package docrevision

import (
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"regexp"
	"strings"
)

var (
	headingRe   = regexp.MustCompile(`^(#{1,6})\s+(.+)$`)
	reqIDRe     = regexp.MustCompile(`\b(REQ-[A-Z0-9-]+)\b`)
	listItemRe  = regexp.MustCompile(`^(\s*[-*+]|\s*\d+\.)\s+`)
	tableRowRe  = regexp.MustCompile(`^\|`)
	fenceRe     = regexp.MustCompile("^```")
	mermaidRe   = regexp.MustCompile(`(?i)^mermaid\b`)
)

// BuildBlockMap parses markdown into stable block nodes.
// When previous is provided, nodes are aligned by business key → heading path → fingerprint → order.
func BuildBlockMap(markdown string, previous []BlockNode) []BlockNode {
	lines := strings.Split(strings.TrimSuffix(markdown, "\n"), "\n")
	var nodes []BlockNode
	offset := 0
	headingPath := []string{}
	inFence := false
	fenceLang := ""
	var fenceBuf []string
	fenceStart := 0
	order := 0

	flushFence := func() {
		if !inFence {
			return
		}
		body := strings.Join(fenceBuf, "\n")
		typ := "code"
		if mermaidRe.MatchString(fenceLang) {
			typ = "mermaid"
		}
		nodes = append(nodes, makeNode(typ, fenceStart, offset, strings.Join(headingPath, " / "), "", body, order, previous))
		order++
		inFence = false
		fenceBuf = nil
		fenceLang = ""
	}

	for i, line := range lines {
		lineLen := len(line)
		if i < len(lines)-1 || strings.HasSuffix(markdown, "\n") {
			lineLen++ // account for newline except possibly last without NL; we trimSuffix above so always +1 except last
		}
		// recompute offset carefully
		_ = lineLen
	}

	offset = 0
	for i, line := range lines {
		start := offset
		end := start + len(line)
		if i < len(lines)-1 {
			end++ // newline
		} else if strings.HasSuffix(markdown, "\n") {
			end++
		}

		if fenceRe.MatchString(line) {
			if !inFence {
				inFence = true
				fenceLang = strings.TrimSpace(strings.TrimPrefix(line, "```"))
				fenceStart = start
				fenceBuf = nil
			} else {
				flushFence()
			}
			offset = end
			continue
		}
		if inFence {
			fenceBuf = append(fenceBuf, line)
			offset = end
			continue
		}

		if m := headingRe.FindStringSubmatch(line); m != nil {
			level := len(m[1])
			title := strings.TrimSpace(m[2])
			if level <= len(headingPath) {
				headingPath = headingPath[:level-1]
			}
			headingPath = append(headingPath, title)
			path := strings.Join(headingPath, " / ")
			bk := ""
			if id := reqIDRe.FindString(title); id != "" {
				bk = id
			}
			nodes = append(nodes, makeNode("heading", start, end, path, bk, line, order, previous))
			order++
			offset = end
			continue
		}

		path := strings.Join(headingPath, " / ")
		switch {
		case tableRowRe.MatchString(line):
			bk := ""
			if id := reqIDRe.FindString(line); id != "" {
				bk = id
			}
			nodes = append(nodes, makeNode("table_row", start, end, path, bk, line, order, previous))
		case listItemRe.MatchString(line):
			bk := ""
			if id := reqIDRe.FindString(line); id != "" {
				bk = id
			}
			nodes = append(nodes, makeNode("list_item", start, end, path, bk, line, order, previous))
		case strings.TrimSpace(line) == "":
			// skip empty
		default:
			bk := ""
			if id := reqIDRe.FindString(line); id != "" {
				bk = id
			}
			nodes = append(nodes, makeNode("paragraph", start, end, path, bk, line, order, previous))
		}
		order++
		offset = end
	}
	flushFence()
	return nodes
}

func makeNode(typ string, start, end int, path, businessKey, text string, order int, previous []BlockNode) BlockNode {
	fp := ContentFingerprint(text)
	id := matchPreviousID(typ, path, businessKey, fp, order, previous)
	if id == "" {
		id = newBlockID()
	}
	return BlockNode{
		ID:          id,
		Type:        typ,
		StartOffset: start,
		EndOffset:   end,
		HeadingPath: path,
		BusinessKey: businessKey,
		Fingerprint: fp,
		Order:       order,
	}
}

func matchPreviousID(typ, path, businessKey, fp string, order int, previous []BlockNode) string {
	if len(previous) == 0 {
		return ""
	}
	if businessKey != "" {
		for _, p := range previous {
			if p.BusinessKey == businessKey && p.Type == typ {
				return p.ID
			}
		}
	}
	for _, p := range previous {
		if p.Type == typ && p.HeadingPath == path && p.Fingerprint == fp {
			return p.ID
		}
	}
	for _, p := range previous {
		if p.Type == typ && p.Fingerprint == fp {
			return p.ID
		}
	}
	for _, p := range previous {
		if p.Order == order && p.Type == typ {
			return p.ID
		}
	}
	return ""
}

func newBlockID() string {
	var b [8]byte
	_, _ = rand.Read(b[:])
	return fmt.Sprintf("blk_%s", hex.EncodeToString(b[:]))
}