Balance calculation

recordTransaction.js

function recordTransaction(txn) {

    // NON FUNCTIONING CODE
    // Let 'friend' be the User's friend, the 'other' party in this Activity/Transaction

    // Record the origin
    var origin = friend.balance;

    // Calculate friend balance
    if(role == Activity.CREDITOR) {
        friend.balance += txn.amount;
    }
    else if(role == Activity.DEBTOR) {
        friend.balance -= txn.amount;
    }
    friend.balance = parseFloat(friend.balance.toFixed(2));

    // Calculate sum totals
    // Algorithm as follows:
    // Sum total (sumFriendsOwe/sumIOwe) is the cumulative representation of debt/credit
    // across all a User's friends. It is wrong to treat sumFriendsOwe/sumIOwe the same
    // way as a single friend's balance, ie. one friend's credit does not cancel out
    // another's debt. Each friend's credit/debit must be taken into account individually
    // first, via the '.balance' field, then its effect factored into the sum totals.
    //
    // (1) In calculating the sum total, we first look at the current friend's balance
    // and if the transaction is a credit or debit against him.
    // If the balance is in credit, ie. positive, and the transaction is a credit,
    // then we simply add the transaction amount towards the sum total, sumFriendsOwe.
    // The same applies in the opposite direction, ie. a debit against a negative balance
    // is simply added to sumIOwe.
    //
    // (2) The tricky bit arises when we cross zero, ie. a debt becomes credit and vice versa
    // When crossing zero, if the transaction amount is larger than the existing balance,
    // we need to first eliminate the balance on one side, then add the remainder to the other.
    //
    // Eg.
    // Transaction type: Credit
    // Transaction amount: 20
    // Balance: -10
    //
    // (A) Eliminate the balance: sumIOwe -= 10 (making balance zero)
    // (B) Add the remainder to the other side: sumFriendsOwe += (20-10) (making balance +10)
    //
    if(role == Activity.CREDITOR) {
        if(origin >= 0) { // See above: (1)
            this.sumFriendsOwe += txn.amount;
        }

        else if(origin < 0) {
            // Here we take into account the case where the difference provided by
            // txn.amount will cause a shift from credit to debt or vice versa.
            // We first eliminate excess credit/debt, then add the remainder to the
            // other side.
            if(txn.amount > Math.abs(origin)) { // See above: (2)
                this.sumFriendsOwe += txn.amount - Math.abs(origin);
                this.sumIOwe -= Math.abs(origin);
            }
            else {
                this.sumIOwe -= txn.amount;
            }
        }
    }
    else if(role == Activity.DEBTOR) {
        if(origin <= 0) { // See above: (1)
            this.sumIOwe += txn.amount;
        }

        else if(origin > 0) {
            // Here we take into account the case where the difference provided by
            // txn.amount will cause a shift from credit to debt or vice versa.
            // We first eliminate excess credit/debt, then add the remainder to the
            // other side.
            if(txn.amount > origin) { // See above: (2)
                this.sumFriendsOwe -= origin;
                this.sumIOwe += txn.amount - Math.abs(origin);
            }
            else {
                this.sumFriendsOwe -= txn.amount;
            }
        }
    }


    // Re-jigger the balances
    friend.balance = Math.round(friend.balance * 100) / 100;
    this.sumIOwe = Math.round(this.sumIOwe * 100) / 100;
    this.sumFriendsOwe = Math.round(this.sumFriendsOwe * 100) / 100;


}

voidTransaction.js

function voidTransaction() {

    // NON FUNCTIONING CODE
    // Let 'friend' be the User's friend, the 'other' party in this Activity/Transaction

    // Undo the transaction
    var origin = friend.balance;
    if(role == Activity.CREDITOR) {
        friend.balance -= txn.amount;

        if(origin <= 0) {
            this.sumIOwe += txn.amount;
        }
        else if(origin > 0) {
            if(txn.amount > Math.abs(origin)) {
                var balance = 0;
                if(this.sumFriendsOwe >= Math.abs(origin)) {
                    this.sumFriendsOwe -= Math.abs(origin);
                }
                else {
                    balance = Math.abs(origin) - this.sumFriendsOwe;
                }

                this.sumIOwe += (txn.amount - Math.abs(origin)) + balance;
            }
            else {
                this.sumFriendsOwe -= txn.amount;
            }
        }
    }
    else if(role == Activity.DEBTOR) {
        friend.balance += txn.amount;

        if(origin >= 0) {
            this.sumFriendsOwe += txn.amount;
        }
        else if(origin < 0) {
            if(txn.amount > Math.abs(origin)) {

                var balance = 0;
                if(this.sumIOwe >= Math.abs(origin)) {
                    this.sumIOwe -= Math.abs(origin);
                }
                else {
                    balance = Math.abs(origin) - this.sumIOwe;
                }

                this.sumFriendsOwe += (txn.amount - Math.abs(origin)) + balance;
            }
            else {
                this.sumIOwe -= txn.amount;
            }
        }
    }

    // Re-jigger the balances
    friend.balance = Math.round(friend.balance * 100) / 100;
    this.sumIOwe = Math.round(this.sumIOwe * 100) / 100;
    this.sumFriendsOwe = Math.round(this.sumFriendsOwe * 100) / 100;


}