Match credit-array indices against impressions in descending order

api.bs

@@ -724,7 +724,8 @@ contribute to the histogram, i.e., will be uniformly zero.
 
       <xmp highlight=js>
         const attributionDetails = {
-          credit: [.25, .25, .5]
+          // top impression's histogram index gets 50% of value, the next two 25% each
+          credit: [.5, .25, .25],
           value: 3,
           maxValue: 7,
         };
@@ -1815,11 +1816,11 @@ To <dfn>fill a histogram with last-n-touch attribution</dfn>, given a [=set=] of
   a [=list=] of doubles |credit|:
 
 1.  [=Assert=]: |matchedImpressions| is [=set/is empty|not empty=].
-1.  Let |sortedImpressions| be |matchedImpressions|, [=list/sorted in ascending order=]
+1.  Let |sortedImpressions| be |matchedImpressions|, [=list/sorted in descending order=]
     by [=impression/priority=], then by [=impression/timestamp=].
 1.  Let |N| be the minimum of the [=list/size=] of |credit|, and the [=list/size=] of |sortedImpressions|.
-1.  Let |lastNImpressions| be the last |N| entries of |sortedImpressions|.
-1.  [=list/remove|Remove=] all but the last |N| entries of |credit|.
+1.  Let |lastNImpressions| be the first |N| entries of |sortedImpressions|.
+1.  [=list/remove|Remove=] all but the first |N| entries of |credit|.
 1.  Let |normalizedCredit| be the result of [=fairly allocate credit|fairly allocating credit=] with |credit| and |value|.
 1.  Let |histogram| be the result of invoking [=create an all-zero histogram=]
     with |histogramSize|.

impl/e2e-tests/credit-longer-than-impressions.json

@@ -0,0 +1,29 @@
+{
+  "events": [
+    {
+      "seconds": 1,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 0 }
+    },
+    {
+      "seconds": 2,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 1 }
+    },
+    {
+      "seconds": 3,
+      "site": "advertiser.example",
+      "event": "measureConversion",
+      "options": {
+        "aggregationService": "https://agg-service.example",
+        "histogramSize": 4,
+        "value": 12,
+        "maxValue": 12,
+        "credit": [6, 3, 3]
+      },
+      "expected": [4, 8, 0, 0]
+    }
+  ]
+}

impl/e2e-tests/multi-touch-divides-evenly-unordered-credit.json

@@ -0,0 +1,36 @@
+{
+  "events": [
+    {
+      "seconds": 1,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 0 }
+    },
+    {
+      "seconds": 2,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 1 }
+    },
+    {
+      "seconds": 3,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 2 }
+    },
+    {
+      "seconds": 4,
+      "site": "advertiser.example",
+      "event": "measureConversion",
+      "options": {
+        "aggregationService": "https://agg-service.example",
+        "histogramSize": 4,
+        "value": 8,
+        "maxValue": 10,
+        "credit": [1, 2, 1]
+      },
+      "$comment": "not necessarily useful, but demonstrates the ability to prefer the second-to-last over the last",
+      "expected": [2, 4, 2, 0]
+    }
+  ]
+}

impl/e2e-tests/multi-touch-divides-evenly.json

@@ -0,0 +1,35 @@
+{
+  "events": [
+    {
+      "seconds": 1,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 0 }
+    },
+    {
+      "seconds": 2,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 1 }
+    },
+    {
+      "seconds": 3,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": { "histogramIndex": 2 }
+    },
+    {
+      "seconds": 4,
+      "site": "advertiser.example",
+      "event": "measureConversion",
+      "options": {
+        "aggregationService": "https://agg-service.example",
+        "histogramSize": 4,
+        "value": 8,
+        "maxValue": 10,
+        "credit": [2, 1, 1]
+      },
+      "expected": [2, 2, 4, 0]
+    }
+  ]
+}

impl/e2e-tests/priority.json

@@ -0,0 +1,45 @@
+{
+  "events": [
+    {
+      "seconds": 1,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": {
+        "histogramIndex": 0,
+        "priority": 200
+      }
+    },
+    {
+      "seconds": 2,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": {
+        "histogramIndex": 1,
+        "priority": 300
+      }
+    },
+    {
+      "seconds": 3,
+      "site": "publisher.example",
+      "event": "saveImpression",
+      "options": {
+        "histogramIndex": 2,
+        "priority": 200
+      }
+    },
+    {
+      "seconds": 4,
+      "site": "advertiser.example",
+      "event": "measureConversion",
+      "options": {
+        "aggregationService": "https://agg-service.example",
+        "histogramSize": 4,
+        "value": 8,
+        "maxValue": 10,
+        "credit": [6, 2]
+      },
+      "$comment": "index 1 has the highest priority; the others have equal priority but index 2 is most recent",
+      "expected": [0, 6, 2, 0]
+    }
+  ]
+}

impl/src/backend.ts

@@ -532,10 +532,10 @@ export class Backend {
     const sortedImpressions = Array.from(matchedImpressions).toSorted(
       (a, b) => {
         if (a.priority < b.priority) {
-          return -1;
+          return 1;
         }
         if (a.priority > b.priority) {
-          return 1;
+          return -1;
         }
         return Temporal.Instant.compare(b.timestamp, a.timestamp);
       },