better previous indicator and funnel

packages/common/src/get-previous-metric.ts

@@ -0,0 +1,39 @@
+import { isNil } from 'ramda';
+
+import type { PreviousValue } from '@openpanel/validation';
+
+import { round } from './math';
+
+export function getPreviousMetric(
+  current: number,
+  previous: number | null | undefined
+): PreviousValue {
+  if (isNil(previous)) {
+    return undefined;
+  }
+
+  const diff = round(
+    ((current > previous
+      ? current / previous
+      : current < previous
+        ? previous / current
+        : 0) -
+      1) *
+      100,
+    1
+  );
+
+  return {
+    diff:
+      Number.isNaN(diff) || !Number.isFinite(diff) || current === previous
+        ? null
+        : diff,
+    state:
+      current > previous
+        ? 'positive'
+        : current < previous
+          ? 'negative'
+          : 'neutral',
+    value: previous,
+  };
+}