fix: only negative transfer time as an "uncertain transfer"

src/transfer-risk/__tests__/transfer-risk.test.ts

@@ -33,8 +33,8 @@ describe('getTransferRisk', () => {
     expect(getTransferRisk(600)).toBeUndefined();
   });
 
-  it('treats a zero gap as uncertain', () => {
-    expect(getTransferRisk(0)).toBe('uncertain');
+  it('passes on a zero gap, which the planner treats as feasible', () => {
+    expect(getTransferRisk(0)).toBeUndefined();
   });
 
   it('is uncertain at any negative gap, however large', () => {
@@ -81,6 +81,14 @@ describe('getLegTransferRisk', () => {
     expect(getLegTransferRisk(legs, 1)).toBeUndefined();
   });
 
+  it('passes when the connection leaves the instant you arrive', () => {
+    const legs = [
+      transitLeg({expectedEndTime: '2024-01-01T10:10:00.000Z'}),
+      transitLeg({expectedStartTime: '2024-01-01T10:10:00.000Z'}),
+    ];
+    expect(getLegTransferRisk(legs, 1)).toBeUndefined();
+  });
+
   it('passes on the first leg, which nothing precedes', () => {
     expect(getLegTransferRisk([transitLeg(), transitLeg()], 0)).toBeUndefined();
   });

src/transfer-risk/transfer-risk.ts

@@ -2,13 +2,12 @@ import type {TransferLeg} from './types';
 import {TransferRisk} from './types';
 
 /**
- * Classifies the gap between arriving and the next departure: any gap that is
- * not positive is uncertain, however large. Zero counts, because arriving
- * exactly as the service leaves is not a transfer you can rely on. A
- * non-finite gap yields undefined.
+ * Classifies the gap between arriving and the next departure: a negative gap is
+ * uncertain, however small. A zero-second transfer is treated as feasible by Entur and may be
+ * returned by the trip planner, so it is not treated as a risky transfer. Non-finite values yield undefined.
  */
 export const getTransferRisk = (seconds: number): TransferRisk | undefined => {
-  if (!Number.isFinite(seconds) || seconds > 0) {
+  if (!Number.isFinite(seconds) || seconds >= 0) {
     return undefined;
   }
   return TransferRisk.Uncertain;
@@ -24,9 +23,9 @@ export const isTransitLeg = (leg: TransferLeg): boolean =>
  * before, so an intervening walk counts.
  *
  * Undefined when the leg is not transit, when no transit leg precedes it, or
- * when the transfer still holds. The transit check also keeps the warning off
- * the leg leading *into* a walk: those gaps are commonly re-anchored to exactly
- * zero, which would otherwise fire on every transfer.
+ * when the transfer still holds. The transit check keeps the warning off the leg
+ * leading *into* a walk, where it would sit on a leg clients are free to filter
+ * out of the display.
  */
 export const getLegTransferRisk = (
   legs: TransferLeg[],

src/transfer-risk/types.ts

@@ -1,7 +1,6 @@
 /**
- * Transfer risk classification for transfers with no slack (0s) or already
- * missed (negative gaps). Kept as a string (vs boolean) because additional
- * levels (e.g. `shortWait`) may be added later.
+ * Transfer risk classification for transfers. Kept as a string (vs boolean)
+ * because additional levels may be added later (e.g. `shortWait`).
  */
 export const TransferRisk = {
   Uncertain: 'uncertain',