// Package classes generates the sitemap: it walks the definition items,
// resolves registry items through the framework's menu item type registry
// and writes the PHP-identical XML.
package classes

import (
	"context"
	"time"

	"git.golem15.com/golem15/sm-sitemap-plugin/models"
	"git.golem15.com/golem15/summercms/modules/festival"
	"git.golem15.com/golem15/summercms/modules/pact"
)

// MaxURLs is the sitemap protocol's URL limit (PHP Definition::MAX_URLS):
// the 50000th URL is written, the 50001st is not.
const MaxURLs = 50000

// lastmodLayout is PHP date('c') in UTC: "+00:00", never "Z".
const lastmodLayout = "2006-01-02T15:04:05-07:00"

// Resolver resolves a registry item; *signpost.Registry implements it.
type Resolver interface {
	Resolve(ctx context.Context, key, reference, itemPattern string, nesting bool, unprefixedLocale string) (*pact.ResolvedMenuItem, error)
}

// Generator writes the sitemap of a definition, following PHP
// Definition::generateSitemap and addItemToSet.
//
// Root is the backend root URL (scheme and host without a trailing slash)
// that Url::to builds absolute URLs from. Resolver resolves registry items;
// nil skips them. Settings are the sitemap settings (tiers, priorities,
// frontend default locale, manifest path), Tiers the compiled tier rules,
// and Events the bus BeforeAddItem and AddItem are fired on (nil fires
// nothing). Manifest, when set, is used instead of reading the settings'
// updates_manifest_path.
type Generator struct {
	Root     string
	Resolver Resolver
	Settings models.Settings
	Tiers    Tiers
	Events   *festival.Bus
	Manifest Manifest

	manifestLoaded bool
	entries        []URLEntry
	count          int
}

// itemInfo is PHP's $itemInfo: the URL (set marks it present), the record's
// modification time and the hreflang alternates.
type itemInfo struct {
	url        string
	set        bool
	mtime      *time.Time
	alternates []Alternate
}

// Generate returns the sitemap XML of items, or "" when there are none (PHP
// generateSitemap). A url item is added with its URL. A registry item is
// resolved with its URL pattern, its nesting and the settings' frontend
// default locale as the unprefixed locale; its URL is added when it has one,
// then its sub-items: when the item has no URL of its own (a list type),
// each sub-item is added once per alternate, with that alternate's URL as
// loc and every alternate as hreflang, as the PHP list types returned them;
// otherwise each sub-item is added once with its own URL. A type the
// registry does not know, or a Resolve that finds nothing, adds nothing.
//
// Generate does not apply the frontend_url rewrite; the route does
// (Rewrite).
func (g *Generator) Generate(ctx context.Context, items []models.DefinitionItem) (string, error) {
	if len(items) == 0 {
		return "", nil
	}
	if ctx == nil {
		ctx = context.Background()
	}
	g.entries, g.count = nil, 0
	for i := range items {
		item := &items[i]
		if item.Type == models.ItemTypeURL {
			if err := g.addItem(ctx, item, itemInfo{}); err != nil {
				return "", err
			}
			continue
		}
		resolved, err := g.resolve(ctx, item)
		if err != nil {
			return "", err
		}
		if resolved == nil {
			continue
		}
		if resolved.URL != "" {
			if err := g.addItem(ctx, item, infoOf(*resolved)); err != nil {
				return "", err
			}
			if err := g.addChildren(ctx, item, resolved.Items); err != nil {
				return "", err
			}
			continue
		}
		if err := g.addPerLocale(ctx, item, resolved.Items); err != nil {
			return "", err
		}
	}
	return Serialize(g.Stylesheet(), g.entries), nil
}

// Stylesheet is the stylesheet href PHP built:
// Url::buildUrl(Url::current(), ['path' => 'sitemap.xsl']).
func (g *Generator) Stylesheet() string {
	return replacePath(URLTo(g.Root, "/sitemap.xml"), "sitemap.xsl")
}

func (g *Generator) resolve(ctx context.Context, item *models.DefinitionItem) (*pact.ResolvedMenuItem, error) {
	if g.Resolver == nil || item.Type == "" {
		return nil, nil
	}
	if reg, ok := g.Resolver.(interface {
		Type(string) (pact.MenuItemType, bool)
	}); ok {
		if _, known := reg.Type(item.Type); !known {
			return nil, nil
		}
	}
	return g.Resolver.Resolve(ctx, item.Type, models.Text(item.Reference), models.Text(item.URLPattern), item.Nesting, models.Text(g.Settings.FrontendDefaultLocale))
}

// addChildren adds every child with a URL, then its own children.
func (g *Generator) addChildren(ctx context.Context, item *models.DefinitionItem, children []pact.ResolvedMenuItem) error {
	for _, child := range children {
		if child.URL != "" {
			if err := g.addItem(ctx, item, infoOf(child)); err != nil {
				return err
			}
		}
		if err := g.addChildren(ctx, item, child.Items); err != nil {
			return err
		}
	}
	return nil
}

// addPerLocale adds the children of a list type: each once per alternate
// (once with its URL when it has no alternates), then its own children.
func (g *Generator) addPerLocale(ctx context.Context, item *models.DefinitionItem, children []pact.ResolvedMenuItem) error {
	for _, child := range children {
		info := infoOf(child)
		if len(child.Alternates) == 0 {
			if child.URL != "" {
				if err := g.addItem(ctx, item, info); err != nil {
					return err
				}
			}
			if err := g.addChildren(ctx, item, child.Items); err != nil {
				return err
			}
			continue
		}
		for _, alt := range child.Alternates {
			perLocale := info
			perLocale.url = alt.URL
			if perLocale.url != "" {
				if err := g.addItem(ctx, item, perLocale); err != nil {
					return err
				}
			}
			if err := g.addChildren(ctx, item, child.Items); err != nil {
				return err
			}
		}
	}
	return nil
}

func infoOf(r pact.ResolvedMenuItem) itemInfo {
	info := itemInfo{url: r.URL, set: true, mtime: r.MTime}
	for _, a := range r.Alternates {
		info.alternates = append(info.alternates, Alternate{Hreflang: a.Locale, Href: a.URL})
	}
	return info
}

func (g *Generator) manifest() Manifest {
	if !g.manifestLoaded {
		if g.Manifest == nil {
			g.Manifest = LoadManifest(models.Text(g.Settings.UpdatesManifestPath))
		}
		g.manifestLoaded = true
	}
	return g.Manifest
}

// addItem is PHP Definition::addItemToSet, in its order: the URL cap, a
// missing URL filled through Url::to, lastmod from the record or the
// manifest, BeforeAddItem, an empty URL dropped and the URL passed through
// Url::to again, the count, the tier or item priority, the elements and
// AddItem. Unlike PHP, a URL or alternate that does not come out as an
// http or https URL is dropped.
func (g *Generator) addItem(ctx context.Context, item *models.DefinitionItem, info itemInfo) error {
	if g.count >= MaxURLs {
		return nil
	}
	if !info.set {
		info.url = URLTo(g.Root, models.Text(item.URL))
	}
	var lastmod time.Time
	hasLastmod := false
	if info.mtime != nil {
		lastmod, hasLastmod = *info.mtime, true
	} else if t, ok := g.manifest().Lastmod(info.url); ok {
		lastmod, hasLastmod = t, true
	}
	lastmodText := ""
	if hasLastmod && lastmod.Unix() != 0 {
		lastmodText = lastmod.UTC().Format(lastmodLayout)
	}
	if g.Events != nil {
		before := &BeforeAddItem{Item: *item, Info: ItemInfo{URL: info.url, MTime: info.mtime, Lastmod: lastmodText, Alternates: append([]Alternate(nil), info.alternates...)}}
		skipped, err := g.Events.UntilHandled(ctx, before)
		if err != nil {
			return err
		}
		if skipped {
			return nil
		}
	}
	if info.url == "" || info.url == "0" {
		return nil
	}
	loc := URLTo(g.Root, info.url)
	if !httpURL(loc) {
		// Only http and https URLs are written: a javascript: or other
		// scheme that an import, a registry type or a pattern let through
		// would be a clickable link in sitemap.xsl.
		return nil
	}
	g.count++
	priority, ok := g.Tiers.Priority(loc, g.Settings)
	if !ok {
		priority = models.Text(item.Priority)
	}
	entry := URLEntry{
		Loc:        loc,
		Lastmod:    lastmodText,
		Changefreq: models.Text(item.Changefreq),
		Priority:   priority,
	}
	if len(info.alternates) > 1 {
		for _, a := range info.alternates {
			href := URLTo(g.Root, a.Href)
			if !httpURL(href) {
				continue
			}
			entry.Alternates = append(entry.Alternates, Alternate{Hreflang: a.Hreflang, Href: href})
		}
	}
	if g.Events != nil {
		added := &AddItem{Item: *item, Info: ItemInfo{URL: loc, MTime: info.mtime, Lastmod: lastmodText, Alternates: append([]Alternate(nil), entry.Alternates...)}}
		if err := g.Events.Fire(ctx, added); err != nil {
			return err
		}
		entry.Extra = added.Extra
	}
	g.entries = append(g.entries, entry)
	return nil
}