// Package web renders lectio's daily readings as an HTML fragment and
// serves a set of "colour only" theme stylesheets (see docs/THEMES.md) that
// restyle a fixed set of role classes/variables; it never lays out or types
// anything itself -- that lives in the embedded base.css. It consumes
// liturgy.Section and render.GatherVersion, never the reverse.
package web

import (
	"bytes"
	"embed"
	"fmt"
	"html/template"
	"io/fs"
	"os"
	"path"
	"path/filepath"
	"regexp"
	"sort"
	"strings"

	"github.com/lukaszkasprzak/lectio/internal/bible"
	"github.com/lukaszkasprzak/lectio/internal/liturgy"
	"github.com/lukaszkasprzak/lectio/internal/render"
)

//go:embed templates
var templatesFS embed.FS

//go:embed static
var staticFS embed.FS

// tmpl holds every parsed templates/*.html, keyed by base file name (so
// "readings.html" is looked up as "readings.html").
var tmpl = template.Must(template.ParseFS(templatesFS, "templates/*.html"))

// verseNumRe matches a bible verse block's "chapter:verse  " prefix, as
// produced by render.GatherVersion (e.g. "3:16  Tak bowiem..."). Mirrors
// internal/tui's verseNumRe.
var verseNumRe = regexp.MustCompile(`^(\d+:\d+)  (.*)$`)

// sectionView, columnView and blockView are the data readings.html ranges
// over: one sectionView per liturgy.Section, one columnView per requested
// version, one blockView per render.GatherVersion block.
type sectionView struct {
	Heading, Subtitle, PartID string
	Columns                   []columnView
}

type columnView struct {
	Label  string
	Blocks []blockView
}

// blockView is one paragraph or verse line. VNum is set (and Text holds
// only the verse text) when the block was a bible verse line; otherwise
// VNum is empty and Text holds the whole block, with Refrain set for a
// responsorial psalm's deduped first ("pl") block.
type blockView struct {
	VNum, Text string
	Refrain    bool
}

// ilSectionView, ilVerseView and ilLineView are what
// templates/readings-interlinear.html ranges over: one ilSectionView per
// liturgy.Section, one ilVerseView per chapter:verse key (in ordered-union
// order, see buildInterlinearViews), one ilLineView per version that carries
// that verse. Note holds a short escaped message in place of Verses when no
// requested version could be interleaved for the section.
type ilSectionView struct {
	Heading, Subtitle, PartID string
	Verses                    []ilVerseView
	Note                      string
}

type ilVerseView struct {
	VNum  string
	Lines []ilLineView
}

type ilLineView struct {
	Label, Text string
}

// RenderReadings builds the reading pane fragment for one of three layouts:
//
//   - "horizontal" (or anything unrecognized): the original stacked layout,
//     one column per version rendered under a shared heading, unchanged.
//   - "vertical": the same per-version columns side by side in a
//     ".display-vertical" grid, like the CLI `compare` view.
//   - "interlinear": versions interleaved verse-by-verse by chapter:verse
//     (see buildInterlinearViews); "pl" cannot participate (no verse
//     numbers) and is substituted/dropped via render.OfflineVersions'
//     pl->wuj transform before gathering.
//
// In every mode, heading, citation (subtitle), verse-number and refrain
// text are wrapped in class="heading|citation|vnum|refrain|version-label"
// spans so theme CSS can restyle them; verse/paragraph text is escaped by
// html/template. lang localises the version-column labels and each section
// heading's part-label word (render.LocalizeHeading, brief §3b); the
// citation/verse text is never touched.
func RenderReadings(secs []liturgy.Section, versions []string, lectionary, display, lang string) template.HTML {
	switch display {
	case "vertical":
		return renderTemplate("readings-vertical.html", buildColumnViews(secs, versions, lectionary, lang))
	case "interlinear":
		return renderTemplate("readings-interlinear.html", buildInterlinearViews(secs, versions, lectionary, lang))
	default:
		return renderTemplate("readings.html", buildColumnViews(secs, versions, lectionary, lang))
	}
}

// renderTemplate executes the named embedded template with data, degrading
// to a visible, escaped error paragraph on failure (should be unreachable:
// the templates are embedded and fixed at build time) rather than panicking
// a request handler in the caller.
func renderTemplate(name string, data any) template.HTML {
	var buf bytes.Buffer
	if err := tmpl.ExecuteTemplate(&buf, name, data); err != nil {
		return template.HTML("<p class=\"error\">" + template.HTMLEscapeString(err.Error()) + "</p>")
	}
	return template.HTML(buf.String())
}

// buildColumnViews gathers each section's per-version columns via
// render.GatherVersion -- the shared data both the "horizontal"
// (readings.html) and "vertical" (readings-vertical.html) templates range
// over; only the surrounding markup differs between the two layouts. lang
// localises the column labels and the heading's part-label word (see
// RenderReadings).
func buildColumnViews(secs []liturgy.Section, versions []string, lectionary, lang string) []sectionView {
	views := make([]sectionView, 0, len(secs))
	for _, sec := range secs {
		isPsalm := sec.PartID == "psalm"

		cols := make([]columnView, 0, len(versions))
		for _, v := range versions {
			label, blocks := render.GatherVersion(v, sec, lectionary, lang)

			bviews := make([]blockView, 0, len(blocks))
			for bi, b := range blocks {
				// A bible verse line always carries its "chapter:verse  "
				// prefix; only text without one (i.e. the "pl" version's
				// paragraphs) can be the deduped psalm refrain -- matches
				// internal/tui's styleBlock precedence.
				if m := verseNumRe.FindStringSubmatch(b); m != nil {
					bviews = append(bviews, blockView{VNum: m[1], Text: m[2]})
					continue
				}
				bviews = append(bviews, blockView{Text: b, Refrain: isPsalm && bi == 0})
			}
			cols = append(cols, columnView{Label: label, Blocks: bviews})
		}

		views = append(views, sectionView{
			Heading:  render.LocalizeHeading(sec.Heading, sec.PartID, lang),
			Subtitle: sec.Subtitle,
			PartID:   sec.PartID,
			Columns:  cols,
		})
	}
	return views
}

// interlinearVersions maps versions through the same pl->wuj substitution
// render.OfflineVersions performs for offline mode: "pl" (niedziela.pl
// paragraph text) carries no verse numbers and cannot interleave, so it is
// dropped, substituting "wuj" (the Polish-language bible version) in its
// place unless "wuj" was already selected. Reuses render.OfflineVersions
// rather than duplicating its two-line transform.
func interlinearVersions(versions []string) []string {
	return render.OfflineVersions(versions)
}

// buildInterlinearViews gathers each requested version's verses via
// render.GatherVerses (after the pl->wuj substitution, see
// interlinearVersions) and interleaves them by chapter:verse: the ordered
// union of keys is taken from the first versified version's own verse
// order, then any keys that only appear in a later version are appended in
// that version's order (stable, no duplicates). A version that comes back
// unversified (a bible lookup miss) is simply skipped -- the remaining
// versions still render. A section where nothing could be interleaved gets
// a short escaped Note instead of an empty Verses list. lang localises the
// per-version labels and the heading's part-label word (see RenderReadings).
func buildInterlinearViews(secs []liturgy.Section, versions []string, lectionary, lang string) []ilSectionView {
	mapped := interlinearVersions(versions)

	type verseSet struct {
		label  string
		verses []bible.Verse
	}

	views := make([]ilSectionView, 0, len(secs))
	for _, sec := range secs {
		view := ilSectionView{Heading: render.LocalizeHeading(sec.Heading, sec.PartID, lang), Subtitle: sec.Subtitle, PartID: sec.PartID}

		var sets []verseSet
		for _, v := range mapped {
			label, verses, versified := render.GatherVerses(v, sec, lectionary, lang)
			if !versified {
				continue
			}
			sets = append(sets, verseSet{label: label, verses: verses})
		}

		if len(sets) == 0 {
			view.Note = "(brak wersetów do zestawienia interlinearnego)"
			views = append(views, view)
			continue
		}

		type vkey struct{ chapter, verse int }
		order := make([]vkey, 0)
		seen := map[vkey]bool{}
		bySet := make([]map[vkey]bible.Verse, len(sets))
		for i, s := range sets {
			m := make(map[vkey]bible.Verse, len(s.verses))
			for _, v := range s.verses {
				k := vkey{v.Chapter, v.Verse}
				m[k] = v
				if !seen[k] {
					seen[k] = true
					order = append(order, k)
				}
			}
			bySet[i] = m
		}

		vviews := make([]ilVerseView, 0, len(order))
		for _, k := range order {
			var lines []ilLineView
			for i, s := range sets {
				if v, ok := bySet[i][k]; ok {
					lines = append(lines, ilLineView{Label: s.label, Text: v.Text})
				}
			}
			vviews = append(vviews, ilVerseView{
				VNum:  fmt.Sprintf("%d:%d", k.chapter, k.verse),
				Lines: lines,
			})
		}
		view.Verses = vviews
		views = append(views, view)
	}
	return views
}

// Themes returns the sorted, deduplicated union of the embedded theme
// stems (static/themes/*.css) and the *.css stems found in the user theme
// directory (${XDG_CONFIG_HOME:-~/.config}/lectio/themes/); a user theme
// overrides a built-in of the same name but does not add a second entry.
func Themes() []string {
	set := map[string]bool{}
	for _, name := range themeStems(staticFS, "static/themes") {
		set[name] = true
	}
	if dir, err := userThemesDir(); err == nil {
		for _, name := range themeStems(os.DirFS(dir), ".") {
			set[name] = true
		}
	}

	names := make([]string, 0, len(set))
	for name := range set {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}

// themeNameRe is the allowlist a theme name must match: letters, digits,
// underscore, hyphen only. Legitimate theme stems (built-in or user) already
// fit this shape; it is deliberately stricter than "no path separators" so
// it rejects "." / ".." / any other filesystem metacharacter outright
// instead of trying to enumerate what's unsafe.
var themeNameRe = regexp.MustCompile(`^[A-Za-z0-9_-]+$`)

// themeCSS returns one theme's CSS: the user file
// ${XDG_CONFIG_HOME:-~/.config}/lectio/themes/<name>.css if it exists,
// otherwise the embedded static/themes/<name>.css, otherwise an error.
// name must match themeNameRe -- callers (B2's /theme.css?name= handler)
// pass this straight through from an HTTP query parameter, so this rejects
// path traversal rather than trusting it.
func themeCSS(name string) ([]byte, error) {
	if !themeNameRe.MatchString(name) {
		return nil, fmt.Errorf("web: invalid theme name %q", name)
	}

	if dir, err := userThemesDir(); err == nil {
		if b, err := os.ReadFile(filepath.Join(dir, name+".css")); err == nil {
			return b, nil
		}
	}

	b, err := staticFS.ReadFile(path.Join("static", "themes", name+".css"))
	if err != nil {
		return nil, fmt.Errorf("web: unknown theme %q", name)
	}
	return b, nil
}

// userThemesDir resolves ${XDG_CONFIG_HOME:-~/.config}/lectio/themes via
// os.UserConfigDir(), which already honors XDG_CONFIG_HOME on Linux.
func userThemesDir() (string, error) {
	dir, err := os.UserConfigDir()
	if err != nil {
		return "", err
	}
	return filepath.Join(dir, "lectio", "themes"), nil
}

// themeStems lists the *.css stems (base name, no extension) directly in
// dir of fsys, silently returning none if dir doesn't exist or isn't
// readable -- a missing user theme directory is normal, not an error.
func themeStems(fsys fs.FS, dir string) []string {
	entries, err := fs.ReadDir(fsys, dir)
	if err != nil {
		return nil
	}
	var names []string
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".css") {
			continue
		}
		names = append(names, strings.TrimSuffix(e.Name(), ".css"))
	}
	return names
}