scriptcache.h

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// Copyright (c) 2017 The Bitcoin developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#ifndef BITCOIN_SCRIPT_SCRIPTCACHE_H
#define BITCOIN_SCRIPT_SCRIPTCACHE_H
 
#include <array>
#include <cstdint>
 
#include <cuckoocache.h>
#include <uint256.h>
#include <util/hasher.h>
 
class CTransaction;
 
class ScriptCacheKey {
    std::array<uint8_t, 28> data;
 
public:
    ScriptCacheKey() = default;
    ScriptCacheKey(const ScriptCacheKey &rhs) = default;
    ScriptCacheKey(const CTransaction &tx, uint32_t flags, CSHA256 &&hasher);
 
    ScriptCacheKey &operator=(const ScriptCacheKey &rhs) = default;
 
    bool operator==(const ScriptCacheKey &rhs) const {
        return rhs.data == data;
    }
 
    friend class ScriptCacheHasher;
};
 
struct ScriptCacheElement {
    using KeyType = ScriptCacheKey;
 
    KeyType key;
    int nSigChecks;
 
    ScriptCacheElement() = default;
 
    ScriptCacheElement(const KeyType &keyIn, int nSigChecksIn)
        : key(keyIn), nSigChecks(nSigChecksIn) {}
 
    const KeyType &getKey() const { return key; }
};
 
static_assert(sizeof(ScriptCacheElement) == 32,
              "ScriptCacheElement should be 32 bytes");
 
class ScriptCacheHasher {
public:
    template <uint8_t hash_select>
    uint32_t operator()(const ScriptCacheKey &k) const {
        static_assert(hash_select < 8, "only has 8 hashes available.");
 
        const auto &d = k.data;
 
        static_assert(sizeof(d) == 28,
                      "modify the following if key size changes");
 
        uint32_t u;
        static_assert(sizeof(u) == 4 && sizeof(d[0]) == 1, "basic assumptions");
        if (hash_select < 7) {
            std::memcpy(&u, d.data() + 4 * hash_select, 4);
        } else {
            // We are required to produce 8 subhashes, and all bytes have
            // been used once. We re-use the bytes but mix together different
            // entries (and flip the order) to get a new, distinct value.
            uint8_t arr[4];
            arr[0] = d[3] ^ d[7] ^ d[11] ^ d[15];
            arr[1] = d[6] ^ d[10] ^ d[14] ^ d[18];
            arr[2] = d[9] ^ d[13] ^ d[17] ^ d[21];
            arr[3] = d[12] ^ d[16] ^ d[20] ^ d[24];
            std::memcpy(&u, arr, 4);
        }
        return u;
    }
};
 
// DoS prevention: limit cache size to 32MiB (over 1000000 entries on 64-bit
// systems). Due to how we count cache size, actual memory usage is slightly
// more (~32.25 MiB)
static constexpr size_t DEFAULT_SCRIPT_EXECUTION_CACHE_BYTES{32 << 20};
 
#endif // BITCOIN_SCRIPT_SCRIPTCACHE_H