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machine.js

  // Available variables:
  // - Machine
  // - interpret
  // - assign
  // - send
  // - sendParent
  // - spawn
  // - raise
  // - actions
  // - XState (all XState exports)
  
  const createTicketQuantityMachine = (max = 19) => Machine({
    context: {count: 0},
    initial: 'idle',
    states: {
      idle: {
        on: {
          INCREMENT: {
            actions: 'increment',
            cond: 'canIncrement'
          },
          DECREMENT: {
            actions: 'decrement',
            cond: 'canDecrement'
          }
        }
      }
    }}, {
    actions: {
      increment: assign({
        count: (context) => context.count + 1
      }),
      decrement: assign({
        count: (context) => context.count - 1
      }),
    },
    guards: {
      canIncrement: (context) => context.count < max,
      canDecrement: (context) => context.count > 0,
    }
  });
  
  const BuyTicketsMachine = Machine({
    id: 'buyTickets',
    initial: 'quantitySelectable',
    context: {
      adult: 0,
      child: 0,
      senior: 0
    },
    states: {
      quantitySelectable: {
        invoke: [
          {
            id: 'adult',
            src: 'ticketQuantityMachine',
            data: {count: ({adult}) => adult},
            onDone: {
              actions: assign({
                adult: (context, event) => event.data.count
              })
            }
          },
          {
            id: 'child',
            src: 'ticketQuantityMachine',
            data: {count: ({child}) => child},
            onDone: {
              actions: assign({
                child: (context, event) => event.data.count
              })
            }
          },
          {
            id: 'senior',
            src: 'ticketQuantityMachine',
            data: {count: ({senior}) => senior},
            onDone: {
              actions: assign({
                senior: (context, event) => event.data.count
              })
            }
          },
        ],
        on: {
          INCREMENT: {
            actions: send('INCREMENT', {to: 'adult'}),
          },
          NEXT: {
            target: 'dateSelectable',
            cond: 'hasQuantity'
          }
        }
      },
      dateSelectable: {
        on: {
          NEXT: {
            target: 'timeSelectable',
            cond: 'hasDate'
          }
        }
      },
      timeSelectable: {
        on: {
          NEXT: {
            target: 'billingCollectable',
            cond: 'hasTime'
          }
        }
      },
      billingCollectable: {
        on: {
          NEXT: {
            target: 'billable',
            cond: 'hasBillingInfo'
          }
        }
      },
      billable: {},
      confirmed: {
        type: 'final'
      }
    }
  }, {
    guards: {
      hasQuantity: ({adult, child, senior}) => 
        adult || child || senior,
    },
    services: {
      ticketQuantityMachine: createTicketQuantityMachine
    }
  });