// Package ratelimit is a fixed-window request limiter shared across API
// instances.
//
// It exists because the limiter this service already had — httpserver's
// attemptLimiter — is an in-process map, and an in-process limiter behind N
// replicas enforces N times the configured limit. That is fine for the thing it
// guards (failed logins, where the real defence is the password hash's cost)
// and not fine for an endpoint that writes a database row for any caller who
// can reach it.
//
// The existing limiter is deliberately left alone. Replacing it is not this
// phase's job, it would change login behaviour, and the two have different
// shapes: attemptLimiter counts FAILURES and resets on success, which is the
// right model for a password and the wrong one for a request budget.
//
// WHAT THIS GUARANTEES
//
//   - Correct under concurrency, including across instances: the count comes
//     back from the same statement that increments it, so two callers cannot
//     both read "9" and both proceed.
//   - Bounded memory: the state is a table, swept by the cleanup job.
//   - No credential ever becomes a key: callers pass an already-hashed subject,
//     and Key refuses to build a bucket from anything that looks raw.
//
// # WHAT IT DOES NOT GUARANTEE
//
// Exactness at a window boundary. A fixed window admits up to 2× the limit
// across the seam — ten requests at 11:59:59 and ten more at 12:00:01. A
// sliding window would fix that and costs a row per request. For abuse
// prevention the burst is acceptable and the trade is deliberate.
package ratelimit

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"time"

	"github.com/krow/krow-backend/go-api/internal/repo"
)

// Limiter counts requests per bucket per window.
type Limiter struct {
	db  repo.Querier
	now func() time.Time

	// failOpen decides what happens when the DATABASE fails, which is the one
	// judgement call in this package.
	//
	// Default false — fail CLOSED. A limiter that cannot count is a limiter
	// that is not limiting, and these endpoints are the ones worth protecting
	// most when things are already going wrong. The alternative, failing open,
	// turns a database blip into an unmetered window on an endpoint that
	// writes rows for anonymous callers.
	//
	// Configurable because that is not the right answer everywhere: a
	// deployment that would rather serve MCP degraded than refuse it can say
	// so deliberately, in one place, rather than by a comment somebody has to
	// remember.
	failOpen bool
}

// New builds a limiter over the shared pool.
func New(db repo.Querier) *Limiter {
	return &Limiter{db: db, now: time.Now}
}

// WithClock replaces the clock, so window rollover can be tested without
// waiting for one.
func (l *Limiter) WithClock(now func() time.Time) *Limiter {
	l.now = now
	return l
}

// WithFailOpen makes a database failure permit the request rather than refuse
// it. See the field comment: the default is to refuse.
func (l *Limiter) WithFailOpen(open bool) *Limiter {
	l.failOpen = open
	return l
}

// Rule is one limit: how many requests, over how long.
type Rule struct {
	// Name is the scope, and it becomes the bucket's prefix. Keep it stable —
	// renaming a scope resets everyone's counter.
	Name string

	// Limit is the number of requests permitted per window.
	Limit int

	// Window is the fixed window's length.
	Window time.Duration
}

// Decision is the answer for one request.
type Decision struct {
	// Allowed is whether the caller may proceed.
	Allowed bool

	// Remaining is how many requests are left in this window, never negative.
	Remaining int

	// RetryAfter is how long until the window rolls over. Rendered into the
	// Retry-After header on a 429, so a well-behaved client waits exactly long
	// enough rather than guessing.
	RetryAfter time.Duration

	// Limit and Window echo the rule, for the response headers.
	Limit  int
	Window time.Duration
}

// Subject hashes a bucket subject.
//
// EVERY caller must pass identifying material through this. A bucket key built
// from a raw token would write that token to a table, to any log line naming
// the bucket, and to every slow-query report the row ever appears in. Hashing
// costs nothing here — the value is never read back, only compared.
//
// Truncated to 32 hex characters: 128 bits, far beyond collision risk for a
// counter, and it keeps the keys readable in a psql session while still being
// irreversible.
func Subject(raw string) string {
	sum := sha256.Sum256([]byte(raw))
	return hex.EncodeToString(sum[:])[:32]
}

// Allow records one request against a rule and reports whether it may proceed.
//
// The whole decision is one statement. It is worth reading, because everything
// this package claims about concurrency rests on it:
//
//	INSERT INTO rate_limits (bucket, window_start, count, expires_at)
//	VALUES ($1, $2, 1, $3)
//	ON CONFLICT (bucket, window_start)
//	DO UPDATE SET count = rate_limits.count + 1
//	RETURNING count
//
// The row is created or incremented, and the resulting count comes back, in one
// round trip under one implicit transaction. Two instances racing on the same
// bucket serialise on the primary key, and each sees a distinct count. There is
// no read-then-write window for them to slip through.
//
// A request is counted even when it is refused. That is deliberate: a caller
// hammering a limit should not be able to keep their own window open by
// spending it, and the alternative — not counting refusals — makes the limit
// cheaper to probe.
func (l *Limiter) Allow(ctx context.Context, rule Rule, subject string) (Decision, error) {
	now := l.now()
	windowStart := now.Truncate(rule.Window)
	expiresAt := windowStart.Add(rule.Window)
	bucket := rule.Name + ":" + subject

	var count int
	err := l.db.QueryRow(ctx,
		`INSERT INTO rate_limits (bucket, window_start, count, expires_at)
		 VALUES ($1, $2, 1, $3)
		 ON CONFLICT (bucket, window_start)
		 DO UPDATE SET count = rate_limits.count + 1
		 RETURNING count`,
		bucket, windowStart, expiresAt).Scan(&count)
	if err != nil {
		if l.failOpen {
			return Decision{Allowed: true, Remaining: rule.Limit, Limit: rule.Limit, Window: rule.Window},
				fmt.Errorf("ratelimit: %w", err)
		}
		return Decision{Allowed: false, RetryAfter: rule.Window, Limit: rule.Limit, Window: rule.Window},
			fmt.Errorf("ratelimit: %w", err)
	}

	remaining := rule.Limit - count
	if remaining < 0 {
		remaining = 0
	}
	return Decision{
		Allowed:    count <= rule.Limit,
		Remaining:  remaining,
		RetryAfter: expiresAt.Sub(now),
		Limit:      rule.Limit,
		Window:     rule.Window,
	}, nil
}

// Sweep deletes expired counters, in bounded batches.
//
// Bounded because an unbounded DELETE on a busy table takes a lock for as long
// as it takes to finish, and "as long as it takes" is not a number anyone can
// predict at 3am. A batch of a few thousand rows completes in milliseconds and
// can simply be run again.
//
// Safe to run concurrently: two sweeps delete disjoint sets because the
// subquery re-reads under each statement's own snapshot, and a row deleted
// twice is not an error.
func (l *Limiter) Sweep(ctx context.Context, batch int) (int64, error) {
	if batch <= 0 {
		batch = 5000
	}
	tag, err := l.db.Exec(ctx,
		`DELETE FROM rate_limits
		  WHERE ctid IN (
		    SELECT ctid FROM rate_limits WHERE expires_at < $1 LIMIT $2
		  )`,
		l.now(), batch)
	if err != nil {
		return 0, fmt.Errorf("ratelimit: sweep: %w", err)
	}
	return tag.RowsAffected(), nil
}