fix(gki): build the index from the glucose shown beside it

A reading entered in mmol/L is stored as whole mg/dL, and the index
divided the unrounded conversion, so a 5.2 / 0.3 pair printed beside a
17.4 rather than 17.3. Round the glucose to the decimal it is shown with
first and divide that. The tiles also printed a two-decimal ketone
reading to one decimal; show it as recorded.

src/components/day/day-gki-tile.tsx

@@ -4,7 +4,10 @@ import Link from "next/link";
 
 import type { DayResponse } from "@/lib/day/contract";
 import { useFormatters, useTranslations } from "@/lib/i18n/context";
-import { GKI_COMPONENT_UNIT } from "@/lib/measurements/glucose-ketone-index";
+import {
+  GKI_COMPONENT_UNIT,
+  ketoneFractionDigits,
+} from "@/lib/measurements/glucose-ketone-index";
 
 type DayGki = NonNullable<DayResponse["glucoseKetoneIndex"]>[number];
 
@@ -38,7 +41,10 @@ export function DayGkiTile({ points }: { points: readonly DayGki[] }) {
             ? tCount("day.gkiCount", points.length)
             : t("day.gkiPair", {
                 glucose: fmt.number(latest.glucoseMmol, 1),
-                ketones: fmt.number(latest.ketonesMmol, 1),
+                ketones: fmt.number(
+                  latest.ketonesMmol,
+                  ketoneFractionDigits(latest.ketonesMmol),
+                ),
                 unit: GKI_COMPONENT_UNIT,
               })}
         </span>

src/components/insights/ketones/gki-card.tsx

@@ -17,7 +17,10 @@ import {
 } from "@/components/ui/card";
 import { EmptyState } from "@/components/ui/empty-state";
 import { useFormatters, useTranslations } from "@/lib/i18n/context";
-import { GKI_COMPONENT_UNIT } from "@/lib/measurements/glucose-ketone-index";
+import {
+  GKI_COMPONENT_UNIT,
+  ketoneFractionDigits,
+} from "@/lib/measurements/glucose-ketone-index";
 import { useGlucoseKetoneIndexSeries } from "@/lib/queries/use-glucose-ketone-index";
 import { importWithRetry } from "@/lib/retry-import";
 
@@ -78,7 +81,10 @@ export function GkiCard() {
               {t("insights.bloodKetones.gki.latest", {
                 value: fmt.number(latest.value, 1),
                 glucose: fmt.number(latest.gki.glucoseMmol, 1),
-                ketones: fmt.number(latest.gki.ketonesMmol, 1),
+                ketones: fmt.number(
+                  latest.gki.ketonesMmol,
+                  ketoneFractionDigits(latest.gki.ketonesMmol),
+                ),
                 unit: GKI_COMPONENT_UNIT,
                 date: fmt.dateTime(latest.at),
               })}

src/lib/measurements/__tests__/glucose-ketone-index.test.ts

@@ -4,6 +4,7 @@ import { MGDL_PER_MMOL } from "@/lib/glucose";
 import {
   GKI_PAIRING_WINDOW_MINUTES,
   glucoseKetoneIndex,
+  ketoneFractionDigits,
   pairGlucoseKetoneIndex,
   summariseGlucoseKetoneIndex,
   type GkiReading,
@@ -49,13 +50,25 @@ describe("pairGlucoseKetoneIndex", () => {
       glucoseId: "g1",
       at: new Date(T0).toISOString(),
       glucoseAt: new Date(T0).toISOString(),
-      // 81 / 18.0182 = 4.4954 mmol/L; / 1.2 = 3.746 -> 3.7
-      value: 3.7,
+      // 81 / 18.0182 = 4.4954 -> 4.5 mmol/L, the figure shown beside the
+      // index; 4.5 / 1.2 = 3.75 -> 3.8
+      value: 3.8,
       ketonesMmol: 1.2,
       glucoseMmol: 4.5,
     });
   });
 
+  it("builds the index from the glucose it prints, not the unrounded one", () => {
+    // 5.2 mmol/L entered is stored as 94 mg/dL. Unrounded that is 5.217 and
+    // 17.39 over 0.3, shown as 17.4 beside "5.2 / 0.3"; the pair gives 17.3.
+    const [point] = pairGlucoseKetoneIndex(
+      [reading("k1", 0, 0.3)],
+      [reading("g1", 0, 94)],
+    );
+    expect(point?.glucoseMmol).toBe(5.2);
+    expect(point?.value).toBe(17.3);
+  });
+
   it("prefers the same instant over a nearer-looking neighbour", () => {
     const [point] = pairGlucoseKetoneIndex(
       [reading("k1", 0, 1)],
@@ -148,6 +161,16 @@ describe("pairGlucoseKetoneIndex", () => {
   });
 });
 
+describe("ketoneFractionDigits", () => {
+  it("shows a ketone reading as recorded, one or two decimals", () => {
+    expect(ketoneFractionDigits(1.2)).toBe(1);
+    expect(ketoneFractionDigits(0.3)).toBe(1);
+    expect(ketoneFractionDigits(2)).toBe(1);
+    expect(ketoneFractionDigits(0.25)).toBe(2);
+    expect(ketoneFractionDigits(1.05)).toBe(2);
+  });
+});
+
 describe("summariseGlucoseKetoneIndex", () => {
   it("summarises the indices and is null for none", () => {
     expect(summariseGlucoseKetoneIndex([])).toBeNull();
@@ -155,14 +178,14 @@ describe("summariseGlucoseKetoneIndex", () => {
       [reading("k1", 0, 1), reading("k2", 60, 2), reading("k3", 120, 4)],
       [reading("g1", 0, 90), reading("g2", 60, 90), reading("g3", 120, 90)],
     );
-    // 90 mg/dL = 4.995 mmol/L: indices 5.0, 2.5, 1.2.
-    expect(points.map((p) => p.value)).toEqual([5, 2.5, 1.2]);
+    // 90 mg/dL = 4.995 -> 5.0 mmol/L: indices 5.0, 2.5, 1.3 (5.0 / 4 = 1.25).
+    expect(points.map((p) => p.value)).toEqual([5, 2.5, 1.3]);
     expect(summariseGlucoseKetoneIndex(points)).toEqual({
       count: 3,
       mean: 2.9,
-      min: 1.2,
+      min: 1.3,
       max: 5,
-      latest: 1.2,
+      latest: 1.3,
     });
   });
 });

src/lib/measurements/glucose-ketone-index.ts

@@ -5,8 +5,9 @@
  *   GKI = glucose (mmol/L) / beta-hydroxybutyrate (mmol/L)
  *
  * Glucose is stored in mg/dL and converted with {@link MGDL_PER_MMOL}, the one
- * factor the tree uses for that conversion, so the glucose a GKI is built from
- * is the same number every glucose surface shows. The index is computed when
+ * factor the tree uses for that conversion, then rounded to the one decimal
+ * the index is shown beside, so the glucose a GKI is built from is the same
+ * number every glucose surface shows. The index is computed when
  * it is read and never stored: a stored ratio drifts the moment either reading
  * it came from is edited or deleted, which is the failure a hand-typed GKI has.
  *
@@ -50,6 +51,16 @@ export const GKI_PAIRING_WINDOW_MINUTES = 15;
 export const GKI_COMPONENT_UNIT = "mmol/L";
 export const GKI_PAIRING_WINDOW_MS = GKI_PAIRING_WINDOW_MINUTES * 60_000;
 
+/**
+ * The decimals a ketone reading is shown with beside its index: as recorded,
+ * one or two. A meter that reads 0.25 must not print as 0.3, or the pair on
+ * screen no longer divides to the index beside it.
+ */
+export function ketoneFractionDigits(ketonesMmol: number): 1 | 2 {
+  const tenths = ketonesMmol * 10;
+  return Math.abs(tenths - Math.round(tenths)) < 1e-9 ? 1 : 2;
+}
+
 /** One stored reading, as the pairing needs it. */
 export interface GkiReading {
   id: string;
@@ -154,16 +165,20 @@ export function pairGlucoseKetoneIndex(
       }
     }
     if (best === null) continue;
-    const index = glucoseKetoneIndex(best.value, ketone.value);
-    if (index === null) continue;
+    if (glucoseKetoneIndex(best.value, ketone.value) === null) continue;
+    // The index is built from the glucose as it is shown beside it, so the
+    // printed pair divides to the printed index. A reading entered in mmol/L
+    // is stored as whole mg/dL; dividing the unrounded conversion would land
+    // a tenth away from the pair on screen.
+    const glucoseMmol = oneDecimal(best.value / MGDL_PER_MMOL);
     out.push({
       ketoneId: ketone.id,
       glucoseId: best.id,
       at: ketone.measuredAt.toISOString(),
       glucoseAt: best.measuredAt.toISOString(),
-      value: oneDecimal(index),
+      value: oneDecimal(glucoseMmol / ketone.value),
       ketonesMmol: ketone.value,
-      glucoseMmol: oneDecimal(best.value / MGDL_PER_MMOL),
+      glucoseMmol,
     });
   }
   return out;

tests/integration/blood-ketones-gki.test.ts

@@ -207,11 +207,11 @@ describe("glucose-ketone index (#1367)", () => {
         glucoseMmol: p.glucoseMmol,
       })),
     ).toEqual([
-      // 81 mg/dL = 4.5 mmol/L; / 1.2 = 3.7
+      // 81 mg/dL = 4.5 mmol/L; / 1.2 = 3.75 -> 3.8
       {
         at: at(0).toISOString(),
         glucoseAt: at(0).toISOString(),
-        value: 3.7,
+        value: 3.8,
         ketonesMmol: 1.2,
         glucoseMmol: 4.5,
       },
@@ -261,7 +261,7 @@ describe("glucose-ketone index (#1367)", () => {
       };
     };
     expect(gkiBody.data.unit).toBe("ratio");
-    expect(gkiBody.data.points.map((p) => p.value)).toEqual([3.7, 2.5]);
+    expect(gkiBody.data.points.map((p) => p.value)).toEqual([3.8, 2.5]);
     expect(gkiBody.data.points[1]!.gki).toMatchObject({
       glucoseAt: at(130).toISOString(),
       glucoseMmol: 5,
@@ -301,7 +301,7 @@ describe("glucose-ketone index (#1367)", () => {
       };
     };
     expect(body.data.glucoseKetoneIndex?.map((p) => p.value)).toEqual([
-      3.7, 2.5,
+      3.8, 2.5,
     ]);
     expect(body.data.values.some((v) => v.type === "BLOOD_KETONES")).toBe(true);
   });
@@ -324,8 +324,8 @@ describe("glucose-ketone index (#1367)", () => {
       count: 2,
       latest: 2.5,
       min: 2.5,
-      max: 3.7,
-      avg: 3.1,
+      max: 3.8,
+      avg: 3.2,
     });
 
     const ketonesOnly = await collectDoctorReportData(