package functions

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"html"
	"io"
	"strings"
	"text/template"

	"github.com/caddyserver/caddy/v2"
	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
	"github.com/caddyserver/caddy/v2/modules/caddyhttp/templates"
)

func init() {
	caddy.RegisterModule(Gemtext{})
	httpcaddyfile.RegisterDirective(
		"gemtext",
		func(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error) {
			var f Gemtext
			err := f.UnmarshalCaddyfile(h.Dispenser)
			return []httpcaddyfile.ConfigValue{{
				Class: "template_function", Value: f,
			}}, err
		},
	)
}

type Gemtext struct{}

var _ templates.CustomFunctions = (*Gemtext)(nil)

func (f *Gemtext) CustomTemplateFunctions() template.FuncMap {
	return template.FuncMap{
		"gemtext": f.funcGemtext,
	}
}

func (Gemtext) CaddyModule() caddy.ModuleInfo {
	return caddy.ModuleInfo{
		ID:  "http.handlers.templates.functions.gemtext",
		New: func() caddy.Module { return new(Gemtext) },
	}
}

func sanitizeText(str string) string {
	return html.EscapeString(strings.TrimSpace(str))
}

func (*Gemtext) funcGemtext(input any) (string, error) {
	var (
		r         = bufio.NewReader(strings.NewReader(caddy.ToString(input)))
		w         = new(bytes.Buffer)
		pft, list bool
		writeErr  error
	)

	write := func(fmtStr string, args ...any) {
		if writeErr != nil {
			return
		}
		fmt.Fprintf(w, fmtStr, args...)
	}

loop:
	for {
		if writeErr != nil {
			return "", fmt.Errorf("writing line: %w", writeErr)
		}

		line, err := r.ReadString('\n')

		switch {
		case errors.Is(err, io.EOF):
			break loop

		case err != nil:
			return "", fmt.Errorf("reading next line: %w", err)

		case strings.HasPrefix(line, "```"):
			if !pft {
				write("<pre>\n")
				pft = true
			} else {
				write("</pre>\n")
				pft = false
			}
			continue

		case pft:
			write(line)
			continue

		case len(strings.TrimSpace(line)) == 0:
			continue
		}

		// list case is special, because it requires a prefix and suffix tag
		if strings.HasPrefix(line, "*") {
			if !list {
				write("<ul>\n")
			}
			write("<li>%s</li>\n", sanitizeText(line[1:]))
			list = true
			continue
		} else if list {
			write("</ul>\n")
			list = false
		}

		switch {
		case strings.HasPrefix(line, "=>"):
			// TODO convert gemini:// links ?
			var (
				line   = strings.TrimSpace(line[2:])
				urlStr = line
				label  = urlStr
			)
			if i := strings.IndexAny(urlStr, " \t"); i > -1 {
				urlStr, label = urlStr[:i], sanitizeText(urlStr[i:])
			}
			write("<p><a href=\"%s\">%s</a></p>\n", urlStr, label)

		case strings.HasPrefix(line, "###"):
			write("<h3>%s</h3>\n", sanitizeText(line[3:]))

		case strings.HasPrefix(line, "##"):
			write("<h2>%s</h2>\n", sanitizeText(line[2:]))

		case strings.HasPrefix(line, "#"):
			write("<h1>%s</h1>\n", sanitizeText(line[1:]))

		case strings.HasPrefix(line, ">"):
			write("<blockquote>%s</blockquote>\n", sanitizeText(line[1:]))

		default:
			line = strings.TrimSpace(line)
			write("<p>%s</p>\n", line)
		}
	}

	return w.String(), nil
}

// UnmarshalCaddyfile implements caddyfile.Unmarshaler.
func (*Gemtext) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
	d.Next() // consume directive name
	return nil
}