#define _GNU_SOURCE
#include <errno.h>
#include <net/if.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "netlink.h"

#define IFADDRS_HASH_SIZE 64

struct ifnamemap {
  unsigned int hash_next;
  unsigned int index;
  unsigned char namelen;
  char name[IFNAMSIZ];
};

struct ifnameindexctx {
  unsigned int num, allocated, str_bytes;
  struct ifnamemap* list;
  unsigned int hash[IFADDRS_HASH_SIZE];
};

static int netlink_msg_to_nameindex(void* pctx, struct nlmsghdr* h) {
  struct ifnameindexctx* ctx = pctx;
  struct ifnamemap* map;
  struct rtattr* rta;
  unsigned int i;
  int index, type, namelen, bucket;

  if (h->nlmsg_type == RTM_NEWLINK) {
    struct ifinfomsg* ifi = NLMSG_DATA(h);
    index = ifi->ifi_index;
    type = IFLA_IFNAME;
    rta = NLMSG_RTA(h, sizeof(*ifi));
  } else {
    struct ifaddrmsg* ifa = NLMSG_DATA(h);
    index = ifa->ifa_index;
    type = IFA_LABEL;
    rta = NLMSG_RTA(h, sizeof(*ifa));
  }
  for (; NLMSG_RTAOK(rta, h); rta = RTA_NEXT(rta)) {
    if (rta->rta_type != type)
      continue;

    namelen = RTA_DATALEN(rta) - 1;
    if (namelen > IFNAMSIZ)
      return 0;

    /* suppress duplicates */
    bucket = index % IFADDRS_HASH_SIZE;
    i = ctx->hash[bucket];
    while (i) {
      map = &ctx->list[i - 1];
      if (map->index == index && map->namelen == namelen &&
          memcmp(map->name, RTA_DATA(rta), namelen) == 0)
        return 0;
      i = map->hash_next;
    }

    if (ctx->num >= ctx->allocated) {
      size_t a = ctx->allocated ? ctx->allocated * 2 + 1 : 8;
      if (a > SIZE_MAX / sizeof *map)
        return -1;
      map = realloc(ctx->list, a * sizeof *map);
      if (!map)
        return -1;
      ctx->list = map;
      ctx->allocated = a;
    }
    map = &ctx->list[ctx->num];
    map->index = index;
    map->namelen = namelen;
    memcpy(map->name, RTA_DATA(rta), namelen);
    ctx->str_bytes += namelen + 1;
    ctx->num++;
    map->hash_next = ctx->hash[bucket];
    ctx->hash[bucket] = ctx->num;
    return 0;
  }
  return 0;
}

struct if_nameindex* if_nameindex(void) {
  struct ifnameindexctx _ctx, *ctx = &_ctx;
  struct if_nameindex *ifs = 0, *d;
  struct ifnamemap* s;
  char* p;
  int i;

  memset(ctx, 0, sizeof(*ctx));
  if (__rtnetlink_enumerate(AF_UNSPEC, AF_INET, netlink_msg_to_nameindex, ctx) < 0)
    goto err;

  ifs = malloc(sizeof(struct if_nameindex[ctx->num + 1]) + ctx->str_bytes);
  if (!ifs)
    goto err;

  p = (char*)(ifs + ctx->num + 1);
  for (i = ctx->num, d = ifs, s = ctx->list; i; i--, s++, d++) {
    d->if_index = s->index;
    d->if_name = p;
    memcpy(p, s->name, s->namelen);
    p += s->namelen;
    *p++ = 0;
  }
  d->if_index = 0;
  d->if_name = 0;
err:
  free(ctx->list);
  errno = ENOBUFS;
  return ifs;
}