🐛 a spiral's radius now excludes the center

This is more consistent with a ring's radius.

docs/guide/traversing-grids.md

@@ -24,20 +24,20 @@ An example:
 ```typescript
 const Hex = defineHex({ dimensions: 30 })
 const grid = new Grid(Hex, rectangle({ width: 5, height: 5 }))
-const spiralTraverser = spiral({ start: [0, 2], radius: 2 })
+const spiralTraverser = spiral({ start: [0, 2], radius: 1 })
 
 grid.traverse(spiralTraverser)
 ```
 
 * Line 2: create a rectangular grid of 5⨉5 hexes.
-* Line 3: create a traverser with the built-in `spiral()` function (`spiral()` *returns* a traverser). The spiral starts at the hex with coordinates `[0, 2]` and runs outward until its radius of 2 hexes (including the center hex) is reached for a total of 7 hexes.
+* Line 3: create a traverser with the built-in `spiral()` function (`spiral()` *returns* a traverser). The spiral starts at the hex with coordinates `[0, 2]` and runs outward until its radius of 1 hex (excluding the center hex) is reached for a total of 7 hexes.
 * Line 5: [`grid.traverse()`](/api/classes/Grid#traverse) internally calls `spiralTraverser` and loops over the hexes it produces. Only the hexes that are present in `grid` are returned in a new grid. In this example all hexes produced by the traverser are also present in `grid`.
 
 This can be visualized like so:
 
 <TileGrid :grid="grid" :traversal="spiral1" />
 
-When the radius is increased from `2` to `3` it becomes apparent that only the hexes present in `grid` are traversed. You see that the traversal "jumps" from `[-2, 4]` to `[0, 0]`:
+When the radius is increased from `1` to `2` it becomes apparent that only the hexes present in `grid` are traversed. You see that the traversal "jumps" from `[-2, 4]` to `[0, 0]`:
 
 <TileGrid :grid="grid" :traversal="spiral2" />
 
@@ -294,9 +294,9 @@ import TileGrid from '../components/TileGrid.vue';
 
 const Hex = defineHex({ dimensions: 30 })
 const grid = new Grid(Hex, rectangle({ width: 5, height: 5 }))
-const spiral1 = grid.traverse(spiral({ start: [0, 2], radius: 2 }))
-const spiral2 = grid.traverse(spiral({ start: [0, 2], radius: 3 }))
-const spiral3 = grid.traverse(spiral({ start: [0, 2], radius: 3 }), { bail: true })
+const spiral1 = grid.traverse(spiral({ start: [0, 2], radius: 1 }))
+const spiral2 = grid.traverse(spiral({ start: [0, 2], radius: 2 }))
+const spiral3 = grid.traverse(spiral({ start: [0, 2], radius: 2 }), { bail: true })
 const squareOutline1 = grid.traverse([
   line({ direction: Direction.E, length: 4 }),
   line({ direction: Direction.S, length: 3 }),

src/grid/traversers/spiral.test.ts

@@ -9,7 +9,7 @@ const createHex = vi.fn((coordinates?: HexCoordinates) => new Hex(coordinates))
 describe('when called with a radius', () => {
   describe('without cursor', () => {
     test('returns a traverser that returns hexes in a spiral starting at [0, 0]', () => {
-      expect(spiral({ radius: 2 })(createHex)).toMatchInlineSnapshot(`
+      expect(spiral({ radius: 1 })(createHex)).toMatchInlineSnapshot(`
         [
           Hex {
             "q": 0,
@@ -81,7 +81,7 @@ describe('when called with a radius', () => {
 describe('when called with a radius and start', () => {
   describe('without cursor', () => {
     test('returns a traverser that returns hexes in a spiral starting at start', () => {
-      expect(spiral({ radius: 2, start: [1, 0] })(createHex)).toMatchInlineSnapshot(`
+      expect(spiral({ radius: 1, start: [1, 0] })(createHex)).toMatchInlineSnapshot(`
         [
           Hex {
             "q": 1,
@@ -118,7 +118,7 @@ describe('when called with a radius and start', () => {
 
   describe('with cursor', () => {
     test('returns a traverser that returns hexes in a spiral starting at start', () => {
-      expect(spiral({ radius: 2, start: [1, 1] })(createHex, cursor)).toMatchInlineSnapshot(`
+      expect(spiral({ radius: 1, start: [1, 1] })(createHex, cursor)).toMatchInlineSnapshot(`
         [
           Hex {
             "q": 1,
@@ -156,7 +156,7 @@ describe('when called with a radius and start', () => {
 
 describe('when called with a radius and rotation', () => {
   test('returns a traverser that returns hexes in a spiral with the given rotation', () => {
-    expect(spiral({ radius: 2, rotation: Rotation.COUNTERCLOCKWISE })(createHex)).toMatchInlineSnapshot(`
+    expect(spiral({ radius: 1, rotation: Rotation.COUNTERCLOCKWISE })(createHex)).toMatchInlineSnapshot(`
       [
         Hex {
           "q": 0,

src/grid/traversers/spiral.ts

@@ -10,10 +10,10 @@ import { ring } from './ring'
 export function spiral<T extends Hex>({ radius, start, rotation }: SpiralOptions): Traverser<T> {
   return function spiralTraverser(createHex, cursor) {
     const center = createHex(start ?? cursor)
-    return repeatWith<T>(line({ start, direction: Direction.N, length: radius }), ring({ center, rotation }))(
-      createHex,
-      cursor,
-    )
+    // radius excludes the center, so 1 is added to radius
+    // only when there's a cursor but no start, radius can be used as-is, because then line() already increases its length by 1
+    const length = !start && cursor ? radius : radius + 1
+    return repeatWith<T>(line({ start, direction: Direction.N, length }), ring({ center, rotation }))(createHex, cursor)
   }
 }