Factor TimelineCredParameters into new module (#1375)

The `timelineCred.js` file is a bit of a beast. One way to start
slimming it down is to pull the parameters into their own file. This is
especially helpful as I'm planning a followon PR that will colocate the
default parameter values with their declaration.

The naming of everything in the `/timeline/` subdirectory is a bit
wonky: it reflects that at the time of creation, "Timeline" designated
an experimental version of SourceCred. Now, it is becoming canonical,
but the cumbersome naming persists. I haven't made any effort to tackle
the name debt here.

Test plan: `yarn test` passes; since this is merely a code
reorganization, this give me great confidence that the change is
correct. I also added a few small tests to the new module. Although the
behavior in question is already tested, I think setting up test files
liberally is a good practice, as the existence of the test file invites
the creation of more tests.

src/analysis/timeline/params.js

@@ -0,0 +1,58 @@
+// @flow
+
+import {
+  type Weights,
+  type WeightsJSON,
+  toJSON as weightsToJSON,
+  fromJSON as weightsFromJSON,
+} from "../weights";
+
+/**
+ * Parameters for computing TimelineCred
+ *
+ * The parameters are intended to be user-configurable.
+ */
+export type TimelineCredParameters = {|
+  // Determines how quickly cred returns to the PageRank seed vector. If alpha
+  // is high, then cred will tend to "stick" to nodes that are seeded, e.g.
+  // issues and pull requests. Alpha should be between 0 and 1.
+  +alpha: number,
+  // Determines how quickly cred decays. The decay is 1, then cred never
+  // decays, and old nodes and edges will retain full weight forever. (This
+  // would result in cred that is highly biased towards old contributions, as
+  // they would continue earning cred in every timeslice, forever.) If the
+  // decay is 0, then weights go to zero the first week after their node/edge
+  // was created. Should be between 0 and 1.
+  +intervalDecay: number,
+  // The weights. This determines how much cred is assigned based on different
+  // node types, how cred flows across various edge types, and can specify
+  // manual weights directly on individual nodes. See the docs in
+  // `analysis/weights` for details.
+  +weights: Weights,
+|};
+
+export type TimelineCredParametersJSON = {|
+  +alpha: number,
+  +intervalDecay: number,
+  +weights: WeightsJSON,
+|};
+
+export function paramsToJSON(
+  p: TimelineCredParameters
+): TimelineCredParametersJSON {
+  return {
+    alpha: p.alpha,
+    intervalDecay: p.intervalDecay,
+    weights: weightsToJSON(p.weights),
+  };
+}
+
+export function paramsFromJSON(
+  p: TimelineCredParametersJSON
+): TimelineCredParameters {
+  return {
+    alpha: p.alpha,
+    intervalDecay: p.intervalDecay,
+    weights: weightsFromJSON(p.weights),
+  };
+}

src/analysis/timeline/params.test.js

@@ -0,0 +1,19 @@
+// @flow
+
+import {paramsToJSON, paramsFromJSON} from "./params";
+import {defaultWeights} from "../weights";
+import {NodeAddress} from "../../core/graph";
+
+describe("analysis/timeline/params", () => {
+  it("JSON round trip", () => {
+    const weights = defaultWeights();
+    // Ensure it works with non-default weights
+    weights.nodeManualWeights.set(NodeAddress.empty, 33);
+    const p = {alpha: 0.5, intervalDecay: 0.5, weights};
+    const j = paramsToJSON(p);
+    const p_ = paramsFromJSON(j);
+    const j_ = paramsToJSON(p_);
+    expect(j).toEqual(j_);
+    expect(p).toEqual(p_);
+  });
+});

src/analysis/timeline/timelineCred.js

@@ -17,13 +17,14 @@ import {
   type Node,
 } from "../../core/graph";
 import {
-  type Weights,
+  type TimelineCredParameters,
-  type WeightsJSON,
+  paramsToJSON,
-  toJSON as weightsToJSON,
+  paramsFromJSON,
-  fromJSON as weightsFromJSON,
+  type TimelineCredParametersJSON,
-} from "../weights";
+} from "./params";
 
 export type {Interval} from "./interval";
+export type {TimelineCredParameters} from "./params";
 
 /**
  * A Graph Node wrapped with cred information.
@@ -37,30 +38,6 @@ export type CredNode = {|
   +cred: $ReadOnlyArray<number>,
 |};
 
-/**
- * Parameters for computing TimelineCred
- *
- * The parameters are intended to be user-configurable.
- */
-export type TimelineCredParameters = {|
-  // Determines how quickly cred returns to the PageRank seed vector. If alpha
-  // is high, then cred will tend to "stick" to nodes that are seeded, e.g.
-  // issues and pull requests. Alpha should be between 0 and 1.
-  +alpha: number,
-  // Determines how quickly cred decays. The decay is 1, then cred never
-  // decays, and old nodes and edges will retain full weight forever. (This
-  // would result in cred that is highly biased towards old contributions, as
-  // they would continue earning cred in every timeslice, forever.) If the
-  // decay is 0, then weights go to zero the first week after their node/edge
-  // was created. Should be between 0 and 1.
-  +intervalDecay: number,
-  // The weights. This determines how much cred is assigned based on different
-  // node types, how cred flows across various edge types, and can specify
-  // manual weights directly on individual nodes. See the docs in
-  // `analysis/weights` for details.
-  +weights: Weights,
-|};
-
 /**
  * Represents the timeline cred of a graph. This class wraps all the data
  * needed to analyze and interpet cred (ie. it has the Graph and the cred
@@ -287,30 +264,8 @@ const COMPAT_INFO = {type: "sourcecred/timelineCred", version: "0.5.0"};
 
 export opaque type TimelineCredJSON = Compatible<{|
   +graphJSON: GraphJSON,
-  +paramsJSON: ParamsJSON,
+  +paramsJSON: TimelineCredParametersJSON,
   +pluginsJSON: $ReadOnlyArray<PluginDeclaration>,
   +credJSON: {[string]: $ReadOnlyArray<number>},
   +intervalsJSON: $ReadOnlyArray<Interval>,
 |}>;
-
-type ParamsJSON = {|
-  +alpha: number,
-  +intervalDecay: number,
-  +weights: WeightsJSON,
-|};
-
-function paramsToJSON(p: TimelineCredParameters): ParamsJSON {
-  return {
-    alpha: p.alpha,
-    intervalDecay: p.intervalDecay,
-    weights: weightsToJSON(p.weights),
-  };
-}
-
-function paramsFromJSON(p: ParamsJSON): TimelineCredParameters {
-  return {
-    alpha: p.alpha,
-    intervalDecay: p.intervalDecay,
-    weights: weightsFromJSON(p.weights),
-  };
-}