'''
write a python code of ISTA for compressed sensing
can you give me program that will display prime numbers less than or equal to 100?
'''

import numpy as np

def ISTA(y, A, lambda_val, num_iters):
    """
    Implements the ISTA algorithm for compressed sensing

    Args:
    y: compressed measurement vector
    A: sensing matrix
    lambda_val: regularization parameter
    num_iters: number of iterations for the algorithm

    Returns:
    x_hat: recovered signal
    """
    # initialize x
    x = np.zeros(A.shape[1])
    # calculate step size
    L = np.linalg.norm(A, ord=2)**2
    gamma = 1/L
    # iterate
    for i in range(num_iters):
        x = soft_threshold(x + gamma*A.T.dot(y - A.dot(x)), lambda_val*gamma)
    x_hat = x
    return x_hat

def soft_threshold(x, thresh):
    """
    Soft thresholding function

    Args:
    x: input vector
    thresh: threshold value

    Returns:
    output: soft thresholded vector
    """
    output = np.sign(x) * np.maximum(np.abs(x) - thresh, 0)
    return output