var coins = [ // ordered by value (high->low) to ease looping
  ["ref", 19, 1], // format: ["name", count, worth]
  ["rec", 1, 0.33],
  ["scrap", 4, 0.11]
];
function pickCoins(coins, target) {
  var picked = {};
  var remainder = target;

  // for every coin, starting with the most expensive...
  for (var i = 0; i < coins.length; ++i) {
    var name = coins[i][0],
        count = coins[i][1],
        worth = coins[i][2];
   picked[name] = picked[name] || 0;

    // ... ignore if we can't pick it ...
    if (picked[name] >= count) { continue; }

    // ... then check how many times we can pick it.
    // This is either limited by the amount of coins left, or the remainder
    var picks = Math.min(count - picked[name], Math.floor(remainder / worth));

    // Finally update stuff
    picked[name] = picked[name] + picks;
    remainder -= picks * worth;
  }

  return {coins: picked, remainder: remainder};
}