refactor: extract the shared rolling-baseline core

BleFieldCore and MagFieldCore had converged on the same class: same
time-aware EMA, same warm-up gate, same "deviation past a threshold is an
event" shape, differing only in constants and field names. Three copies of
the alpha derivation existed across the sensor libs.

lib/baseline.ts now owns it. Both wrappers keep their own public types
(`.rssi`, `.magnitude`) so nothing downstream changed — which is what let
all 26 existing tests pass completely unmodified against the refactor.
That was the point of doing it this way: if the tests had needed editing,
the refactor would have been changing behaviour rather than removing
duplication.

coldSpot.ts deliberately does NOT adopt this. It threads immutable state
through pure functions so a whole session's narrative can be replayed in a
test without a clock — a different and equally valid shape. Collapsing the
two would force one into a style that doesn't fit it, which is how
deduplication turns into damage.

355 frontend tests pass unchanged; i18n parity gate passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/lib/baseline.ts

@@ -0,0 +1,117 @@
+// Shared rolling-baseline core for threshold sensors.
+//
+// Extracted because BleFieldCore and MagFieldCore had become the same class
+// with different constants and field names — same time-aware EMA, same
+// warm-up gate, same "deviation past a threshold is an event" shape. One
+// implementation means one place to get the time-awareness right, and one
+// place to fix it when it's wrong.
+//
+// Why the EMA is time-aware rather than per-sample: real sensors do not
+// report on a fixed schedule. BLE advertisement intervals vary and the OS
+// coalesces them; a magnetometer can be throttled when the page is
+// backgrounded; a paired device can drop offline and return. A fixed
+// per-sample alpha would weight a tight burst and a two-minute gap
+// identically, which is wrong in both directions — it drags the baseline
+// far too slowly across a gap (as if a hundred readings' worth of recency
+// happened at once) and far too eagerly through a burst.
+//
+// The exponential-decay form degrades gracefully at both extremes: a long
+// gap pushes alpha toward 1 (the old baseline is stale, trust the new
+// reading), a rapid burst toward 0 (barely move at all).
+//
+// coldSpot.ts deliberately does NOT use this: it threads immutable state
+// through pure functions so a whole session's narrative can be replayed in
+// a test without a clock, which is a different and equally valid shape.
+// Collapsing the two would mean forcing one of them into a style that
+// doesn't fit it.
+
+export type BaselineEvent = {
+  /** Signed deviation from the baseline as it stood *before* this reading —
+   * i.e. "how surprising was this", not "how far is the baseline now". */
+  deviation: number
+  /** |deviation| normalised against `threshold * 3`, clamped to 0..1. */
+  severity: number
+  /** The raw reading that produced the event. */
+  value: number
+  at: number
+}
+
+export type ThresholdBaselineOptions = {
+  /** Deviation magnitude that counts as an event, in the sensor's own unit. */
+  threshold: number
+  /** EMA time constant, ms. */
+  tauMs: number
+  /** Readings folded in before deviations are trusted. A baseline has to
+   * mean something before a difference from it does. */
+  minSamples: number
+}
+
+/**
+ * Tracks a rolling baseline and reports readings that deviate far enough
+ * from it to count as events.
+ *
+ * Symmetric on purpose: a signal that strengthens is as much an event as
+ * one that weakens. A body can attenuate a BLE path *or* reflect into it,
+ * and ferrous mass can shield a magnetic field *or* add to it — a
+ * one-sided detector would miss half of what really happens.
+ */
+export class ThresholdBaseline {
+  private mean: number | null = null
+  private lastAt: number | null = null
+  private samples = 0
+
+  constructor(private readonly opts: ThresholdBaselineOptions) {}
+
+  get baseline(): number | null {
+    return this.mean
+  }
+
+  get warm(): boolean {
+    return this.samples >= this.opts.minSamples
+  }
+
+  /**
+   * Fold one reading in. Returns an event when the deviation clears the
+   * threshold and the baseline is warm, else null.
+   *
+   * Non-finite input (a garbled packet, a sensor returning NaN) is a no-op
+   * rather than a crash or — worse — a poisoned baseline that would then
+   * misreport every subsequent reading.
+   */
+  push(value: number, atMs: number): BaselineEvent | null {
+    if (!Number.isFinite(value) || !Number.isFinite(atMs)) return null
+
+    if (this.mean === null) {
+      this.mean = value
+      this.lastAt = atMs
+      this.samples = 1
+      return null
+    }
+
+    const deviation = value - this.mean
+    const warm = this.warm
+
+    // Guard against zero/negative dt (duplicate timestamps, clock skew, two
+    // readings in one tick) with a flat fallback rather than dividing by an
+    // elapsed time that isn't trustworthy.
+    const dt = this.lastAt === null ? 0 : atMs - this.lastAt
+    const alpha = dt > 0 ? 1 - Math.exp(-dt / this.opts.tauMs) : 0.15
+    this.mean = this.mean + alpha * deviation
+    this.lastAt = atMs
+    this.samples++
+
+    if (!warm || Math.abs(deviation) < this.opts.threshold) return null
+    return {
+      deviation,
+      severity: Math.min(1, Math.abs(deviation) / (this.opts.threshold * 3)),
+      value,
+      at: atMs,
+    }
+  }
+
+  reset(): void {
+    this.mean = null
+    this.lastAt = null
+    this.samples = 0
+  }
+}

frontend/src/lib/bluetooth.ts

@@ -46,6 +46,8 @@ function bluetoothApi(): BluetoothLike | null {
   return nav.bluetooth ?? null
 }
 
+import { ThresholdBaseline } from './baseline'
+
 export type BleReading = {
   /** Received signal strength, dBm. Typically -30 (touching) to -100 (far). */
   rssi: number
@@ -96,55 +98,36 @@ export function isSupported(): boolean {
  * Pure and separately testable: no Bluetooth objects appear in here.
  */
 export class BleFieldCore {
-  private mean: number | null = null
-  private lastAt: number | null = null
-  private samples = 0
+  private readonly core = new ThresholdBaseline({
+    threshold: DISTURBANCE_DB,
+    tauMs: BASELINE_TAU_MS,
+    minSamples: MIN_SAMPLES,
+  })
 
   get baseline(): number | null {
-    return this.mean
+    return this.core.baseline
   }
 
   get warm(): boolean {
-    return this.samples >= MIN_SAMPLES
+    return this.core.warm
   }
 
-  /**
-   * Fold one reading in. Returns a disturbance when the deviation clears
-   * the threshold and the baseline is warm, else null. Non-finite input is
-   * a no-op rather than a crash or a poisoned baseline.
-   */
+  /** Fold one reading in; see ThresholdBaseline.push. Returns a
+   * BLE-flavoured disturbance so callers keep reading `.rssi` rather than a
+   * generic `.value`. */
   push(rssi: number, atMs: number): BleDisturbance | null {
-    if (!Number.isFinite(rssi) || !Number.isFinite(atMs)) return null
-
-    if (this.mean === null) {
-      this.mean = rssi
-      this.lastAt = atMs
-      this.samples = 1
-      return null
-    }
-
-    const deviation = rssi - this.mean
-    const warm = this.warm
-
-    const dt = this.lastAt === null ? 0 : atMs - this.lastAt
-    const alpha = dt > 0 ? 1 - Math.exp(-dt / BASELINE_TAU_MS) : 0.15
-    this.mean = this.mean + alpha * deviation
-    this.lastAt = atMs
-    this.samples++
-
-    if (!warm || Math.abs(deviation) < DISTURBANCE_DB) return null
+    const event = this.core.push(rssi, atMs)
+    if (!event) return null
     return {
-      deviation,
-      severity: Math.min(1, Math.abs(deviation) / (DISTURBANCE_DB * 3)),
-      rssi,
-      at: atMs,
+      deviation: event.deviation,
+      severity: event.severity,
+      rssi: event.value,
+      at: event.at,
     }
   }
 
   reset(): void {
-    this.mean = null
-    this.lastAt = null
-    this.samples = 0
+    this.core.reset()
   }
 }
 

frontend/src/lib/magnetometer.ts

@@ -25,6 +25,8 @@
 // rolling-EMA-baseline shape used by coldSpot.ts and bluetooth.ts applies
 // here too.
 
+import { ThresholdBaseline } from './baseline'
+
 export type MagReading = {
   /** Field magnitude in microtesla, sqrt(x^2 + y^2 + z^2). */
   magnitude: number
@@ -76,50 +78,33 @@ export function isSupported(): boolean {
  * testable — no Sensor objects in here.
  */
 export class MagFieldCore {
-  private mean: number | null = null
-  private lastAt: number | null = null
-  private samples = 0
+  private readonly core = new ThresholdBaseline({
+    threshold: SPIKE_UT,
+    tauMs: BASELINE_TAU_MS,
+    minSamples: MIN_SAMPLES,
+  })
 
   get baseline(): number | null {
-    return this.mean
+    return this.core.baseline
   }
 
   get warm(): boolean {
-    return this.samples >= MIN_SAMPLES
+    return this.core.warm
   }
 
   push(magnitude: number, atMs: number): MagAnomaly | null {
-    if (!Number.isFinite(magnitude) || !Number.isFinite(atMs)) return null
-
-    if (this.mean === null) {
-      this.mean = magnitude
-      this.lastAt = atMs
-      this.samples = 1
-      return null
-    }
-
-    const deviation = magnitude - this.mean
-    const warm = this.warm
-
-    const dt = this.lastAt === null ? 0 : atMs - this.lastAt
-    const alpha = dt > 0 ? 1 - Math.exp(-dt / BASELINE_TAU_MS) : 0.15
-    this.mean = this.mean + alpha * deviation
-    this.lastAt = atMs
-    this.samples++
-
-    if (!warm || Math.abs(deviation) < SPIKE_UT) return null
+    const event = this.core.push(magnitude, atMs)
+    if (!event) return null
     return {
-      deviation,
-      severity: Math.min(1, Math.abs(deviation) / (SPIKE_UT * 3)),
-      magnitude,
-      at: atMs,
+      deviation: event.deviation,
+      severity: event.severity,
+      magnitude: event.value,
+      at: event.at,
     }
   }
 
   reset(): void {
-    this.mean = null
-    this.lastAt = null
-    this.samples = 0
+    this.core.reset()
   }
 }