/*
Package main contains command "duplicates", which detects documents containing
a duplicated field in a MongoDB collection.

It takes its configuration from environment variables: refer to file `example.env`
for a sample.

(c) 2024 Ouest Systèmes Informatiques

Licensed under the Apache 2.0 license.
*/
package main

import (
	"context"
	"flag"
	"fmt"
	"io"
	"log"
	"os"
	"slices"

	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
	"gopkg.in/yaml.v3"
)

const (
	defaultMongoDBURI = "mongodb://localhost:27017"
	defaultDatabase   = "test"
	defaultCollection = "test"
	defaultField      = "email"
	defaultCommand    = "check"
	seedCommand       = "seed"
)

type conf struct {
	dbURI    string
	client   *mongo.Client
	dbName   string
	collName string
	command  string
	field    string
}

func configure(ctx context.Context, name string, args []string) (*conf, error) {
	var (
		conf conf
		err  error
		ok   bool
	)
	if conf.dbURI, ok = os.LookupEnv("MONGODB_URI"); !ok {
		conf.dbURI = defaultMongoDBURI
	}
	if conf.dbName, ok = os.LookupEnv("MONGODB_DB"); !ok {
		conf.dbName = defaultDatabase
	}
	if conf.collName, ok = os.LookupEnv("MONGODB_COLLECTION"); !ok {
		conf.collName = defaultCollection
	}
	if conf.field, ok = os.LookupEnv("MONGODB_FIELD"); !ok {
		conf.field = defaultField
	}
	conf.client, err = mongo.Connect(ctx, options.Client().ApplyURI(conf.dbURI))
	if err != nil {
		return nil, fmt.Errorf("failed to connect to MongoDB: %v", err)
	}
	fs := flag.NewFlagSet(name, flag.ContinueOnError)
	fs.StringVar(&conf.command, "command", defaultCommand, "sub-command to run")
	if err := fs.Parse(args); err != nil {
		return nil, fmt.Errorf("failed to parse arguments: %v", err)
	}
	if !slices.Contains([]string{defaultCommand, seedCommand}, conf.command) {
		return nil, fmt.Errorf("unknown command %q", conf.command)
	}
	return &conf, nil
}

func user(n int) string {
	return fmt.Sprintf("user%d@example.com", n)
}

func seed(ctx context.Context, coll *mongo.Collection, field string) error {
	// 1. Ensure empty collection on startup.
	if err := coll.Drop(ctx); err != nil {
		return fmt.Errorf("seed/dropping collection: %w", err)
	}

	// 2. Insert non duplicate elements
	for i := range 5 {
		if _, err := coll.InsertOne(ctx, bson.D{{Key: field, Value: user(i)}}); err != nil {
			return fmt.Errorf("seed/inserting initial doc %d: %w", i, err)
		}
	}
	// 3. Insert duplicate elements: 3*1, 2*2
	for _, i := range []int{1, 1, 2} {
		if _, err := coll.InsertOne(ctx, bson.D{{Key: field, Value: user(i)}}); err != nil {
			return fmt.Errorf("seed/inserting duplicate doc %d: %w", i, err)
		}
	}
	return nil
}

func check(ctx context.Context, coll *mongo.Collection, field string) (map[string]int, error) {
	docs, err := coll.Distinct(ctx, field, bson.D{}, nil)
	dups := make(map[string]int)
	if err != nil {
		return nil, fmt.Errorf("check/distinct: %w", err)
	}
	for _, doc := range docs {
		n, err := coll.CountDocuments(ctx, bson.D{{Key: field, Value: doc}}, nil)
		if err != nil {
			return nil, fmt.Errorf("check/counting: %w", err)
		}
		if n > 1 {
			dups[doc.(string)] = int(n)
		}
	}
	return dups, nil
}

// testableMain is extracted for testability
func testableMain(ctx context.Context, w io.Writer, logger *log.Logger, name string, args []string) (exit int) {
	config, err := configure(ctx, name, args)
	if err != nil {
		exit = 1
		logger.Println(err)
		return
	}
	defer func() {
		if err := config.client.Disconnect(ctx); err != nil {
			exit = 2
			logger.Println(err)
		}
	}()
	coll := config.client.Database(config.dbName).Collection(config.collName)
	if config.command == seedCommand {
		if err := seed(ctx, coll, config.field); err != nil {
			exit = 3
			logger.Println(err)
			return
		}
	}

	dups, err := check(ctx, coll, config.field)
	if err != nil {
		exit = 4
		logger.Println(err)
		return
	}

	if err := yaml.NewEncoder(w).Encode(dups); err != nil {
		exit = 5
		logger.Println(err)
		return
	}

	// Allow a non-zero exit in the deferred disconnect.
	exit = 0
	return
}

func main() {
	ctx := context.Background()
	logger := log.Default()
	name, args := os.Args[0], os.Args[1:]
	out := os.Stdout
	os.Exit(testableMain(ctx, out, logger, name, args))
}