Projection onto the simplex

projection_simplex.py

"""
Implements three algorithms for projecting a vector onto the simplex: sort, pivot and bisection.

For details and references, see the following paper:

Large-scale Multiclass Support Vector Machine Training via Euclidean Projection onto the Simplex
Mathieu Blondel, Akinori Fujino, and Naonori Ueda.
ICPR 2014.
http://www.mblondel.org/publications/mblondel-icpr2014.pdf

"""

import numpy as np


def projection_simplex_sort(v, z=1):
    n_features = v.shape[0]
    u = np.sort(v)[::-1]
    cssv = np.cumsum(u) - z
    ind = np.arange(n_features) + 1
    cond = u - cssv / ind > 0
    rho = ind[cond][-1]
    theta = cssv[cond][-1] / float(rho)
    w = np.maximum(v - theta, 0)
    return w


def projection_simplex_pivot(v, z=1, random_state=None):
    rs = np.random.RandomState(random_state)
    n_features = len(v)
    U = np.arange(n_features)
    s = 0
    rho = 0
    while len(U) > 0:
        G = []
        L = []
        k = U[rs.randint(0, len(U))]
        ds = v[k]
        for j in U:
            if v[j] >= v[k]:
                if j != k:
                    ds += v[j]
                    G.append(j)
            elif v[j] < v[k]:
                L.append(j)
        drho = len(G) + 1
        if s + ds - (rho + drho) * v[k] < z:
            s += ds
            rho += drho
            U = L
        else:
            U = G
    theta = (s - z) / float(rho)
    return np.maximum(v - theta, 0)


def projection_simplex_bisection(v, z=1, tau=0.0001, max_iter=1000):
    lower = 0
    upper = np.max(v)
    current = np.inf

    for it in xrange(max_iter):
        if np.abs(current) / z < tau and current < 0:
            break

        theta = (upper + lower) / 2.0
        w = np.maximum(v - theta, 0)
        current = np.sum(w) - z
        if current <= 0:
            upper = theta
        else:
            lower = theta
    return w


if __name__ == '__main__':
    rs = np.random.RandomState(0)
    v = rs.rand(1000)
    z = np.sum(v) * 0.5
    print z

    w = projection_simplex_sort(v, z)
    print np.sum(w)

    w = projection_simplex_pivot(v, z)
    print np.sum(w)

    w = projection_simplex_bisection(v, z)
    print np.sum(w)