// Package crawl reads a web page: fetch, robots check, HTML to text.
//
// It is the last LOCAL-FIRST step, and the ordering is the whole design.
// "Never phones home" is deprecated, but what replaced it puts local sources
// first. Where this actually sits in querySources (cmd/mavend/actions_query.go):
// after his memory and after his notes, and BEFORE the model answers from what
// it remembers. Not after the model, which is what this comment used to claim.
//
// That position is deliberate. A fetch only happens where he named a URL out
// loud, and a named URL is an instruction, not a guess; letting a 1.7B answer
// about a page it cannot read is how a small model invents contents. Kiwix
// (internal/kiwix) is not in the chain yet, so nothing here describes it.
//
// A page's text is written by whoever owns the page. It reaches PhraseQuery as
// context beside his question, and for a watch it becomes a note. It cannot
// reach a tool or an act — the query path executes nothing — but it can steer
// what she says, which is the same trust level as a mail body and lower than
// anything he said himself.
//
// What never leaves the box: his notes, his facts, the persona block, the
// conversation history. Only the URL is requested and, for the on-demand path,
// only because he said it out loud. Nothing here reads the store.
//
// The limits are not in this package — they are in internal/webfetch, which is
// the only way anything here touches a socket: http(s) only, host allow/deny,
// private-address refusal, size cap, redirect cap, per-host rate limit. What
// this package adds is politeness (robots.txt) and dedup.
package crawl

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"net/url"
	"strings"
	"sync"
	"time"
)

// Errors callers distinguish.
var (
	ErrRobots  = errors.New("crawl: robots.txt disallows this path")
	ErrNotHTML = errors.New("crawl: response is not html or text")

	// ErrFetchRefused — the fetcher would not go: a denied host, a private
	// address, a scheme that is not http(s). The adapter that owns both
	// packages (cmd/mavend/crawls.go) maps webfetch's sentinels onto this one,
	// so this package tells "off limits" from "no robots.txt here" without
	// importing webfetch and without matching on message text.
	ErrFetchRefused = errors.New("crawl: the fetcher refused this url")

	// ErrFetchStatus — the server answered, badly (5xx, and anything else
	// non-2xx). Separate from ErrFetchRefused because robots treats them
	// differently: a broken server is not permission to crawl.
	ErrFetchStatus = errors.New("crawl: the server answered with an error status")
)

// Fetcher is the guarded HTTP door (internal/webfetch adapted by the daemon). An
// interface so this package constructs no http.Client of its own and can be
// tested without a network.
type Fetcher interface {
	Get(ctx context.Context, url string) (*Response, error)
}

// Response is the minimum a crawl needs from a fetch.
type Response struct {
	URL         string
	ContentType string
	Body        []byte
}

// Config — crawler knobs.
type Config struct {
	// UserAgent is the name matched against robots.txt groups. It must be the
	// same string the fetcher sends, or Maven would be claiming one identity
	// and obeying the rules for another.
	UserAgent string
	// MaxRunes caps extracted text. 0 ⇒ DefaultMaxRunes.
	MaxRunes int
	// RobotsTTL — how long a parsed robots.txt is trusted. 0 ⇒ 1h.
	RobotsTTL time.Duration
	// Now is injectable for tests. nil ⇒ time.Now.
	Now func() time.Time
}

// Crawler fetches and extracts pages. Safe for concurrent use.
type Crawler struct {
	fetch  Fetcher
	cfg    Config
	robots *robotsCache

	// mu guards last, the per-host time of the previous fetch. It is what makes
	// Crawl-delay real: the fetcher's own limiter is a flat one request per
	// host per second and knows nothing about what a site asked for.
	mu   sync.Mutex
	last map[string]time.Time
}

// robotsTimeout — the robots fetch gets its own, shorter deadline. It shares the
// caller's budget with the page fetch (30s for the on-demand path, against a 20s
// default fetch timeout each), so a slow robots.txt used to eat the page's half
// and he heard "не получилось прочитать страницу" about a site that was fine.
const robotsTimeout = 8 * time.Second

// New builds a crawler. Returns nil when there is no fetcher, which is how the
// daemon expresses "crawling is off unless configured".
func New(fetch Fetcher, cfg Config) *Crawler {
	if fetch == nil {
		return nil
	}
	if cfg.UserAgent == "" {
		cfg.UserAgent = "Maven"
	}
	if cfg.MaxRunes <= 0 {
		cfg.MaxRunes = DefaultMaxRunes
	}
	if cfg.RobotsTTL <= 0 {
		cfg.RobotsTTL = time.Hour
	}
	if cfg.Now == nil {
		cfg.Now = time.Now
	}
	return &Crawler{fetch: fetch, cfg: cfg, robots: newRobotsCache(cfg.RobotsTTL), last: map[string]time.Time{}}
}

// Page fetches rawURL and returns its text. It checks robots.txt first and
// refuses a disallowed path with ErrRobots — there is no override.
func (c *Crawler) Page(ctx context.Context, rawURL string) (Page, error) {
	u, err := url.Parse(strings.TrimSpace(rawURL))
	if err != nil {
		return Page{}, fmt.Errorf("crawl: bad url %q: %w", rawURL, err)
	}
	rules, err := c.rulesFor(ctx, u)
	if err != nil {
		return Page{}, err
	}
	path := u.EscapedPath()
	if u.RawQuery != "" {
		path += "?" + u.RawQuery
	}
	if !rules.Allowed(path) {
		return Page{}, fmt.Errorf("%w: %s", ErrRobots, u.Path)
	}
	if err := c.waitCrawlDelay(ctx, u.Host, rules.Delay); err != nil {
		return Page{}, err
	}
	resp, err := c.fetch.Get(ctx, u.String())
	c.markFetched(u.Host)
	if err != nil {
		return Page{}, err
	}
	// A PDF or an image is bytes Maven cannot read; saying so beats storing
	// binary garbage as a "note".
	ct := strings.ToLower(resp.ContentType)
	if ct != "" && !strings.Contains(ct, "html") && !strings.Contains(ct, "text/") &&
		!strings.Contains(ct, "xml") && !strings.Contains(ct, "json") {
		return Page{}, fmt.Errorf("%w: %s", ErrNotHTML, resp.ContentType)
	}
	return Extract(resp.URL, resp.Body, c.cfg.MaxRunes), nil
}

// rulesFor consults robots.txt for u's host, reading it at most once per TTL.
//
// A robots.txt that is not there (404) means allow, per the standard. What does
// NOT mean allow: a server that answered with an error. The standard asks for
// the opposite there, and "the site is broken, so crawl it" is the wrong way to
// resolve an unknown.
func (c *Crawler) rulesFor(ctx context.Context, u *url.URL) (Rules, error) {
	host := u.Host
	now := c.cfg.Now()
	rules, ok := c.robots.get(host, now)
	if ok {
		return rules, nil
	}
	robotsURL := u.Scheme + "://" + host + "/robots.txt"
	rctx, cancel := context.WithTimeout(ctx, robotsTimeout)
	defer cancel()
	resp, err := c.fetch.Get(rctx, robotsURL)
	c.markFetched(host)
	switch {
	case err == nil:
		rules = ParseRobots(string(resp.Body), c.cfg.UserAgent)
	case errors.Is(err, ErrFetchRefused):
		// Not swallowed: if the fetcher says this host is denied or private,
		// the page fetch would fail the same way, and the real reason beats a
		// robots verdict we never got.
		return Rules{}, err
	case errors.Is(err, ErrFetchStatus) && isServerError(err):
		return Rules{}, fmt.Errorf("%w: robots.txt at %s could not be read", ErrFetchStatus, host)
	default:
		rules = Rules{}
	}
	c.robots.put(host, rules, now)
	return rules, nil
}

// waitCrawlDelay honours a Crawl-delay the site asked for. The fetcher's flat
// one-per-host-per-second is the floor and cannot express "30 seconds"; without
// this, deploy/README's claim that Crawl-delay is honoured was false.
//
// It waits on ctx, so a delay longer than the caller's budget fails the read
// rather than blocking a turn. That is the honest outcome: a site that wants a
// minute between requests is not a site to answer a voice question from.
func (c *Crawler) waitCrawlDelay(ctx context.Context, host string, delay time.Duration) error {
	if delay <= 0 {
		return nil
	}
	c.mu.Lock()
	last, ok := c.last[host]
	c.mu.Unlock()
	if !ok {
		return nil
	}
	wait := delay - c.cfg.Now().Sub(last)
	if wait <= 0 {
		return nil
	}
	t := time.NewTimer(wait)
	defer t.Stop()
	select {
	case <-ctx.Done():
		return fmt.Errorf("crawl: %s asks for %s between requests, longer than this read has: %w", host, delay, ctx.Err())
	case <-t.C:
		return nil
	}
}

func (c *Crawler) markFetched(host string) {
	c.mu.Lock()
	c.last[host] = c.cfg.Now()
	c.mu.Unlock()
}

// isServerError — a 5xx rather than any other non-2xx. The adapter formats the
// status into the message, which is the only place it survives.
func isServerError(err error) bool {
	s := err.Error()
	for _, code := range []string{" 50", " 51", " 52", " 53"} {
		if strings.Contains(s, code) {
			return true
		}
	}
	return false
}

// Hash is the dedup key for a crawl result: the sha256 of the extracted text,
// hex, first 16 chars. Text and not raw HTML, because a page whose only change
// is a rotating ad slot or a CSRF token has not changed.
func Hash(text string) string {
	sum := sha256.Sum256([]byte(strings.TrimSpace(text)))
	return hex.EncodeToString(sum[:])[:16]
}