<?php

namespace App\Services;

use Illuminate\Database\QueryException;

/** Combines independent, already-authorised sources without loading complete catalogues. */
final class FederatedSearch
{
    /** @param array<string, callable(): array{count:int, fetch:callable(int,int): iterable}> $factories */
    public function page(array $factories, int $requestedPage, int $size = 5, ?callable $score = null): array
    {
        $sources = [];
        $unavailable = [];
        foreach ($factories as $label => $factory) {
            try {
                $sources[$label] = $factory();
            } catch (QueryException $exception) {
                report($exception);
                $unavailable[] = $label;
            }
        }

        // If a source fails after its count, remove it and recompute the page offsets.
        do {
            $failed = false;
            $total = array_sum(array_column($sources, 'count'));
            $last = max(1, (int) ceil($total / $size));
            $page = min(max(1, $requestedPage), $last);
            $first = ($page - 1) * $size;
            $items = [];
            $positions = array_fill_keys(array_keys($sources), 0);
            $buffers = array_fill_keys(array_keys($sources), []);
            for ($position = 0; $position < min($total, $first + $size); $position++) {
                $best = null;
                $bestScore = PHP_INT_MIN;
                foreach ($sources as $label => $source) {
                    if ($buffers[$label] === [] && $positions[$label] < $source['count']) {
                        try {
                            $batch = iterator_to_array((function () use ($source, $positions, $label, $size) {
                                yield from ($source['fetch'])($positions[$label], $size);
                            })(), false);
                            $positions[$label] += count($batch);
                            $buffers[$label] = $batch;
                            if ($batch === []) {
                                $positions[$label] = $source['count'];
                            }
                        } catch (QueryException $exception) {
                            report($exception);
                            $unavailable[] = $label;
                            unset($sources[$label]);
                            $failed = true;
                            break;
                        }
                    }
                    if ($buffers[$label] === []) {
                        continue;
                    }
                    $candidateScore = $score ? $score($buffers[$label][0]) : 0;
                    if ($best === null || $candidateScore > $bestScore) {
                        $best = $label;
                        $bestScore = $candidateScore;
                    }
                }
                if ($failed || $best === null) {
                    break;
                }
                $item = array_shift($buffers[$best]);
                if ($position >= $first) {
                    $items[] = [...$item, 'category' => $best];
                }
            }
        } while ($failed);

        return ['data' => $items, 'total' => $total, 'current_page' => $page, 'last_page' => $last,
            'per_page' => $size, 'unavailable' => $unavailable];
    }
}