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machine.js

const G99_REQUIRED_THRESHOLD = 3.68;
const assignByValue = (context, event) => event.value
const updateExistingPv = assign({existingPv: assignByValue});
const updateProposedPv = assign({proposedPv: assignByValue});
const updateBatteryOutput = assign({batteryOutput: assignByValue});
const updateBatteryEnabled = assign({isBatteryEnabled: assignByValue});
const updateG99DecisionOverridden = assign({hasG99DecisionBeenOverridden: assignByValue});
const calcTotalOnSiteGeneration = assign({
  totalOnSiteGeneration: (context, event) => {
    let result = context.existingPv + context.proposedPv;
    if (context.isBatteryEnabled) {
      result += context.batteryOutput
    }
    return result
  }
})
const calcIsG99Required = assign({
  isG99Required: (context, event) => 
    context.totalOnSiteGeneration >=  G99_REQUIRED_THRESHOLD
})
const calcDecision = assign({
  decision: (context, event) => 
    context.hasG99DecisionBeenOverridden ? true : context.isG99Required
})
const logContext = (context, event) => {
  console.log(context)  
}

const tabG99Machine = Machine({
  id: 'tab_g99',
  initial: 'idle',
  context: {
    existingPv: 0,
    proposedPv: 0,
    batteryOutput: 0,
    isBatteryEnabled: false,
    totalOnSiteGeneration: 0,
    isG99Required: false,
    hasG99DecisionBeenOverridden: false,
    decision: false,
  },
  states: {
    'idle': {
      on: {
        EXISTING_PV: {
          target: 'calculating',
          actions: 'updateExistingPv'
        },
        PROPOSED_PV: {
          target: 'calculating',
          actions: 'updateProposedPv'
        },
        BATTERY: {
          target: 'calculating',
          actions: 'updateBatteryOutput'
        },
        BATTERY_ENABLED: {
          target: 'calculating',
          actions: 'updateBatteryEnabled'
        },
        OVERRIDE_DECISION: {
          target: 'calculating',
          actions: 'updateG99DecisionOverridden'
        }
      }
    },
    'calculating': {
      on: {
        '': { // Transient transition
          target: 'idle',
          actions: [
            'calcTotalOnSiteGeneration',
            'calcIsG99Required',
            'calcDecision',
            'logContext'
          ]
        }
      }
    },
    
  }
},
{
  actions: {
    updateExistingPv,
    updateProposedPv,
    updateBatteryOutput,
    updateBatteryEnabled,
    updateG99DecisionOverridden,
    calcTotalOnSiteGeneration,
    calcIsG99Required,
    calcDecision,
    logContext
  }
});