Evaluate edge weight based on node and edge type (#486)

Summary:
PageRank wants an _edge evaluator_: a function mapping an edge to its
to-weight and fro-weight. This commit provides functions for creating
edge evaluators based on the high-level, coarse notions of node and edge
types. We use [the formulation described in a comment on #476][1].

Paired with @decentralion.

[1]: https://github.com/sourcecred/sourcecred/issues/476#issuecomment-402576435

Test Plan:
None. We will add tests later. We promise!

wchargin-branch: edge-weight-generators

src/app/credExplorer/edgeWeights.js

@@ -0,0 +1,56 @@
+// @flow
+import {
+  type Edge,
+  type EdgeAddressT,
+  type NodeAddressT,
+  EdgeAddress,
+  NodeAddress,
+} from "../../core/graph";
+import type {EdgeEvaluator} from "../../core/attribution/pagerank";
+
+export function byEdgeType(
+  prefixes: $ReadOnlyArray<{|
+    +prefix: EdgeAddressT,
+    +weight: number,
+    +directionality: number,
+  |}>
+): EdgeEvaluator {
+  return function evaluator(edge: Edge) {
+    const datum = prefixes.find(({prefix}) =>
+      EdgeAddress.hasPrefix(edge.address, prefix)
+    );
+    if (datum == null) {
+      throw new Error(EdgeAddress.toString(edge.address));
+    }
+    const {weight, directionality} = datum;
+    return {
+      toWeight: directionality * weight,
+      froWeight: (1 - directionality) * weight,
+    };
+  };
+}
+
+export function byNodeType(
+  base: EdgeEvaluator,
+  prefixes: $ReadOnlyArray<{|
+    +prefix: NodeAddressT,
+    +weight: number,
+  |}>
+): EdgeEvaluator {
+  return function evaluator(edge: Edge) {
+    const srcDatum = prefixes.find(({prefix}) =>
+      NodeAddress.hasPrefix(edge.src, prefix)
+    );
+    const dstDatum = prefixes.find(({prefix}) =>
+      NodeAddress.hasPrefix(edge.dst, prefix)
+    );
+    if (srcDatum == null || dstDatum == null) {
+      throw new Error(EdgeAddress.toString(edge.address));
+    }
+    const baseResult = base(edge);
+    return {
+      toWeight: dstDatum.weight * baseResult.toWeight,
+      froWeight: srcDatum.weight * baseResult.froWeight,
+    };
+  };
+}

src/core/attribution/pagerank.js

@@ -18,6 +18,9 @@ export type PagerankOptions = {|
   +maxIterations?: number,
 |};
 
+export type {EdgeWeight} from "./graphToMarkovChain";
+export type EdgeEvaluator = (Edge) => EdgeWeight;
+
 function defaultOptions(): PagerankOptions {
   return {
     verbose: false,
@@ -29,7 +32,7 @@ function defaultOptions(): PagerankOptions {
 
 export function pagerank(
   graph: Graph,
-  edgeWeight: (Edge) => EdgeWeight,
+  edgeWeight: EdgeEvaluator,
   options?: PagerankOptions
 ): PagerankResult {
   const fullOptions = {