More flow coverage of geo submodule

(re: #3744)

also:
- Move types to index and export/import them
- add return types

modules/geo/vector.js

@@ -1,8 +1,8 @@
 // @flow
-type Vec2 = [number, number];
+import type { Vec2 } from '.';
 
 // vector equals
-export function geoVecEqual(a: Vec2, b: Vec2, epsilon?: number) {
+export function geoVecEqual(a: Vec2, b: Vec2, epsilon?: number): boolean {
     if (epsilon) {
         return (Math.abs(a[0] - b[0]) <= epsilon) && (Math.abs(a[1] - b[1]) <= epsilon);
     } else {
@@ -11,27 +11,27 @@ export function geoVecEqual(a: Vec2, b: Vec2, epsilon?: number) {
 }
 
 // vector addition
-export function geoVecAdd(a: Vec2, b: Vec2) {
+export function geoVecAdd(a: Vec2, b: Vec2): Vec2 {
     return [ a[0] + b[0], a[1] + b[1] ];
 }
 
 // vector subtraction
-export function geoVecSubtract(a: Vec2, b: Vec2) {
+export function geoVecSubtract(a: Vec2, b: Vec2): Vec2 {
     return [ a[0] - b[0], a[1] - b[1] ];
 }
 
 // vector scaling
-export function geoVecScale(a: Vec2, mag: number) {
+export function geoVecScale(a: Vec2, mag: number): Vec2 {
     return [ a[0] * mag, a[1] * mag ];
 }
 
 // vector rounding (was: geoRoundCoordinates)
-export function geoVecFloor(a: Vec2) {
+export function geoVecFloor(a: Vec2): Vec2 {
     return [ Math.floor(a[0]), Math.floor(a[1]) ];
 }
 
 // linear interpolation
-export function geoVecInterp(a: Vec2, b: Vec2, t: number) {
+export function geoVecInterp(a: Vec2, b: Vec2, t: number): Vec2 {
     return [
         a[0] + (b[0] - a[0]) * t,
         a[1] + (b[1] - a[1]) * t
@@ -39,7 +39,7 @@ export function geoVecInterp(a: Vec2, b: Vec2, t: number) {
 }
 
 // http://jsperf.com/id-dist-optimization
-export function geoVecLength(a: Vec2, b: Vec2) {
+export function geoVecLength(a: Vec2, b: Vec2): number {
     var x: number = a[0] - b[0];
     var y: number = a[1] - b[1];
     return Math.sqrt((x * x) + (y * y));
@@ -47,12 +47,12 @@ export function geoVecLength(a: Vec2, b: Vec2) {
 
 // Return the counterclockwise angle in the range (-pi, pi)
 // between the positive X axis and the line intersecting a and b.
-export function geoVecAngle(a: Vec2, b: Vec2) {
+export function geoVecAngle(a: Vec2, b: Vec2): number {
     return Math.atan2(b[1] - a[1], b[0] - a[0]);
 }
 
 // dot product
-export function geoVecDot(a: Vec2, b: Vec2, origin?: Vec2) {
+export function geoVecDot(a: Vec2, b: Vec2, origin?: Vec2): number {
     origin = origin || [0, 0];
     return (a[0] - origin[0]) * (b[0] - origin[0]) +
         (a[1] - origin[1]) * (b[1] - origin[1]);
@@ -61,7 +61,7 @@ export function geoVecDot(a: Vec2, b: Vec2, origin?: Vec2) {
 // 2D cross product of OA and OB vectors, returns magnitude of Z vector
 // Returns a positive value, if OAB makes a counter-clockwise turn,
 // negative for clockwise turn, and zero if the points are collinear.
-export function geoVecCross(a: Vec2, b: Vec2, origin?: Vec2) {
+export function geoVecCross(a: Vec2, b: Vec2, origin?: Vec2): number {
     origin = origin || [0, 0];
     return (a[0] - origin[0]) * (b[1] - origin[1]) -
         (a[1] - origin[1]) * (b[0] - origin[0]);