mimir/src/heap.cpp

//
//  heap.cpp
//  Kraken Engine
//
//  Copyright 2026 Kearwood Gilbert. All rights reserved.
//
//  Redistribution and use in source and binary forms, with or without modification, are
//  permitted provided that the following conditions are met:
//
//  1. Redistributions of source code must retain the above copyright notice, this list of
//  conditions and the following disclaimer.
//
//  2. 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 KEARWOOD GILBERT ''AS IS'' AND ANY EXPRESS OR IMPLIED
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//

#include "../include/mimir.h"
#include "mimir_impl.h"

#include <cassert>
#include <cstring>
#include <bit>

namespace mimir {

// Two-Level Segregated Fit (TLSF): http://www.gii.upv.es/tlsf/files/papers/ecrts04_tlsf.pdf

const size_t kMinblockSize = 32;

struct TLSFBlock
{
  size_t size; // LSB: T, F: T = Last physical block, F = Free block
  TLSFBlock* prevPhys;
  TLSFBlock* prevFree;
  TLSFBlock* nextFree;
};
static_assert(sizeof(TLSFBlock) == kMinblockSize);

struct TLSFIndex
{
  uint64_t firstLevelFreeBitMap;
  uint8_t secondLevelFreeBitMap[60];
  TLSFBlock* secondLevelFreeBlocks[60][16];
};

// First Level Ranges:
// 0:  0b00000000010000  (    1 ~   16 )
// 1:  0b00000000100000  (   17 ~   32 )
// 2:  0b00000001000000  (   33 ~   64 )
// 3:  0b00000010000000  (   65 ~  128 )
// 4:  0b00000100000000  (  129 ~  256 )
// 5:  0b00001000000000  (  257 ~  512 )
// 6:  0b00010000000000  (  513 ~ 1024 )
// 7:  0b00100000000000  ( 1025 ~ 2048 )
//
// bit_width(15): 4    bit_floor(15): 0b000001000
// bit_width(16): 5    bit_floor(16): 0b000010000
// bit_width(31): 5    bit_floor(31): 0b000010000
// bit_width(32): 6    bit_floor(32): 0b000100000
// bit_width(63): 6    bit_floor(63): 0b000100000
// bit_width(64): 7    bit_floor(64): 0b001000000

/*
uint64_t blockSizeToIndex(uint64_t size)
{
  // MSB 60 bits are the first level index
  // LSB 4 bits is the second level index

  // size              0b0000001101011010000
  // bit_width         13
  // bit_width - 5     8
  // size >> 8         0b0000000000000011010
  //   & 0b1111        0b1010

  size_t firstLevelIndex = std::bit_width(size) - 5;
  size_t secondLevelIndex = (size >> firstLevelIndex) & 0b1111;
  return (firstLevelIndex << 4) | secondLevelIndex;
}
*/

Heap::Heap()
{
}

Heap::~Heap()
{
}

bool Heap::init(size_t maxSize)
{
  if (!m_region.init(maxSize)) {
    return false;
  }
  if (!m_region.resize(sizeof(TLSFIndex) + 16)) {
    return false;
  }
  TLSFIndex* index = (TLSFIndex*)m_region.getAddress();
  memset(index, 0, sizeof(TLSFIndex));

  // Start with one free block, filling the entire Region
  TLSFBlock* block = (TLSFBlock*)(m_region.getAddress() + sizeof(TLSFIndex));
  block->nextFree = nullptr;
  block->prevFree = nullptr;
  block->size = m_region.getMaxSize() - sizeof(TLSFIndex);
  block->size |= 0b11; // T=1: Last Block, F=1: Free Block

  insertFreeBlock(block);

  return true;
}

void Heap::insertFreeBlock(TLSFBlock* block)
{
  size_t usableSize = (block->size & ~0b11) - 16;

  // Add the block to the index

  TLSFIndex* index = (TLSFIndex*)m_region.getAddress();

  size_t firstLevelIndex = std::bit_width(usableSize) - 5;
  size_t secondLevelIndex = (usableSize >> firstLevelIndex) & 0b1111;
  index->firstLevelFreeBitMap |= std::bit_floor(usableSize);
  index->secondLevelFreeBitMap[firstLevelIndex] |= secondLevelIndex & 0b1111;
  index->secondLevelFreeBlocks[firstLevelIndex][secondLevelIndex] = block;
}

void Heap::removeFreeBlock(TLSFBlock* block)
{
  // Remove the block from the index
  TLSFIndex* index = (TLSFIndex*)m_region.getAddress();

  size_t usableSize = (block->size & ~0b11) - 16;
}

// Allocate `size` bytes
std::byte* Heap::alloc(size_t size)
{
  TLSFIndex* index = (TLSFIndex*)m_region.getAddress();


  // size:           0b0000001110011101 (925)
  // bit_ceil(size): 0b0000010000000000
  // - 1 ...         0b0000001111111111
  // ~ ...           0b1111110000000000

  uint64_t possibleFirstLevels = ~(std::bit_ceil(size) - 1);
  uint64_t freeFirstLevels = index->firstLevelFreeBitMap & possibleFirstLevels;
  uint64_t selectedFirstLevel = std::countr_zero(freeFirstLevels);
  if (selectedFirstLevel < 5) {
    selectedFirstLevel = 0;
  } else {
    selectedFirstLevel -= 5;
  }



  return nullptr; // not implemented
}

// Allocate `size` bytes, aligned to 16 bytes and padded to next 16-byte offset.
std::byte* Heap::allocA16(size_t size)
{
  return nullptr; // not implemented
}

// Allocate `size` bytes, aligned to 64 bytes and padded to next 64-byte offset.
std::byte* Heap::allocA64(size_t size)
{
  return nullptr; // not implemented
}

// Free the allocation at `address`
void Heap::free(std::byte* address)
{
}

} // namespace mimir