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SparseFHT Code repo

This is the code for SparseFHT algorithm as presented in the following papers. The code is hosted on github.

[Long] R. Scheibler, S.Haghighatshoar, and M. Vetterli, A Fast Hadamard Transform for Signals with Sub-linear Sparsity in the Transform Domain, IEEE Trans. Inf. Theory, vol. 61, 2015.

[Short] R. Scheibler, S. Haghighatshoar, and M. Vetterli, A Fast Hadamard Transform for Signals with Sub-linear Sparsity, Allerton Conference on Communication, Control and Computing, 2013.


In this paper, we design a new iterative low-complexity algorithm for computing the Walsh-Hadamard transform (WHT) of an N dimensional signal with a K-sparse WHT. We suppose that N is a power of two and K = O(N^α), scales sub-linearly in N for some α ∈ (0,1). Assuming a random support model for the nonzero transform-domain components, our algorithm reconstructs the WHT of the signal with a sample complexity O(K log_(N/K)) and a computational complexity O(K log_2(K)log_2(N/K)). Moreover, the algorithm succeeds with a high probability approaching 1 for large dimension N.

Our approach is mainly based on the subsampling (aliasing) property of the WHT, where by a carefully designed subsampling of the time-domain signal, a suitable aliasing pattern is induced in the transform domain. We treat the resulting aliasing patterns as parity-check constraints and represent them by a bipartite graph. We analyze the properties of the resulting bipartite graphs and borrow ideas from codes defined over sparse bipartite graphs to formulate the recovery of the nonzero spectral values as a peeling decoding algorithm for a specific sparse-graph code transmitted over a binary erasure channel (BEC). This enables us to use tools from coding theory (belief-propagation analysis) to characterize the asymptotic performance of our algorithm in the very sparse (α ∈ (0,1/3]) and the less sparse (α ∈ (1/3,1)) regime. Comprehensive simulation results are provided to assess the empirical performance of the proposed algorithm.


Robin Scheibler (email) (homepage)

Please do not hesitate to contact me for help and support! I would be happy to help you run the code.


The code has been tested on Mac OS X 10.7, 10.8, 10.9, and on Ubuntu linux.

Python and Matlab mex wrappers were used for the code generating the figures in the paper. The core of the algorithm is implemented in C.

The code needs a C99 compatible compiler, which seems not to be the case for the windows version of Matlab currently. There seems to be some workaround, one possibility being to rename all the .c files into .cpp and use a C++ compiler. All of this is untested as of now.

Run the code in Python

A python wrapper for the C code was written and is available in python/pysparsefht. The wrapper must be compiled as follows

cd python/pysparsefht
python build_ext --inplace


The code relies on numpy, scipy, matplotlib, seaborn, pandas, and ipyparallel for the parallel simulation. It is also possible to run all the code serially, but this would take a long time.

Reproduce the figures from the paper

A script is provided to reproduce all the figures in the paper. Assuming the python module has been built as explained above, do the following.

cd python/

And that's it pretty much. The simulation scripts are suffixed with _sim and produce a data file stored in data that can be reused later to generate the figure using a second script suffixed with _plot. The figures will be stored in the figures folder.

The scripts has a few options for testing and number of cores used.

./ [OPTS]
  -t    Runs a single loop only for test purpose
  -s    Runs all the code in a simple for loop. No parallelism
  -n x  Runs the loops in parallel using x workers. This option is ignored if -s is used

The number of workers is in general set to the number of threads available minus one. That is twice the number of cores, minus one.

Running ./ -t -n 7 on an Intel Core i7 2.8 GHz took 5 minutes.

Pysparsefht module

A user-friendly python module was written around the C code and can be used to run the FHT and SparseFHT.

import numpy as np
import pysparsefht

# Create a sparse Hadamard domain vector
y = np.zeros(512)
y[[13,72,121, 384]] = [12., -123., 91.5, -37.]

# We can generate the dense time domain by
# applying conventional FHT
x = pysparsefht.fht(y)

# Then, we use the sparse FHT to get back the
# original sparse vector
# y_val, y_loc contain the magnitude and locations, respectively, of the sparse signal
y_val, y_loc = pysparsefht.sparse_fht(xs, 4)

# The output of the transform is not normalize,
# so we need to divide by the square root of the length
y_hat = np.zeros(512)
y_hat[y_loc] = y_val / np.sqrt(512)

# says True
np.allclose(y_hat, y)

The docstring for sparse_fht:

Signature: pysparsefht.sparse_fht(x, K, B, C, max_iter=20, algo=3, req_loops=False, req_unsat=False, seed=0)
Wrapper for the Sparse Fast Hadamard Transform

x: ndarray (1D or 2D)
    Input vector, the size should be a power of two.
    K: int
      The sparsity expected
    B: int
      The number of buckets
    C: int
      The oversampling factor
    max_iter: int, optional
      The maximum number of iterations of decoder
    algo: int, optional
      The variant of the algorithm to use:
        * ALGO_RANDOM : Uses random hash functions
        * ALGO_DETERMINISTIC: Uses deterministic hash functions
        * ALGO_OPTIMIZED: Uses deterministic hash functions with optimized implementation (default)
    req_loops: bool, optional
      Requests to return the number of loops executed by the decoded
    req_unsat: bool, optional
      Requests to return the number of unsatisfied check nodes when the algorithm terminates
    seed: unsigned int, optional
      A seed for the random number generator to pass to the C code

y: ndarray (1D or 2D)
  The output vector of magnitudes (size K)
support: ndarray (1D or 2D)
  The output vector of locations of non-zero coefficients (size K)
unsat: int or ndarray
  The number of unsatisfied checks (if req_unsat == True)
loops: int or ndarray
  The number of loops run by decoder (if req_loops == True)

The docstring for fht:

Signature: pysparsefht.fht(x)
Fast Hadamard Transform

This is a wrapper that calls a C implementation of the fast hadamard transform

If x is a 2D array, the transform operates on the rows.

The length of the transform should be a power of two.

x: ndarray (1D or 2D)
    The data to transform

    A new ndarray containing the Hadamard transform of x

Run the code in Matlab

Reproduce the figures from the paper

To reproduce the figures from the paper, type in the following in a matlab shell:

cd <path_to_SparseFHT>/matlab/


  1. The simulation is fairly time-consuming.
  2. To speed-up the simulation, the parallel toolbox was used. If you do not have the parallel toolbox, replace the parfor instructions by for in all the Sim scripts.

Mex files info

  • SparseFHT The fast sparse Hadamard transform algorithm.

       [Y, S, U, I] = SparseFHT(X, K, B, C, L, T)
       Wrapper for the Sparse Fast Hadamard Transform
       Input arguments:
       X: input vector  (size n)
       K: the sparsity (and size of y)
       B: number of buckets
       C: oversampling factor
       L: maximum number of iterations of decoder
       T: Type of algorithm to use ('Random' / 'Deterministic' / 'Optimized')
       Output arguments: 
       Y: output vector (size K)
       S: support vector (size K)
       U: the number of unsatisfied checks (optional)
       I: the number of loops run (optional)
  • FastHadamard A straighforward implementation of the conventional fast Hadamard transform. No scaling factor is applied, i.e. applying the algorithm twice will result in the input vector weighted by its length.

      [Y] = FastHadamard(X)
      Input arguments:
      X: input vector (size has to be a power of two)
      Output arguments:
      Y: output vector, the Hadamard transform of X.
  • HadamardBenchmark Calls a C routine performing a timing comparison of SparseFHT and FastHadamard.

      [Tfht Tsfht] = HadamardBenchmark(N, K, B, C, L, R, SEED)
      Input arguments:
      N: transform size to investigate (scalar, power of two)
      K: sparsity parameter, number of non-zero tranform domain coefficients (vector, power of two)
      B: number of buckets to use in SparseFHT (vector, same size as K, power of two)
      C: oversampling factor
      L: maximum number of iterations of decoder
      R: a length 4 vector containing the following parameters
          1. Number of repetitions of one measurement
          2. Number of warm-up run 
          3. Number of iterations for one measurement
          4. Maximum magnitude of non-zero components in the sparse signal
      SEED: A seed for the C random number generator
      Output arguments:
      Tfht: The runtime measurement of FastHadamard
      Tsfht: An array containing the runtime measurement of SparseFHT for every value of K

Reuse the C code

The makefile in ./C folder will compile all the code as well as a number of example/test files. This can be used as a basis to reuse the C code directly.

cd ../C
make all

The libraries included are:

  • SparseFHT,
  • Fast Hadamard tranform,
  • Fast linear algebra in GF2 (boolean matrices and vectors algebra),
  • Test files.


2013-2015 (c) Robin Scheibler, Saeid Haghighatshoar, Martin, Vetterli.

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. To view a copy of this license, visit

The code is free to reuse for non-commercial and academic purposes. However, please acknowledge its use with the following citation.

   author      = {Scheibler, Robin and Haghighatshoar, Saeid and Vetterli, Martin},
   title       = {A {F}ast {H}adamard {T}ransform for {S}ignals with
                 {S}ub-linear {S}parsity in the {T}ransform {D}omain},
   journal     = {IEEE Trans. Inf. Theory}
   volume      = 61,
   year        = 2015,
   ee          = {}

For any other purposes, please contact the authors.


A Fast Hadamard Transform for Signals with Sub-linear Sparsity in the Transform Domain -- Implementation



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