/*
 * M*LIB - Integer Generator (GENINT) module
 *
 * Copyright (c) 2017-2021, Patrick Pelissier
 * All rights reserved.
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * + Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * + Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE REGENTS AND CONTRIBUTORS BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#ifndef MSTARLIB_GENINT_H
#define MSTARLIB_GENINT_H

#include "m-core.h"
#include "m-atomic.h"

M_BEGIN_PROTECTED_CODE

/* GENINT is a container providing unique integers.
  It has the following properties:
   - it stores integer from [0..N) (N is fixed).
   - an integer can have only one occurrence in the container.
   - you can atomically push in / pop out integer from this container
   provided that it is not already in the container.
   - there are no order (like FIFO or stack)

   This can be used to map integers to index of resources in a table.
   At most we can support N = 2048 with the master limb usage.
   For the typical usage of this container
   (mapping hardware or software limited resources), this should be
   enough.
*/

// TO DO: We may want a specialization with constant N value.

// Define the limb size used by genint
typedef unsigned long long genint_limb_ct;

/* Define a generator of unique integer (Lock Free) */
typedef struct genint_s {
  unsigned int n;            // size of the container
  unsigned int max;          // number of allocated limb - 1
  genint_limb_ct mask0;       // mask of the last limb (constant)
  genint_limb_ct mask_master; // mask of the master limb that controls others (constant)
  atomic_ullong master;      // master bitfield (which informs if a limb is full or not)
  atomic_ullong *data;       // the bitfield which informs if an integer is used or not
} genint_t[1];

// Define the max absolute supported value
#define GENINT_MAX_ALLOC (GENINT_LIMBSIZE * (GENINT_LIMBSIZE - GENINT_ABA_CPT))

// Define the size of a limb in bits.
#define GENINT_LIMBSIZE ((unsigned)(sizeof(genint_limb_ct) * CHAR_BIT))

// Define the contract of a genint
#define GENINT_CONTRACT(s)                              do {                  \
    M_ASSERT (s != NULL);                                                     \
    M_ASSERT (s->n > 0 && s->n <= GENINT_MAX_ALLOC);                          \
    M_ASSERT ((s->max+1) * GENINT_LIMBSIZE >= s->n);                          \
    M_ASSERT (s->data != NULL);                                               \
  } while (0)

// Define the limb one
#define GENINT_ONE  ((genint_limb_ct)1)

#define GENINT_FULL_MASK ULLONG_MAX

// Value returned in case of error (not integer available).
#define GENINT_ERROR (UINT_MAX)

/* 32 bits of the master mask are kept for handling the ABA problem.
 * NOTE: May be too much. 16 bits should be more than enough. TBC
 */
#define GENINT_ABA_CPT 32
#define GENINT_ABA_CPT_T uint32_t

// Set the bit 'i' of the master limb, and increase ABA counter.
#define GENINT_MASTER_SET(master, i)                                          \
  ((((master)& (~((GENINT_ONE<< GENINT_ABA_CPT)-1))) | (GENINT_ONE << (GENINT_LIMBSIZE - 1 - i))) \
   |((GENINT_ABA_CPT_T)((master) + 1)))

// Reset the bit i of the master limb, and increase ABA counter.
#define GENINT_MASTER_RESET(master, i)                                        \
  (((master) & (~((GENINT_ONE<< GENINT_ABA_CPT)-1)) & ~(GENINT_ONE << (GENINT_LIMBSIZE - 1 - i))) \
   |((GENINT_ABA_CPT_T)((master) + 1)))

/* Initialize an integer generator (CONSTRUCTOR).
 * Initialy, the container is full of all the integers up to 'n-1'
 * The typical sequence is to initialize the container, and pop
 * the integer from it. Each pop integer is **unique** for all threads,
 * meaning it can be used to index global unique resources shared 
 * for all threads.
 */
static inline void
genint_init(genint_t s, unsigned int n)
{
  M_ASSERT (s != NULL && n > 0 && n <= GENINT_MAX_ALLOC);
  const size_t alloc = (n + GENINT_LIMBSIZE - 1) / GENINT_LIMBSIZE;
  const unsigned int index  = n % GENINT_LIMBSIZE;
  atomic_ullong *ptr = M_MEMORY_REALLOC (atomic_ullong, NULL, alloc);
  if (M_UNLIKELY (ptr == NULL)) {
    M_MEMORY_FULL(alloc);
    return;
  }
  s->n = n;
  s->data = ptr;
  s->max = (unsigned int) (alloc-1);
  s->mask0 = (index == 0) ? GENINT_FULL_MASK : ~((GENINT_ONE<<(GENINT_LIMBSIZE-index))-1);
  s->mask_master = (((GENINT_ONE << alloc) - 1) << (GENINT_LIMBSIZE-alloc)) >> GENINT_ABA_CPT;
  atomic_init (&s->master, (genint_limb_ct)0);
  for(unsigned int i = 0; i < alloc; i++)
    atomic_init(&s->data[i], (genint_limb_ct)0);
  GENINT_CONTRACT(s);
}

/* Clear an integer generator (Destructor) */
static inline void
genint_clear(genint_t s)
{
  GENINT_CONTRACT(s);
  M_MEMORY_FREE(s->data);
  s->data = NULL;
}

/* Return the maximum integer that the generator will provide */
static inline size_t
genint_size(genint_t s)
{
  GENINT_CONTRACT(s);
  return s->n;
}

/* Get an unique integer from the integer generator.
 * NOTE: For a typical case, the amortized cost is one CAS per pop. */
static inline unsigned int
genint_pop(genint_t s)
{
  GENINT_CONTRACT(s);
  // First read master to see which limb is not full.
  genint_limb_ct master = atomic_load(&s->master);
  // While master is not full
  while ((master >> GENINT_ABA_CPT) != s->mask_master) {
    // Let's get the index i of the first not full limb according to master.
    unsigned int i = m_core_clz64(~master);
    M_ASSERT (i < GENINT_LIMBSIZE);
    // Let's compute the mask of this limb representing the limb as being full
    genint_limb_ct mask = s->mask0;
    mask = (i == s->max) ? mask : GENINT_FULL_MASK;
    unsigned int bit;
    // Let's load this limb,
    genint_limb_ct next, org = atomic_load(&s->data[i]);
    do {
      // If it is now full, we have been preempted by another.
      if (M_UNLIKELY (org == mask))
        goto next_element;
      M_ASSERT (org != GENINT_FULL_MASK);
      // At least one bit is free in the limb. Find one.
      bit = GENINT_LIMBSIZE - 1 - m_core_clz64(~org);
      M_ASSERT (bit < GENINT_LIMBSIZE);
      M_ASSERT ((org & (GENINT_ONE<<bit)) == 0);
      M_ASSERT (i * GENINT_LIMBSIZE + GENINT_LIMBSIZE - 1 - bit < s->n);
      // Set the integer as being used.
      next = org | (GENINT_ONE << bit);
      // Try to reserve the integer 
    } while (!atomic_compare_exchange_weak (&s->data[i], &org, next));
    // We have reserved the integer.
    // If the limb is now full, try to update master
    if (M_UNLIKELY(next == mask)) {
      while (true) {
        genint_limb_ct newMaster;
        if (next == mask) {
          newMaster = GENINT_MASTER_SET(master, i);
        } else {
          newMaster = GENINT_MASTER_RESET(master, i);
        }
        if (atomic_compare_exchange_weak (&s->master, &master, newMaster))
          break;
        // Fail to update. Reload limb to check if it is still full.
        next = atomic_load(&s->data[i]);
      }
    }
    // Return the new number
    GENINT_CONTRACT(s);
    return i * GENINT_LIMBSIZE + GENINT_LIMBSIZE - 1 - bit;
  next_element:
    // Reload master
    master = atomic_load(&s->master);
  }
  GENINT_CONTRACT(s);
  return GENINT_ERROR; // No more resource available
}

/* Restore a used integer in the integer generator.
 * NOTE: For a typical case, the amortized cost is one CAS per pop */
static inline void
genint_push(genint_t s, unsigned int n)
{
  GENINT_CONTRACT(s);
  M_ASSERT (n < s->n);
  const unsigned int i    = n / GENINT_LIMBSIZE;
  const unsigned int bit  = GENINT_LIMBSIZE - 1 - (n % GENINT_LIMBSIZE);
  genint_limb_ct master = atomic_load(&s->master);
  // Load the limb
  genint_limb_ct next, org = atomic_load(&s->data[i]);
  do {
    M_ASSERT ((org & (GENINT_ONE << bit)) != 0);
    // Reset it
    next = org & (~(GENINT_ONE << bit));
    //  Try to unreserve it.
  } while (!atomic_compare_exchange_weak (&s->data[i], &org, next));
  // if the limb  was marked as full by master
  genint_limb_ct mask = s->mask0;
  mask = (i == s->max) ? mask : GENINT_FULL_MASK;
  if (M_UNLIKELY (next != mask)) {
    // Let's compute the mask of this limb representing the limb as being full
    // Let's try to update master to say that this limb is not full
    while (true) {
      genint_limb_ct newMaster;
      if (next == mask) {
        newMaster = GENINT_MASTER_SET(master, i);
      } else {
        newMaster = GENINT_MASTER_RESET(master, i);
      }
      if (atomic_compare_exchange_weak (&s->master, &master, newMaster))
        break;
      // Fail to update. Reload limb to check if it is still full.
      next = atomic_load(&s->data[i]);
    }
  }
  GENINT_CONTRACT(s);
}

M_END_PROTECTED_CODE

#endif