// mautrix-discord - A Matrix-Discord puppeting bridge.
// Copyright (C) 2024 Tulir Asokan
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.

package msgconv

import (
	"fmt"
	"regexp"
	"strings"

	"github.com/yuin/goldmark"
	"github.com/yuin/goldmark/extension"
	"github.com/yuin/goldmark/parser"
	"github.com/yuin/goldmark/util"
	"maunium.net/go/mautrix/bridgev2"
	"maunium.net/go/mautrix/format"
	"maunium.net/go/mautrix/format/mdext"
)

// escapeFixer is a hacky partial fix for the difference in escaping markdown, used with escapeReplacement
//
// Discord allows escaping with just one backslash, e.g. \__a__,
// but standard markdown requires both to be escaped (\_\_a__)
var escapeFixer = regexp.MustCompile(`\\(__[^_]|\*\*[^*])`)

func escapeReplacement(s string) string {
	return s[:2] + `\` + s[2:]
}

// indentableParagraphParser is the default paragraph parser with CanAcceptIndentedLine.
// Used when disabling CodeBlockParser (as disabling it without a replacement will make indented blocks disappear).
type indentableParagraphParser struct {
	parser.BlockParser
}

var defaultIndentableParagraphParser = &indentableParagraphParser{BlockParser: parser.NewParagraphParser()}

func (b *indentableParagraphParser) CanAcceptIndentedLine() bool {
	return true
}

var removeFeaturesExceptLinks = []any{
	parser.NewListParser(), parser.NewListItemParser(), parser.NewHTMLBlockParser(), parser.NewRawHTMLParser(),
	parser.NewSetextHeadingParser(), parser.NewThematicBreakParser(),
	parser.NewCodeBlockParser(),
}
var removeFeaturesAndLinks = append(removeFeaturesExceptLinks, parser.NewLinkParser())
var fixIndentedParagraphs = goldmark.WithParserOptions(parser.WithBlockParsers(util.Prioritized(defaultIndentableParagraphParser, 500)))
var discordExtensions = goldmark.WithExtensions(extension.Strikethrough, mdext.SimpleSpoiler, mdext.DiscordUnderline, ExtDiscordEveryone, ExtDiscordTag)

var discordRenderer = goldmark.New(
	goldmark.WithParser(mdext.ParserWithoutFeatures(removeFeaturesAndLinks...)),
	fixIndentedParagraphs, format.HTMLOptions, discordExtensions,
)
var discordRendererWithInlineLinks = goldmark.New(
	goldmark.WithParser(mdext.ParserWithoutFeatures(removeFeaturesExceptLinks...)),
	fixIndentedParagraphs, format.HTMLOptions, discordExtensions,
)

// renderDiscordMarkdownOnlyHTML converts Discord-flavored Markdown text to HTML.
//
// After conversion, if the text is surrounded by a single outermost paragraph
// tag, it is unwrapped.
func (mc *MessageConverter) renderDiscordMarkdownOnlyHTML(portal *bridgev2.Portal, text string, allowInlineLinks bool) string {
	return format.UnwrapSingleParagraph(mc.renderDiscordMarkdownOnlyHTMLNoUnwrap(portal, text, allowInlineLinks))
}

// renderDiscordMarkdownOnlyHTMLNoUnwrap converts Discord-flavored Markdown text to HTML.
func (mc *MessageConverter) renderDiscordMarkdownOnlyHTMLNoUnwrap(portal *bridgev2.Portal, text string, allowInlineLinks bool) string {
	text = escapeFixer.ReplaceAllStringFunc(text, escapeReplacement)

	var buf strings.Builder
	ctx := parser.NewContext()
	ctx.Set(parserContextPortal, portal)
	renderer := discordRenderer
	if allowInlineLinks {
		renderer = discordRendererWithInlineLinks
	}
	err := renderer.Convert([]byte(text), &buf, parser.WithContext(ctx))
	if err != nil {
		panic(fmt.Errorf("markdown parser errored: %w", err))
	}
	return buf.String()
}

// TODO(skip): Stopping here for now. Continue at formatterContextPortalKey.