Partage des frais

frais.ts

interface Friend {
  id: number;
  name: string;
  expense: number;
}

interface Transaction {
  from: number;
  to: number;
  value: number;
}

const friends: Array<Friend> = [
  { id: 1, name: "paul", expense: 13 },
  { id: 2, name: "marc", expense: 21 },
  { id: 3, name: "marie", expense: 30 },
  { id: 4, name: "jane", expense: 15 },
  { id: 5, name: "cresus", expense: 77 }
];

function averageExpense(friends: Array<Friend>): number {
  return friends.reduce(
    (acc: number, friend: Friend) => acc + friend.expense / friends.length,
    0
  );
}

function addExpense(friend: Friend, expense: number): Friend {
  return { ...friend, expense: friend.expense + expense };
}

function groupById(friends: Array<Friend>): { [id: number]: Friend } {
  return friends.reduce(
    (acc, friend) => ({
      ...acc,
      [friend.id]: friend
    }),
    {}
  );
}

function friendTransactions(
  friend: Friend,
  friends: { [id: number]: Friend },
  avg: number
): { transactions: Array<Transaction>; friends: { [id: number]: Friend } } {
  let due = avg - friend.expense;
  if (due <= 0) return { transactions: [], friends };
  let transactions = [];

  friends = Object.keys(friends)
    .map(id => parseInt(id, 10))
    .reduce((acc, fid) => {
      const value = Math.min(
        avg - acc[friend.id].expense,
        acc[fid].expense - avg
      );
      if (acc[fid].expense < avg || value === 0) return acc;
      transactions = [...transactions, { from: friend.id, to: fid, value }];

      return {
        ...acc,
        [fid]: addExpense(acc[fid], -value),
        [friend.id]: addExpense(acc[friend.id], value)
      };
    }, friends);

  return { transactions, friends };
}

function transactions(friends: Array<Friend>): Array<Transaction> {
  let transactions = [];
  const avg = averageExpense(friends);
  let fdb = groupById(friends);

  friends.forEach((friend: Friend) => {
    let tf = friendTransactions(friend, fdb, avg);
    fdb = tf["friends"];
    transactions = [...transactions, ...tf["transactions"]];
  });

  return transactions;
}

console.log(friends, transactions(friends));