package usage

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"time"

	"github.com/carlosframework/platform/internal/release"
)

// Statement is usage/accounts/<acct>/<month>.json — one account's
// month-to-date totals per meter, the durable input a future billing engine
// consumes (statement × price sheet = invoice; spec: billing readiness).
// Meters is keyed by the Meter* vocabulary. CoveredDays/DaysInMonth carry
// the honesty label every rendered total must show.
type Statement struct {
	V           int                `json:"v"` // 1
	Account     string             `json:"account"`
	Month       string             `json:"month"` // "2006-01"
	Meters      map[string]float64 `json:"meters"`
	CoveredDays int                `json:"covered_days"`
	DaysInMonth int                `json:"days_in_month"`
	UpdatedAt   time.Time          `json:"updated_at"`
}

// StatementKey is the bucket key for an account's month statement.
func StatementKey(acct, month string) string {
	return "usage/accounts/" + acct + "/" + month + ".json"
}

// StatementsPrefix lists an account's statements.
func StatementsPrefix(acct string) string { return "usage/accounts/" + acct + "/" }

// AccountsPrefix discovers accounts that have statements.
const AccountsPrefix = "usage/accounts/"

// UpsertStatement CAS-replaces the account's month statement (3 attempts).
func UpsertStatement(ctx context.Context, s release.Store, st Statement) error {
	key := StatementKey(st.Account, st.Month)
	body, err := json.Marshal(st)
	if err != nil {
		return err
	}
	for attempt := 0; attempt < 3; attempt++ {
		_, etag, err := s.Get(ctx, key)
		if err != nil {
			if !errors.Is(err, os.ErrNotExist) {
				return err
			}
			etag = ""
		}
		if _, err := s.Put(ctx, key, body, etag); err == nil {
			return nil
		} else if !errors.Is(err, release.ErrCASConflict) {
			return err
		}
	}
	return fmt.Errorf("usage: %s: lost CAS race three times", key)
}

// CreateDayObject writes key create-only. (false, nil) means another writer
// won today — the daily-pass idempotence rule (spec: error handling).
func CreateDayObject(ctx context.Context, s release.Store, key string, v any) (bool, error) {
	body, err := json.Marshal(v)
	if err != nil {
		return false, err
	}
	if _, err := s.Put(ctx, key, body, ""); err != nil {
		if errors.Is(err, release.ErrCASConflict) {
			return false, nil
		}
		return false, err
	}
	return true, nil
}