<?php
// SPDX-License-Identifier: GPL-3.0-or-later
declare(strict_types=1);

namespace Aiwend\Chat;

/**
 * Optional chat colours, set separately for the light and dark schemes. An empty slot keeps the
 * widget's built-in colour for that scheme, so assistants saved before 0.7 look unchanged.
 * Values are validated here (6-digit HEX only). Contrast is advisory: a weak pair produces a
 * warning with a readable alternative, and nothing is changed automatically.
 */
final class Palette
{
    /** slot => label. Stored as appearance.colors.{light|dark}.{slot}. */
    public const SLOTS = [
        'panel_bg' => 'Panel background',
        'header_bg' => 'Header background',
        'header_fg' => 'Header text',
        'user_bg' => 'Visitor message background',
        'user_fg' => 'Visitor message text',
        'bot_bg' => 'Assistant message background',
        'bot_fg' => 'Assistant message text',
        'composer_bg' => 'Message box background',
        'composer_fg' => 'Message box text',
        'button_bg' => 'Primary button background',
        'button_fg' => 'Primary button text and icon',
        'launcher_bg' => 'Launcher button background',
        'launcher_fg' => 'Launcher logo backing',
        'link' => 'Links and focus outline',
    ];

    public const SCHEMES = ['light', 'dark'];

    /** [foreground slot, background slot, minimum ratio]: text 4.5:1, non-text UI parts 3:1. */
    public const PAIRS = [
        ['header_fg', 'header_bg', 4.5],
        ['user_fg', 'user_bg', 4.5],
        ['bot_fg', 'bot_bg', 4.5],
        ['composer_fg', 'composer_bg', 4.5],
        ['button_fg', 'button_bg', 4.5],
        ['launcher_fg', 'launcher_bg', 3.0],
        ['link', 'panel_bg', 4.5],
    ];

    /**
     * @return array{light: array<string, string>, dark: array<string, string>} only the set slots
     * @throws \InvalidArgumentException on an unknown scheme or slot, or a value that is not #rrggbb
     */
    public static function normalize(mixed $input): array
    {
        $out = ['light' => [], 'dark' => []];
        if ($input === null || $input === '' || $input === []) {
            return $out;
        }
        if (!is_array($input)) {
            throw new \InvalidArgumentException('Invalid colours.');
        }
        foreach ($input as $scheme => $slots) {
            if (!in_array($scheme, self::SCHEMES, true) || !is_array($slots)) {
                throw new \InvalidArgumentException('Unknown colour scheme.');
            }
            foreach ($slots as $slot => $value) {
                if (!isset(self::SLOTS[$slot]) || !is_string($value)) {
                    throw new \InvalidArgumentException('Unknown colour slot.');
                }
                $value = strtolower(trim($value));
                if ($value === '') {
                    continue;
                }
                if (!self::isHex($value)) {
                    throw new \InvalidArgumentException('Colours must be 6-digit HEX values such as #1a2b3c.');
                }
                $out[$scheme][$slot] = $value;
            }
        }
        return $out;
    }

    public static function isHex(string $value): bool
    {
        return preg_match('/^#[0-9a-f]{6}$/D', strtolower($value)) === 1;
    }

    /** The widget's built-in colours for a scheme, given the brand accent (mirrors chat-widget.css). */
    public static function defaults(string $scheme, string $accent): array
    {
        $accent = self::isHex($accent) ? strtolower($accent) : '#4f46e5';
        if ($scheme === 'dark') {
            $panel = '#16191f';
            return [
                'panel_bg' => $panel, 'header_bg' => $panel, 'header_fg' => '#eceff4',
                'user_bg' => self::mix($accent, $panel, .72), 'user_fg' => '#ffffff',
                'bot_bg' => '#1f232b', 'bot_fg' => '#eceff4',
                'composer_bg' => $panel, 'composer_fg' => '#eceff4',
                'button_bg' => $accent, 'button_fg' => '#ffffff',
                'launcher_bg' => $accent, 'launcher_fg' => '#ffffff',
                'link' => self::mix($accent, '#ffffff', .45),
            ];
        }
        return [
            'panel_bg' => '#ffffff', 'header_bg' => '#ffffff', 'header_fg' => '#151b2c',
            'user_bg' => self::mix($accent, '#ffffff', .11), 'user_fg' => '#151b2c',
            'bot_bg' => '#f5f6fa', 'bot_fg' => '#151b2c',
            'composer_bg' => '#ffffff', 'composer_fg' => '#151b2c',
            'button_bg' => $accent, 'button_fg' => '#ffffff',
            'launcher_bg' => $accent, 'launcher_fg' => '#ffffff',
            'link' => $accent,
        ];
    }

    /**
     * Pairs below their minimum contrast, with the effective colours and a readable alternative
     * for the foreground (black-ish or white, whichever reads better), or null if neither does.
     *
     * @param array<string, string> $set the scheme's set slots
     * @return list<array{fg: string, bg: string, ratio: float, min: float, fg_color: string, bg_color: string, suggestion: ?string}>
     */
    public static function warnings(string $scheme, array $set, string $accent): array
    {
        $c = $set + self::defaults($scheme, $accent);
        // Unset header/composer backgrounds follow the panel background.
        foreach (['header_bg', 'composer_bg'] as $s) {
            if (!isset($set[$s]) && isset($set['panel_bg'])) {
                $c[$s] = $set['panel_bg'];
            }
        }
        $out = [];
        foreach (self::PAIRS as [$fg, $bg, $min]) {
            $ratio = self::contrast($c[$fg], $c[$bg]);
            if ($ratio + 1e-9 >= $min) {
                continue;
            }
            $dark = self::contrast('#151b2c', $c[$bg]);
            $light = self::contrast('#ffffff', $c[$bg]);
            $best = $dark >= $light ? '#151b2c' : '#ffffff';
            $out[] = ['fg' => $fg, 'bg' => $bg, 'ratio' => round($ratio, 2), 'min' => $min, 'fg_color' => $c[$fg], 'bg_color' => $c[$bg], 'suggestion' => max($dark, $light) >= $min ? $best : null];
        }
        return $out;
    }

    /** WCAG 2 contrast ratio of two #rrggbb colours (1–21). */
    public static function contrast(string $a, string $b): float
    {
        $la = self::luminance($a);
        $lb = self::luminance($b);
        return (max($la, $lb) + .05) / (min($la, $lb) + .05);
    }

    private static function luminance(string $hex): float
    {
        $l = 0.0;
        foreach ([[1, .2126], [3, .7152], [5, .0722]] as [$at, $w]) {
            $v = hexdec(substr($hex, $at, 2)) / 255;
            $l += $w * ($v <= .04045 ? $v / 12.92 : (($v + .055) / 1.055) ** 2.4);
        }
        return $l;
    }

    /** sRGB mix like CSS color-mix(in srgb, $a $p, $b). */
    public static function mix(string $a, string $b, float $p): string
    {
        $out = '#';
        foreach ([1, 3, 5] as $at) {
            $out .= sprintf('%02x', (int) round(hexdec(substr($a, $at, 2)) * $p + hexdec(substr($b, $at, 2)) * (1 - $p)));
        }
        return $out;
    }
}