Factor out _intervalResult in timelinePagerank (#1799)

As part of work for #1773, I want to add a lot more complexity to the
logic for computing individual time-slices of pagerank scores, so that
we can trace how much score flowed on each individual edge. This means
adding more complexity to the _computeTimelineDistribution function;
however, that function is an untested wrapper, so I'm hesitant to add
more complexity directly.

Instead, I'm first factoring out an _intervalResult method and a
corresponding type, which computes the scores for a given timeline
interval. I've also added sanity checking tests for this method. In a
followon commit, I'll add more logic for tracking edge-level score
flows.

Test plan: This is just a refactor, maintaining existing behavior and
adding tests. `yarn test --full` passes. Since our sharness tests
include doing a full load (including timeline cred computation) on
realistic data from GitHub, this gives us confidence that there hasn't
been any change to cred semantics.

src/core/algorithm/timelinePagerank.js

@@ -24,10 +24,7 @@ import {
 } from "./graphToMarkovChain";
 import {findStationaryDistribution, type PagerankParams} from "./markovChain";
 
-/**
- * Represents raw PageRank distributions on a graph over time.
- */
-export type TimelineDistributions = $ReadOnlyArray<{|
+export type IntervalResult = {|
   // The interval for this slice
   +interval: Interval,
   // The total node weight within this interval (normalized to account for the
@@ -37,7 +34,11 @@ export type TimelineDistributions = $ReadOnlyArray<{|
   // The raw score distribution over nodes for this interval (i.e. sums to 1).
   // Uses the canonical graph node order.
   +distribution: Distribution,
-|}>;
+|};
+/**
+ * Represents raw PageRank distributions on a graph over time.
+ */
+export type TimelineDistributions = $ReadOnlyArray<IntervalResult>;
 
 export const SYNTHETIC_LOOP_WEIGHT = 1e-3;
 
@@ -206,6 +207,30 @@ export async function _computeTimelineDistribution(
     const nodeToConnections = NullUtil.get(
       nodeToConnectionsIterator.next().value
     );
+    const result = await _intervalResult(
+      nodeWeights,
+      nodeToConnections,
+      nodeOrder,
+      interval,
+      pi0,
+      alpha
+    );
+    results.push(result);
+    // Use the latest convergce results as the starting point for the next run
+    // of PageRank
+    pi0 = result.distribution;
+  }
+  return results;
+}
+
+export async function _intervalResult(
+  nodeWeights: Map<NodeAddressT, number>,
+  nodeToConnections: NodeToConnections,
+  nodeOrder: $ReadOnlyArray<NodeAddressT>,
+  interval: Interval,
+  pi0: Distribution | null,
+  alpha: number
+): Promise<IntervalResult> {
   const {chain} = createOrderedSparseMarkovChain(nodeToConnections);
 
   const seed = weightedDistribution(nodeOrder, nodeWeights);
@@ -220,14 +245,9 @@ export async function _computeTimelineDistribution(
     yieldAfterMs: 30,
   });
   const intervalWeight = sum(nodeWeights.values());
-    results.push({
+  return {
     interval,
     intervalWeight,
     distribution: distributionResult.pi,
-    });
-    // Use the latest convergce results as the starting point for the next run
-    // of PageRank
-    pi0 = distributionResult.pi;
-  }
-  return results;
+  };
 }

src/core/algorithm/timelinePagerank.test.js

@@ -2,12 +2,13 @@
 
 import {sum} from "d3-array";
 import * as NullUtil from "../../util/null";
-import {node, edge} from "../graphTestUtil";
+import {node, edge, advancedGraph} from "../graphTestUtil";
 import {Graph, type EdgeAddressT, type Edge} from "../graph";
 import {
   _timelineNodeWeights,
   _timelineNodeToConnections,
   SYNTHETIC_LOOP_WEIGHT,
+  _intervalResult,
 } from "./timelinePagerank";
 import {
   createConnections,
@@ -107,4 +108,67 @@ describe("src/core/algorithm/timelinePagerank", () => {
       expect(chains[3]).toEqual(chain4);
     });
   });
+
+  describe("_intervalResult", () => {
+    async function example() {
+      const {graph1, nodes} = advancedGraph();
+      const g = graph1();
+      const nodeWeights = new Map()
+        .set(nodes.src.address, 1)
+        .set(nodes.isolated.address, 2);
+      const edgeFn = (_unused_edge) => ({forwards: 1, backwards: 0.5});
+      const nodeToConnections = createConnections(g, edgeFn, 1e-3);
+      const nodeOrder = Array.from(g.nodes()).map((x) => x.address);
+      const interval = {endTimeMs: 1000, startTimeMs: 0};
+      const pi0 = null;
+      const alpha = 0.05;
+      const result = await _intervalResult(
+        nodeWeights,
+        nodeToConnections,
+        nodeOrder,
+        interval,
+        pi0,
+        alpha
+      );
+      return {
+        graph: g,
+        nodes,
+        nodeOrder,
+        nodeWeights,
+        edgeFn,
+        nodeToConnections,
+        interval,
+        pi0,
+        alpha,
+        result,
+      };
+    }
+    it("passes through the interval", async () => {
+      const {result, interval} = await example();
+      expect(result.interval).toEqual(interval);
+    });
+    it("computes the summed nodeWeight", async () => {
+      const {result, nodeWeights} = await example();
+      const actualIntervalWeight = sum(nodeWeights.values());
+      expect(result.intervalWeight).toEqual(actualIntervalWeight);
+    });
+    it("produces sane score distribution on an example graph", async () => {
+      const {result, nodes, nodeOrder} = await example();
+      function getScore(a) {
+        const idx = nodeOrder.indexOf(a.address);
+        if (idx === -1) {
+          throw new Error("bad address");
+        }
+        return result.distribution[idx];
+      }
+      const isoScore = getScore(nodes.isolated);
+      const srcScore = getScore(nodes.src);
+      const dstScore = getScore(nodes.dst);
+      expect(isoScore + srcScore + dstScore).toBeCloseTo(1);
+      // It has 2/3rd weight, and is isolated, so it's simple
+      expect(isoScore).toBeCloseTo(2 / 3);
+      // src has the weight, and dst doesnt, so it should have a higher score
+      expect(srcScore).toBeGreaterThan(dstScore);
+    });
+  });
 });