<?php

/*
   The Computer Language Benchmarks Game
   https://salsa.debian.org/benchmarksgame-team/benchmarksgame/

   contributed by Nikita Popov
 */


function A($i, $j)
{
    return 1.0 / (((($i + $j) * ($i + $j + 1)) >> 1) + $i + 1);
}

function Av($n, $v, $start, $end)
{
    global $_tpl;
    $Av = $_tpl;
    for ($i = $start; $i < $end; ++$i) {
        $sum = 0.0;
        foreach ($v as $j => $v_j) {
            $sum += A($i, $j) * $v_j;
        }
        $Av[$i] = $sum;
    }
    return $Av;
}

function Atv($n, $v, $start, $end)
{
    global $_tpl;
    $Atv = $_tpl;
    for ($i = $start; $i < $end; ++$i) {
        $sum = 0.0;
        foreach ($v as $j => $v_j) {
            $sum += A($j, $i) * $v_j;
        }
        $Atv[$i] = $sum;
    }
    return $Atv;
}

function AtAv($n, $v, $start, $end, $sync)
{

    $tmp = Av($n, $v, $start, $end);
    if ($sync) $tmp = sync($tmp);

    $tmp = Atv($n, $tmp, $start, $end);
    if ($sync) $tmp = sync($tmp);

    return $tmp;
}

function sync($tmp)
{
    global $parent, $chunk_data_size, $total_data_size, $pipe, $pipes;

    if (!$parent) {
        $data = call_user_func_array('pack', array_merge(['d*'], $tmp));
        safe_write($pipe, $data);
        return array_values(unpack('d*', safe_read($pipe, $total_data_size)));
    } else {
        $tmps = array();
        foreach ($pipes as $_pipe) {
            $tmps[] = unpack('d*', safe_read($_pipe, $chunk_data_size));
        }
        $tmps[] = $tmp;
        $tmp = call_user_func_array('array_merge', $tmps);

        $data = call_user_func_array('pack', array_merge(['d*'], $tmp));
        foreach ($pipes as $_pipe) {
            safe_write($_pipe, $data);
        }
        return $tmp;
    }
}

function safe_write($fd, $data)
{
    $len = strlen($data);
    do {
        $w = fwrite($fd, $data);
        $len -= $w;
    } while ($len && ($data = substr($data, $w)) !== FALSE);
}

function safe_read($fd, $len)
{
    $data = '';
    while ($len > 0) {
        $d = fread($fd, $len);
        $len -= strlen($d);
        $data .= $d;
    }
    return $data;
}

function pipe()
{
    return stream_socket_pair(STREAM_PF_UNIX, STREAM_SOCK_STREAM, 0);
}

function main()
{
    $begin = microtime(TRUE);
    global $argc, $argv, $parent, $chunk_data_size, $total_data_size, $pipe, $pipes, $_tpl;
    $n = $argc > 2 ? $argv[2] : 1;

    $procs = 1;
    if (file_exists('/proc/cpuinfo')) {
        $procs = preg_match_all('/^processor\s/m', file_get_contents('/proc/cpuinfo'), $discard);
    }

    if ($n < $procs) {
        $procs = 1;
    }

    $chunk_size = (int)($n / $procs);
    $double_size = strlen(pack('d', 0.0));
    $chunk_data_size = $double_size * $chunk_size;
    $total_data_size = $double_size * $n;

    $pipes = array();
    $parent = FALSE;
    for ($i = 0; $i < $procs; ++$i) {
        $range_begin = $i * $chunk_size;
        if ($i < ($procs - 1)) {
            $pipe = pipe();
            if (!$pipe) {
                die('could not create pipe');
            }
            $pipes[] = $pipe[0];
            $pipe = $pipe[1];
            $range_end = $range_begin + $chunk_size;
            $pid = pcntl_fork();
            if ($pid === -1) {
                die('could not fork');
            } else if ($pid) {
                continue;
            }
            break;
        } else {
            $range_end = $n;
            $parent = TRUE;
        }
    }

    $u = array_fill(0, $n, 1.0);
    $_tpl = array_fill($range_begin, $range_end - $range_begin, 0.0);
    $sync = $procs > 0;

    for ($i = 0; $i < 10; $i++) {
        $v = AtAv($n, $u, $range_begin, $range_end, $sync);
        $u = AtAv($n, $v, $range_begin, $range_end, $sync);
    }

    if (!$parent) {
        exit(0);
    }

    $childs = $procs - 1;
    while ($childs--) {
        pcntl_wait($s);
    }

    $vBv = 0.0;
    $vv = 0.0;
    $i = 0;
    foreach ($v as $val) {
        $vBv += $u[$i] * $val;
        $vv += $val * $val;
        ++$i;
    }
    printf("%0.9f\n", sqrt($vBv / $vv));
    printf("Time: %.9f\n", microtime(TRUE) - $begin);
}