columns_to_rows.php

<?php

/*
    Extract the data represented as an array of columns and represent it as an
    array of rows.

    Assert that the data in all non-empty columns is of the same length.
    Represent empty columns with an array of $zeros.

 */

function columns_to_rows($wanted_columns, $data_as_columns, $zero) {
    // find the length of non-empty columns
    // check if wanted columns are missing or empty
    // crash if non-empty columns are of a different length
    $length = null;
    $some_columns_are_missing = false;
    foreach ($wanted_columns as $col) {
        if (!isset($data_as_columns[$col])) {
            $some_columns_are_missing = true;
        } else {
            if ($length === null && isset($data_as_columns[$col]) && count($data_as_columns[$col]) > 0) {
                $length = count($data_as_columns[$col]);
            } else {
                if ($length != count($data_as_columns[$col])) {
                    throw new LogicException('Columns must be of the same length');
                }
            }
        }
    }

    // return early if no data
    if ($length === null) {
        return array();
    }

    // represent missing data with $zero
    if ($some_columns_are_missing) {
        $zeros = array_fill(0, $length, $zero);
    }

    // use array_map(null, col[1], col[2]...) to zip arrays
    $args_for_array_map = array(null);
    foreach ($wanted_columns as $col) {
        $args_for_array_map[] = isset($data_as_columns[$col]) ? $data_as_columns[$col] : $zeros;
    }
    $data_as_rows = call_user_func_array('array_map', $args_for_array_map);

    // handle quirk
    if (count($wanted_columns) == 1) {
        $t = array();
        foreach ($data_as_rows as $value) {
            $t[] = array($value);
        }
        $data_as_rows = $t;
    }

    return $data_as_rows;
}


# TEST
$wanted_columns = array('quuxes', 'foos', 'bars', 'bazzes');
#$wanted_columns = array('foos');

$data_as_columns = array(
    'foos' => array(5, 3, 9),
    'bars' => array(0, 1, 2),
    'noise' => "such baz"
);

print_r($wanted_columns);
echo "\n";

print_r($data_as_columns);
echo "\n";

print_r(columns_to_rows($wanted_columns, $data_as_columns, 0));
echo "\n";