fix(main.ts): import correct functions from utils module

feat(stage.ts): implement stage navigation functionality
feat(stageNav.ts): implement stage navigation overlay functionality
feat(state.ts): implement state management for index and threshold
feat(utils.ts): add utility functions for increment and decrement

assets/ts/state.ts

@@ -0,0 +1,68 @@
+import { increment, decrement } from './utils'
+import { updateIndexText, updateThresholdText } from './nav'
+
+const thresholds = [
+  { threshold: 20, trailLength: 20 },
+  { threshold: 40, trailLength: 10 },
+  { threshold: 80, trailLength: 5 },
+  { threshold: 140, trailLength: 5 },
+  { threshold: 200, trailLength: 5 }
+]
+
+const defaultState = {
+  index: -1,
+  length: 0,
+  threshold: thresholds[2].threshold,
+  trailLength: thresholds[2].trailLength
+}
+
+export type State = typeof defaultState
+
+let state = defaultState
+
+export function getState(): State {
+  // return a copy of state
+  return Object.create(
+    Object.getPrototypeOf(state),
+    Object.getOwnPropertyDescriptors(state)
+  )
+}
+
+export function initState(length: number): void {
+  state.length = length
+}
+
+export function setIndex(index: number): void {
+  state.index = index
+  updateIndexText()
+}
+
+export function incIndex(): void {
+  state.index = increment(state.index, state.length)
+  updateIndexText()
+}
+
+export function decIndex(): void {
+  state.index = decrement(state.index, state.length)
+  updateIndexText()
+}
+
+export function incThreshold(): void {
+  state = updateThreshold(state, 1)
+  updateThresholdText()
+}
+
+export function decThreshold(): void {
+  state = updateThreshold(state, -1)
+  updateThresholdText()
+}
+
+// helper
+
+function updateThreshold(state: State, inc: number): State {
+  const i = thresholds.findIndex((t) => state.threshold === t.threshold)
+  const newItems = thresholds[i + inc]
+  // out of range
+  if (!newItems) return state
+  return { ...state, ...newItems }
+}