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/*
* Copyright © 2012 Google, Inc.
*
* This is part of HarfBuzz, a text shaping library.
*
* Permission is hereby granted, without written agreement and without
* license or royalty fees, to use, copy, modify, and distribute this
* software and its documentation for any purpose, provided that the
* above copyright notice and the following two paragraphs appear in
* all copies of this software.
*
* IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
* DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
* ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
* IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
* DAMAGE.
*
* THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
* BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
* FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS
* ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
* PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
*
* Google Author(s): Behdad Esfahbod
*/
#ifndef HB_CACHE_HH
#define HB_CACHE_HH
#include "hb.hh"
/* Implements a lockfree and thread-safe cache for int->int functions,
* using (optionally) _relaxed_ atomic integer operations.
*
* The cache is a fixed-size array of 16-bit or 32-bit integers,
* typically 256 elements.
*
* The key is split into two parts: the cache index (high bits)
* and the rest (low bits).
*
* The cache index is used to index into the array. The array
* member is a 16-bit or 32-bit integer that is used *both*
* to store the low bits of the key, and the value.
*
* The value is stored in the least significant bits of the integer.
* The low bits of the key are stored in the most significant bits
* of the integer.
*
* A cache hit is detected by comparing the low bits of the key
* with the high bits of the integer at the array position indexed
* by the high bits of the key. If they match, the value is extracted
* from the least significant bits of the integer and returned.
* Otherwise, a cache miss is reported.
*
* Cache operations (storage and retrieval) involve just a few
* arithmetic operations and a single memory access.
*/
template <unsigned int key_bits=16,
unsigned int value_bits=8 + 32 - key_bits,
unsigned int cache_bits=8,
bool thread_safe=true>
struct hb_cache_t
{
using item_t = typename std::conditional<thread_safe,
typename std::conditional<key_bits + value_bits - cache_bits <= 16,
hb_atomic_t<unsigned short>,
hb_atomic_t<unsigned int>>::type,
typename std::conditional<key_bits + value_bits - cache_bits <= 16,
unsigned short,
unsigned int>::type
>::type;
static_assert ((key_bits >= cache_bits), "");
static_assert ((key_bits + value_bits <= cache_bits + 8 * sizeof (item_t)), "");
static constexpr unsigned MAX_VALUE = (1u << value_bits) - 1;
hb_cache_t () { clear (); }
void clear ()
{
for (auto &v : values)
v = -1;
}
HB_HOT
bool get (unsigned int key, unsigned int *value) const
{
unsigned int k = key & ((1u<<cache_bits)-1);
unsigned int v = values[k];
if ((key_bits + value_bits - cache_bits == 8 * sizeof (item_t) && v == (unsigned int) -1) ||
(v >> value_bits) != (key >> cache_bits))
return false;
*value = v & ((1u<<value_bits)-1);
return true;
}
HB_HOT
void set (unsigned int key, unsigned int value)
{
if (unlikely ((key >> key_bits) || (value >> value_bits)))
return; /* Overflows */
set_unchecked (key, value);
}
HB_HOT
void set_unchecked (unsigned int key, unsigned int value)
{
unsigned int k = key & ((1u<<cache_bits)-1);
unsigned int v = ((key>>cache_bits)<<value_bits) | value;
values[k] = v;
}
private:
item_t values[1u<<cache_bits];
};
#endif /* HB_CACHE_HH */