	ec := NewEngineCache(c, nil)
+
+	ctx := context.Background()
+	urlStr := "https://example.com"
+	resp := contracts.SearchResponse{
+		Results: []contracts.MainResult{
+			{Title: "Test", URL: &urlStr},
+		},
+	}
+
+	// Should not panic on disabled cache
+	ec.Set(ctx, "wikipedia", "abc123", resp)
+}