// Package thread builds local Usenet conversation trees from article headers.
package thread

import (
	"sort"
	"strings"

	"aegis/internal/nntp"
)

type Node struct {
	Header   nntp.ArticleHeader
	Parent   *Node
	Children []*Node
}

func Build(headers []nntp.ArticleHeader) []*Node {
	ordered := append([]nntp.ArticleHeader(nil), headers...)
	sort.SliceStable(ordered, func(i, j int) bool {
		if ordered[i].Number == ordered[j].Number {
			return ordered[i].MessageID < ordered[j].MessageID
		}
		return ordered[i].Number < ordered[j].Number
	})
	nodes := make(map[string]*Node, len(ordered))
	for _, header := range ordered {
		node := &Node{Header: header}
		if key := canonicalID(header.MessageID); key != "" {
			nodes[key] = node
		}
	}
	var roots []*Node
	for _, node := range nodesInOrder(ordered, nodes) {
		parentID := lastReference(node.Header.References)
		parent := nodes[canonicalID(parentID)]
		if parent == nil || parent == node {
			roots = append(roots, node)
			continue
		}
		node.Parent = parent
		parent.Children = append(parent.Children, node)
	}
	for _, node := range nodesInOrder(ordered, nodes) {
		sort.SliceStable(node.Children, func(i, j int) bool { return node.Children[i].Header.Number < node.Children[j].Header.Number })
	}
	return roots
}

func nodesInOrder(headers []nntp.ArticleHeader, nodes map[string]*Node) []*Node {
	result := make([]*Node, 0, len(headers))
	seen := make(map[*Node]struct{}, len(headers))
	for _, header := range headers {
		node := nodes[canonicalID(header.MessageID)]
		if node == nil {
			continue
		}
		if _, ok := seen[node]; ok {
			continue
		}
		seen[node] = struct{}{}
		result = append(result, node)
	}
	return result
}

func lastReference(value string) string {
	parts := strings.Fields(value)
	if len(parts) == 0 {
		return ""
	}
	return parts[len(parts)-1]
}

func canonicalID(value string) string {
	return strings.ToLower(strings.TrimSpace(value))
}