/**
 * @file ethernet.h
 * @brief Ethernet subsystem.
 *
 * Provides a unified abstraction over individual Ethernet NIC drivers.
 * Each NIC driver registers itself with the ethernet subsystem, which
 * then creates the `/dev/ethN` character device and exposes interface
 * information via `/sys/net`.
 *
 * Higher-level protocols (IPv4, ARP, etc.) use this subsystem to send
 * and receive Ethernet frames without knowing which NIC driver is in use.
 */

#ifndef ETHERNET_H
#define ETHERNET_H

#include <stdint.h>

/** Maximum number of Ethernet interfaces. */
#define ETH_MAX_IFACES 8

/** Ethernet MAC address length. */
#define ETH_ALEN 6

/** Ethernet header size (dst + src + ethertype). */
#define ETH_HLEN 14

/** Maximum Ethernet payload size. */
#define ETH_MTU 1500

/** Maximum Ethernet frame size (header + payload). */
#define ETH_FRAME_LEN (ETH_HLEN + ETH_MTU)

/* ================================================================
 * EtherType constants
 * ================================================================ */
#define ETHERTYPE_IPV4  0x0800  /**< IPv4 */
#define ETHERTYPE_ARP   0x0806  /**< ARP */

/* ================================================================
 * Ethernet header (14 bytes)
 * ================================================================ */

/**
 * Ethernet frame header.
 */
typedef struct __attribute__((packed)) eth_header {
    uint8_t  dst[ETH_ALEN];   /**< Destination MAC address. */
    uint8_t  src[ETH_ALEN];   /**< Source MAC address. */
    uint16_t ethertype;        /**< EtherType (big-endian). */
} eth_header_t;

/* ================================================================
 * NIC driver callbacks
 * ================================================================ */

/**
 * Callback to send a raw Ethernet frame (complete with header).
 *
 * @param dev_data  Driver-specific device pointer.
 * @param frame     Complete Ethernet frame (header + payload).
 * @param len       Frame length in bytes.
 * @return 0 on success, -1 on failure.
 */
typedef int (*eth_send_fn)(void *dev_data, const void *frame, uint32_t len);

/**
 * Callback to receive a raw Ethernet frame (complete with header).
 *
 * @param dev_data  Driver-specific device pointer.
 * @param buf       Buffer for received frame.
 * @param bufsize   Buffer size.
 * @return Number of bytes received, 0 if none, -1 on error.
 */
typedef int (*eth_recv_fn)(void *dev_data, void *buf, uint32_t bufsize);

/* ================================================================
 * Ethernet interface
 * ================================================================ */

/**
 * Represents one Ethernet network interface.
 */
typedef struct eth_iface {
    char name[16];             /**< Interface name (e.g., "eth1"). */
    uint8_t mac[ETH_ALEN];    /**< MAC address. */
    uint8_t active;            /**< 1 if registered, 0 if free. */
    uint8_t link_up;           /**< 1 if link is up. */
    uint32_t ip_addr;          /**< IPv4 address (0 if not configured). */
    uint32_t netmask;          /**< Subnet mask (0 if not configured). */
    uint32_t gateway;          /**< Default gateway (0 if not configured). */

    eth_send_fn send;          /**< NIC driver send callback. */
    eth_recv_fn recv;          /**< NIC driver receive callback. */
    void *dev_data;            /**< Opaque pointer to NIC driver state. */
} eth_iface_t;

/* ================================================================
 * Public API
 * ================================================================ */

/**
 * Initialize the ethernet subsystem.
 * Registers sysfs namespace "net" and prepares the interface table.
 */
void ethernet_init(void);

/**
 * Register a NIC with the ethernet subsystem.
 *
 * Creates a `/dev/ethN` character device and adds the interface to the
 * internal table. Called by NIC drivers during initialization.
 *
 * @param mac       6-byte MAC address.
 * @param send      Send callback function.
 * @param recv      Receive callback function.
 * @param dev_data  Opaque NIC device pointer (passed to callbacks).
 * @return Pointer to the registered interface, or NULL on failure.
 */
eth_iface_t *ethernet_register(const uint8_t *mac,
                               eth_send_fn send,
                               eth_recv_fn recv,
                               void *dev_data);

/**
 * Send an Ethernet frame through an interface.
 *
 * Constructs the Ethernet header (src MAC from interface, dst from caller)
 * and dispatches through the NIC driver.
 *
 * @param iface     Interface to send through.
 * @param dst_mac   Destination MAC address (6 bytes).
 * @param ethertype EtherType (host byte order, will be converted to big-endian).
 * @param payload   Frame payload.
 * @param len       Payload length (max ETH_MTU).
 * @return 0 on success, -1 on failure.
 */
int ethernet_send(eth_iface_t *iface, const uint8_t *dst_mac,
                  uint16_t ethertype, const void *payload, uint32_t len);

/**
 * Receive an Ethernet frame from an interface.
 *
 * @param iface     Interface to receive from.
 * @param buf       Buffer for frame payload (Ethernet header is stripped).
 * @param bufsize   Buffer size.
 * @param src_mac   Output: source MAC of received frame (6 bytes, can be NULL).
 * @param ethertype Output: EtherType of received frame (host byte order, can be NULL).
 * @return Payload length in bytes, 0 if no packet, -1 on error.
 */
int ethernet_recv(eth_iface_t *iface, void *buf, uint32_t bufsize,
                  uint8_t *src_mac, uint16_t *ethertype);

/**
 * Get an interface by index.
 *
 * @param index  0-based index.
 * @return Pointer to interface, or NULL if index out of range.
 */
eth_iface_t *ethernet_get_iface(uint32_t index);

/**
 * Get the number of registered interfaces.
 *
 * @return Number of active Ethernet interfaces.
 */
uint32_t ethernet_get_iface_count(void);

/**
 * Find an interface by name (e.g., "eth1").
 *
 * @param name  Interface name.
 * @return Pointer to interface, or NULL if not found.
 */
eth_iface_t *ethernet_find_iface(const char *name);

/**
 * Convert a 16-bit value from host to network byte order (big-endian).
 */
static inline uint16_t htons(uint16_t h) {
    return (uint16_t)((h >> 8) | (h << 8));
}

/**
 * Convert a 16-bit value from network to host byte order.
 */
static inline uint16_t ntohs(uint16_t n) {
    return htons(n);
}

/**
 * Convert a 32-bit value from host to network byte order (big-endian).
 */
static inline uint32_t htonl(uint32_t h) {
    return ((h >> 24) & 0x000000FF) |
           ((h >>  8) & 0x0000FF00) |
           ((h <<  8) & 0x00FF0000) |
           ((h << 24) & 0xFF000000);
}

/**
 * Convert a 32-bit value from network to host byte order.
 */
static inline uint32_t ntohl(uint32_t n) {
    return htonl(n);
}

#endif /* ETHERNET_H */