Determine Taker Price from Bid/Ask Order Book - php

Question

I am working on a bitcoin trading script in PHP. To run paper trading on live data, I have to determine buy/sell price from order book i.e taker price.
Order book live json data looks like this
Order book has two main arrays - bids & asks. Each bid/ask array has price [0], quantity [1] and the third parameter [2] is irrelevant:
Bid/Ask Sample Array
[0] => Array
(
[0] => 8848.99
[1] => 9.89850469
[2] => 7
)
[1] => Array
(
[0] => 8848.2
[1] => 0.05
[2] => 1
)
[2] => Array
(
[0] => 8848.02
[1] => 0.274203
[2] => 1
)
[3] => Array
(
[0] => 8848.01
[1] => 0.0012
[2] => 1
)
[4] => Array
(
[0] => 8847.47
[1] => 0.5
[2] => 1
)
[5] => Array
(
[0] => 8846.99
[1] => 0.28345
[2] => 1
)
[6] => Array
(
[0] => 8846.81
[1] => 0.75
[2] => 1
)
[7] => Array
(
[0] => 8846
[1] => 0.75181214
[2] => 2
)
[8] => Array
(
[0] => 8845.99
[1] => 26.57694043
[2] => 28
)
From the above data, how to calculate average price for 15 or n number of coins in PHP? Considering that orders will be filled/taken from top to bottom as available.

Answer 1

I solved it with this Excel formula:
Avg. Cost = sumproduct(price series, qty series) / sum(qty series)
Here is the PHP code:
$order_book = json_decode($order_book, true);
$bids = ($order_book['bids']);
$ordered = 15; //n number of coins
$filled = 0;
$fill_array = array();
foreach ($bids as $PriceQty) {
$price = $PriceQty[0];
$qty = $PriceQty[1];
if ($ordered != $filled){
$required = $ordered - $filled;
if ($qty >= $required){
$fill_array[] = array($price,$required);
$filled = $filled + $required;
break;
}else{
$filled = $filled + $qty;
$fill_array[] = array($price,$qty);
}
}
}
$totalQty = 0;
$totalSum = 0;
foreach ($fill_array as $PriceQty) {
$totalSum = $totalSum + ($PriceQty[0] * $PriceQty[1]);
$totalQty = $totalQty + $PriceQty[1];
}
echo "$totalSum/$totalQty = ".($totalSum/$totalQty);
There might be simpler way of doing this, someone could improve this answer. For now it produces expected average.