read_mnist8m_libsvm_parallel.m

function [labels, images] = read_mnist8m_libsvm_parallel(filename, num_samples)
    % Open file and read all lines in one go (fast)
    fid = fopen(filename, 'r');
    if fid == -1
        error('Could not open file.');
    end
    data = textscan(fid, '%s', num_samples, 'Delimiter', '\n');
    fclose(fid);
    
    data = data{1};  % Extract cell array of strings
    labels = zeros(num_samples, 1, 'uint8');  % Labels
    images = zeros(num_samples, 784, 'uint8'); % Preallocate matrix for images

    % Start parallel processing
    parfor i = 1:num_samples
        line = data{i};  % Extract current line
        parts = strsplit(line, ' '); % Split line into parts

        % First part is the label
        labels(i) = str2double(parts{1});

        % Remaining parts are pixel index:value pairs
        pixel_data = parts(2:end);
        pixel_info = split(pixel_data, ':'); % Efficient parsing of indices and values

        indices = str2double(pixel_info(:, :, 1)); % Convert indices to numeric
        values = str2double(pixel_info(:, :, 2)); % Convert values to numeric

        % Assign pixel values in a local temporary array
        temp_img = zeros(1, 784, 'uint8');
        temp_img(indices) = values;

        % Store the processed image in the main matrix
        images(i, :) = temp_img;
    end
end