Add serialization to TimelineCredScores (#1656)

* Add serialization to TimelineCredScores

Right now the serialization is super trivial, we just attach a compat
header. In the future, we can try encoding the float64 arrays as
bytestrings to save space.

Test plan: Unit tests included; `yarn test` passes.

* Fix up serialization per @Beanow's review

src/core/algorithm/distributionToCred.js

@@ -5,6 +5,7 @@
  */
 
 import {sum} from "d3-array";
+import {toCompat, fromCompat, type Compatible} from "../../util/compat";
 import {type Interval} from "../interval";
 import {type TimelineDistributions} from "./timelinePagerank";
 import {NodeAddress, type NodeAddressT} from "../../core/graph";
@@ -81,3 +82,28 @@ export function distributionToCred(
   });
   return {intervalCredScores, intervals};
 }
+
+const COMPAT_INFO = {type: "sourcecred/timelineCredScores", version: "0.1.0"};
+
+export type TimelineCredScoresJSON = Compatible<{|
+  +intervals: $ReadOnlyArray<Interval>,
+  // TODO: Serializing floats as strings is space-inefficient. We can likely
+  // get space savings if we base64 encode a byte representation of the
+  // floats.
+  +intervalCredScores: $ReadOnlyArray<$ReadOnlyArray<number>>,
+|}>;
+
+export function toJSON(s: TimelineCredScores): TimelineCredScoresJSON {
+  return toCompat(COMPAT_INFO, {
+    intervals: s.intervals,
+    intervalCredScores: s.intervalCredScores.map((x) => Array.from(x)),
+  });
+}
+
+export function fromJSON(j: TimelineCredScoresJSON): TimelineCredScores {
+  const {intervals, intervalCredScores} = fromCompat(COMPAT_INFO, j);
+  return {
+    intervals,
+    intervalCredScores: intervalCredScores.map((x) => new Float64Array(x)),
+  };
+}

src/core/algorithm/distributionToCred.test.js

@@ -1,7 +1,7 @@
 // @flow
 
 import {NodeAddress} from "../graph";
-import {distributionToCred} from "./distributionToCred";
+import {distributionToCred, toJSON, fromJSON} from "./distributionToCred";
 
 describe("src/core/algorithm/distributionToCred", () => {
   const na = (...parts) => NodeAddress.fromParts(parts);
@@ -128,4 +128,59 @@ describe("src/core/algorithm/distributionToCred", () => {
       });
     });
   });
+  describe("to/from JSON", () => {
+    const exampleCred = () => {
+      const ds = [
+        {
+          interval: {startTimeMs: 0, endTimeMs: 10},
+          intervalWeight: 2,
+          distribution: new Float64Array([0.5, 0.5]),
+        },
+        {
+          interval: {startTimeMs: 10, endTimeMs: 20},
+          intervalWeight: 10,
+          distribution: new Float64Array([0.9, 0.1]),
+        },
+      ];
+      const nodeOrder = [na("foo"), na("bar")];
+      return distributionToCred(ds, nodeOrder, [na("bar")]);
+    };
+    it("satisfies round-trip equality", () => {
+      const json = toJSON(exampleCred());
+      const result = fromJSON(json);
+      expect(result).toEqual(exampleCred());
+    });
+    it("snapshots as expected", () => {
+      expect(toJSON(exampleCred())).toMatchInlineSnapshot(`
+        Array [
+          Object {
+            "type": "sourcecred/timelineCredScores",
+            "version": "0.1.0",
+          },
+          Object {
+            "intervalCredScores": Array [
+              Array [
+                2,
+                2,
+              ],
+              Array [
+                90,
+                10,
+              ],
+            ],
+            "intervals": Array [
+              Object {
+                "endTimeMs": 10,
+                "startTimeMs": 0,
+              },
+              Object {
+                "endTimeMs": 20,
+                "startTimeMs": 10,
+              },
+            ],
+          },
+        ]
+      `);
+    });
+  });
 });