// mavend/crawls.go — the driver for reading web pages (Vikunja #259,
// docs/plans/14-web-crawler.md). The crawler is pure and lives in
// internal/crawl; this is the impure half: the guarded fetcher, a ticker for the
// scheduled watches, and the fact-backed dedup hashes.
//
// Two paths, one config block, both off unless configured:
//
//   - ON DEMAND — he names a URL out loud and she reads it. That is the
//     `queryWeb` source in actions_query.go, LAST in the chain: after his
//     memory, after the notes, and (once Kiwix is wired into the chain) after
//     the local ZIMs. A local read costs nothing and leaks nothing; a fetch puts
//     a URL in someone's log, so it goes last.
//   - SCHEDULED — a watched page is re-read on its interval, and a page whose
//     text changed is written as a note. It does NOT announce itself. Same rule
//     as the feed poller: notes, never nudges.
//
// Only the URL goes out. Nothing here reads a note, a fact, the persona block or
// the history, and internal/crawl has no access to the store at all.
package main

import (
	"context"
	"errors"
	"fmt"
	"log"
	"net/url"
	"time"

	"github.com/kami/maven/internal/config"
	"github.com/kami/maven/internal/crawl"
	"github.com/kami/maven/internal/ipc"
	"github.com/kami/maven/internal/router"
	"github.com/kami/maven/internal/webfetch"
)

// newCrawler builds the crawler from the `crawl` block, or returns nil when
// there is none. Every caller checks for nil, and nil means no page is ever
// fetched.
func newCrawler(cfg *config.Config) *crawl.Crawler {
	if cfg.Crawl == nil {
		return nil
	}
	cc := cfg.Crawl
	// The WATCH crawler, and only it, reaches the watched hosts. webfetch reads
	// a non-empty allow list as "these and nothing else", so folding the watch
	// hosts in turned a single watch into an allowlist for everything: a config
	// with one watch and on_demand true silently refused every other page he
	// pasted, with "не получилось прочитать страницу." and no clue why.
	return crawlerWithHosts(cc, crawlHosts(cc, true))
}

// crawlHosts — the allowlist for one of the two crawlers. forWatches adds the
// watched pages' own hosts, so a watch does not have to be allowlisted by hand.
//
// The on-demand crawler gets his allow_hosts and nothing else. webfetch reads a
// non-empty list as "these and nothing else", so adding the watch hosts there
// would silently narrow on-demand reading to the watched sites.
func crawlHosts(cc *config.CrawlConfig, forWatches bool) []string {
	hosts := append([]string(nil), cc.AllowHosts...)
	if !forWatches {
		return hosts
	}
	for _, w := range cc.Watches {
		if u, err := url.Parse(w.URL); err == nil && u.Hostname() != "" {
			hosts = append(hosts, u.Hostname())
		}
	}
	return hosts
}

// crawlerWithHosts builds a crawler over one allowlist. Two callers, two lists:
// see newCrawler and onDemandCrawler.
func crawlerWithHosts(cc *config.CrawlConfig, hosts []string) *crawl.Crawler {
	ua := cc.UserAgent
	if ua == "" {
		ua = webfetch.DefaultUserAgent
	}
	fetcher := webfetch.New(webfetch.Config{
		AllowHosts: hosts,
		DenyHosts:  cc.DenyHosts,
		Timeout:    time.Duration(cc.Timeout),
		MaxBytes:   cc.MaxBytes,
		UserAgent:  ua,
	})
	// The user-agent handed to the crawler is the one the fetcher sends: obeying
	// robots rules written for a different name would be a lie.
	return crawl.New(&crawlFetcher{f: fetcher}, crawl.Config{
		UserAgent: ua,
		MaxRunes:  cc.MaxRunes,
	})
}

// onDemandCrawler returns a crawler for the answer path, or nil when on-demand
// reading is off. The scheduled watches can be on while this is off: reading a
// fixed list of pages on a timer and reading whatever URL is in an utterance are
// different permissions, and the config keeps them separate.
func onDemandCrawler(cfg *config.Config) *crawl.Crawler {
	if cfg.Crawl == nil || !cfg.Crawl.OnDemand {
		return nil
	}
	cc := cfg.Crawl
	// His own allow_hosts, and nothing added behind his back. Empty means "any
	// host that is not denied and not private", which is what on-demand reading
	// of a URL he just said out loud has to mean.
	if len(cc.AllowHosts) > 0 {
		log.Printf("crawl: allow_hosts is set, so on-demand reading is limited to those %d host(s)", len(cc.AllowHosts))
	}
	return crawlerWithHosts(cc, crawlHosts(cc, false))
}

// crawlWorker — ticker + watcher for the scheduled half.
type crawlWorker struct {
	watcher  *crawl.Watcher
	interval time.Duration
}

// crawlTickInterval — how often the worker asks what is due. Per-watch cadence
// is the watcher's business.
const crawlTickInterval = 15 * time.Minute

// newCrawlWorker wires the scheduled crawls, or nil when nothing is watched.
func newCrawlWorker(c *crawl.Crawler, api ipc.CoreAPI, emb router.Embedder, cfg *config.Config) *crawlWorker {
	if c == nil || cfg.Crawl == nil || len(cfg.Crawl.Watches) == 0 {
		return nil
	}
	watches := make([]crawl.WatchConfig, 0, len(cfg.Crawl.Watches))
	for _, w := range cfg.Crawl.Watches {
		watches = append(watches, crawl.WatchConfig{
			Name:     w.Name,
			URL:      w.URL,
			Interval: time.Duration(w.Interval),
		})
	}
	watcher := crawl.NewWatcher(c, watches, api, &factHashes{api: api},
		crawlEmbedder(emb), time.Duration(cfg.Crawl.Interval))
	if watcher == nil {
		log.Printf("crawl: configured but nothing watchable — scheduled crawls disabled")
		return nil
	}
	log.Printf("crawl: watching %d page(s), checking what is due every %s", len(watches), crawlTickInterval)
	return &crawlWorker{watcher: watcher, interval: crawlTickInterval}
}

// run checks what is due until ctx is canceled. The first round runs
// immediately; it writes notes only, so an early round startles nobody.
func (w *crawlWorker) run(ctx context.Context) {
	w.watcher.CheckDue(ctx, time.Now())
	t := time.NewTicker(w.interval)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case now := <-t.C:
			w.watcher.CheckDue(ctx, now)
		}
	}
}

// crawlFetcher adapts webfetch to crawl.Fetcher, which is the seam that keeps
// net/http out of the crawler package.
type crawlFetcher struct{ f *webfetch.Fetcher }

// Get maps webfetch's sentinels onto crawl's. This adapter is the one place
// that imports both packages, so the mapping belongs here; the crawler used to
// match on three substrings of a message it could not see the definition of,
// and a reworded error would have quietly turned a blocked host into "there is
// no robots.txt here".
func (a *crawlFetcher) Get(ctx context.Context, u string) (*crawl.Response, error) {
	resp, err := a.f.Get(ctx, u)
	if err != nil {
		switch {
		case errors.Is(err, webfetch.ErrBlocked), errors.Is(err, webfetch.ErrPrivate), errors.Is(err, webfetch.ErrScheme):
			return nil, fmt.Errorf("%w: %v", crawl.ErrFetchRefused, err)
		case errors.Is(err, webfetch.ErrStatus):
			// Carry the code across the seam. The crawler needs to tell a 5xx
			// from a 404 to decide what a failed robots.txt means, and it must
			// not learn that by reading this sentence.
			var se *webfetch.StatusError
			if errors.As(err, &se) {
				return nil, &crawl.StatusError{Code: se.Code}
			}
			return nil, fmt.Errorf("%w: %v", crawl.ErrFetchStatus, err)
		}
		return nil, err
	}
	return &crawl.Response{URL: resp.URL, ContentType: resp.ContentType, Body: resp.Body}, nil
}

// factHashes stores each watch's last content hash as a config fact, so a
// restart does not re-note an unchanged page. Same mechanism the feed reader
// uses for its marks, and inspectable on /dash.
type factHashes struct{ api ipc.CoreAPI }

func hashKey(name string) string { return "crawl:hash:" + name }

func (h *factHashes) LastHash(ctx context.Context, name string) (string, error) {
	f, err := h.api.LatestFact(ctx, hashKey(name))
	if err != nil {
		// No hash yet is not an error: the watcher treats "" as "never read".
		return "", nil
	}
	return f.Value, nil
}

func (h *factHashes) SetHash(ctx context.Context, name, hash string) error {
	_, err := h.api.WriteFact(ctx, ipc.WriteFactReq{
		Ts:         time.Now(),
		Kind:       "config",
		Key:        hashKey(name),
		Value:      hash,
		Source:     "poll:crawl",
		Confidence: 1.0,
	})
	return err
}

// crawlEmbedder adapts router.Embedder for the watcher, embedding with
// EmbedPassage (a page is text being searched FOR, and the e5 embedder is
// asymmetric).
func crawlEmbedder(emb router.Embedder) crawl.Embedder {
	if emb == nil {
		return nil
	}
	return passageEmbedder{emb}
}