Add "toFloatRatio" helper method (#1750)

Add a numerically-naive method for calculating the floating point ratio
between grain values, as it is needed in #1743.

Following discussion in [this review], we hope that @wchargin will
re-write this method later to have better precision.

Test plan: Attached unit tests should pass.

[this review]: https://github.com/sourcecred/sourcecred/pull/1715#discussion_r396909459

Paired with @hammadj

src/grain/grain.js

@@ -139,3 +139,15 @@ export function multiplyFloat(grain: Grain, num: number): Grain {
 export function fromApproximateFloat(f: number): Grain {
   return multiplyFloat(ONE, f);
 }
+
+/**
+ * Approximates the division of two grain values
+ *
+ * This naive implementation of grain division converts the given values
+ * to floats and performs simple floating point division.
+ *
+ * Do not assume this will be precise!
+ */
+export function toFloatRatio(numerator: Grain, denominator: Grain): number {
+  return Number(numerator) / Number(denominator);
+}

src/grain/grain.test.js

@@ -7,6 +7,7 @@ import {
   ZERO,
   fromApproximateFloat,
   multiplyFloat,
+  toFloatRatio,
 } from "./grain";
 
 describe("src/grain/grain", () => {
@@ -117,4 +118,32 @@ describe("src/grain/grain", () => {
       expect(fromApproximateFloat(0.1)).toEqual(ONE / 10n);
     });
   });
+
+  describe("toFloatRatio", () => {
+    it("handles a one-to-one ratio", () => {
+      expect(toFloatRatio(ONE, ONE)).toEqual(1);
+    });
+    it("handles a larger numerator", () => {
+      // $ExpectFlowError
+      expect(toFloatRatio(ONE * 2n, ONE)).toEqual(2);
+    });
+    it("handles fractional numbers", () => {
+      // $ExpectFlowError
+      expect(toFloatRatio(ONE * 5n, ONE * 2n)).toEqual(2.5);
+    });
+    it("calculates repeating decimal ratios", () => {
+      // $ExpectFlowError
+      expect(toFloatRatio(ONE * 5n, ONE * 3n)).toEqual(5 / 3);
+    });
+    it("approximates correctly when Grain values are not exactly equal", () => {
+      // $ExpectFlowError
+      const almostOne = ONE - 1n;
+      expect(toFloatRatio(ONE, almostOne)).toEqual(1);
+    });
+    it("handles irrational numbers", () => {
+      const bigPi = multiplyFloat(ONE, Math.PI);
+      // $ExpectFlowError
+      expect(toFloatRatio(bigPi, ONE * 2n)).toEqual(Math.PI / 2);
+    });
+  });
 });