package felhomsshd

import "fmt"

// Candidates is the ordered OOB-port candidate list (spike §2). First free wins; NEVER :22 or a
// random port. Package-var (not const) so tests can shrink it.
var Candidates = []int{8822, 2222, 8022, 62222}

// ErrPortsExhausted is returned when every candidate is busy — a LOUD failure [SF-4/trap 6], never a
// silent fallback to :22 or a random high port.
var ErrPortsExhausted = fmt.Errorf("felhomsshd: all candidate OOB ports are busy — refusing to fall back to :22 or a random port")

// portProbe reports whether a TCP port is free (nothing listening AND a real bind succeeds). Injected
// for tests; production impl = probeFree (ss + net.Listen).
type portProbe func(port int) bool

// claimPort returns the OOB port, mirroring the spike's shell algorithm:
//   - if a persisted port exists AND is still free → keep it (idempotent, no thrash),
//   - else the FIRST free candidate → persist + return,
//   - else ErrPortsExhausted (LOUD).
//
// persist writes PortFile; readPersisted reads it. isFree is the probe. All injected for tests.
func claimPort(candidates []int, isFree portProbe, readPersisted func() (int, bool), persist func(int) error) (int, error) {
	if cur, ok := readPersisted(); ok && cur != 22 && isFree(cur) {
		return cur, nil
	}
	for _, p := range candidates {
		if p == 22 {
			continue // defensive: never :22
		}
		if isFree(p) {
			if err := persist(p); err != nil {
				return 0, fmt.Errorf("felhomsshd: persist claimed port %d: %w", p, err)
			}
			return p, nil
		}
	}
	return 0, ErrPortsExhausted
}