// Copyright (C) 2019 Storj Labs, Inc.
// See LICENSE for copying information.

package stripecoinpayments

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/shopspring/decimal"
	"github.com/spacemonkeygo/monkit/v3"
	"github.com/stripe/stripe-go/v72"
	"github.com/zeebo/errs"
	"go.uber.org/zap"

	"storj.io/common/currency"
	"storj.io/common/uuid"
	"storj.io/storj/satellite/accounting"
	"storj.io/storj/satellite/console"
	"storj.io/storj/satellite/payments"
	"storj.io/storj/satellite/payments/billing"
	"storj.io/storj/satellite/payments/storjscan"
)

var (
	// Error defines stripecoinpayments service error.
	Error = errs.Class("stripecoinpayments service")

	mon = monkit.Package()
)

// hoursPerMonth is the number of months in a billing month. For the purpose of billing, the billing month is always 30 days.
const hoursPerMonth = 24 * 30

// Config stores needed information for payment service initialization.
type Config struct {
	StripeSecretKey        string `help:"stripe API secret key" default:""`
	StripePublicKey        string `help:"stripe API public key" default:""`
	StripeFreeTierCouponID string `help:"stripe free tier coupon ID" default:""`
	AutoAdvance            bool   `help:"toggle autoadvance feature for invoice creation" default:"false"`
	ListingLimit           int    `help:"sets the maximum amount of items before we start paging on requests" default:"100" hidden:"true"`
	SkipEmptyInvoices      bool   `help:"if set, skips the creation of empty invoices for customers with zero usage for the billing period" default:"true"`
}

// Service is an implementation for payment service via Stripe and Coinpayments.
//
// architecture: Service
type Service struct {
	log *zap.Logger

	db        DB
	walletsDB storjscan.WalletsDB
	billingDB billing.TransactionsDB

	projectsDB   console.Projects
	usersDB      console.Users
	usageDB      accounting.ProjectAccounting
	stripeClient StripeClient

	usagePrices         payments.ProjectUsagePriceModel
	usagePriceOverrides map[string]payments.ProjectUsagePriceModel
	packagePlans        map[string]payments.PackagePlan
	partnerNames        []string
	// BonusRate amount of percents
	BonusRate int64
	// Coupon Values
	StripeFreeTierCouponID string

	// Stripe Extended Features
	AutoAdvance bool

	listingLimit      int
	skipEmptyInvoices bool
	nowFn             func() time.Time
}

// NewService creates a Service instance.
func NewService(log *zap.Logger, stripeClient StripeClient, config Config, db DB, walletsDB storjscan.WalletsDB, billingDB billing.TransactionsDB, projectsDB console.Projects, usersDB console.Users, usageDB accounting.ProjectAccounting, usagePrices payments.ProjectUsagePriceModel, usagePriceOverrides map[string]payments.ProjectUsagePriceModel, packagePlans map[string]payments.PackagePlan, bonusRate int64) (*Service, error) {
	var partners []string
	for partner := range usagePriceOverrides {
		partners = append(partners, partner)
	}

	return &Service{
		log:                    log,
		db:                     db,
		walletsDB:              walletsDB,
		billingDB:              billingDB,
		projectsDB:             projectsDB,
		usersDB:                usersDB,
		usageDB:                usageDB,
		stripeClient:           stripeClient,
		usagePrices:            usagePrices,
		usagePriceOverrides:    usagePriceOverrides,
		packagePlans:           packagePlans,
		partnerNames:           partners,
		BonusRate:              bonusRate,
		StripeFreeTierCouponID: config.StripeFreeTierCouponID,
		AutoAdvance:            config.AutoAdvance,
		listingLimit:           config.ListingLimit,
		skipEmptyInvoices:      config.SkipEmptyInvoices,
		nowFn:                  time.Now,
	}, nil
}

// Accounts exposes all needed functionality to manage payment accounts.
func (service *Service) Accounts() payments.Accounts {
	return &accounts{service: service}
}

// PrepareInvoiceProjectRecords iterates through all projects and creates invoice records if none exist.
func (service *Service) PrepareInvoiceProjectRecords(ctx context.Context, period time.Time) (err error) {
	defer mon.Task()(&ctx)(&err)

	now := service.nowFn().UTC()
	utc := period.UTC()

	start := time.Date(utc.Year(), utc.Month(), 1, 0, 0, 0, 0, time.UTC)
	end := time.Date(utc.Year(), utc.Month()+1, 1, 0, 0, 0, 0, time.UTC)

	if end.After(now) {
		return Error.New("allowed for past periods only")
	}

	var numberOfCustomers, numberOfRecords int
	customersPage, err := service.db.Customers().List(ctx, 0, service.listingLimit, end)
	if err != nil {
		return Error.Wrap(err)
	}
	numberOfCustomers += len(customersPage.Customers)

	records, err := service.processCustomers(ctx, customersPage.Customers, start, end)
	if err != nil {
		return Error.Wrap(err)
	}
	numberOfRecords += records

	for customersPage.Next {
		if err = ctx.Err(); err != nil {
			return Error.Wrap(err)
		}

		customersPage, err = service.db.Customers().List(ctx, customersPage.NextOffset, service.listingLimit, end)
		if err != nil {
			return Error.Wrap(err)
		}
		numberOfCustomers += len(customersPage.Customers)

		records, err := service.processCustomers(ctx, customersPage.Customers, start, end)
		if err != nil {
			return Error.Wrap(err)
		}
		numberOfRecords += records
	}

	service.log.Info("Number of processed entries.", zap.Int("Customers", numberOfCustomers), zap.Int("Projects", numberOfRecords))
	return nil
}

func (service *Service) processCustomers(ctx context.Context, customers []Customer, start, end time.Time) (int, error) {
	var allRecords []CreateProjectRecord
	for _, customer := range customers {
		projects, err := service.projectsDB.GetOwn(ctx, customer.UserID)
		if err != nil {
			return 0, err
		}

		records, err := service.createProjectRecords(ctx, customer.ID, projects, start, end)
		if err != nil {
			return 0, err
		}

		allRecords = append(allRecords, records...)
	}

	return len(allRecords), service.db.ProjectRecords().Create(ctx, allRecords, start, end)
}

// createProjectRecords creates invoice project record if none exists.
func (service *Service) createProjectRecords(ctx context.Context, customerID string, projects []console.Project, start, end time.Time) (_ []CreateProjectRecord, err error) {
	defer mon.Task()(&ctx)(&err)

	var records []CreateProjectRecord
	for _, project := range projects {
		if err = ctx.Err(); err != nil {
			return nil, err
		}

		if err = service.db.ProjectRecords().Check(ctx, project.ID, start, end); err != nil {
			if errors.Is(err, ErrProjectRecordExists) {
				service.log.Warn("Record for this project already exists.", zap.String("Customer ID", customerID), zap.String("Project ID", project.ID.String()))
				continue
			}

			return nil, err
		}

		usage, err := service.usageDB.GetProjectTotal(ctx, project.ID, start, end)
		if err != nil {
			return nil, err
		}

		// TODO: account for usage data.
		records = append(records,
			CreateProjectRecord{
				ProjectID: project.ID,
				Storage:   usage.Storage,
				Egress:    usage.Egress,
				Segments:  usage.SegmentCount,
			},
		)
	}

	return records, nil
}

// InvoiceApplyProjectRecords iterates through unapplied invoice project records and creates invoice line items
// for stripe customer.
func (service *Service) InvoiceApplyProjectRecords(ctx context.Context, period time.Time) (err error) {
	defer mon.Task()(&ctx)(&err)

	now := service.nowFn().UTC()
	utc := period.UTC()

	start := time.Date(utc.Year(), utc.Month(), 1, 0, 0, 0, 0, time.UTC)
	end := time.Date(utc.Year(), utc.Month()+1, 1, 0, 0, 0, 0, time.UTC)

	if end.After(now) {
		return Error.New("allowed for past periods only")
	}

	var totalRecords int
	var totalSkipped int

	recordsPage, err := service.db.ProjectRecords().ListUnapplied(ctx, 0, service.listingLimit, start, end)
	if err != nil {
		return Error.Wrap(err)
	}
	totalRecords += len(recordsPage.Records)

	skipped, err := service.applyProjectRecords(ctx, recordsPage.Records)
	if err != nil {
		return Error.Wrap(err)
	}
	totalSkipped += skipped

	for recordsPage.Next {
		if err = ctx.Err(); err != nil {
			return Error.Wrap(err)
		}

		// we are always starting from offset 0 because applyProjectRecords is changing project record state to applied
		recordsPage, err = service.db.ProjectRecords().ListUnapplied(ctx, 0, service.listingLimit, start, end)
		if err != nil {
			return Error.Wrap(err)
		}
		totalRecords += len(recordsPage.Records)

		skipped, err := service.applyProjectRecords(ctx, recordsPage.Records)
		if err != nil {
			return Error.Wrap(err)
		}
		totalSkipped += skipped
	}

	service.log.Info("Processed project records.",
		zap.Int("Total", totalRecords),
		zap.Int("Skipped", totalSkipped))
	return nil
}

// InvoiceApplyTokenBalance iterates through customer storjscan wallets and creates invoice credit notes
// for stripe customers with invoices on or after the given date.
func (service *Service) InvoiceApplyTokenBalance(ctx context.Context, createdOnAfter time.Time) (err error) {
	defer mon.Task()(&ctx)(&err)

	// get all wallet entries
	wallets, err := service.walletsDB.GetAll(ctx)
	if err != nil {
		return Error.New("unable to get users in the wallets table")
	}

	var errGrp errs.Group

	for _, wallet := range wallets {
		// get the user token balance, if it's not > 0, don't bother with the rest
		monetaryTokenBalance, err := service.billingDB.GetBalance(ctx, wallet.UserID)
		// truncate here since stripe only has cent level precision for invoices.
		// The users account balance will still maintain the full precision monetary value!
		tokenBalance := currency.AmountFromDecimal(monetaryTokenBalance.AsDecimal().Truncate(2), currency.USDollars)
		if err != nil {
			errGrp.Add(Error.New("unable to compute balance for user ID %s", wallet.UserID.String()))
			continue
		}
		if tokenBalance.BaseUnits() <= 0 {
			continue
		}
		// get the stripe customer invoice balance
		cusID, err := service.db.Customers().GetCustomerID(ctx, wallet.UserID)
		if err != nil {
			errGrp.Add(Error.New("unable to get stripe customer ID for user ID %s", wallet.UserID.String()))
			continue
		}
		invoices, err := service.getInvoices(ctx, cusID, createdOnAfter)
		if err != nil {
			errGrp.Add(Error.New("unable to get invoice balance for stripe customer ID %s", cusID))
			continue
		}
		for _, invoice := range invoices {
			// if no balance due, do nothing
			if invoice.AmountRemaining <= 0 {
				continue
			}

			var tokenCreditAmount int64
			if invoice.AmountRemaining >= tokenBalance.BaseUnits() {
				tokenCreditAmount = tokenBalance.BaseUnits()
			} else {
				tokenCreditAmount = invoice.AmountRemaining
			}

			txID, err := service.createTokenPaymentBillingTransaction(ctx, wallet.UserID, invoice.ID, wallet.Address.Hex(), -tokenCreditAmount)
			if err != nil {
				errGrp.Add(Error.New("unable to create token payment billing transaction for user %s", wallet.UserID.String()))
				continue
			}

			creditNoteID, err := service.addCreditNoteToInvoice(ctx, invoice.ID, cusID, wallet.Address.Hex(), tokenCreditAmount, txID)
			if err != nil {
				errGrp.Add(Error.New("unable to create token payment credit note for user %s", wallet.UserID.String()))
				continue
			}

			metadata, err := json.Marshal(map[string]interface{}{
				"Credit Note ID": creditNoteID,
			})

			if err != nil {
				errGrp.Add(Error.New("unable to marshall credit note ID %s", creditNoteID))
				continue
			}

			err = service.billingDB.UpdateMetadata(ctx, txID, metadata)
			if err != nil {
				errGrp.Add(Error.New("unable to add credit note ID to billing transaction for user %s", wallet.UserID.String()))
				continue
			}

			err = service.billingDB.UpdateStatus(ctx, txID, billing.TransactionStatusCompleted)
			if err != nil {
				errGrp.Add(Error.New("unable to update status for billing transaction for user %s", wallet.UserID.String()))
				continue
			}
		}
	}
	return errGrp.Err()
}

// getInvoices returns the stripe customer's open finalized invoices created on or after the given date.
func (service *Service) getInvoices(ctx context.Context, cusID string, createdOnAfter time.Time) (_ []stripe.Invoice, err error) {
	defer mon.Task()(&ctx)(&err)

	params := &stripe.InvoiceListParams{
		Customer: stripe.String(cusID),
		Status:   stripe.String(string(stripe.InvoiceStatusOpen)),
	}
	params.Filters.AddFilter("created", "gte", strconv.FormatInt(createdOnAfter.Unix(), 10))
	invoicesIterator := service.stripeClient.Invoices().List(params)
	var stripeInvoices []stripe.Invoice
	for invoicesIterator.Next() {
		stripeInvoice := invoicesIterator.Invoice()
		if stripeInvoice != nil {
			stripeInvoices = append(stripeInvoices, *stripeInvoice)
		}
	}
	return stripeInvoices, nil
}

// addCreditNoteToInvoice creates a credit note for the user token payment.
func (service *Service) addCreditNoteToInvoice(ctx context.Context, invoiceID, cusID, wallet string, amount, txID int64) (_ string, err error) {
	defer mon.Task()(&ctx)(&err)

	var lineParams []*stripe.CreditNoteLineParams

	lineParam := stripe.CreditNoteLineParams{
		Description: stripe.String("Storjscan Token payment"),
		Type:        stripe.String("custom_line_item"),
		UnitAmount:  stripe.Int64(amount),
		Quantity:    stripe.Int64(1),
	}

	lineParams = append(lineParams, &lineParam)

	params := &stripe.CreditNoteParams{
		Invoice: stripe.String(invoiceID),
		Lines:   lineParams,
		Memo:    stripe.String("Storjscan Token Payment - Wallet: 0x" + wallet),
	}
	params.AddMetadata("txID", "0x"+strconv.FormatInt(txID, 10))
	params.AddMetadata("wallet address", wallet)
	creditNote, err := service.stripeClient.CreditNotes().New(params)
	if err != nil {
		service.log.Warn("unable to add credit note for stripe customer", zap.String("Customer ID", cusID))
		return "", Error.Wrap(err)
	}
	return creditNote.ID, nil
}

// createTokenPaymentBillingTransaction creates a billing DB entry for the user token payment.
func (service *Service) createTokenPaymentBillingTransaction(ctx context.Context, userID uuid.UUID, invoiceID, wallet string, amount int64) (_ int64, err error) {
	defer mon.Task()(&ctx)(&err)

	metadata, err := json.Marshal(map[string]interface{}{
		"InvoiceID": invoiceID,
		"Wallet":    wallet,
	})

	transaction := billing.Transaction{
		UserID:      userID,
		Amount:      currency.AmountFromBaseUnits(amount, currency.USDollars),
		Description: "Paid Stripe Invoice",
		Source:      "stripe",
		Status:      billing.TransactionStatusPending,
		Type:        billing.TransactionTypeDebit,
		Metadata:    metadata,
		Timestamp:   time.Now(),
	}
	txID, err := service.billingDB.Insert(ctx, transaction)
	if err != nil {
		service.log.Warn("unable to add transaction to billing DB for user", zap.String("User ID", userID.String()))
		return 0, Error.Wrap(err)
	}
	return txID, nil
}

// applyProjectRecords applies invoice intents as invoice line items to stripe customer.
func (service *Service) applyProjectRecords(ctx context.Context, records []ProjectRecord) (skipCount int, err error) {
	defer mon.Task()(&ctx)(&err)

	for _, record := range records {
		if err = ctx.Err(); err != nil {
			return 0, errs.Wrap(err)
		}

		proj, err := service.projectsDB.Get(ctx, record.ProjectID)
		if err != nil {
			// This should never happen, but be sure to log info to further troubleshoot before exiting.
			service.log.Error("project ID for corresponding project record not found", zap.Stringer("Record ID", record.ID), zap.Stringer("Project ID", record.ProjectID))
			return 0, errs.Wrap(err)
		}

		cusID, err := service.db.Customers().GetCustomerID(ctx, proj.OwnerID)
		if err != nil {
			if errors.Is(err, ErrNoCustomer) {
				service.log.Warn("Stripe customer does not exist for project owner.", zap.Stringer("Owner ID", proj.OwnerID), zap.Stringer("Project ID", proj.ID))
				continue
			}

			return 0, errs.Wrap(err)
		}

		if skipped, err := service.createInvoiceItems(ctx, cusID, proj.Name, record); err != nil {
			return 0, errs.Wrap(err)
		} else if skipped {
			skipCount++
		}
	}

	return skipCount, nil
}

// createInvoiceItems creates invoice line items for stripe customer.
func (service *Service) createInvoiceItems(ctx context.Context, cusID, projName string, record ProjectRecord) (skipped bool, err error) {
	defer mon.Task()(&ctx)(&err)

	if err = service.db.ProjectRecords().Consume(ctx, record.ID); err != nil {
		return false, err
	}

	if service.skipEmptyInvoices && doesProjectRecordHaveNoUsage(record) {
		return true, nil
	}

	usages, err := service.usageDB.GetProjectTotalByPartner(ctx, record.ProjectID, service.partnerNames, record.PeriodStart, record.PeriodEnd)
	if err != nil {
		return false, err
	}

	items := service.InvoiceItemsFromProjectUsage(projName, usages)
	for _, item := range items {
		item.Currency = stripe.String(string(stripe.CurrencyUSD))
		item.Customer = stripe.String(cusID)
		item.AddMetadata("projectID", record.ProjectID.String())

		_, err = service.stripeClient.InvoiceItems().New(item)
		if err != nil {
			return false, err
		}
	}

	return false, nil
}

// InvoiceItemsFromProjectUsage calculates Stripe invoice item from project usage.
func (service *Service) InvoiceItemsFromProjectUsage(projName string, partnerUsages map[string]accounting.ProjectUsage) (result []*stripe.InvoiceItemParams) {
	var partners []string
	if len(partnerUsages) == 0 {
		partners = []string{""}
		partnerUsages = map[string]accounting.ProjectUsage{"": {}}
	} else {
		for partner := range partnerUsages {
			partners = append(partners, partner)
		}
		sort.Strings(partners)
	}

	for _, partner := range partners {
		usage := partnerUsages[partner]
		priceModel := service.Accounts().GetProjectUsagePriceModel(partner)

		prefix := "Project " + projName
		if partner != "" {
			prefix += " (" + partner + ")"
		}

		projectItem := &stripe.InvoiceItemParams{}
		projectItem.Description = stripe.String(prefix + " - Segment Storage (MB-Month)")
		projectItem.Quantity = stripe.Int64(storageMBMonthDecimal(usage.Storage).IntPart())
		storagePrice, _ := priceModel.StorageMBMonthCents.Float64()
		projectItem.UnitAmountDecimal = stripe.Float64(storagePrice)
		result = append(result, projectItem)

		projectItem = &stripe.InvoiceItemParams{}
		projectItem.Description = stripe.String(prefix + " - Egress Bandwidth (MB)")
		projectItem.Quantity = stripe.Int64(egressMBDecimal(usage.Egress).IntPart())
		egressPrice, _ := priceModel.EgressMBCents.Float64()
		projectItem.UnitAmountDecimal = stripe.Float64(egressPrice)
		result = append(result, projectItem)

		projectItem = &stripe.InvoiceItemParams{}
		projectItem.Description = stripe.String(prefix + " - Segment Fee (Segment-Month)")
		projectItem.Quantity = stripe.Int64(segmentMonthDecimal(usage.SegmentCount).IntPart())
		segmentPrice, _ := priceModel.SegmentMonthCents.Float64()
		projectItem.UnitAmountDecimal = stripe.Float64(segmentPrice)
		result = append(result, projectItem)
	}

	service.log.Info("invoice items", zap.Any("result", result))

	return result
}

// ApplyFreeTierCoupons iterates through all customers in Stripe. For each customer,
// if that customer does not currently have a Stripe coupon, the free tier Stripe coupon
// is applied.
func (service *Service) ApplyFreeTierCoupons(ctx context.Context) (err error) {
	defer mon.Task()(&ctx)(&err)

	customers := service.db.Customers()

	appliedCoupons := 0
	failedUsers := []string{}
	morePages := true
	nextOffset := int64(0)
	listingLimit := 100
	end := time.Now()
	for morePages {
		customersPage, err := customers.List(ctx, nextOffset, listingLimit, end)
		if err != nil {
			return err
		}
		morePages = customersPage.Next
		nextOffset = customersPage.NextOffset

		for _, c := range customersPage.Customers {
			stripeCust, err := service.stripeClient.Customers().Get(c.ID, nil)
			if err != nil {
				service.log.Error("Failed to get customer", zap.Error(err))
				failedUsers = append(failedUsers, c.ID)
				continue
			}
			// if customer does not have a coupon, apply the free tier coupon
			if stripeCust.Discount == nil || stripeCust.Discount.Coupon == nil {
				params := &stripe.CustomerParams{
					Coupon: stripe.String(service.StripeFreeTierCouponID),
				}
				_, err := service.stripeClient.Customers().Update(c.ID, params)
				if err != nil {
					service.log.Error("Failed to update customer with free tier coupon", zap.Error(err))
					failedUsers = append(failedUsers, c.ID)
					continue
				}
				appliedCoupons++
			}
		}
	}

	if len(failedUsers) > 0 {
		service.log.Warn("Failed to get or apply free tier coupon to some customers:", zap.String("idlist", strings.Join(failedUsers, ", ")))
	}
	service.log.Info("Finished", zap.Int("number of coupons applied", appliedCoupons))

	return nil
}

// CreateInvoices lists through all customers and creates invoices.
func (service *Service) CreateInvoices(ctx context.Context, period time.Time) (err error) {
	defer mon.Task()(&ctx)(&err)

	now := service.nowFn().UTC()
	utc := period.UTC()

	start := time.Date(utc.Year(), utc.Month(), 1, 0, 0, 0, 0, time.UTC)
	end := time.Date(utc.Year(), utc.Month()+1, 1, 0, 0, 0, 0, time.UTC)

	if end.After(now) {
		return Error.New("allowed for past periods only")
	}

	var nextOffset int64
	var draft, scheduled int
	for {
		cusPage, err := service.db.Customers().List(ctx, nextOffset, service.listingLimit, end)
		if err != nil {
			return Error.Wrap(err)
		}

		for _, cus := range cusPage.Customers {
			if err = ctx.Err(); err != nil {
				return Error.Wrap(err)
			}

			stripeInvoice, err := service.createInvoice(ctx, cus.ID, start)
			if err != nil {
				return Error.Wrap(err)
			}

			switch {
			case stripeInvoice == nil:
			case stripeInvoice.AutoAdvance:
				scheduled++
			default:
				draft++
			}
		}

		if !cusPage.Next {
			break
		}
		nextOffset = cusPage.NextOffset
	}

	service.log.Info("Number of created invoices", zap.Int("Draft", draft), zap.Int("Scheduled", scheduled))
	return nil
}

// createInvoice creates invoice for stripe customer. Returns nil error and nil invoice
// if there are no pending invoice line items for customer.
func (service *Service) createInvoice(ctx context.Context, cusID string, period time.Time) (stripeInvoice *stripe.Invoice, err error) {
	defer mon.Task()(&ctx)(&err)

	description := fmt.Sprintf("Storj DCS Cloud Storage for %s %d", period.Month(), period.Year())

	stripeInvoice, err = service.stripeClient.Invoices().New(
		&stripe.InvoiceParams{
			Customer:    stripe.String(cusID),
			AutoAdvance: stripe.Bool(service.AutoAdvance),
			Description: stripe.String(description),
		},
	)

	if err != nil {
		var stripErr *stripe.Error
		if errors.As(err, &stripErr) {
			if stripErr.Code == stripe.ErrorCodeInvoiceNoCustomerLineItems {
				return nil, nil
			}
		}
		return nil, err
	}

	// auto advance the invoice if nothing is due from the customer
	if !stripeInvoice.AutoAdvance && stripeInvoice.AmountDue == 0 {
		stripeInvoice, err = service.stripeClient.Invoices().Update(
			stripeInvoice.ID,
			&stripe.InvoiceParams{AutoAdvance: stripe.Bool(true)},
		)
		if err != nil {
			return nil, err
		}
	}

	return stripeInvoice, nil
}

// GenerateInvoices performs all tasks necessary to generate Stripe invoices.
// This is equivalent to invoking ApplyFreeTierCoupons, PrepareInvoiceProjectRecords,
// InvoiceApplyProjectRecords, and CreateInvoices in order.
func (service *Service) GenerateInvoices(ctx context.Context, period time.Time) (err error) {
	defer mon.Task()(&ctx)(&err)

	for _, subFn := range []struct {
		Description string
		Exec        func(context.Context, time.Time) error
	}{
		{"Applying free tier coupons", func(ctx context.Context, _ time.Time) error {
			return service.ApplyFreeTierCoupons(ctx)
		}},
		{"Preparing invoice project records", service.PrepareInvoiceProjectRecords},
		{"Applying invoice project records", service.InvoiceApplyProjectRecords},
		{"Creating invoices", service.CreateInvoices},
	} {
		service.log.Info(subFn.Description)
		if err := subFn.Exec(ctx, period); err != nil {
			return err
		}
	}

	return nil
}

// FinalizeInvoices transitions all draft invoices to open finalized invoices in stripe. No payment is to be collected yet.
func (service *Service) FinalizeInvoices(ctx context.Context) (err error) {
	defer mon.Task()(&ctx)(&err)

	params := &stripe.InvoiceListParams{
		Status: stripe.String("draft"),
	}

	invoicesIterator := service.stripeClient.Invoices().List(params)
	for invoicesIterator.Next() {
		stripeInvoice := invoicesIterator.Invoice()
		if stripeInvoice.AutoAdvance {
			continue
		}

		err := service.finalizeInvoice(ctx, stripeInvoice.ID)
		if err != nil {
			return Error.Wrap(err)
		}
	}

	return Error.Wrap(invoicesIterator.Err())
}

func (service *Service) finalizeInvoice(ctx context.Context, invoiceID string) (err error) {
	defer mon.Task()(&ctx)(&err)

	params := &stripe.InvoiceFinalizeParams{AutoAdvance: stripe.Bool(false)}
	_, err = service.stripeClient.Invoices().FinalizeInvoice(invoiceID, params)
	return err
}

// PayInvoices attempts to transition all open finalized invoices created on or after a certain time to "paid"
// by charging the customer according to subscriptions settings.
func (service *Service) PayInvoices(ctx context.Context, createdOnAfter time.Time) (err error) {
	defer mon.Task()(&ctx)(&err)

	params := &stripe.InvoiceListParams{
		Status: stripe.String("open"),
	}
	params.Filters.AddFilter("created", "gte", strconv.FormatInt(createdOnAfter.Unix(), 10))

	var errGrp errs.Group

	invoicesIterator := service.stripeClient.Invoices().List(params)
	for invoicesIterator.Next() {
		stripeInvoice := invoicesIterator.Invoice()
		if stripeInvoice.DueDate > 0 {
			service.log.Info("Skipping invoice marked for manual payment",
				zap.String("id", stripeInvoice.ID),
				zap.String("number", stripeInvoice.Number),
				zap.String("customer", stripeInvoice.Customer.ID))
			continue
		}

		params := &stripe.InvoicePayParams{}
		_, err = service.stripeClient.Invoices().Pay(stripeInvoice.ID, params)
		if err != nil {
			errGrp.Add(Error.New("unable to pay invoice %s", stripeInvoice.ID))
			continue
		}
	}
	return errGrp.Err()
}

// projectUsagePrice represents pricing for project usage.
type projectUsagePrice struct {
	Storage  decimal.Decimal
	Egress   decimal.Decimal
	Segments decimal.Decimal
}

// Total returns project usage price total.
func (price projectUsagePrice) Total() decimal.Decimal {
	return price.Storage.Add(price.Egress).Add(price.Segments)
}

// Total returns project usage price total.
func (price projectUsagePrice) TotalInt64() int64 {
	return price.Storage.Add(price.Egress).Add(price.Segments).IntPart()
}

// calculateProjectUsagePrice calculate project usage price.
func (service *Service) calculateProjectUsagePrice(egress int64, storage, segments float64, pricing payments.ProjectUsagePriceModel) projectUsagePrice {
	return projectUsagePrice{
		Storage:  pricing.StorageMBMonthCents.Mul(storageMBMonthDecimal(storage)).Round(0),
		Egress:   pricing.EgressMBCents.Mul(egressMBDecimal(egress)).Round(0),
		Segments: pricing.SegmentMonthCents.Mul(segmentMonthDecimal(segments)).Round(0),
	}
}

// SetNow allows tests to have the Service act as if the current time is whatever
// they want. This avoids races and sleeping, making tests more reliable and efficient.
func (service *Service) SetNow(now func() time.Time) {
	service.nowFn = now
}

// storageMBMonthDecimal converts storage usage from Byte-Hours to Megabyte-Months.
// The result is rounded to the nearest whole number, but returned as Decimal for convenience.
func storageMBMonthDecimal(storage float64) decimal.Decimal {
	return decimal.NewFromFloat(storage).Shift(-6).Div(decimal.NewFromInt(hoursPerMonth)).Round(0)
}

// egressMBDecimal converts egress usage from bytes to Megabytes
// The result is rounded to the nearest whole number, but returned as Decimal for convenience.
func egressMBDecimal(egress int64) decimal.Decimal {
	return decimal.NewFromInt(egress).Shift(-6).Round(0)
}

// segmentMonthDecimal converts segments usage from Segment-Hours to Segment-Months.
// The result is rounded to the nearest whole number, but returned as Decimal for convenience.
func segmentMonthDecimal(segments float64) decimal.Decimal {
	return decimal.NewFromFloat(segments).Div(decimal.NewFromInt(hoursPerMonth)).Round(0)
}

// doesProjectRecordHaveNoUsage returns true if the given project record
// represents a billing cycle where there was no usage.
func doesProjectRecordHaveNoUsage(record ProjectRecord) bool {
	return record.Storage == 0 && record.Egress == 0 && record.Segments == 0
}