Fix one-off hashrate calculation

src/models/StratumV1ClientStatistics.spec.ts

@@ -32,7 +32,30 @@ describe('StratumV1ClientStatistics', () => {
             await statistics.addShares(client, 64);
         }
 
-        expect(statistics.hashRate).toBeGreaterThan(0);
+        expect(statistics.hashRate).toBeCloseTo((64 * 4294967296) / 62);
+    });
+
+    it('keeps exactly the configured number of share samples', async () => {
+        for (let i = 0; i < 31; i++) {
+            jest.setSystemTime(new Date(Date.parse('2026-05-06T12:00:00Z') + (i * 31000)));
+            await statistics.addShares(client, 64);
+        }
+
+        expect((statistics as any).submissionCache).toHaveLength(30);
+        expect((statistics as any).submissionCacheDifficultySum).toBe(30 * 64);
+    });
+
+    it('excludes pre-window work and corrects share-terminated sampling bias', async () => {
+        for (let i = 0; i < 30; i++) {
+            jest.setSystemTime(new Date(Date.parse('2026-05-06T12:00:00Z') + (i * 31000)));
+            await statistics.addShares(client, 64);
+        }
+
+        const elapsedSeconds = 29 * 31;
+        const expectedUnbiasedDifficulty = 28 * 64;
+        expect(statistics.hashRate).toBeCloseTo(
+            (expectedUnbiasedDifficulty * 4294967296) / elapsedSeconds,
+        );
     });
 
     it('should not suggest a difficulty change before enough time or shares have passed', () => {
@@ -51,7 +74,7 @@ describe('StratumV1ClientStatistics', () => {
             await statistics.addShares(client, 64);
         }
 
-        expect(statistics.getSuggestedDifficulty(64)).toBe(2048);
+        expect(statistics.getSuggestedDifficulty(64)).toBe(1024);
     });
 
     it('does not retarget when accepted shares have a zero-duration sample window', async () => {
@@ -76,7 +99,7 @@ describe('StratumV1ClientStatistics', () => {
             await statistics.addShares(client, 64);
         }
 
-        expect(statistics.getSuggestedDifficulty(128)).toBe(16);
+        expect(statistics.getSuggestedDifficulty(128)).toBe(8);
     });
 
     it('should not suggest a difficulty below the configured minimum', () => {

src/models/StratumV1ClientStatistics.ts

@@ -16,9 +16,9 @@ export class StratumV1ClientStatistics {
     }
 
     public async addShares(_client: ClientEntity, targetDifficulty: number) {
-        var date = new Date();
+        const date = new Date();
 
-        if (this.submissionCache.length > CACHE_SIZE) {
+        if (this.submissionCache.length >= CACHE_SIZE) {
             this.submissionCacheDifficultySum -= this.submissionCache[0].difficulty;
             this.submissionCache.shift();
         }
@@ -28,9 +28,12 @@ export class StratumV1ClientStatistics {
         });
         this.submissionCacheDifficultySum += targetDifficulty;
 
-        const time = new Date().getTime() - this.submissionCache[0].time.getTime();
+        const elapsedSeconds = (date.getTime() - this.submissionCache[0].time.getTime()) / 1000;
-        if(time > 60000 && this.submissionCache.length > 2) { 
+        if (elapsedSeconds > 60) {
-            this.hashRate = (this.submissionCacheDifficultySum * 4294967296) / (time / 1000);
+            const difficultyPerSecond = this.getDifficultyPerSecond(elapsedSeconds);
+            if (difficultyPerSecond != null) {
+                this.hashRate = difficultyPerSecond * 4294967296;
+            }
         }
     }
 
@@ -46,17 +49,12 @@ export class StratumV1ClientStatistics {
             }
         }
 
-        const sum = this.submissionCache.reduce((pre, cur) => {
-            pre += cur.difficulty;
-            return pre;
-        }, 0);
         const diffSeconds = (this.submissionCache[this.submissionCache.length - 1].time.getTime() - this.submissionCache[0].time.getTime()) / 1000;
-        if (!Number.isFinite(diffSeconds) || diffSeconds <= 0) {
+        const difficultyPerSecond = this.getDifficultyPerSecond(diffSeconds);
+        if (difficultyPerSecond == null) {
             return null;
         }
 
-        const difficultyPerSecond = sum / diffSeconds;
-
         const targetDifficulty = difficultyPerSecond * this.targetSubmitShareEveryNSeconds;
         if (!Number.isFinite(targetDifficulty) || targetDifficulty <= 0) {
             return null;
@@ -69,6 +67,32 @@ export class StratumV1ClientStatistics {
         return null;
     }
 
+    /**
+     * Estimate work rate from a share-terminated sample window.
+     *
+     * A cache of N shares contains N - 1 observed inter-share intervals. The
+     * first share's work predates the window and must not be counted. Because
+     * the window closes on a share arrival, the reciprocal elapsed time also
+     * has the usual finite-sample Poisson bias; multiplying by
+     * (intervalCount - 1) / intervalCount removes it.
+     */
+    private getDifficultyPerSecond(elapsedSeconds: number): number | null {
+        const sampleCount = this.submissionCache.length;
+        if (sampleCount <= 2 || !Number.isFinite(elapsedSeconds) || elapsedSeconds <= 0) {
+            return null;
+        }
+
+        const intervalCount = sampleCount - 1;
+        const observedDifficulty = this.submissionCacheDifficultySum
+            - this.submissionCache[0].difficulty;
+        const unbiasedDifficulty = observedDifficulty * (intervalCount - 1) / intervalCount;
+        const difficultyPerSecond = unbiasedDifficulty / elapsedSeconds;
+
+        return Number.isFinite(difficultyPerSecond) && difficultyPerSecond > 0
+            ? difficultyPerSecond
+            : null;
+    }
+
     private nearestPowerOfTwo(val: number): number {
         if (!Number.isFinite(val) || val <= 0) {
             return null;