package config

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"net"
	"os"
	"path/filepath"
	"strconv"
	"strings"

	"gopkg.in/yaml.v3"
)

type Config struct {
	ListenAddr             string
	DatabaseDSN            string
	SigningPrivateKeyFile  string
	SigningCertificateFile string
}

type fileConfig struct {
	Server struct {
		ListenAddr strictString `yaml:"listen_addr"`
	} `yaml:"server"`
	Database struct {
		DSN strictString `yaml:"dsn"`
	} `yaml:"database"`
	Signing struct {
		PrivateKeyFile  strictString `yaml:"private_key_file"`
		CertificateFile strictString `yaml:"certificate_file"`
	} `yaml:"signing"`
}

type strictString string

func (value *strictString) UnmarshalYAML(node *yaml.Node) error {
	if node.Kind != yaml.ScalarNode || node.Tag != "!!str" {
		return errors.New("必须是字符串")
	}
	*value = strictString(node.Value)
	return nil
}

func LoadFile(filePath string) (Config, error) {
	if filePath == "" {
		return Config{}, errors.New("配置文件路径不能为空")
	}

	content, err := os.ReadFile(filePath)
	if err != nil {
		return Config{}, fmt.Errorf("读取配置文件 %q: %w", filePath, err)
	}
	if err := rejectYAMLMergeKeys(content); err != nil {
		return Config{}, err
	}

	var raw fileConfig
	decoder := yaml.NewDecoder(bytes.NewReader(content))
	decoder.KnownFields(true)
	if err := decoder.Decode(&raw); err != nil {
		if errors.Is(err, io.EOF) {
			return Config{}, errors.New("配置文件不能为空")
		}
		return Config{}, fmt.Errorf("解析配置文件: %w", err)
	}

	var extra any
	if err := decoder.Decode(&extra); err == nil {
		return Config{}, errors.New("配置文件只允许包含一个 YAML 文档")
	} else if !errors.Is(err, io.EOF) {
		return Config{}, fmt.Errorf("解析配置文件的尾部内容: %w", err)
	}

	return raw.build()
}

func rejectYAMLMergeKeys(content []byte) error {
	var document yaml.Node
	decoder := yaml.NewDecoder(bytes.NewReader(content))
	if err := decoder.Decode(&document); err != nil {
		if errors.Is(err, io.EOF) {
			return nil
		}
		return fmt.Errorf("解析配置文件: %w", err)
	}

	return rejectYAMLMergeNode(&document, make(map[*yaml.Node]struct{}))
}

func rejectYAMLMergeNode(node *yaml.Node, visited map[*yaml.Node]struct{}) error {
	if node == nil {
		return nil
	}
	if _, exists := visited[node]; exists {
		return nil
	}
	visited[node] = struct{}{}

	if node.Kind == yaml.MappingNode {
		for index := 0; index+1 < len(node.Content); index += 2 {
			key := node.Content[index]
			if key.Value == "<<" || key.Tag == "!!merge" || key.Tag == "tag:yaml.org,2002:merge" {
				return fmt.Errorf("配置文件不允许使用 YAML merge key <<（第 %d 行，第 %d 列）", key.Line, key.Column)
			}
			if err := rejectYAMLMergeNode(key, visited); err != nil {
				return err
			}
			if err := rejectYAMLMergeNode(node.Content[index+1], visited); err != nil {
				return err
			}
		}
	} else {
		for _, child := range node.Content {
			if err := rejectYAMLMergeNode(child, visited); err != nil {
				return err
			}
		}
	}

	if node.Kind == yaml.AliasNode {
		return rejectYAMLMergeNode(node.Alias, visited)
	}
	return nil
}

func (raw fileConfig) build() (Config, error) {
	listenAddr, err := requireValue("server.listen_addr", raw.Server.ListenAddr)
	if err != nil {
		return Config{}, err
	}
	if err := validateLoopbackListenAddr(listenAddr); err != nil {
		return Config{}, err
	}

	databaseDSN, err := requireValue("database.dsn", raw.Database.DSN)
	if err != nil {
		return Config{}, err
	}
	signingPrivateKeyFile, err := requireAbsolutePath("signing.private_key_file", raw.Signing.PrivateKeyFile)
	if err != nil {
		return Config{}, err
	}
	signingCertificateFile, err := requireAbsolutePath("signing.certificate_file", raw.Signing.CertificateFile)
	if err != nil {
		return Config{}, err
	}

	return Config{
		ListenAddr:             listenAddr,
		DatabaseDSN:            databaseDSN,
		SigningPrivateKeyFile:  signingPrivateKeyFile,
		SigningCertificateFile: signingCertificateFile,
	}, nil
}

func requireValue(name string, value strictString) (string, error) {
	text := string(value)
	if strings.TrimSpace(text) == "" {
		return "", fmt.Errorf("配置项 %s 不能为空", name)
	}
	return text, nil
}

func requireAbsolutePath(name string, value strictString) (string, error) {
	filePath, err := requireValue(name, value)
	if err != nil {
		return "", err
	}
	if !filepath.IsAbs(filePath) {
		return "", fmt.Errorf("配置项 %s 必须是绝对路径", name)
	}
	return filePath, nil
}

func validateLoopbackListenAddr(listenAddr string) error {
	host, port, err := net.SplitHostPort(listenAddr)
	if err != nil {
		return fmt.Errorf("配置项 server.listen_addr 必须是主机和端口: %w", err)
	}

	ip := net.ParseIP(host)
	if ip == nil || !ip.IsLoopback() {
		return errors.New("配置项 server.listen_addr 必须使用回环 IP 地址")
	}

	portNumber, err := strconv.ParseUint(port, 10, 16)
	if err != nil || portNumber == 0 {
		return errors.New("配置项 server.listen_addr 的端口无效")
	}
	return nil
}