Castor rainbow

castor.php

<?php

use Castor\Attribute\AsTask;
use Symfony\Component\Console\Helper\Helper;
use Symfony\Component\Console\Terminal;

use function Castor\io;

#[AsTask()]
function rainbow(string $text): void
{
    $termWidth = (new Terminal())->getWidth();
    $maxLen = 0;
    $lines = [];
    foreach (preg_split('/\r?\n/', $text) as $line) {
        foreach (mb_str_split($line, $termWidth) as $line) {
            $lines[] = $line;
            $len = Helper::width($line);
            $maxLen = max($len, $maxLen);
        }
    }

    $colors = $bgColors = [];

    foreach ($lines as $line) {
        $line = str_pad($line, $maxLen, ' ', \STR_PAD_RIGHT);
        $columns = mb_str_split($line, 1);

        foreach ($columns as $i => $column) {
            [$r, $g, $b] = $colors[$i] ??= getColor($i, $maxLen);
            [$textR, $textG, $textB] = $bgColors[$i] ??= getContrastingTextColor($r, $g, $b);

            io()->write(sprintf("\033[48;2;%d;%d;%dm", $r, $g, $b));
            io()->write(sprintf("\033[38;2;%d;%d;%dm", $textR, $textG, $textB));
            io()->write(sprintf("%s\033[0m", $column));
        }
        io()->writeln('');
    }
}

function getColor(int $i, int $maxLen): array
{
    $r = 255 - ($i * 255 / $maxLen);
    $g = ($i * 510 / $maxLen);
    if ($g > 255) {
        $g = 510 - $g;
    }
    $b = ($i * 255 / $maxLen);

    return [$r, $g, $b];
}

function getContrastingTextColor(float $r, float $g, float $b): array
{
    $r /= 255;
    $g /= 255;
    $b /= 255;

    $luminance = 0.2126 * $r + 0.7152 * $g + 0.0722 * $b;

    return ($luminance > 0.5) ? [0, 0, 0] : [255, 255, 255];
}