Lookup Table: Static Array vs. Stack Array

Question 8 / 51 • Correct so far: 0 (0 answered)

Snippet A

Constexpr Table

constexpr std::array<uint8_t, 256> makeTable() {
    std::array<uint8_t, 256> t{};
    for (int i = 0; i < 256; ++i)
        t[i] = static_cast<uint8_t>((i * 73 + 19) & 0xFF);
    return t;
}

uint8_t transform(uint8_t c) {
    static constexpr auto TABLE = makeTable();
    return TABLE[c];
}

uint32_t sum = 0;
for (uint8_t q : QUERIES)
    sum += transform(q);
Snippet B

Runtime Init Table

uint8_t transform(uint8_t c) {
    std::array<uint8_t, 256> table;
    for (int i = 0; i < 256; ++i)
        table[i] = static_cast<uint8_t>((i * 73 + 19) & 0xFF);
    return table[c];
}

uint32_t sum = 0;
for (uint8_t q : QUERIES)
    sum += transform(q);
Shared test data (shared-setup) 
constexpr int kQueries = 65536;

static std::vector<uint8_t> makeQueries() {
    std::vector<uint8_t> qs(kQueries);
    for (int i = 0; i < kQueries; ++i)
        qs[i] = static_cast<uint8_t>(i & 0xFF);
    return qs;
}

static const std::vector<uint8_t> QUERIES = makeQueries();

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