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Speed up lookup by ~13% (template callbacks + root dispatch table) #17

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Merged
yhirose merged 1 commit into from
Jul 13, 2026
Merged

Speed up lookup by ~13% (template callbacks + root dispatch table) #17

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73 changes: 62 additions & 11 deletions fstlib.h
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@

#include <algorithm>
#include <any>
#include <array>
#include <cassert>
#include <cstdint>
#include <cstring>
Expand Down Expand Up @@ -1950,6 +1951,8 @@ template <typename output_t> class matcher {
}

is_valid_ = true;

build_root_dispatch();
}

operator bool() const { return is_valid_; }
Expand All @@ -1961,10 +1964,14 @@ template <typename output_t> class matcher {
}

protected:
bool match(
const char *str, size_t len,
std::function<void(const output_t &)> outputs = nullptr,
std::function<void(size_t, const output_t &)> prefixes = nullptr) const {
// The callbacks are template parameters (with std::nullptr_t defaults), so
// no std::function is constructed per query and the calls are inlined.
template <typename OutputsFn = std::nullptr_t,
typename PrefixesFn = std::nullptr_t>
bool match(const char *str, size_t len, OutputsFn outputs = nullptr,
PrefixesFn prefixes = nullptr) const {
constexpr auto has_outputs = !std::is_null_pointer_v<OutputsFn>;
constexpr auto has_prefixes = !std::is_null_pointer_v<PrefixesFn>;

if (trace_) {
std::cout << "Char\tAddress\tArc\tN F L\tNxtAddr\tOutput\tStOuts\tSize"
Expand All @@ -1979,6 +1986,16 @@ template <typename output_t> class matcher {
auto address = header_.start_address;
auto i = 0u;
auto arc_in_jump_table = false;

// Every query passes through the root state; resolve the first
// character with the precomputed dispatch table instead of the root's
// jump table binary search.
if (has_root_dispatch_ && len > 0) {
address = root_dispatch_[static_cast<uint8_t>(str[0])];
if (!address) { return false; }
arc_in_jump_table = true;
}

while (i < len) {
auto ch = static_cast<uint8_t>(str[i]);
auto state_output = output_t{};
Expand Down Expand Up @@ -2116,7 +2133,7 @@ template <typename output_t> class matcher {
output += output_suffix;
i++;
if (ope.data.final) {
if (prefixes) {
if constexpr (has_prefixes) {
if (OutputTraits<output_t>::empty(state_output)) {
prefixes(i, output);
} else {
Expand All @@ -2125,7 +2142,7 @@ template <typename output_t> class matcher {
ret = true;
}
if (i == len) {
if (outputs) {
if constexpr (has_outputs) {
if (OutputTraits<output_t>::empty(state_output)) {
outputs(output);
} else {
Expand Down Expand Up @@ -2287,13 +2304,48 @@ template <typename output_t> class matcher {
}
}

// If the root state has a jump table with labels, precompute a direct
// 256 entry 'label -> record address' table for it. The byte code is
// not affected; this only trades 1KB of memory per matcher for the
// root's binary search on every query.
void build_root_dispatch() {
auto address = header_.start_address;
auto p = byte_code_ + address;

auto ope = FstOpe(*p--);
if (!ope.has_jump_table() || !header_.flags.data.jump_table_labels) {
return;
}

auto element_size = ope.jump_table_element_size();
size_t count = 0;
auto vb_len = vb_decode_value_reverse(p, count);
p -= vb_len;
p -= count * element_size;

auto jump_table = p;
auto labels = reinterpret_cast<const uint8_t *>(p) + 1 - count;
auto jump_table_byte_size = 1 + vb_len + count * element_size + count;

root_dispatch_.fill(0);
for (size_t i = 0; i < count; i++) {
auto offset = lookup_jump_table(jump_table, i, element_size);
root_dispatch_[labels[i]] =
static_cast<uint32_t>(address - (offset + jump_table_byte_size));
}
has_root_dispatch_ = true;
}

const char *byte_code_;
const size_t byte_code_size_;

FstHeader header_;
bool is_valid_ = false;
bool trace_ = false;

bool has_root_dispatch_ = false;
std::array<uint32_t, 256> root_dispatch_{};

// Suggestion
template <typename T>
decltype(auto) suggest_core(std::string_view word, const T &matcher) const {
Expand Down Expand Up @@ -2493,9 +2545,8 @@ template <typename output_t> class map : public matcher<output_t> {
[&](const auto &_) { output = _; });
}

bool common_prefix_search(
std::string_view sv,
std::function<void(size_t, const output_t &)> prefixes) const {
template <typename PrefixesFn>
bool common_prefix_search(std::string_view sv, PrefixesFn prefixes) const {
return matcher<output_t>::match(sv.data(), sv.size(), nullptr, prefixes);
}

Expand Down Expand Up @@ -2590,8 +2641,8 @@ class set : public matcher<none_t> {

static const bool has_output = false;

bool common_prefix_search(std::string_view sv,
std::function<void(size_t)> prefixes) const {
template <typename PrefixesFn>
bool common_prefix_search(std::string_view sv, PrefixesFn prefixes) const {
return matcher<none_t>::match(
sv.data(), sv.size(), nullptr,
[&](size_t len, const none_t &) { prefixes(len); });
Expand Down
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