# Serial-to-ESP-NOW bridge: receives from Pi on UART, forwards to ESP-NOW peers.
# Wire format: first 6 bytes = destination MAC, rest = payload. Address is always 6 bytes.
from machine import Pin, UART
import espnow
import network
import time

UART_BAUD = 912000
BROADCAST = b"\xff\xff\xff\xff\xff\xff"
MAX_PEERS = 20

network.WLAN(network.STA_IF).active(True)
esp = espnow.ESPNow()
esp.active(True)
esp.add_peer(BROADCAST)

uart = UART(1, UART_BAUD, tx=Pin(21), rx=Pin(6))

# Track last send time per peer for LRU eviction (remove oldest when at limit).
last_used = {BROADCAST: time.ticks_ms()}


# ESP_ERR_ESPNOW_EXIST: peer already registered (ignore when adding).
ESP_ERR_ESPNOW_EXIST = -12395


def ensure_peer(addr):
    """Ensure addr is in the peer list. When at 20 peers, remove the oldest-used (LRU)."""
    peers = esp.get_peers()
    peer_macs = [p[0] for p in peers]
    if addr in peer_macs:
        return
    if len(peer_macs) >= MAX_PEERS:
        # Remove the peer we used least recently (oldest).
        oldest_mac = None
        oldest_ts = time.ticks_ms()
        for mac in peer_macs:
            if mac == BROADCAST:
                continue
            ts = last_used.get(mac, 0)
            if ts <= oldest_ts:
                oldest_ts = ts
                oldest_mac = mac
        if oldest_mac is not None:
            esp.del_peer(oldest_mac)
            last_used.pop(oldest_mac, None)
    try:
        esp.add_peer(addr)
    except OSError as e:
        if e.args[0] != ESP_ERR_ESPNOW_EXIST:
            raise


while True:
    if uart.any():
        data = uart.read()
        if not data or len(data) < 6:
            continue
        addr = data[:6]
        payload = data[6:]
        ensure_peer(addr)
        esp.send(addr, payload)
        last_used[addr] = time.ticks_ms()