🐛 Fix slice explicit 0 index (#346)

* ✅ Add a test

* 🐛 fix #345 slice nav explicit 0 index

* ♻️ Rename resolveNegativeIndex to resolveIndex

packages/immutadot/src/nav/sliceNav.js

@@ -1,7 +1,7 @@
 import { ArrayNav } from './arrayNav'
 import { isNil } from 'util/lang'
 
-const resolveNegativeIndex = (index, length) => index > 0 ? index : Math.max(length + index, 0)
+const resolveIndex = (index, length) => index >= 0 ? index : Math.max(length + index, 0)
 
 class SliceNav extends ArrayNav {
   constructor(value, params, next) {
@@ -12,14 +12,14 @@ class SliceNav extends ArrayNav {
   get start() {
     const { length, params: [start] } = this
     if (length === 0) return 0
-    if (start !== undefined) return resolveNegativeIndex(start, length)
+    if (start !== undefined) return resolveIndex(start, length)
     return this.step > 0 ? 0 : length - 1
   }
 
   get end() {
     const { length, params: [, end] } = this
     if (length === 0) return 0
-    if (end !== undefined) return resolveNegativeIndex(end, length)
+    if (end !== undefined) return resolveIndex(end, length)
     return this.step > 0 ? length : -1
   }
 

packages/immutadot/src/nav/sliceNav.spec.js

@@ -0,0 +1,23 @@
+/* eslint-env jest */
+import { sliceNav } from './sliceNav'
+
+describe('SliceNav', () => {
+
+  it('should update a slice', () => {
+    const input = [0, 1]
+    const next = value => ({
+      update: updater => updater(value),
+    })
+    const updater = value => value + 1
+
+    const test = (params, expected) => {
+      const output = sliceNav(params, next)(input).update(updater)
+
+      expect(input).toEqual([0, 1])
+      expect(output).not.toBe(input)
+      expect(output).toEqual(expected)
+    }
+
+    test([0, undefined], [1, 2])
+  })
+})