package com.atlassian.vcache.internal.core.metrics;
   
   import com.atlassian.vcache.internal.MetricLabel;
   import com.atlassian.vcache.internal.RequestContext;
   
   import java.util.function.Predicate;
   import java.util.function.Supplier;
   
   import static java.util.Objects.requireNonNull;
  
  /**
   * Metrics collector that collects metrics only for specified requests.
   *
   * @since 1.4.0
   */
  public class SamplingMetricsCollector extends DefaultMetricsCollector {
  
      private static final NoOpRequestMetrics NOOP_REQUEST_METRICS = new NoOpRequestMetrics();
  
      private final Predicate<RequestContext> collectMetrics;
  
      public SamplingMetricsCollector(
              Supplier<RequestContext> contextSupplier,
              Predicate<RequestContext> collectMetrics) {
          super(contextSupplier);
          this.collectMetrics = requireNonNull(collectMetrics);
      }
  
      @Override
      protected MutableRequestMetrics obtainMetrics(RequestContext context) {
          return context.computeIfAbsent(this, () ->
                  collectMetrics.test(context) ? new DefaultRequestMetrics() : NOOP_REQUEST_METRICS);
      }
  
      private static class NoOpRequestMetrics extends EmptyRequestMetrics implements MutableRequestMetrics {
          @Override
          public void record(String cacheName, CacheType cacheType, MetricLabel metricLabel, long sample) {
              // do nothing
          }
      }
  }