/*
  StarForth — Steady-State Virtual Machine Runtime

  Copyright (c) 2023–2025 Robert A. James
  All rights reserved.

  This file is part of the StarForth project.

  Licensed under the StarForth License, Version 1.0 (the "License");
  you may not use this file except in compliance with the License.

  You may obtain a copy of the License at:
      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt

  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
  express or implied, including but not limited to the warranties of
  merchantability, fitness for a particular purpose, and noninfringement.

 See the License for the specific language governing permissions and
 limitations under the License.

 */

/**
 * vm_uuid.c - 128-bit VM identifiers (FABRIC-0.md punch list item 3.8)
 *
 * See vm_uuid.h for the design rationale (no RNG source exists on any of
 * the three ISAs uniformly, so this is deterministic, not random).
 */

#include "starkernel/vm_uuid.h"

#ifdef __STARKERNEL__

#include <stddef.h>

#define VM_UUID_POOL_SIZE 16

static uint64_t vm_uuid_prng_state  = 0;
static int      vm_uuid_seeded      = 0;
static VMUuid   vm_uuid_pool[VM_UUID_POOL_SIZE];
static size_t   vm_uuid_pool_pos    = VM_UUID_POOL_SIZE; /* "empty": first next() call refills */

VMUuid vm_uuid_hera(void) {
    VMUuid id;
    id.hi = 0;
    id.lo = 0;
    return id;
}

int vm_uuid_is_hera(VMUuid id) {
    return id.hi == 0 && id.lo == 0;
}

VMUuid vm_uuid_none(void) {
    VMUuid id;
    id.hi = 0xFFFFFFFFFFFFFFFFULL;
    id.lo = 0xFFFFFFFFFFFFFFFFULL;
    return id;
}

int vm_uuid_is_none(VMUuid id) {
    return id.hi == 0xFFFFFFFFFFFFFFFFULL && id.lo == 0xFFFFFFFFFFFFFFFFULL;
}

int vm_uuid_equal(VMUuid a, VMUuid b) {
    return a.hi == b.hi && a.lo == b.lo;
}

/* splitmix64 -- public-domain, well-known, minimal. Deterministic given a
 * seed; not cryptographically random, not claimed to be. */
static uint64_t splitmix64_next(uint64_t *state) {
    uint64_t z = (*state += 0x9E3779B97F4A7C15ULL);
    z = (z ^ (z >> 30)) * 0xBF58476D1CE4E5B9ULL;
    z = (z ^ (z >> 27)) * 0x94D049BB133111EBULL;
    return z ^ (z >> 31);
}

static void vm_uuid_pool_refill(void) {
    size_t i;
    for (i = 0; i < VM_UUID_POOL_SIZE; i++) {
        uint64_t hi = splitmix64_next(&vm_uuid_prng_state);
        uint64_t lo = splitmix64_next(&vm_uuid_prng_state);
        /* RFC 4122 version 4 / variant bits -- cosmetic shape only, not a
         * claim these are cryptographically random. */
        hi = (hi & 0xFFFFFFFFFFFF0FFFULL) | 0x0000000000004000ULL;
        lo = (lo & 0x3FFFFFFFFFFFFFFFULL) | 0x8000000000000000ULL;
        vm_uuid_pool[i].hi = hi;
        vm_uuid_pool[i].lo = lo;
    }
    vm_uuid_pool_pos = 0;
}

void vm_uuid_pool_init(uint64_t seed) {
    vm_uuid_prng_state = seed;
    vm_uuid_seeded = 1;
    vm_uuid_pool_refill();
}

VMUuid vm_uuid_next(void) {
    VMUuid id;

    if (!vm_uuid_seeded) {
        /* Safety net, not the intended path -- callers should seed from the
         * Mama capsule's content hash before the first non-Hera VM birth. */
        vm_uuid_pool_init(0x9E3779B97F4A7C15ULL);
    }
    if (vm_uuid_pool_pos >= VM_UUID_POOL_SIZE) {
        vm_uuid_pool_refill();
    }

    id = vm_uuid_pool[vm_uuid_pool_pos];
    vm_uuid_pool_pos++;
    return id;
}

void vm_uuid_format(VMUuid id, char *buf) {
    static const char hex[] = "0123456789abcdef";
    int pos = 0;
    int i;
    uint64_t halves[2];
    /* Group boundaries per RFC 4122: 8-4-4-4-12 hex digits, dashes after
     * digit 8, 12, 16, 20 (of 32 total). hi supplies digits 0-15, lo
     * supplies digits 16-31. */
    halves[0] = id.hi;
    halves[1] = id.lo;

    for (i = 0; i < 32; i++) {
        uint64_t v = (i < 16) ? halves[0] : halves[1];
        int shift = (15 - (i % 16)) * 4;
        buf[pos++] = hex[(v >> shift) & 0xF];
        if (i == 7 || i == 11 || i == 15 || i == 19) buf[pos++] = '-';
    }
    buf[pos] = '\0';
}

#endif /* __STARKERNEL__ */