Add concurrent city lookup benchmark

Add BenchmarkCityLookupConcurrent to demonstrate string cache performance
improvements under concurrent load. Tests 1, 4, 16, and 64 goroutines
performing realistic city lookups, providing clear metrics for concurrent
scaling behavior.

Author: oschwald

reader_test.go

@@ -9,6 +9,7 @@ import (
 	"net/netip"
 	"os"
 	"path/filepath"
+	"sync"
 	"testing"
 	"time"
 
@@ -1007,6 +1008,61 @@ func BenchmarkDecodePathCountryCode(b *testing.B) {
 	require.NoError(b, db.Close(), "error on close")
 }
 
+// BenchmarkCityLookupConcurrent tests concurrent city lookups to demonstrate
+// string cache performance under concurrent load.
+func BenchmarkCityLookupConcurrent(b *testing.B) {
+	db, err := Open("GeoLite2-City.mmdb")
+	require.NoError(b, err)
+	defer func() {
+		require.NoError(b, db.Close(), "error on close")
+	}()
+
+	// Test with different numbers of concurrent goroutines
+	goroutineCounts := []int{1, 4, 16, 64}
+
+	for _, numGoroutines := range goroutineCounts {
+		b.Run(fmt.Sprintf("goroutines_%d", numGoroutines), func(b *testing.B) {
+			// Each goroutine performs 100 lookups
+			const lookupsPerGoroutine = 100
+			b.ResetTimer()
+
+			for range b.N {
+				var wg sync.WaitGroup
+				wg.Add(numGoroutines)
+
+				for range numGoroutines {
+					go func() {
+						defer wg.Done()
+
+						//nolint:gosec // this is a test
+						r := rand.New(rand.NewSource(time.Now().UnixNano()))
+						s := make(net.IP, 4)
+						var result fullCity
+
+						for range lookupsPerGoroutine {
+							ip := randomIPv4Address(r, s)
+							err := db.Lookup(ip).Decode(&result)
+							if err != nil {
+								b.Error(err)
+								return
+							}
+							// Access string fields to exercise the cache
+							_ = result.City.Names
+							_ = result.Country.Names
+						}
+					}()
+				}
+
+				wg.Wait()
+			}
+
+			// Report operations per second
+			totalOps := int64(b.N) * int64(numGoroutines) * int64(lookupsPerGoroutine)
+			b.ReportMetric(float64(totalOps)/b.Elapsed().Seconds(), "lookups/sec")
+		})
+	}
+}
+
 func randomIPv4Address(r *rand.Rand, ip []byte) netip.Addr {
 	num := r.Uint32()
 	ip[0] = byte(num >> 24)