// Package librarydiff 比较两个已归档整库版本的知识文档集合。
//
// 身份用稳定文档 ID（不是标题、不是顺序），内容用排除易变字段后的整行摘要：
// 只有真实内容/元数据变化才算"更新"，updated_at 之类的噪声不会引发假差异。
// 撤销（生产有、目标没有）单列，不能当作"无变化"。
package librarydiff

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"sort"
)

// volatileFields 是每次保存都会变、但不代表知识内容变化的列。
var volatileFields = []string{"updated_at", "updated_by", "last_edited_at"}

type Document struct {
	ID     string `json:"id"`
	Title  string `json:"title"`
	Digest string `json:"digest"`
}

type Result struct {
	Added   []Document `json:"added"`
	Updated []Document `json:"updated"`
	Removed []Document `json:"removed"`
	// BaseCount/TargetCount 是两侧文档数，便于核对"差异之和是否自洽"。
	BaseCount   int `json:"base_count"`
	TargetCount int `json:"target_count"`
}

// ReadDocuments 读取归档里 knowledges 部件的行数组。
func ReadDocuments(r io.Reader) ([]map[string]any, error) {
	decoder := json.NewDecoder(r)
	token, err := decoder.Token()
	if err != nil || token != json.Delim('[') {
		return nil, errors.New("knowledge component must be a JSON row array")
	}
	rows := make([]map[string]any, 0, 64)
	for decoder.More() {
		var row map[string]any
		if err = decoder.Decode(&row); err != nil {
			return nil, fmt.Errorf("decode knowledge row: %w", err)
		}
		rows = append(rows, row)
	}
	if _, err = decoder.Token(); err != nil {
		return nil, fmt.Errorf("close knowledge array: %w", err)
	}
	return rows, nil
}

func documentOf(row map[string]any) (Document, error) {
	id, _ := row["id"].(string)
	if id == "" {
		return Document{}, errors.New("knowledge row without id")
	}
	canonical := make(map[string]any, len(row))
	for key, value := range row {
		canonical[key] = value
	}
	for _, key := range volatileFields {
		delete(canonical, key)
	}
	encoded, err := json.Marshal(canonical)
	if err != nil {
		return Document{}, fmt.Errorf("encode knowledge row: %w", err)
	}
	sum := sha256.Sum256(encoded)
	title, _ := row["title"].(string)
	return Document{ID: id, Title: title, Digest: hex.EncodeToString(sum[:])}, nil
}

// Compare 以 base 为基准（通常是生产当前版本），target 为拟发布版本。
func Compare(base, target []map[string]any) (Result, error) {
	baseDocs, err := index(base)
	if err != nil {
		return Result{}, err
	}
	targetDocs, err := index(target)
	if err != nil {
		return Result{}, err
	}
	result := Result{Added: []Document{}, Updated: []Document{}, Removed: []Document{},
		BaseCount: len(baseDocs), TargetCount: len(targetDocs)}
	for id, doc := range targetDocs {
		previous, ok := baseDocs[id]
		if !ok {
			result.Added = append(result.Added, doc)
			continue
		}
		if previous.Digest != doc.Digest {
			result.Updated = append(result.Updated, doc)
		}
	}
	for id, doc := range baseDocs {
		if _, ok := targetDocs[id]; !ok {
			result.Removed = append(result.Removed, doc)
		}
	}
	sort.Slice(result.Added, func(i, j int) bool { return result.Added[i].ID < result.Added[j].ID })
	sort.Slice(result.Updated, func(i, j int) bool { return result.Updated[i].ID < result.Updated[j].ID })
	sort.Slice(result.Removed, func(i, j int) bool { return result.Removed[i].ID < result.Removed[j].ID })
	return result, nil
}

func index(rows []map[string]any) (map[string]Document, error) {
	docs := make(map[string]Document, len(rows))
	for _, row := range rows {
		doc, err := documentOf(row)
		if err != nil {
			return nil, err
		}
		if _, exists := docs[doc.ID]; exists {
			return nil, fmt.Errorf("duplicate knowledge id %s", doc.ID)
		}
		docs[doc.ID] = doc
	}
	return docs, nil
}