bi = np.floor(np.arange(n) / batch_size).astype(np.int)

This line of code creates an array 'bi' that assigns a batch index to each element in a range of numbers from 0 to n-1.

The first part of the line 'np.arange(n)' creates an array of numbers from 0 to n-1.

The second part of the line '/ batch_size' divides each element in the array by the batch size. This performs integer division, so any remainder is discarded.

The final part of the line 'np.floor(...).astype(np.int)' takes the floor of each element (rounds down to the nearest integer) and converts the resulting array to an integer data type. This creates an array 'bi' where each element corresponds to the batch index of the original array of numbers.

For example, if n=10 and batch_size=3, the 'bi' array would be [0, 0, 0, 1, 1, 1, 2, 2, 2, 3]. This assigns the first three numbers to batch 0, the next three numbers to batch 1, and so on.