add level/stage/step selectors

Author: ShMcK

web-app/package-lock.json

@@ -38,6 +38,7 @@
     "react": "^16.9.0",
     "react-dom": "^16.9.0",
     "react-scripts": "^3.1.1",
+    "reselect": "^4.0.0",
     "typescript": "^3.6.2",
     "xstate": "^4.6.7"
   },

web-app/package.json

@@ -0,0 +1,44 @@
+import {MachineContext} from 'typings'
+import * as G from 'typings/graphql'
+import {createSelector} from 'reselect'
+
+export const currentLevel = ({tutorial, position}: MachineContext): G.Level => {
+	if (!tutorial) {
+		throw new Error('Tutorial not found when selecting level')
+	}
+	// merge in the updated position
+	// sent with the test to ensure consistency
+	const levels: G.Level[] = tutorial.version.levels
+
+	const level: G.Level | undefined = levels.find((l: G.Level) => l.id === position.levelId)
+
+	if (!level) {
+		throw new Error('Level not found when selecting level')
+	}
+	return level
+}
+
+export const currentStage = (context: MachineContext): G.Stage => createSelector(
+	currentLevel,
+	(level: G.Level): G.Stage => {
+		const stages: G.Stage[] = level.stages
+		const stage: G.Stage | undefined = stages.find((s: G.Stage) => s.id === context.position.stageId)
+		if (!stage) {
+			throw new Error('No Stage found')
+		}
+		return stage
+	}
+)(context)
+
+export const currentStep = (context: MachineContext): G.Step => createSelector(
+	currentStage,
+	(stage: G.Stage): G.Step => {
+		const steps: G.Step[] = stage.steps
+		const step: G.Step | undefined = steps.find((s: G.Step) => s.id === context.position.stepId)
+		if (!step) {
+			throw new Error('No Step found')
+		}
+		return step
+	}
+)(context)
+