// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package biblio turns the structured `refs` frontmatter of a post into
// a rendered reference list, machine-readable citations, and the
// resolver links a scholar expects. It is a leaf: it reads plain
// metadata values and writes HTML and JSON shapes, importing no other
// domain package.
//
// The shape is measured on the author's own volumes: numbered `[n]`
// entries, inline `[n]` citations, and `doi:` and `arXiv:` identifiers
// embedded in free text. The engine keeps that convention and makes it
// live: each entry gets a target the inline citations point at, and
// every identifier becomes a link to its resolver.
package biblio

import (
	stdhtml "html"
	"regexp"
	"strconv"
	"strings"
)

// Author is one cited author: a name, and the author's ORCID when the
// reference records one.
type Author struct {
	Name  string `json:"name"`
	ORCID string `json:"orcid,omitempty"`
}

// Entry is one reference. A hand-written entry that does not break down
// into the structured fields keeps its verbatim text in Raw, which the
// renderer prints as given, with any identifier inside it linked: a
// reference is never dropped or rewritten into something the author did
// not write.
type Entry struct {
	Num     int      `json:"num"`
	Authors []Author `json:"authors,omitempty"`
	Title   string   `json:"title,omitempty"`
	Venue   string   `json:"venue,omitempty"`
	Year    string   `json:"year,omitempty"`
	Volume  string   `json:"volume,omitempty"`
	Pages   string   `json:"pages,omitempty"`
	DOI     string   `json:"doi,omitempty"`
	ArXiv   string   `json:"arxiv,omitempty"`
	URL     string   `json:"url,omitempty"`
	Raw     string   `json:"raw,omitempty"`
	// Internal is the same-instance link: the API URL of the post whose
	// DOI this entry cites, set by the caller that knows the instance
	// (post.RefsLinked annotates each post's entries). Empty when the cited
	// work is not published here, or is the citing post itself.
	Internal string `json:"internal,omitempty"`
}

// href is the address the rendered link points at: the internal post
// when the entry cites a work published in this instance, otherwise
// the external resolver.
func (e Entry) href() string {
	if e.Internal != "" {
		return e.Internal
	}
	return e.Link()
}

// Marker is the token an author places where the reference list should
// be rendered; a body without it gets the list appended.
const Marker = "[[refs]]"

var (
	markerRe     = regexp.MustCompile(`(?s)<p>\s*\Q` + Marker + `\E\s*</p>`)
	doiTokenRe   = regexp.MustCompile(`(?i)\bdoi:\s*(10\.[0-9]{4,9}/[^\s]+)`)
	arxivTokenRe = regexp.MustCompile(`(?i)\barXiv:\s*([A-Za-z0-9][A-Za-z0-9./:-]*)`)
	yearRe       = regexp.MustCompile(`\b(1[89][0-9]{2}|20[0-9]{2})\b`)
	inlineRefRe  = regexp.MustCompile(`\[(\d{1,3})\]`)
)

// Parse reads the refs array out of flattened frontmatter tables. Each
// element may carry authors, title, venue, year, volume, pages, doi,
// arxiv, url or raw; authors is a list of strings or of {name, orcid}
// tables. Entries without an explicit number are numbered by position.
func Parse(refs []map[string]any) []Entry {
	var out []Entry
	for i, raw := range refs {
		entry := Entry{
			Num:    numOr(raw["num"], i+1),
			Title:  strings.TrimSpace(str(raw["title"])),
			Venue:  strings.TrimSpace(str(raw["venue"])),
			Year:   strings.TrimSpace(str(raw["year"])),
			Volume: strings.TrimSpace(str(raw["volume"])),
			Pages:  strings.TrimSpace(str(raw["pages"])),
			DOI:    cleanDOI(str(raw["doi"])),
			ArXiv:  cleanArxiv(str(raw["arxiv"])),
			URL:    strings.TrimSpace(str(raw["url"])),
			Raw:    strings.TrimSpace(str(raw["raw"])),
		}
		entry.Authors = parseAuthors(raw["authors"])
		entry.enrichFromText()
		out = append(out, entry)
	}
	return out
}

func parseAuthors(v any) []Author {
	list, ok := v.([]any)
	if !ok {
		return nil
	}
	var out []Author
	for _, item := range list {
		switch value := item.(type) {
		case string:
			if name := strings.TrimSpace(value); name != "" {
				out = append(out, Author{Name: name})
			}
		case map[string]any:
			author := Author{
				Name:  strings.TrimSpace(str(value["name"])),
				ORCID: strings.TrimSpace(str(value["orcid"])),
			}
			if author.Name != "" {
				out = append(out, author)
			}
		}
	}
	return out
}

func numOr(v any, fallback int) int {
	switch n := v.(type) {
	case int64:
		return int(n)
	case int:
		return n
	case float64:
		return int(n)
	case string:
		if parsed, err := strconv.Atoi(strings.TrimSpace(n)); err == nil {
			return parsed
		}
	}
	return fallback
}

func str(v any) string {
	s, _ := v.(string)
	return s
}

// cleanDOI accepts a bare identifier, a doi: prefix or a resolver URL
// and returns the bare form.
func cleanDOI(s string) string {
	s = strings.TrimSpace(s)
	if s == "" {
		return ""
	}
	if m := doiTokenRe.FindStringSubmatch(s); m != nil {
		return strings.TrimRight(m[1], ".,;)")
	}
	s = strings.TrimPrefix(strings.TrimPrefix(s, "https://doi.org/"), "doi:")
	return strings.TrimRight(strings.TrimSpace(s), ".,;)")
}

func cleanArxiv(s string) string {
	s = strings.TrimSpace(s)
	if s == "" {
		return ""
	}
	if m := arxivTokenRe.FindStringSubmatch(s); m != nil {
		s = m[1]
	}
	s = strings.TrimRight(strings.TrimPrefix(strings.TrimPrefix(s, "arXiv:"), "https://arxiv.org/abs/"), ".,;)")
	if !strings.HasPrefix(s, "arXiv:") && s != "" {
		// Already a bare id; accept anything identifier-shaped.
		if strings.ContainsAny(s, " \t<\"") {
			return ""
		}
		return s
	}
	return ""
}

// stripTokens removes the identifier tokens from the entry's own text,
// used once an entry is structured: the renderer puts the identifier at
// the end as a link, and it must not also sit in the title as prose.
func (e *Entry) stripTokens() {
	if e.Title == "" && len(e.Authors) == 0 {
		return // a raw entry prints its tokens inline as links instead
	}
	e.Title = tidyTokens(e.Title)
	e.Venue = tidyTokens(e.Venue)
}

func tidyTokens(s string) string {
	s = doiTokenRe.ReplaceAllString(s, "")
	s = arxivTokenRe.ReplaceAllString(s, "")
	s = strings.ReplaceAll(s, ", ,", ",")
	s = strings.TrimSpace(s)
	s = strings.TrimRight(s, ",;")
	return strings.TrimSpace(s)
}

// enrichFromText recovers identifiers and a year a hand-written entry
// kept in its free text, so a migrated volume gains live links without
// every field being split by hand.
func (e *Entry) enrichFromText() {
	joined := strings.Join([]string{e.Title, e.Venue, e.Raw}, " ")
	if e.DOI == "" {
		if m := doiTokenRe.FindStringSubmatch(joined); m != nil {
			e.DOI = strings.TrimRight(m[1], ".,;)")
			e.stripTokens()
		}
	}
	if e.ArXiv == "" {
		if m := arxivTokenRe.FindStringSubmatch(joined); m != nil {
			e.ArXiv = m[1]
			e.stripTokens()
		}
	}
	// The year is a structured field only when the entry is structured:
	// a raw entry already prints its year inside its own text.
	if e.Year == "" && (e.Title != "" || len(e.Authors) > 0) {
		if m := yearRe.FindStringSubmatch(joined); m != nil {
			e.Year = m[1]
		}
	}
}

// Link returns the resolver URL for this entry: DOI first, then arXiv,
// then a plain URL. Empty when the entry carries no identifier.
func (e Entry) Link() string {
	switch {
	case e.DOI != "":
		return "https://doi.org/" + e.DOI
	case e.ArXiv != "":
		return "https://arxiv.org/abs/" + e.ArXiv
	case e.URL != "":
		return e.URL
	default:
		return ""
	}
}

// ListHTML renders the numbered reference list. Each entry is anchored
// so an inline citation can point at it.
func ListHTML(entries []Entry) string {
	if len(entries) == 0 {
		return ""
	}
	var b strings.Builder
	b.WriteString(`<section class="refs" id="references">` + "\n<ol>\n")
	for _, e := range entries {
		b.WriteString(`<li id="ref-` + strconv.Itoa(e.Num) + `">`)
		b.WriteString(e.line())
		b.WriteString("</li>\n")
	}
	b.WriteString("</ol>\n</section>\n")
	return b.String()
}

// line renders one entry: authors, title, the venue tail, and the
// identifier link. A purely raw entry prints its verbatim text with
// embedded identifiers made clickable, and nothing else: the author's
// own wording already carries the year and venue.
func (e Entry) line() string {
	if e.Raw != "" && e.Title == "" && len(e.Authors) == 0 {
		return e.identifierLinks()
	}
	var b strings.Builder
	if len(e.Authors) > 0 {
		names := make([]string, 0, len(e.Authors))
		for _, a := range e.Authors {
			name := stdhtml.EscapeString(a.Name)
			if a.ORCID != "" {
				name += " " + link("https://orcid.org/"+a.ORCID, "orcid:"+a.ORCID)
			}
			names = append(names, name)
		}
		// An author string that already ends in a period ("Riess, A. G.
		// a kol.") must not gain a second one at the segment boundary.
		authorText := strings.Join(names, ", ")
		b.WriteString(authorText)
		if !strings.HasSuffix(authorText, ".") {
			b.WriteString(".")
		}
		b.WriteString(" ")
	}
	if e.Title != "" {
		b.WriteString(stdhtml.EscapeString(e.Title) + ".")
	}
	tail := make([]string, 0, 4)
	if e.Venue != "" {
		tail = append(tail, stdhtml.EscapeString(e.Venue))
	}
	if e.Volume != "" {
		tail = append(tail, "vol. "+stdhtml.EscapeString(e.Volume))
	}
	if e.Pages != "" {
		tail = append(tail, "pp. "+stdhtml.EscapeString(e.Pages))
	}
	if e.Year != "" {
		tail = append(tail, stdhtml.EscapeString(e.Year))
	}
	if len(tail) > 0 {
		if e.Title != "" {
			b.WriteString(" ")
		}
		b.WriteString(strings.Join(tail, ", "))
		b.WriteString(".")
	}
	if url := e.href(); url != "" {
		b.WriteString(" " + link(url, label(e)))
	}
	return b.String()
}

// label names the identifier link by whichever resolver it uses.
func label(e Entry) string {
	switch {
	case e.DOI != "":
		return "doi:" + e.DOI
	case e.ArXiv != "":
		return "arXiv:" + e.ArXiv
	default:
		return "url"
	}
}

// identifierLinks prints a raw entry with each doi:/arXiv: token it
// embeds replaced by a live link, keeping the author's own wording
// around it untouched. A token equal to the entry's cited DOI takes the
// internal link when the entry has one; every other token keeps its
// resolver.
func (e Entry) identifierLinks() string {
	raw, doi, arxiv := e.Raw, e.DOI, e.ArXiv
	html := stdhtml.EscapeString(raw)
	if doi != "" {
		html = doiTokenRe.ReplaceAllStringFunc(html, func(match string) string {
			id := strings.TrimRight(doiTokenRe.FindStringSubmatch(match)[1], ".,;)")
			trail := match[len("doi:"):]
			trail = trail[strings.Index(trail, id)+len(id):]
			href := "https://doi.org/" + id
			if e.Internal != "" && id == doi {
				// The slug comes from the store, so it is escaped like
				// every other href: a hand-edited file whose slug carries
				// a quote must not open an attribute here.
				href = stdhtml.EscapeString(e.Internal)
			}
			return `<a class="refs-link" href="` + href + `">doi:` + id + `</a>` + trail
		})
	}
	if arxiv != "" {
		html = arxivTokenRe.ReplaceAllStringFunc(html, func(match string) string {
			id := arxivTokenRe.FindStringSubmatch(match)[1]
			body := "arXiv:" + id
			trail := match[len(body):]
			return `<a class="refs-link" href="https://arxiv.org/abs/` + id + `">arXiv:` + id + `</a>` + trail
		})
	}
	return html
}

// Citations renders the entries as schema.org citation objects for the
// post's JSON-LD block, so a machine reading the article also reads the
// works it cites, with each author and identifier resolved.
func Citations(entries []Entry) []map[string]any {
	var out []map[string]any
	for _, e := range entries {
		name := e.Title
		if name == "" {
			name = e.Raw
		}
		if name == "" {
			continue
		}
		c := map[string]any{"@type": "ScholarlyArticle", "position": e.Num, "name": name}
		if len(e.Authors) > 0 {
			persons := make([]map[string]any, 0, len(e.Authors))
			for _, a := range e.Authors {
				person := map[string]any{"@type": "Person", "name": a.Name}
				if a.ORCID != "" {
					person["identifier"] = "https://orcid.org/" + a.ORCID
				}
				persons = append(persons, person)
			}
			c["author"] = persons
		}
		if e.Venue != "" {
			c["isPartOf"] = map[string]any{"@type": "PublicationJournal", "name": e.Venue}
		}
		if e.Year != "" {
			c["datePublished"] = e.Year
		}
		if url := e.Link(); url != "" {
			c["identifier"] = url
		}
		if e.Internal != "" {
			c["url"] = e.Internal
		}
		out = append(out, c)
	}
	return out
}

func link(href, text string) string {
	return `<a class="refs-link" href="` + stdhtml.EscapeString(href) + `">` +
		stdhtml.EscapeString(text) + "</a>"
}

// Place splices the list into the rendered body: at the marker
// paragraph when the author wrote one, otherwise appended.
func Place(bodyHTML string, entries []Entry) string {
	list := ListHTML(entries)
	if list == "" {
		return bodyHTML
	}
	if markerRe.MatchString(bodyHTML) {
		return markerRe.ReplaceAllString(bodyHTML, list)
	}
	return strings.TrimRight(bodyHTML, "\n") + "\n" + list
}

// LinkCitations rewrites inline `[n]` markers in the rendered HTML into
// links to the numbered entries. It walks the markup and touches text
// only: tags and attribute values are copied verbatim, and the inside
// of <code>, <pre> and <a> elements is left alone, so code that holds
// bracketed numbers and the reference list itself keep their text.
func LinkCitations(html string, entries []Entry) string {
	if len(entries) == 0 {
		return html
	}
	highest := 0
	for _, e := range entries {
		if e.Num > highest {
			highest = e.Num
		}
	}
	var b strings.Builder
	b.Grow(len(html) + 2*len(html)/8)
	i := 0
	skipping := "" // non-empty while inside a protected element
	for i < len(html) {
		if html[i] != '<' {
			j := i
			for j < len(html) && html[j] != '<' {
				j++
			}
			b.WriteString(citeText(html[i:j], highest, skipping != ""))
			i = j
			continue
		}
		end := strings.IndexByte(html[i:], '>')
		if end < 0 {
			b.WriteString(html[i:])
			break
		}
		tag := html[i : i+end+1]
		b.WriteString(tag)
		switch {
		case skipping != "":
			// Inside a protected element: wait for its close.
			closing := "</" + skipping + ">"
			if pos := strings.Index(strings.ToLower(tag), closing); pos >= 0 {
				skipping = ""
			}
		case strings.HasPrefix(strings.ToLower(tag), "<pre"),
			strings.HasPrefix(strings.ToLower(tag), "<code"),
			strings.HasPrefix(strings.ToLower(tag), "<a"):
			if !strings.HasSuffix(tag, "/>") {
				kind := "a"
				switch {
				case strings.HasPrefix(strings.ToLower(tag), "<pre"):
					kind = "pre"
				case strings.HasPrefix(strings.ToLower(tag), "<code"):
					kind = "code"
				}
				skipping = kind
			}
		}
		i += end + 1
	}
	return b.String()
}

// citeText replaces the bracketed numbers within one text run when
// linking is allowed there.
func citeText(text string, highest int, protected bool) string {
	if protected || highest == 0 {
		return text
	}
	return inlineRefRe.ReplaceAllStringFunc(text, func(tok string) string {
		n, err := strconv.Atoi(strings.Trim(tok, "[]"))
		if err != nil || n < 1 || n > highest {
			return tok
		}
		return `<a class="ref-cite" href="#ref-` + strconv.Itoa(n) + `">` + tok + "</a>"
	})
}