// Copyright 2022 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package math

import (
	"bytes"

	"github.com/yuin/goldmark/ast"
	"github.com/yuin/goldmark/parser"
	"github.com/yuin/goldmark/text"
)

type inlineParser struct {
	trigger              []byte
	endBytesSingleDollar []byte
	endBytesDoubleDollar []byte
	endBytesParentheses  []byte
	enableInlineDollar   bool
}

func NewInlineDollarParser(enableInlineDollar bool) parser.InlineParser {
	return &inlineParser{
		trigger:              []byte{'$'},
		endBytesSingleDollar: []byte{'$'},
		endBytesDoubleDollar: []byte{'$', '$'},
		enableInlineDollar:   enableInlineDollar,
	}
}

var defaultInlineParenthesesParser = &inlineParser{
	trigger:             []byte{'\\', '('},
	endBytesParentheses: []byte{'\\', ')'},
}

func NewInlineParenthesesParser() parser.InlineParser {
	return defaultInlineParenthesesParser
}

// Trigger triggers this parser on $ or \
func (parser *inlineParser) Trigger() []byte {
	return parser.trigger
}

func isPunctuation(b byte) bool {
	return b == '.' || b == '!' || b == '?' || b == ',' || b == ';' || b == ':'
}

func isParenthesesClose(b byte) bool {
	return b == ')'
}

func isAlphanumeric(b byte) bool {
	return (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || (b >= '0' && b <= '9')
}

func isInMarkdownLinkText(block text.Reader, lineAfter []byte) bool {
	return block.PrecendingCharacter() == '[' && bytes.HasPrefix(lineAfter, []byte("]("))
}

// Parse parses the current line and returns a result of parsing.
func (parser *inlineParser) Parse(parent ast.Node, block text.Reader, pc parser.Context) ast.Node {
	line, _ := block.PeekLine()

	if !bytes.HasPrefix(line, parser.trigger) {
		// We'll catch this one on the next time round
		return nil
	}

	var startMarkLen int
	var stopMark []byte
	checkSurrounding := true
	if line[0] == '$' {
		startMarkLen = 1
		stopMark = parser.endBytesSingleDollar
		if len(line) > 1 {
			switch line[1] {
			case '$':
				startMarkLen = 2
				stopMark = parser.endBytesDoubleDollar
			case '`':
				pos := 1
				for ; pos < len(line) && line[pos] == '`'; pos++ {
				}
				startMarkLen = pos
				stopMark = bytes.Repeat([]byte{'`'}, pos)
				stopMark[len(stopMark)-1] = '$'
				checkSurrounding = false
			}
		}
	} else {
		startMarkLen = 2
		stopMark = parser.endBytesParentheses
	}

	if line[0] == '$' && !parser.enableInlineDollar && (len(line) == 1 || line[1] != '`') {
		return nil
	}

	if checkSurrounding {
		precedingCharacter := block.PrecendingCharacter()
		if precedingCharacter < 256 && (isAlphanumeric(byte(precedingCharacter)) || isPunctuation(byte(precedingCharacter))) {
			// need to exclude things like `a$` from being considered a start
			return nil
		}
	}

	// move the opener marker point at the start of the text
	opener := startMarkLen

	// Now look for an ending line
	depth := 0
	ender := -1
	for i := opener; i < len(line); i++ {
		if depth == 0 && bytes.HasPrefix(line[i:], stopMark) {
			succeedingCharacter := byte(0)
			if i+len(stopMark) < len(line) {
				succeedingCharacter = line[i+len(stopMark)]
			}
			// check valid ending character
			isValidEndingChar := isPunctuation(succeedingCharacter) || isParenthesesClose(succeedingCharacter) ||
				succeedingCharacter == ' ' || succeedingCharacter == '\n' || succeedingCharacter == 0 ||
				succeedingCharacter == '$' ||
				isInMarkdownLinkText(block, line[i+len(stopMark):])
			if checkSurrounding && !isValidEndingChar {
				break
			}
			ender = i
			break
		}
		if line[i] == '\\' {
			i++
			continue
		}
		switch line[i] {
		case '{':
			depth++
		case '}':
			depth--
		}
	}
	if ender == -1 {
		return nil
	}

	block.Advance(opener)
	_, pos := block.Position()
	node := NewInline()

	segment := pos.WithStop(pos.Start + ender - opener)
	node.AppendChild(node, ast.NewRawTextSegment(segment))
	block.Advance(ender - opener + len(stopMark))
	trimBlock(node, block)
	return node
}

func trimBlock(node *Inline, block text.Reader) {
	if node.IsBlank(block.Source()) {
		return
	}

	// trim first space and last space
	first := node.FirstChild().(*ast.Text)
	if !(!first.Segment.IsEmpty() && block.Source()[first.Segment.Start] == ' ') {
		return
	}

	last := node.LastChild().(*ast.Text)
	if !(!last.Segment.IsEmpty() && block.Source()[last.Segment.Stop-1] == ' ') {
		return
	}

	first.Segment = first.Segment.WithStart(first.Segment.Start + 1)
	last.Segment = last.Segment.WithStop(last.Segment.Stop - 1)
}
