Google spreadsheet custom function settling up debts in group of people.

SETTLE_UP.js

function SETTLE_UP(data) {
  var min = 0;
  var max = 0;
  var debter = 0;
  var crediter = 0;
  var amount = 0;
  
  var names = data[0];
  var values = data[1];
  
  for(var i=0; i < values.length; i++){
    if (values[i] > max){
      max = values[i];
      debter = i;
    }
    if (values[i] < min){
      min = values[i];
      crediter = i;
    }
  }
  
  if (max * min == 0){
    return [];
  }
  
  min = Math.abs(min);
  if (max > min){
    amount = min;
  } else {
    amount = max;
  }
  
  values[crediter] += amount;
  values[debter] -= amount;
  var transactions = SETTLE_UP([names, values]);
  transactions.unshift([names[debter], names[crediter], amount]);
  return transactions;
}