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#include "slang-perfect-hash.h"
#include "../core/slang-string-util.h"
#include "../core/slang-writer.h"
namespace Slang
{
// Implemented according to "Hash, displace, and compress"
// https://cmph.sourceforge.net/papers/esa09.pdf
HashFindResult minimalPerfectHash(const List<String>& ss, HashParams& hashParams)
{
// Check for uniqueness
for (Index i = 0; i < ss.getCount(); ++i)
{
for (Index j = i + 1; j < ss.getCount(); ++j)
{
if (ss[i] == ss[j])
{
return HashFindResult::NonUniqueKeys;
}
}
}
SLANG_ASSERT(UIndex(ss.getCount()) < std::numeric_limits<UInt32>::max());
const UInt32 nBuckets = UInt32(ss.getCount());
List<List<String>> initialBuckets;
initialBuckets.setCount(nBuckets);
const auto hash = [&](const String& s, const HashCode32 salt = 0) -> UInt32
{
//
// The current getStableHashCode is susceptible to patterns of
// collisions causing the search to fail for the SPIR-V opnames; it
// performs poorly on short strings, taking over 300000 iterations to
// diverge on "Ceil" and "FMix" (and place them in already unoccupied
// slots)!
//
// Use FNV Hash here which seem perform much better on these short inputs
// https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
//
// If you change this, don't forget to also sync the version below in
// the printing code.
UInt32 h = salt;
for (const char c : s)
h = (h * 0x01000193) ^ c;
return h % nBuckets;
};
// Assign the inputs into their buckets according to the hash without salt.
// Sort the buckets according to size, so that later we can make these have
// unique destinations starting with the largest ones first as they are at
// most risk of collision.
for (const auto& s : ss)
{
initialBuckets[hash(s)].add(s);
}
initialBuckets.stableSort([](const List<String>& a, const List<String>& b) { return a.getCount() > b.getCount(); });
// These are our outputs, the salts are calculated such that for all input
// word, x, hash(x, salt[hash(x, 0)]) is unique
//
// We keep the final table as we need to detect when we've been given a
// word not in our language.
hashParams.saltTable.setCount(nBuckets);
for (auto& s : hashParams.saltTable)
s = 0;
hashParams.destTable.setCount(nBuckets);
for (auto& s : hashParams.destTable)
s.reduceLength(0);
// This mask will, in each salt tryout, be used to prevent collisions
// within a single bucket.
List<bool> bucketDestinations = List<bool>::makeRepeated(false, nBuckets);
for (const auto& b : initialBuckets)
{
// Break if we've reached the empty buckets
if (!b.getCount())
{
break;
}
// Try out all the salts until we get one which has no internal
// collisions for this bucket and also no collisions with the buckets
// we've processed so far.
UInt32 salt = 1;
while (true)
{
bool collision = false;
for (auto& d : bucketDestinations)
{
d = false;
}
for (const auto& s : b)
{
const auto i = hash(s, salt);
if (hashParams.destTable[i].getLength() || bucketDestinations[i])
{
collision = true;
break;
}
bucketDestinations[i] = true;
}
if (!collision)
{
break;
}
salt++;
// If we fail to find a solution after some massive amount of tries
// it's almost certainly because of some property of the hash
// function and language causing an irresolvable collision.
if (salt > 10000 * nBuckets)
{
return HashFindResult::UnavoidableHashCollision;
}
}
for (const auto& s : b)
{
hashParams.saltTable[hash(s)] = salt;
hashParams.destTable[hash(s, salt)] = s;
}
}
return HashFindResult::Success;
}
String perfectHashToEmbeddableCpp(
const HashParams& hashParams,
const UnownedStringSlice& valueType,
const UnownedStringSlice& funcName,
const List<String>& values)
{
SLANG_ASSERT(hashParams.saltTable.getCount() == hashParams.destTable.getCount());
SLANG_ASSERT(hashParams.saltTable.getCount() == values.getCount());
StringBuilder sb;
StringWriter writer(&sb, WriterFlags(0));
WriterHelper w(&writer);
const auto line = [&](const char* l){
w.put(l);
w.put("\n");
};
w.print("bool %s(const UnownedStringSlice& str, %s& value)\n", String(funcName).getBuffer(), String(valueType).getBuffer());
line("{");
w.print(" static const unsigned tableSalt[%d] = {\n", (int)hashParams.saltTable.getCount());
w.print(" ");
for (Index i = 0; i < hashParams.saltTable.getCount(); ++i)
{
const auto salt = hashParams.saltTable[i];
if (i != hashParams.saltTable.getCount() - 1)
{
w.print(" %d,", salt);
if (i % 16 == 15)
{
w.print("\n ");
}
}
else
{
w.print(" %d", salt);
}
}
line("\n };");
line("");
w.print(" using KV = std::pair<const char*, %s>;\n", String(valueType).getBuffer());
line("");
w.print(" static const KV words[%d] =\n", (int)hashParams.destTable.getCount());
line(" {");
for (Index i = 0; i < hashParams.destTable.getCount(); ++i)
{
const auto& s = hashParams.destTable[i];
const auto& v = values[i];
w.print(
" {\"%s\", %s},\n",
s.getBuffer(),
v.getBuffer()
);
}
line(" };");
line("");
// Make sure to update the hash function in the search function above if
// you change this.
line(" static const auto hash = [](const UnownedStringSlice& str, UInt32 salt){");
line(" UInt32 h = salt;");
line(" for (const char c : str)");
line(" h = (h * 0x01000193) ^ c;");
w.print(" return h %% %d;\n", (int)hashParams.saltTable.getCount());
line(" };");
line("");
line(" const auto i = hash(str, tableSalt[hash(str, 0)]);");
line(" if(str == words[i].first)");
line(" {");
line(" value = words[i].second;");
line(" return true;");
line(" }");
line(" else");
line(" {");
line(" return false;");
line(" }");
line("}");
line("");
return sb.produceString();
}
}
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