// Package ledger implements append-only double-entry accounting.
//
// Invariants, enforced here and again by database constraints and triggers:
//   - every transaction's postings sum to exactly zero
//   - no account balance may go negative
//   - rows are never updated or deleted; corrections are compensating entries
//
// Every balance change is explained by a posting that records what happened,
// when, which round it belonged to, and the balance either side of it.
package ledger

import (
	"context"
	"errors"
	"fmt"
	"sort"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgxpool"
)

var (
	ErrUnbalanced        = errors.New("ledger: postings do not sum to zero")
	ErrInsufficientFunds = errors.New("ledger: insufficient funds")
	ErrEmptyTransaction  = errors.New("ledger: transaction has no postings")
	ErrNonPositiveAmount = errors.New("ledger: amount must be positive")
)

// Posting is a single leg of a transaction. Positive credits, negative debits.
type Posting struct {
	AccountID  int64
	AmountMsat int64
}

// Entry is a posting as seen from one account's history.
type Entry struct {
	TransactionID int64
	Kind          string
	RoundID       *int64
	AmountMsat    int64
	BalanceBefore int64
	BalanceAfter  int64
	CreatedAt     time.Time
}

type Ledger struct{ pool *pgxpool.Pool }

func New(pool *pgxpool.Pool) *Ledger { return &Ledger{pool: pool} }

// Post writes one balanced transaction atomically.
//
// Accounts are locked in ascending id order so that concurrent transactions
// touching the same accounts cannot deadlock, and so a balance read cannot be
// stale by the time the posting is written.
func (l *Ledger) Post(ctx context.Context, kind string, roundID *int64, postings []Posting) (int64, error) {
	if len(postings) == 0 {
		return 0, ErrEmptyTransaction
	}
	var sum int64
	for _, p := range postings {
		sum += p.AmountMsat
	}
	if sum != 0 {
		return 0, fmt.Errorf("%w: sum is %d", ErrUnbalanced, sum)
	}

	tx, err := l.pool.Begin(ctx)
	if err != nil {
		return 0, err
	}
	defer tx.Rollback(ctx)

	var txID int64
	if err := tx.QueryRow(ctx,
		`INSERT INTO transactions (kind, round_id) VALUES ($1, $2) RETURNING id`,
		kind, roundID).Scan(&txID); err != nil {
		return 0, err
	}

	ordered := append([]Posting(nil), postings...)
	sort.Slice(ordered, func(i, j int) bool {
		return ordered[i].AccountID < ordered[j].AccountID
	})

	for _, p := range ordered {
		// Lock the account row first, then read its latest balance. Taking the
		// lock before the read is what serializes concurrent spends.
		var allowNegative bool
		if err := tx.QueryRow(ctx,
			`SELECT allow_negative FROM accounts WHERE id = $1 FOR UPDATE`,
			p.AccountID).Scan(&allowNegative); err != nil {
			return 0, fmt.Errorf("locking account %d: %w", p.AccountID, err)
		}

		var before int64
		if err := tx.QueryRow(ctx,
			`SELECT COALESCE(
			     (SELECT balance_after FROM postings
			       WHERE account_id = $1 ORDER BY id DESC LIMIT 1), 0)`,
			p.AccountID).Scan(&before); err != nil {
			return 0, fmt.Errorf("reading balance of account %d: %w", p.AccountID, err)
		}

		after := before + p.AmountMsat
		if after < 0 && !allowNegative {
			return 0, fmt.Errorf("%w: account %d holds %d, needs %d",
				ErrInsufficientFunds, p.AccountID, before, -p.AmountMsat)
		}

		if _, err := tx.Exec(ctx,
			`INSERT INTO postings
			   (transaction_id, account_id, amount_msat, balance_before, balance_after)
			 VALUES ($1, $2, $3, $4, $5)`,
			txID, p.AccountID, p.AmountMsat, before, after); err != nil {
			return 0, err
		}
	}

	if err := tx.Commit(ctx); err != nil {
		return 0, err
	}
	return txID, nil
}

// Transfer moves funds between two accounts. This is the peer-to-peer path.
func (l *Ledger) Transfer(ctx context.Context, from, to int64, amountMsat int64) (int64, error) {
	if amountMsat <= 0 {
		return 0, ErrNonPositiveAmount
	}
	return l.Post(ctx, "transfer", nil, []Posting{
		{AccountID: from, AmountMsat: -amountMsat},
		{AccountID: to, AmountMsat: amountMsat},
	})
}

// Deposit credits a player from the Lightning bridge account.
func (l *Ledger) Deposit(ctx context.Context, player int64, amountMsat int64) (int64, error) {
	if amountMsat <= 0 {
		return 0, ErrNonPositiveAmount
	}
	bridge, err := l.AccountByName(ctx, "lightning_bridge")
	if err != nil {
		return 0, err
	}
	return l.Post(ctx, "deposit", nil, []Posting{
		{AccountID: bridge, AmountMsat: -amountMsat},
		{AccountID: player, AmountMsat: amountMsat},
	})
}

// Withdraw debits a player back to the Lightning bridge account.
func (l *Ledger) Withdraw(ctx context.Context, player int64, amountMsat int64) (int64, error) {
	if amountMsat <= 0 {
		return 0, ErrNonPositiveAmount
	}
	bridge, err := l.AccountByName(ctx, "lightning_bridge")
	if err != nil {
		return 0, err
	}
	return l.Post(ctx, "withdraw", nil, []Posting{
		{AccountID: player, AmountMsat: -amountMsat},
		{AccountID: bridge, AmountMsat: amountMsat},
	})
}

// Balance returns the account's current balance in millisatoshis.
func (l *Ledger) Balance(ctx context.Context, accountID int64) (int64, error) {
	var bal int64
	err := l.pool.QueryRow(ctx,
		`SELECT COALESCE(
		     (SELECT balance_after FROM postings
		       WHERE account_id = $1 ORDER BY id DESC LIMIT 1), 0)`,
		accountID).Scan(&bal)
	return bal, err
}

// History returns an account's postings, newest first.
func (l *Ledger) History(ctx context.Context, accountID int64, limit int) ([]Entry, error) {
	rows, err := l.pool.Query(ctx,
		`SELECT p.transaction_id, t.kind, t.round_id,
		        p.amount_msat, p.balance_before, p.balance_after, p.created_at
		   FROM postings p
		   JOIN transactions t ON t.id = p.transaction_id
		  WHERE p.account_id = $1
		  ORDER BY p.id DESC
		  LIMIT $2`, accountID, limit)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var out []Entry
	for rows.Next() {
		var e Entry
		if err := rows.Scan(&e.TransactionID, &e.Kind, &e.RoundID,
			&e.AmountMsat, &e.BalanceBefore, &e.BalanceAfter, &e.CreatedAt); err != nil {
			return nil, err
		}
		out = append(out, e)
	}
	return out, rows.Err()
}

// EnsurePlayer returns the account id for a public key, creating it if needed.
func (l *Ledger) EnsurePlayer(ctx context.Context, pubkey []byte) (int64, error) {
	var id int64
	err := l.pool.QueryRow(ctx,
		`INSERT INTO accounts (kind, pubkey) VALUES ('player', $1)
		 ON CONFLICT (pubkey) DO UPDATE SET pubkey = EXCLUDED.pubkey
		 RETURNING id`, pubkey).Scan(&id)
	return id, err
}

// AccountByName resolves a system account such as "house_pot".
func (l *Ledger) AccountByName(ctx context.Context, name string) (int64, error) {
	var id int64
	err := l.pool.QueryRow(ctx,
		`SELECT id FROM accounts WHERE name = $1`, name).Scan(&id)
	if errors.Is(err, pgx.ErrNoRows) {
		return 0, fmt.Errorf("ledger: no account named %q", name)
	}
	return id, err
}

// TotalIssued is the value held inside the system by players and the house —
// every account except the external Lightning bridge. It changes only when
// funds genuinely enter or leave, never through internal play.
func (l *Ledger) TotalIssued(ctx context.Context) (int64, error) {
	var total int64
	err := l.pool.QueryRow(ctx,
		`SELECT COALESCE(SUM(b.balance_msat), 0)
		   FROM account_balances b
		   JOIN accounts a ON a.id = b.account_id
		  WHERE NOT a.allow_negative`).Scan(&total)
	return total, err
}

// ConservationCheck sums every account including the bridge. Because each
// transaction sums to zero, this must always be exactly zero. A non-zero
// result means the books are corrupt, and is the top-level audit alarm.
func (l *Ledger) ConservationCheck(ctx context.Context) (int64, error) {
	var total int64
	err := l.pool.QueryRow(ctx,
		`SELECT COALESCE(SUM(balance_msat), 0) FROM account_balances`).Scan(&total)
	return total, err
}