package validation

import (
	"context"
	"fmt"
	"sort"
	"strconv"
	"strings"
	"unicode"

	"github.com/yuin/goldmark/v2/ast"
	"github.com/yuin/goldmark/v2/parser"
)

type markdownLocation struct {
	Line   int
	Column int
}

type markdownLink struct {
	Destination string
	Title       string
	Image       bool
	Reference   bool
	Location    markdownLocation
}

type markdownDocument struct {
	links   []markdownLink
	anchors map[string]struct{}
}

type markdownCache struct {
	tree       *CandidateTree
	byPath     map[string]*markdownDocument
	byObjectID map[string]*markdownDocument
}

func newMarkdownCache(tree *CandidateTree) *markdownCache {
	return &markdownCache{
		tree:       tree,
		byPath:     make(map[string]*markdownDocument),
		byObjectID: make(map[string]*markdownDocument),
	}
}

func (cache *markdownCache) document(ctx context.Context, path string) (*markdownDocument, error) {
	if document, ok := cache.byPath[path]; ok {
		return document, nil
	}
	entry, ok := cache.tree.Entry(path)
	if !ok || !isRegularFile(entry) {
		return nil, fmt.Errorf("committed path %q is not a regular file", path)
	}
	if document, ok := cache.byObjectID[entry.ObjectID]; ok {
		cache.byPath[path] = document
		return document, nil
	}
	source, err := cache.tree.ReadFile(ctx, path)
	if err != nil {
		return nil, err
	}
	document := parseMarkdown(source)
	source = nil
	cache.byObjectID[entry.ObjectID] = document
	cache.byPath[path] = document
	return document, nil
}

func parseMarkdown(source []byte) *markdownDocument {
	root := parser.New().Parse(source)
	document := &markdownDocument{
		anchors: make(map[string]struct{}),
	}
	lineStarts := markdownLineStarts(source)
	usedAnchors := make(map[string]struct{})
	nextSuffix := make(map[string]int)
	_ = ast.Walk(root, func(node ast.Node, entering bool) (ast.WalkStatus, error) {
		if !entering {
			return ast.WalkContinue, nil
		}
		switch node := node.(type) {
		case *ast.Link:
			document.links = append(document.links, markdownLink{
				Destination: strings.Clone(string(node.Destination.Value(source))),
				Title:       strings.Clone(string(node.Title.Value(source))),
				Reference:   node.Reference != nil,
				Location:    markdownLocationAt(lineStarts, node.Pos()),
			})
		case *ast.Image:
			document.links = append(document.links, markdownLink{
				Destination: strings.Clone(string(node.Destination.Value(source))),
				Title:       strings.Clone(string(node.Title.Value(source))),
				Image:       true,
				Reference:   node.Reference != nil,
				Location:    markdownLocationAt(lineStarts, node.Pos()),
			})
			// Image labels contribute alternative text, not navigable destinations.
			return ast.WalkSkipChildren, nil
		case *ast.AutoLink:
			document.links = append(document.links, markdownLink{
				Destination: strings.Clone(string(node.Destination.Value(source))),
				Location:    markdownLocationAt(lineStarts, node.Pos()),
			})
		case *ast.Heading:
			base := markdownAnchor(markdownHeadingText(node, source))
			anchor := base
			if _, exists := usedAnchors[anchor]; exists {
				suffix, ok := nextSuffix[base]
				if !ok {
					suffix = 1
				}
				for {
					anchor = base + "-" + strconv.Itoa(suffix)
					suffix++
					if _, exists := usedAnchors[anchor]; !exists {
						break
					}
				}
				nextSuffix[base] = suffix
			}
			usedAnchors[anchor] = struct{}{}
			document.anchors[anchor] = struct{}{}
		}
		return ast.WalkContinue, nil
	})
	return document
}

func markdownHeadingText(heading *ast.Heading, source []byte) string {
	var result strings.Builder
	var appendNode func(ast.Node)
	appendNode = func(node ast.Node) {
		switch node := node.(type) {
		case *ast.RawHTML:
			return
		case *ast.Text:
			result.WriteString(node.Value.Value(source))
			if node.SoftLineBreak() || node.HardLineBreak() {
				result.WriteByte(' ')
			}
			return
		case *ast.CodeSpan:
			result.WriteString(node.Value.Value(source))
			return
		case *ast.AutoLink:
			result.WriteString(node.Label.Value(source))
			return
		}
		for child := node.FirstChild(); child != nil; child = child.NextSibling() {
			appendNode(child)
		}
	}
	for child := heading.FirstChild(); child != nil; child = child.NextSibling() {
		appendNode(child)
	}
	return result.String()
}

func markdownAnchor(heading string) string {
	heading = strings.ToLower(heading)
	var result strings.Builder
	for _, r := range heading {
		if unicode.IsLetter(r) || unicode.IsNumber(r) || r == '_' || r == '-' || r == ' ' {
			result.WriteRune(r)
		}
	}
	return strings.ReplaceAll(result.String(), " ", "-")
}

func markdownLineStarts(source []byte) []int {
	starts := []int{0}
	for offset, value := range source {
		if value == '\n' {
			starts = append(starts, offset+1)
		}
	}
	return starts
}

func markdownLocationAt(lineStarts []int, offset int) markdownLocation {
	if offset < 0 {
		return markdownLocation{}
	}
	lineIndex := sort.Search(len(lineStarts), func(index int) bool {
		return lineStarts[index] > offset
	}) - 1
	return markdownLocation{Line: lineIndex + 1, Column: offset - lineStarts[lineIndex] + 1}
}