package clone

import (
	"context"
	"crypto/sha256"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"strings"
	"sync"

	"github.com/tamnd/kage/asset"
	"github.com/tamnd/kage/browser"
	"github.com/tamnd/kage/robots"
	"github.com/tamnd/kage/sanitize"
	"github.com/tamnd/kage/urlx"
	"golang.org/x/net/html"
	"golang.org/x/time/rate"
)

// Logf is an optional sink for human-readable progress lines.
type Logf func(format string, args ...any)

// Cloner runs one clone. Build it with New, then call Run.
type Cloner struct {
	cfg      Config
	seed     *url.URL
	seedHost string
	outRoot  string
	statePth string

	pool   *browser.Pool
	dl     *asset.Downloader
	httpC  *http.Client
	robots *robots.Matcher
	front  *frontier
	stats  stats
	logf   Logf

	mu         sync.Mutex
	seenAssets map[string]bool
	enqueued   int // pages offered to the queue

	crawlLimiter *rate.Limiter

	wg        sync.WaitGroup
	pageJobs  chan pageItem
	assetJobs chan assetItem

	muContent   sync.Mutex
	seenContent map[string]string // sha-256 of page bytes -> first path written
}

type pageItem struct {
	u     *url.URL
	depth int
}

type assetItem struct {
	u       *url.URL
	referer string
}

// New builds a Cloner for seed under cfg. It does not touch the network until
// Run is called.
func New(seed *url.URL, cfg Config, logf Logf) *Cloner {
	if logf == nil {
		logf = func(string, ...any) {}
	}
	host := seed.Hostname()
	outRoot := cfg.HostDir(host)
	return &Cloner{
		cfg:         cfg,
		seed:        seed,
		seedHost:    host,
		outRoot:     outRoot,
		statePth:    filepath.Join(outRoot, cfg.Reserved, "state.json"),
		dl:          asset.NewDownloader(cfg.UserAgent, cfg.Timeout, cfg.MaxAssetBytes),
		httpC:       &http.Client{Timeout: cfg.Timeout},
		robots:      robots.AllowAll(),
		front:       newFrontier(),
		logf:        logf,
		seenAssets:  map[string]bool{},
		seenContent: map[string]string{},
		pageJobs:    make(chan pageItem),
		assetJobs:   make(chan assetItem),
	}
}

// Snapshot returns the current progress, for a CLI ticker.
func (c *Cloner) Snapshot() Progress { return c.stats.snapshot() }

// pageKey is the dedup identity for a page: its output file. Two URLs that would
// write to the same file (http vs https, "/" vs "/index.html", a trailing
// slash) are the same page and must be crawled only once.
func (c *Cloner) pageKey(u *url.URL) string {
	return urlx.LocalPath(c.seedHost, u, urlx.Page, c.cfg.Reserved)
}

// assetKey is the dedup identity for an asset: its output file, so the same
// bytes referenced over http and https download once.
func (c *Cloner) assetKey(u *url.URL) string {
	return urlx.LocalPath(c.seedHost, u, urlx.Asset, c.cfg.Reserved)
}

// pagePathKey is the identity of a page ignoring its query string, used to tell
// a real page apart from its ?q=…/?page=… variants for the progress display.
// Each variant writes its own file (so the crawl stays complete), but they all
// fold to one path here.
func (c *Cloner) pagePathKey(u *url.URL) string {
	if u.RawQuery == "" {
		return c.pageKey(u)
	}
	cp := *u
	cp.RawQuery = ""
	return c.pageKey(&cp)
}

// Run executes the clone until the frontier drains, MaxPages is hit, or ctx is
// cancelled (which flushes the resume state). It returns the final Result.
func (c *Cloner) Run(ctx context.Context) (Result, error) {
	if c.cfg.Force {
		_ = os.RemoveAll(c.outRoot)
	}
	// Refresh re-renders every page in place, so it deliberately skips loading
	// the prior visited set; everything else resumes from where it left off.
	if c.cfg.Resume && !c.cfg.Refresh {
		if err := c.front.load(c.statePth); err != nil {
			c.logf("resume: could not load state: %v", err)
		} else if n := c.front.visitedCount(); n > 0 {
			c.logf("resume: %d pages already done", n)
		}
	}

	c.pool = browser.New(browser.Options{
		Headless:      c.cfg.Headless,
		Workers:       c.cfg.BrowserPages,
		Settle:        c.cfg.Settle,
		RenderTimeout: c.cfg.RenderTimeout,
		Scroll:        c.cfg.Scroll,
		ChromeBin:     c.cfg.ChromeBin,
		ControlURL:    c.cfg.ControlURL,
	})
	defer func() { _ = c.pool.Close() }()

	c.loadRobots(ctx)
	c.setupCrawlDelayLimiter()

	// Start workers.
	var workers sync.WaitGroup
	for range max1(c.cfg.Workers) {
		workers.Go(func() {
			for j := range c.pageJobs {
				c.processPage(ctx, j)
				c.wg.Done()
			}
		})
	}
	for range max1(c.cfg.AssetWorkers) {
		workers.Go(func() {
			for j := range c.assetJobs {
				c.processAsset(ctx, j)
				c.wg.Done()
			}
		})
	}

	// Seed.
	c.enqueuePage(ctx, c.seed, 0)
	if c.cfg.FollowSitemap {
		c.seedSitemaps(ctx)
	}

	// Close the job channels once every outstanding item is processed.
	go func() {
		c.wg.Wait()
		close(c.pageJobs)
		close(c.assetJobs)
	}()
	workers.Wait()

	if c.cfg.Persist {
		if err := c.front.save(c.statePth); err != nil {
			c.logf("could not save resume state: %v", err)
		}
	}

	res := Result{Progress: c.stats.snapshot(), OutDir: c.outRoot, Failures: c.stats.recordedFailures()}
	if ctx.Err() != nil {
		return res, ctx.Err()
	}
	return res, nil
}

// loadRobots fetches and parses robots.txt (unless disabled) and seeds the
// sitemap list it advertises.
func (c *Cloner) loadRobots(ctx context.Context) {
	if !c.cfg.RespectRobots {
		return
	}
	robotsURL := c.seed.Scheme + "://" + c.seed.Host + "/robots.txt"
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, robotsURL, nil)
	if err != nil {
		return
	}
	req.Header.Set("User-Agent", c.cfg.UserAgent)
	resp, err := c.httpC.Do(req)
	if err != nil {
		return
	}
	defer func() { _ = resp.Body.Close() }()
	if resp.StatusCode != http.StatusOK {
		return
	}
	data, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return
	}
	c.robots = robots.Parse(string(data), "kage")
}

func (c *Cloner) setupCrawlDelayLimiter() {
	delay := c.cfg.CrawlDelay
	if delay <= 0 && c.cfg.RespectRobots && c.robots != nil {
		delay = c.robots.CrawlDelay
	}
	if delay <= 0 {
		c.crawlLimiter = nil
		return
	}

	c.crawlLimiter = rate.NewLimiter(rate.Every(delay), 1)
}

// seedSitemaps adds in-scope sitemap URLs (from robots and the default path) to
// the frontier.
func (c *Cloner) seedSitemaps(ctx context.Context) {
	seeds := append([]string{}, c.robots.Sitemaps...)
	seeds = append(seeds, c.seed.Scheme+"://"+c.seed.Host+"/sitemap.xml")
	locs := collectSitemaps(ctx, c.httpC, c.cfg.UserAgent, seeds)
	added := 0
	for _, loc := range locs {
		u, err := urlx.Normalize(c.seed, loc)
		if err != nil {
			continue
		}
		if urlx.InScope(c.seed, u, c.cfg.scope()) && urlx.LikelyPage(u) {
			if c.enqueuePage(ctx, u, 1) {
				added++
			}
		}
	}
	if added > 0 {
		c.logf("sitemap: seeded %d URLs", added)
	}
}

// processPage renders one page, rewrites its references to local paths, strips
// every script, and writes the result.
func (c *Cloner) processPage(ctx context.Context, j pageItem) {
	if ctx.Err() != nil {
		return
	}
	key := c.pageKey(j.u)
	if c.cfg.RespectRobots && !c.robots.Allowed(j.u.Path) {
		c.stats.skipped.Add(1)
		return
	}
	if !c.waitForCrawlDelay(ctx) {
		return
	}

	res, err := c.pool.Render(ctx, j.u.String())
	if err != nil {
		var notHTML *browser.ErrNotHTML
		if errors.As(err, &notHTML) {
			// The URL is not a page but a file (a zip, a CSV, a bare image) that
			// reached the page worker through an extensionless link. Hand it to the
			// asset downloader, where the size and media policy decides whether to
			// localise it or leave it remote, rather than saving a broken page or
			// letting Chrome download it to the user's Downloads folder (issue #32).
			c.front.markVisited(key)
			if c.wantAsset(j.u) {
				c.enqueueAsset(ctx, j.u, "")
				c.logf("not a page, fetching as asset (%s): %s", notHTML.ContentType, j.u.String())
			} else {
				c.logf("not a page, left on the live web (%s): %s", notHTML.ContentType, j.u.String())
			}
			return
		}
		c.failPage(j.u.String(), fmt.Errorf("render: %w", err))
		return
	}

	root, err := html.Parse(strings.NewReader(res.HTML))
	if err != nil {
		c.failPage(j.u.String(), fmt.Errorf("parse: %w", err))
		return
	}

	localFile := urlx.LocalPath(c.seedHost, j.u, urlx.Page, c.cfg.Reserved)
	fileDir := urlx.Dir(localFile)

	sink := func(u *url.URL, kind urlx.Kind) string {
		switch kind {
		case urlx.Page:
			if urlx.InScope(c.seed, u, c.cfg.scope()) {
				c.enqueuePage(ctx, u, j.depth+1)
				local := urlx.LocalPath(c.seedHost, u, urlx.Page, c.cfg.Reserved)
				return urlx.Rel(fileDir, local)
			}
			return u.String() // external page link stays on the live web
		default: // Asset
			if !c.wantAsset(u) {
				return u.String() // off-domain or bulk media: leave it on the live web
			}
			c.enqueueAsset(ctx, u, j.u.String())
			local := urlx.LocalPath(c.seedHost, u, urlx.Asset, c.cfg.Reserved)
			return urlx.Rel(fileDir, local)
		}
	}

	asset.RewriteHTML(root, j.u, sink)
	sanitize.CleanTree(root, sanitize.Options{
		KeepNoscript:   c.cfg.KeepNoscript,
		MobileReadable: c.cfg.MobileReadable,
		Banner:         "cloned by kage from " + j.u.String(),
	})

	var buf strings.Builder
	if err := html.Render(&buf, root); err != nil {
		c.failPage(j.u.String(), fmt.Errorf("render html: %w", err))
		return
	}
	deduped, err := c.writePage(localFile, []byte(buf.String()))
	if err != nil {
		c.failPage(j.u.String(), fmt.Errorf("write %s: %w", localFile, err))
		return
	}
	c.front.markVisited(key)
	c.stats.recordPage(c.pagePathKey(j.u), deduped)
}

// waitForCrawlDelay spaces page render starts.
func (c *Cloner) waitForCrawlDelay(ctx context.Context) bool {
	if c.crawlLimiter == nil {
		return true
	}

	return c.crawlLimiter.Wait(ctx) == nil
}

// processAsset downloads one asset, rewriting CSS references on the way, and
// writes it to its deterministic local path.
func (c *Cloner) processAsset(ctx context.Context, j assetItem) {
	if ctx.Err() != nil {
		return
	}
	res, err := c.dl.Get(ctx, j.u, j.referer)
	if err != nil {
		if errors.Is(err, asset.ErrTooLarge) {
			// Over the size cap: leave it out rather than save a truncated
			// fragment. Count it as skipped, not failed, so the run is clean.
			c.stats.assetSkipped.Add(1)
			c.logf("asset skipped (over %d MB): %s", c.cfg.MaxAssetBytes>>20, j.u.String())
			return
		}
		c.failAsset(j.u.String(), j.referer, err)
		return
	}

	localFile := urlx.LocalPath(c.seedHost, j.u, urlx.Asset, c.cfg.Reserved)
	body := res.Body
	if res.IsCSS {
		fileDir := urlx.Dir(localFile)
		cssSink := func(u *url.URL, _ urlx.Kind) string {
			if !c.wantAsset(u) {
				return u.String() // off-domain or bulk media: leave it on the live web
			}
			c.enqueueAsset(ctx, u, j.u.String())
			local := urlx.LocalPath(c.seedHost, u, urlx.Asset, c.cfg.Reserved)
			return urlx.Rel(fileDir, local)
		}
		body = asset.RewriteCSS(body, j.u, cssSink)
	}
	if err := c.writeFile(localFile, body); err != nil {
		c.failAsset(j.u.String(), j.referer, fmt.Errorf("write %s: %w", localFile, err))
		return
	}
	c.stats.assets.Add(1)
}

// failAsset records and logs a failed asset, naming the page that referenced it
// so a 403 or 404 is traceable back to where it came from. The reason is
// classified (HTTP status, timeout, or other) for a readable line.
func (c *Cloner) failAsset(u, referer string, err error) {
	c.stats.assetErrors.Add(1)
	reason := classifyError(err)
	c.stats.recordFailure(Failure{Kind: "asset", URL: u, Referer: referer, Reason: reason})
	if referer != "" {
		c.logf("asset error: %s\n    %s\n    referenced by %s", reason, u, referer)
	} else {
		c.logf("asset error: %s\n    %s", reason, u)
	}
}

// failPage records and logs a failed page.
func (c *Cloner) failPage(u string, err error) {
	c.stats.pageErrors.Add(1)
	reason := classifyError(err)
	c.stats.recordFailure(Failure{Kind: "page", URL: u, Reason: reason})
	c.logf("page error: %s\n    %s", reason, u)
}

// classifyError turns an error into a short, human-readable reason for the log
// and the final report: an HTTP status with its name, a timeout, a cancellation,
// or the underlying message otherwise.
func classifyError(err error) string {
	if err == nil {
		return ""
	}
	var se *asset.StatusError
	if errors.As(err, &se) {
		return se.Error()
	}
	switch {
	case errors.Is(err, context.DeadlineExceeded):
		return "timed out"
	case errors.Is(err, context.Canceled):
		return "cancelled"
	}
	return err.Error()
}

// enqueuePage offers a page URL to the frontier, honouring the visited set, the
// depth cap, and the page budget. It reports whether the page was newly queued.
func (c *Cloner) enqueuePage(ctx context.Context, u *url.URL, depth int) bool {
	if c.cfg.MaxDepth > 0 && depth > c.cfg.MaxDepth {
		return false
	}
	key := c.pageKey(u)
	if c.front.isVisited(key) {
		return false
	}
	if !c.front.offer(key) {
		return false
	}
	c.mu.Lock()
	if c.cfg.MaxPages > 0 && c.enqueued >= c.cfg.MaxPages {
		c.mu.Unlock()
		return false
	}
	c.enqueued++
	c.mu.Unlock()

	c.wg.Add(1)
	go func() {
		select {
		case c.pageJobs <- pageItem{u: u, depth: depth}:
		case <-ctx.Done():
			c.wg.Done()
		}
	}()
	return true
}

// wantAsset reports whether an asset should be downloaded and localized, or left
// pointing at its live URL. kage skips two classes by default: assets on a host
// outside the seed's registrable domain (a third-party tracker, an unrelated
// CDN), and bulk media, installers, and archives whose extension is in the skip
// set. Both are decisions a page worker can make from the URL alone, before any
// download, so the rewritten HTML simply keeps the remote link.
func (c *Cloner) wantAsset(u *url.URL) bool {
	if c.cfg.AssetSameDomain && !urlx.SameRegistrableDomain(c.seed, u) {
		return false
	}
	if c.cfg.SkipAssetExts[urlx.Ext(u)] {
		return false
	}
	return true
}

// enqueueAsset offers an asset URL for download, deduping by canonical URL.
func (c *Cloner) enqueueAsset(ctx context.Context, u *url.URL, referer string) {
	key := c.assetKey(u)
	c.mu.Lock()
	if c.seenAssets[key] {
		c.mu.Unlock()
		return
	}
	c.seenAssets[key] = true
	c.mu.Unlock()

	c.wg.Add(1)
	go func() {
		select {
		case c.assetJobs <- assetItem{u: u, referer: referer}:
		case <-ctx.Done():
			c.wg.Done()
		}
	}()
}

// writePage writes a rendered page, deduping by content. The first page with a
// given byte content is written normally; a later page with identical bytes is
// stored as a hard link to that first file, so the same content never occupies
// disk twice (a faceted site whose ?q=… variants all render the same page is the
// motivating case). It reports whether this write was deduped. If hard links are
// unsupported, it falls back to writing the bytes, so correctness never depends
// on the link succeeding.
func (c *Cloner) writePage(relSlash string, data []byte) (bool, error) {
	sum := sha256.Sum256(data)
	h := string(sum[:])

	c.muContent.Lock()
	canon, seen := c.seenContent[h]
	if !seen {
		c.seenContent[h] = relSlash
	}
	c.muContent.Unlock()

	if seen && canon != relSlash {
		if err := c.linkFile(canon, relSlash); err == nil {
			return true, nil
		}
		// The canonical file may not be on disk yet (a concurrent first write)
		// or the filesystem may not support links; write the bytes instead.
	}
	return false, c.writeFile(relSlash, data)
}

// linkFile hard-links the already-written canonical file to target, replacing
// any file already at target. Both paths are confined to the mirror root.
func (c *Cloner) linkFile(canonSlash, targetSlash string) error {
	canonFull := filepath.Join(c.outRoot, filepath.FromSlash(canonSlash))
	targetFull := filepath.Join(c.outRoot, filepath.FromSlash(targetSlash))
	if !strings.HasPrefix(targetFull, filepath.Clean(c.outRoot)+string(os.PathSeparator)) {
		return fmt.Errorf("refusing to link outside mirror root: %s", targetSlash)
	}
	if err := os.MkdirAll(filepath.Dir(targetFull), 0o755); err != nil {
		return err
	}
	_ = os.Remove(targetFull)
	return os.Link(canonFull, targetFull)
}

// writeFile writes data to a slash path relative to the mirror root, creating
// parent directories. The path is cleaned so it can never escape the root.
func (c *Cloner) writeFile(relSlash string, data []byte) error {
	rel := filepath.FromSlash(relSlash)
	full := filepath.Join(c.outRoot, rel)
	if !strings.HasPrefix(full, filepath.Clean(c.outRoot)+string(os.PathSeparator)) && full != c.outRoot {
		return fmt.Errorf("refusing to write outside mirror root: %s", relSlash)
	}
	if err := os.MkdirAll(filepath.Dir(full), 0o755); err != nil {
		return err
	}
	return os.WriteFile(full, data, 0o644)
}

func max1(n int) int {
	if n < 1 {
		return 1
	}
	return n
}