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machine.js

const fetchMachine = Machine({
  id: "fetch",
  initial: "idle",
  context: {
    // constants
    reproductiveNumber: 0.4,
    deathRate: 0.15,
    startingPopulation: 1000,
    judgmentDay: 7,
    
    // the current numbers
    day: 1,
    population: 990,
    neverContracted: 990,
    contractedAll: 10,
    recoveredToday: 0,
    diedAll: 0,
    diedToday: 0,
    contractedHistory: [10],
  },
  states: {
    idle: {
      on: {
        NEXT: {
          internal: true,
          actions: assign((ctx) => {
            let newDied = 0;
            let recovered = 0;
            const day = ctx.day + 1;
            
            if (day >= 7) {
              const contracted7DaysAgo = ctx.contractedHistory[ctx.day-ctx.judgmentDay+1] || 0;
              newDied = Math.round(contracted7DaysAgo * ctx.deathRate);
              recovered = contracted7DaysAgo - newDied;
            }
            
            const diedYesterday = ctx.diedToday;
            const recoveredYesterday = ctx.recoveredToday;
            const contractedAllYesterday = ctx.contractedAll;
            const newContracted =  Math.round((contractedAllYesterday - recoveredYesterday - diedYesterday)*ctx.reproductiveNumber*ctx.neverContracted/ctx.startingPopulation);
            
            const contractedHistory = [...ctx.contractedHistory, newContracted];
            
            return {
              population: ctx.population - newDied,
              contractedAll: newContracted + contractedAllYesterday - recovered - newDied,
              diedAll: ctx.diedAll + newDied,
              diedToday: newDied,
              neverContracted: ctx.neverContracted - newContracted,
              day,
              contractedHistory,
              recoveredToday: recovered,
            };
          }),
        },
      },
    },
  },
});