BalancedConditionalProbabilityTree.cpp

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/*
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * Written (W) 2012 Chiyuan Zhang
 * Copyright (C) 2012 Chiyuan Zhang
 */

#include <shogun/multiclass/tree/BalancedConditionalProbabilityTree.h>

using namespace shogun;

CBalancedConditionalProbabilityTree::CBalancedConditionalProbabilityTree()
    :m_alpha(0.4)
{
    SG_ADD(&m_alpha, "m_alpha", "Trade-off parameter of tree balance", MS_NOT_AVAILABLE);
}

void CBalancedConditionalProbabilityTree::set_alpha(float64_t alpha)
{
    if (alpha < 0 || alpha > 1)
        SG_ERROR("expect 0 <= alpha <= 1, but got %g\n", alpha);
    m_alpha = alpha;
}

bool CBalancedConditionalProbabilityTree::which_subtree(node_t *node, SGVector<float32_t> ex)
{
    float64_t pred = predict_node(ex, node);
    float64_t depth_left = tree_depth(node->left());
    float64_t depth_right = tree_depth(node->right());

    float64_t cnt_left = CMath::pow(2.0, depth_left);
    float64_t cnt_right = CMath::pow(2.0, depth_right);

    float64_t obj_val = (1-m_alpha) * 2 * (pred-0.5) + m_alpha * CMath::log2(cnt_left/cnt_right);

    if (obj_val > 0)
        return false; // go right
    return true; // go left
}

int32_t CBalancedConditionalProbabilityTree::tree_depth(node_t *node)
{
    int32_t depth = 0;
    while (node != NULL)
    {
        depth++;
        node = node->left();
    }

    return depth;
}