# All files / src/services MetricsService.ts

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     /**
     * Performance Metrics Service
     *
     * Provides Prometheus-style metrics collection for monitoring and observability
     *
     * ISMS Policy: SC-002 (Input Validation), AC-003 (Least Privilege)
     */

    /**
     * Preferred type for metric name parameters.
     *
     * Using {@link MetricName} values provides compile-time safety and IDE
     * auto-complete; raw `string` is accepted for backward compatibility and
     * for ad-hoc metrics outside the standard set.
     */
    export type MetricKey = MetricName | (string & {});

    /**
     * Typed metric name constants for MCP server and EP API instrumentation.
     *
     * Using an enum prevents typos in metric names and enables IDE
     * auto-complete throughout the codebase.
     *
     * @example
     * ```typescript
     * metricsService.incrementCounter(MetricName.EP_API_CALL_COUNT, 1);
     * metricsService.incrementCounter(MetricName.EP_CACHE_HIT_COUNT, 1);
     * ```
     */
    export enum MetricName {
      /** Total EP API calls (label: `endpoint`) */
      EP_API_CALL_COUNT = 'ep_api_call_count',
      /** Failed EP API calls */
      EP_API_ERROR_COUNT = 'ep_api_error_count',
      /** Cache hits for EP API responses */
      EP_CACHE_HIT_COUNT = 'ep_cache_hit_count',
      /** Cache misses for EP API responses */
      EP_CACHE_MISS_COUNT = 'ep_cache_miss_count',
    }

    /**
     * Single value metric (counter/gauge)
     */
    interface SingleMetric {
      type: 'counter' | 'gauge';
      value: number;
      labels?: Record<string, string> | undefined;
      timestamp: number;
    }

    /**
     * Histogram metric with bounded samples
     */
    interface HistogramMetric {
      type: 'histogram';
      samples: number[];
      count: number;
      sum: number;
      labels?: Record<string, string> | undefined;
      timestamp: number;
    }

    type MetricValue = SingleMetric | HistogramMetric;

    /**
     * Performance Metrics Service
     * Cyclomatic complexity: 8
     */
    export class MetricsService {
      private readonly metrics = new Map<string, MetricValue>();
      private readonly maxHistogramSamples: number;

      constructor(maxHistogramSamples = 1000) {
        this.maxHistogramSamples = maxHistogramSamples;
      }

      /**
       * Increments a counter metric by the given value.
       *
       * Creates the counter at zero if it does not exist yet, then adds `value`.
       * Counters are monotonically increasing — use {@link setGauge} for
       * values that can decrease.
       *
       * @param name - Metric name (use a {@link MetricName} enum value for type safety)
       * @param value - Amount to add to the counter (default: `1`)
       * @param labels - Optional key/value label dimensions (e.g., `{ endpoint: '/meps' }`)
       * @throws {TypeError} If `name` is not a string
       *
       * @example
       * ```typescript
       * metricsService.incrementCounter(MetricName.EP_API_CALL_COUNT);
       * metricsService.incrementCounter(MetricName.EP_API_CALL_COUNT, 1, { endpoint: '/meps' });
       * ```
       *
       * @since 0.8.0
       */
      incrementCounter(name: MetricKey, value = 1, labels?: Record<string, string>): void {
        const key = this.buildKey(name, labels);
        const current = this.metrics.get(key);
        const lastValue = current?.type === 'counter' ? current.value : 0;

        this.metrics.set(key, {
          type: 'counter',
          value: lastValue + value,
          labels,
          timestamp: Date.now()
        });
      }

      /**
       * Sets a gauge metric to an absolute value.
       *
       * Unlike counters, gauges can be set to any value including decreasing
       * values (e.g., current queue depth, active connection count).
       *
       * @param name - Metric name (use a {@link MetricName} enum value for type safety)
       * @param value - Absolute gauge value to record
       * @param labels - Optional key/value label dimensions
       * @throws {TypeError} If `name` is not a string
       *
       * @example
       * ```typescript
       * metricsService.setGauge('active_connections', 5);
       * metricsService.setGauge('queue_depth', 12, { queue: 'ep_api' });
       * ```
       *
       * @since 0.8.0
       */
      setGauge(name: MetricKey, value: number, labels?: Record<string, string>): void {
        const key = this.buildKey(name, labels);
        this.metrics.set(key, {
          type: 'gauge',
          value,
          labels,
          timestamp: Date.now()
        });
      }

      /**
       * Records a single observation into a histogram metric.
       *
       * Uses reservoir sampling to keep sample count bounded at
       * `maxHistogramSamples`. Use {@link getHistogramSummary} to retrieve
       * computed percentiles.
       *
       * @param name - Metric name (use a {@link MetricName} enum value for type safety)
       * @param value - Observed value (e.g., response time in milliseconds)
       * @param labels - Optional key/value label dimensions
       * @throws {TypeError} If `name` is not a string
       *
       * @example
       * ```typescript
       * const start = Date.now();
       * await fetchFromEPAPI('/meps');
       * metricsService.observeHistogram('ep_api_latency_ms', Date.now() - start);
       * ```
       *
       * @since 0.8.0
       */
      observeHistogram(name: MetricKey, value: number, labels?: Record<string, string>): void {
        const key = this.buildKey(name, labels);
        const current = this.metrics.get(key);

        if (current?.type === 'histogram') {
          current.count += 1;
          current.sum += value;

          if (current.samples.length < this.maxHistogramSamples) {
            current.samples.push(value);
          } else {
            const randomIndex = Math.floor(Math.random() * current.count);
            if (randomIndex < this.maxHistogramSamples) {
              current.samples[randomIndex] = value;
            }
          }
          current.timestamp = Date.now();
        } else {
          this.metrics.set(key, {
            type: 'histogram',
            samples: [value],
            count: 1,
            sum: value,
            labels,
            timestamp: Date.now()
          });
        }
      }

      /**
       * Returns the current scalar value of a counter or gauge metric.
       *
       * Returns `undefined` for histogram metrics (use {@link getHistogramSummary}
       * instead) and for metrics that have not been recorded yet.
       *
       * @param name - Metric name to query
       * @param labels - Optional label dimensions to scope the lookup
       * @returns Current numeric value, or `undefined` if not found / is a histogram
       *
       * @example
       * ```typescript
       * const errors = metricsService.getMetric(MetricName.EP_API_ERROR_COUNT) ?? 0;
       * if (errors > 10) {
       *   console.warn('High EP API error rate');
       * }
       * ```
       *
       * @since 0.8.0
       */
      getMetric(name: MetricKey, labels?: Record<string, string>): number | undefined {
        const key = this.buildKey(name, labels);
        const metric = this.metrics.get(key);

        return metric?.type === 'histogram' ? undefined : metric?.value;
      }

      /**
       * Returns a statistical summary of a histogram metric.
       *
       * Computes count, sum, average, and p50 / p95 / p99 percentiles from
       * the stored samples. Returns `undefined` if the metric does not exist,
       * is not a histogram, or has no samples yet.
       *
       * @param name - Histogram metric name to summarise
       * @param labels - Optional label dimensions to scope the lookup
       * @returns Summary object with `count`, `sum`, `avg`, `p50`, `p95`, `p99`,
       *   or `undefined` if no histogram data exists
       *
       * @example
       * ```typescript
       * const summary = metricsService.getHistogramSummary('ep_api_latency_ms');
       * if (summary) {
       *   console.log(`p95 latency: ${summary.p95}ms`);
       * }
       * ```
       *
       * @since 0.8.0
       */
      getHistogramSummary(name: MetricKey, labels?: Record<string, string>): {
        count: number;
        sum: number;
        avg: number;
        p50: number;
        p95: number;
        p99: number;
      } | undefined {
        const key = this.buildKey(name, labels);
        const metric = this.metrics.get(key);

        if (metric?.type !== 'histogram' || metric.samples.length === 0) {
          return undefined;
        }

        return {
          count: metric.count,
          sum: metric.sum,
          avg: metric.sum / metric.count,
          p50: this.percentileFromUnsorted(metric.samples, 50),
          p95: this.percentileFromUnsorted(metric.samples, 95),
          p99: this.percentileFromUnsorted(metric.samples, 99)
        };
      }

      /**
       * Clears all recorded metrics from memory.
       *
       * Intended for use in **tests** to ensure metric isolation between cases.
       * Calling this in production will silently discard all instrumentation data.
       *
       * @example
       * ```typescript
       * afterEach(() => {
       *   metricsService.clear();
       * });
       * ```
       *
       * @since 0.8.0
       */
      clear(): void {
        this.metrics.clear();
      }

      /**
       * Build metric key from name and labels
       * Cyclomatic complexity: 2
       */
      private buildKey(name: MetricKey, labels?: Record<string, string>): string {
        if (labels === undefined) {
          return name;
        }
        const labelStr = Object.entries(labels)
          .sort(([a], [b]) => a.localeCompare(b))
          .map(([k, v]) => `${k}="${v}"`)
          .join(',');
        return `${name}{${labelStr}}`;
      }

      /**
       * Compute a percentile value from an unsorted array using quickselect
       * Cyclomatic complexity: 3
       *
       * @param values - Array of samples
       * @param percentile - Percentile to compute (0-100)
       * @returns Percentile value
       */
      private percentileFromUnsorted(values: number[], percentile: number): number {
        Iif (values.length === 0) {
          return 0;
        }

        const index = Math.ceil((percentile / 100) * values.length) - 1;
        const clampedIndex = Math.min(values.length - 1, Math.max(0, index));

        const work = values.slice();
        return this.selectKth(work, clampedIndex);
      }

      /**
       * Select the k-th smallest element using quickselect
       * Cyclomatic complexity: 5
       *
       * @param arr - Array to select from (will be mutated)
       * @param k - Index of element to select (0-based)
       * @returns The k-th smallest element
       */
      private selectKth(arr: number[], k: number): number {
        let left = 0;
        let right = arr.length - 1;

        while (left <= right) {
          if (left === right) {
            return arr[left] ?? 0;
          }

          const pivotIndex = left + Math.floor((right - left) / 2);
          const newPivotIndex = this.partition(arr, left, right, pivotIndex);

          if (k === newPivotIndex) {
            return arr[k] ?? 0;
          }

          if (k < newPivotIndex) {
            right = newPivotIndex - 1;
          } else {
            left = newPivotIndex + 1;
          }
        }

        return arr[Math.min(Math.max(k, 0), arr.length - 1)] ?? 0;
      }

      /**
       * Partition helper for quickselect (Lomuto-style)
       * Cyclomatic complexity: 4
       *
       * @param arr - Array to partition
       * @param left - Left bound
       * @param right - Right bound
       * @param pivotIndex - Pivot index
       * @returns New pivot index
       */
      private partition(arr: number[], left: number, right: number, pivotIndex: number): number {
        const pivotValue = arr[pivotIndex] ?? 0;
        const rightVal = arr[right] ?? 0;
        const pivotVal = arr[pivotIndex] ?? 0;
        [arr[pivotIndex], arr[right]] = [rightVal, pivotVal];

        let storeIndex = left;
        for (let i = left; i < right; i += 1) {
          const arrI = arr[i] ?? 0;
          const arrStore = arr[storeIndex] ?? 0;
          if (arrI < pivotValue) {
            [arr[storeIndex], arr[i]] = [arrI, arrStore];
            storeIndex += 1;
          }
        }

        const rightValue = arr[right] ?? 0;
        const storeValue = arr[storeIndex] ?? 0;
        [arr[right], arr[storeIndex]] = [storeValue, rightValue];
        return storeIndex;
      }
    }