Performance Issues with _distinct_but_small

[pl-fuchs]

Hi, I noticed that the current implementation of _distinct_but_small affects the performance of Allegro from allegro_jax.

The following replacement works better for me and avoids using lax.scan:

def _distinct_but_small(x: jax.Array):
    """Maps the entries of x into integers from 0 to n-1 denoting unique values."""

    shape = x.shape
    x = x.ravel()
    sorted_idx = jnp.argsort(x)

    # Each segment of equal numbers gets a unique index
    new_group = jnp.concat([jnp.zeros(1), jnp.diff(x[sorted_idx]) > 0], axis=0)
    group_idx = jnp.cumsum(new_group)

    # Assigns each entry of x to its corresponding unique element
    x = x.at[sorted_idx].set(group_idx)
    return x.reshape(shape)

Potentially, changing this snippet could also improve the performance in this issue: mariogeiger/allegro-jax#3