зеркало из https://github.com/mozilla/gecko-dev.git
Bug 1579862 - Introducing HuffmanTableImplementationSaturated;r=arai
We introduce a new implementation of Huffman Tables that trades space for a fast lookup. For the time being, this implementation is reserved for tables with a max bitlength of 8 or less. Depends on D45643 Differential Revision: https://phabricator.services.mozilla.com/D45644 --HG-- extra : moz-landing-system : lando
This commit is contained in:
David Teller
Родитель
0475ac490b
Коммит
01cf344b6c
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@ -58,6 +58,15 @@ static_assert(
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// more bites than necessary, just to be on the safe side.
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const uint8_t MAX_PREFIX_BIT_LENGTH = 32;
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// Maximal bit length acceptable in a `HuffmanTableSaturated`.
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//
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// As `HuffmanTableSaturated` require O(2 ^ max bit len) space, we
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// cannot afford to use them for all tables. Whenever the max bit
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// length in the table is <= MAX_BIT_LENGTH_IN_SATURATED_TABLE,
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// we use a `HuffmanTableSaturated`. Otherwise, we fall back to
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// a slower but less memory-hungry solution.
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const uint8_t MAX_BIT_LENGTH_IN_SATURATED_TABLE = 8;
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// The length of the bit buffer, in bits.
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const uint8_t BIT_BUFFER_SIZE = 64;
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@ -1631,6 +1640,11 @@ FlatHuffmanKey::FlatHuffmanKey(const HuffmanKey* key)
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// ---- Implementation of Huffman Tables
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template <typename T>
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HuffmanTableImplementationGeneric<T>::Iterator::Iterator(
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typename HuffmanTableImplementationSaturated<T>::Iterator&& iterator)
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: implementation(std::move(iterator)) {}
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template <typename T>
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HuffmanTableImplementationGeneric<T>::Iterator::Iterator(
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typename HuffmanTableImplementationMap<T>::Iterator&& iterator)
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@ -1639,6 +1653,9 @@ HuffmanTableImplementationGeneric<T>::Iterator::Iterator(
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template <typename T>
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void HuffmanTableImplementationGeneric<T>::Iterator::operator++() {
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implementation.match(
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[](typename HuffmanTableImplementationSaturated<T>::Iterator& iterator) {
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iterator.operator++();
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},
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[](typename HuffmanTableImplementationMap<T>::Iterator& iterator) {
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iterator.operator++();
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});
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@ -1648,6 +1665,12 @@ template <typename T>
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bool HuffmanTableImplementationGeneric<T>::Iterator::operator==(
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const HuffmanTableImplementationGeneric<T>::Iterator& other) const {
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return implementation.match(
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[other](const typename HuffmanTableImplementationSaturated<T>::Iterator&
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iterator) {
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return iterator ==
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other.implementation.template as<
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typename HuffmanTableImplementationSaturated<T>::Iterator>();
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},
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[other](
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const typename HuffmanTableImplementationMap<T>::Iterator& iterator) {
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return iterator ==
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@ -1660,6 +1683,12 @@ template <typename T>
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bool HuffmanTableImplementationGeneric<T>::Iterator::operator!=(
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const HuffmanTableImplementationGeneric<T>::Iterator& other) const {
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return implementation.match(
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[other](const typename HuffmanTableImplementationSaturated<T>::Iterator&
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iterator) {
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return iterator !=
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other.implementation.template as<
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typename HuffmanTableImplementationSaturated<T>::Iterator>();
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},
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[other](
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const typename HuffmanTableImplementationMap<T>::Iterator& iterator) {
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return iterator !=
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@ -1671,6 +1700,8 @@ bool HuffmanTableImplementationGeneric<T>::Iterator::operator!=(
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template <typename T>
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const T* HuffmanTableImplementationGeneric<T>::Iterator::operator*() const {
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return implementation.match(
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[](const typename HuffmanTableImplementationSaturated<T>::Iterator&
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iterator) { return iterator.operator*(); },
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[](const typename HuffmanTableImplementationMap<T>::Iterator& iterator) {
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return iterator.operator*();
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});
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@ -1685,6 +1716,9 @@ HuffmanTableImplementationGeneric<T>::HuffmanTableImplementationGeneric(
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template <typename T>
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void HuffmanTableImplementationGeneric<T>::selfCheck() {
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this->implementation.match(
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[](HuffmanTableImplementationSaturated<T>& implementation) {
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implementation.selfCheck();
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},
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[](HuffmanTableImplementationMap<T>& implementation) {
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implementation.selfCheck();
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},
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@ -1698,6 +1732,10 @@ template <typename T>
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typename HuffmanTableImplementationGeneric<T>::Iterator
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HuffmanTableImplementationGeneric<T>::begin() const {
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return this->implementation.match(
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[](const HuffmanTableImplementationSaturated<T>& implementation)
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-> HuffmanTableImplementationGeneric<T>::Iterator {
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return implementation.begin();
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},
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[](const HuffmanTableImplementationMap<T>& implementation)
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-> HuffmanTableImplementationGeneric<T>::Iterator {
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return implementation.begin();
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@ -1712,6 +1750,10 @@ template <typename T>
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typename HuffmanTableImplementationGeneric<T>::Iterator
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HuffmanTableImplementationGeneric<T>::end() const {
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return this->implementation.match(
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[](const HuffmanTableImplementationSaturated<T>& implementation)
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-> HuffmanTableImplementationGeneric<T>::Iterator {
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return implementation.end();
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},
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[](const HuffmanTableImplementationMap<T>& implementation)
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-> HuffmanTableImplementationGeneric<T>::Iterator {
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return implementation.end();
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@ -1725,14 +1767,16 @@ HuffmanTableImplementationGeneric<T>::end() const {
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template <typename T>
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JS::Result<Ok> HuffmanTableImplementationGeneric<T>::initWithSingleValue(
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JSContext* cx, T&& value) {
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// Placeholder implementation.
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// Future versions will pick between several implementations of
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// Huffman Tables depending on `numberOfSymbols`/`maxBitLength`.
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this->implementation = {
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mozilla::VariantType<HuffmanTableImplementationMap<T>>{}, cx};
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MOZ_TRY(this->implementation.template as<HuffmanTableImplementationMap<T>>()
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.initWithSingleValue(cx, std::move(value)));
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// Only one value: use HuffmanImplementationSaturated
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MOZ_ASSERT(this->implementation.template is<
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HuffmanTableUnreachable>()); // Make sure that we're initializing.
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this->implementation = {
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mozilla::VariantType<HuffmanTableImplementationSaturated<T>>{}, cx};
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MOZ_TRY(
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this->implementation.template as<HuffmanTableImplementationSaturated<T>>()
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.initWithSingleValue(cx, std::move(value)));
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return Ok();
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}
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@ -1741,14 +1785,18 @@ JS::Result<Ok> HuffmanTableImplementationGeneric<T>::init(
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JSContext* cx, size_t numberOfSymbols, uint8_t maxBitLength) {
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MOZ_ASSERT(this->implementation.template is<
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HuffmanTableUnreachable>()); // Make sure that we're initializing.
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// Placeholder implementation.
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// Future versions will pick between several implementations of
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// Huffman Tables depending on `numberOfSymbols`/`maxBitLength`.
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this->implementation = {
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mozilla::VariantType<HuffmanTableImplementationMap<T>>{}, cx};
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MOZ_TRY(
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this->implementation.template as<HuffmanTableImplementationMap<T>>().init(
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cx, numberOfSymbols, maxBitLength));
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if (maxBitLength > MAX_BIT_LENGTH_IN_SATURATED_TABLE) {
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this->implementation = {
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mozilla::VariantType<HuffmanTableImplementationMap<T>>{}, cx};
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MOZ_TRY(this->implementation.template as<HuffmanTableImplementationMap<T>>()
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.init(cx, numberOfSymbols, maxBitLength));
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} else {
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this->implementation = {
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mozilla::VariantType<HuffmanTableImplementationSaturated<T>>{}, cx};
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MOZ_TRY(this->implementation
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.template as<HuffmanTableImplementationSaturated<T>>()
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.init(cx, numberOfSymbols, maxBitLength));
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}
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return Ok();
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}
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@ -1756,6 +1804,12 @@ template <typename T>
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JS::Result<Ok> HuffmanTableImplementationGeneric<T>::addSymbol(
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uint32_t bits, uint8_t bitLength, T&& value) {
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return this->implementation.match(
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[bits, bitLength, value = std::move(value)](
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HuffmanTableImplementationSaturated<T>&
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implementation) mutable /* discard implicit const */
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-> JS::Result<Ok> {
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return implementation.addSymbol(bits, bitLength, std::move(value));
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},
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[bits, bitLength, value = std::move(value)](
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HuffmanTableImplementationMap<T>&
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implementation) mutable /* discard implicit const */
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@ -1772,6 +1826,8 @@ template <typename T>
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HuffmanEntry<const T*> HuffmanTableImplementationGeneric<T>::lookup(
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HuffmanLookup lookup) const {
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return this->implementation.match(
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[lookup](const HuffmanTableImplementationSaturated<T>& implementation)
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-> HuffmanEntry<const T*> { return implementation.lookup(lookup); },
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[lookup](const HuffmanTableImplementationMap<T>& implementation)
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-> HuffmanEntry<const T*> { return implementation.lookup(lookup); },
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[](const HuffmanTableUnreachable&) -> HuffmanEntry<const T*> {
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@ -1917,6 +1973,153 @@ HuffmanEntry<const T*> HuffmanTableImplementationMap<T>::lookup(
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return HuffmanEntry<const T*>(0, 0, nullptr);
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}
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template <typename T>
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HuffmanTableImplementationSaturated<T>::Iterator::Iterator(
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const HuffmanEntry<T>* position)
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: position(position) {}
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template <typename T>
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void HuffmanTableImplementationSaturated<T>::Iterator::operator++() {
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position++;
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}
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template <typename T>
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const T* HuffmanTableImplementationSaturated<T>::Iterator::operator*() const {
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return &position->value;
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}
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template <typename T>
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bool HuffmanTableImplementationSaturated<T>::Iterator::operator==(
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const Iterator& other) const {
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return position == other.position;
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}
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template <typename T>
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bool HuffmanTableImplementationSaturated<T>::Iterator::operator!=(
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const Iterator& other) const {
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return position != other.position;
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}
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template <typename T>
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JS::Result<Ok> HuffmanTableImplementationSaturated<T>::initWithSingleValue(
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JSContext* cx, T&& value) {
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MOZ_ASSERT(values.empty()); // Make sure that we're initializing.
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if (!values.emplaceBack(0, 0, std::move(value))) {
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return cx->alreadyReportedError();
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}
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if (!saturated.emplaceBack(0)) {
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return cx->alreadyReportedError();
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}
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this->maxBitLength = 0;
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return Ok();
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}
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template <typename T>
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JS::Result<Ok> HuffmanTableImplementationSaturated<T>::init(
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JSContext* cx, size_t numberOfSymbols, uint8_t maxBitLength) {
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MOZ_ASSERT(maxBitLength <= MAX_BIT_LENGTH_IN_SATURATED_TABLE);
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MOZ_ASSERT(values.empty()); // Make sure that we're initializing.
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this->maxBitLength = maxBitLength;
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if (!values.initCapacity(numberOfSymbols)) {
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return cx->alreadyReportedError();
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}
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const size_t saturatedLength = 1 << maxBitLength;
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if (!saturated.initCapacity(saturatedLength)) {
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return cx->alreadyReportedError();
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}
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// Enlarge `saturated`, as we're going to fill it in random order.
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for (size_t i = 0; i < saturatedLength; ++i) {
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// We use -1 (which is always invalid) to detect implementation errors.
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saturated.infallibleAppend(
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size_t(-1)); // Capacity reserved in this method.
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}
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return Ok();
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}
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#ifdef DEBUG
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template <typename T>
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void HuffmanTableImplementationSaturated<T>::selfCheck() {
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MOZ_ASSERT(
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this->maxBitLength <=
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MAX_CODE_BIT_LENGTH); // Double-check that we've initialized properly.
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bool foundMaxBitLength = false;
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for (size_t i = 0; i < saturated.length(); ++i) {
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// Check that all indices have been properly initialized.
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// Note: this is an explicit `for(;;)` loop instead of
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// a `for(:)` range loop, as knowing `i` should simplify
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// debugging.
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const size_t index = saturated[i];
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MOZ_ASSERT(index != size_t(-1));
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if (values[index].key.bitLength == maxBitLength) {
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foundMaxBitLength = true;
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}
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}
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MOZ_ASSERT(foundMaxBitLength);
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}
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#endif // DEBUG
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template <typename T>
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JS::Result<Ok> HuffmanTableImplementationSaturated<T>::addSymbol(
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uint32_t bits, uint8_t bitLength, T&& value) {
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MOZ_ASSERT(bitLength != 0,
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"Adding a symbol with a bitLength of 0 doesn't make sense.");
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MOZ_ASSERT(values.empty() || values.back().key.bitLength <= bitLength,
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"Symbols must be ranked by increasing bits length");
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MOZ_ASSERT_IF(bitLength != 32 /* >> 32 is UB */, bits >> bitLength == 0);
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MOZ_ASSERT(bitLength <= maxBitLength);
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const size_t index = values.length();
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// First add the value to `values`.
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if (!values.emplaceBack(bits, bitLength, std::move(value))) {
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MOZ_CRASH(); // Memory was reserved in `init()`.
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}
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// Notation: in the following, unless otherwise specified, we consider
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// values with `maxBitLength` bits exactly.
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//
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// When we perform lookup, we will extract `maxBitLength` bits from the key
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// into a value `0bB...B`. We have a match for `value` if and only if
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// `0bB...B` may be decomposed into `0bC...CX...X` such that
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// - `0bC...C` is `bitLength` bits long;
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// - `0bC...C == bits`.
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//
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// To perform a fast lookup, we precompute all possible values of `0bB...B`
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// for which this condition is true. That's all the values of segment
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// `[0bC...C0...0, 0bC...C1...1]`.
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const uint8_t padding = maxBitLength - bitLength;
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const size_t begin = bits << padding; // `0bC...C0...0` above
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const size_t length =
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((padding != 0) // `0bC...C1...1` above - `0bC...C0...0` above.
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? size_t(-1) >> (8 * sizeof(size_t) - padding)
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: 0) +
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1;
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for (size_t i = begin; i < begin + length; ++i) {
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saturated[i] = index;
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}
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return Ok();
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}
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template <typename T>
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HuffmanEntry<const T*> HuffmanTableImplementationSaturated<T>::lookup(
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HuffmanLookup key) const {
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// Take the `maxBitLength` highest weight bits of `key`.
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// In the documentation of `addSymbol`, this is
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// `0bB...B`.
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const uint32_t bits = key.leadingBits(maxBitLength);
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const size_t index =
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saturated[bits]; // Invariants: `saturated.length() == 1 << maxBitLength`
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// and `bits <= 1 << maxBitLength`.
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const auto& entry =
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values[index]; // Invariants: `saturated[i] < values.length()`.
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return HuffmanEntry<const T*>(entry.key.bits, entry.key.bitLength,
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&entry.value);
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}
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// -----
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// The number of possible interfaces in each sum, indexed by
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|
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@ -325,6 +325,158 @@ class HuffmanTableImplementationMap {
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friend class HuffmanPreludeReader;
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};
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// An implementation of Huffman Tables as a vector designed to allow
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// constant-time lookups at the expense of high space complexity.
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//
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// # Time complexity
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//
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// Lookups take constant time, which essentially consists in two
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// simple vector lookups.
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//
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// # Space complexity
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//
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// After initialization, a `HuffmanTableImplementationSaturated`
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// requires O(2 ^ max bit length in the table) space:
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//
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// - A vector `values` containing one entry per symbol.
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// - A vector `saturated` containing exactly 2 ^ (max bit length in the
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// table) entries, which we use to map any combination of `maxBitLength`
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// bits onto the only `HuffmanEntry` that may be reached by a prefix
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// of these `maxBitLength` bits. See below for more details.
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//
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// # Algorithm
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//
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// Consider the following Huffman table
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//
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// Symbol | Binary Code | Int value of Code | Bit Length
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// ------ | ------------ | ----------------- | ----------
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// A | 00111 | 7 | 5
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// B | 00110 | 6 | 5
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// C | 0010 | 2 | 4
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// D | 011 | 3 | 3
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// E | 010 | 2 | 3
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// F | 000 | 0 | 3
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// G | 11 | 3 | 2
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// H | 10 | 2 | 2
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//
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// By definition of a Huffman Table, the Binary Codes represent
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// paths in a Huffman Tree. Consequently, padding these codes
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// to the end would not change the result.
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//
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// Symbol | Binary Code | Int value of Code | Bit Length
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// ------ | ------------ | ----------------- | ----------
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// A | 00111 | 7 | 5
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// B | 00110 | 6 | 5
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// C | 0010? | [4...5] | 4
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// D | 011?? | [12...15] | 3
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// E | 010?? | [8..11] | 3
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// F | 000?? | [0..3] | 3
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// G | 11??? | [24...31] | 2
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// H | 10??? | [16...23] | 2
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//
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// Row "Int value of Code" now contains all possible values
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// that may be expressed in 5 bits. By using these values
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// as array indices, we may therefore represent the
|
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// Huffman table as an array:
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//
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// Index | Symbol | Bit Length
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// -------- | ---------- | -------------
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||||
// [0..3] | F | 3
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||||
// [4..5] | C | 4
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// 6 | B | 5
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// 7 | A | 5
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// [8..11] | E | 3
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// [12..15] | D | 3
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// [16..23] | H | 2
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// [24..31] | G | 2
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//
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// By using the next 5 bits in the bit buffer, we may, in
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// a single lookup, determine the symbol and the bit length.
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//
|
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// In the current implementation, to save some space, we have
|
||||
// two distinct arrays, one (`values`) with a single instance of each
|
||||
// symbols bit length, and one (`saturated`) with indices into that
|
||||
// array.
|
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template <typename T>
|
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class HuffmanTableImplementationSaturated {
|
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public:
|
||||
explicit HuffmanTableImplementationSaturated(JSContext* cx)
|
||||
: values(cx), saturated(cx), maxBitLength(-1) {}
|
||||
HuffmanTableImplementationSaturated(
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HuffmanTableImplementationSaturated&& other) = default;
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// Initialize a Huffman table containing a single value.
|
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JS::Result<Ok> initWithSingleValue(JSContext* cx, T&& value);
|
||||
|
||||
// Initialize a Huffman table containing `numberOfSymbols`.
|
||||
// Symbols must be added with `addSymbol`.
|
||||
JS::Result<Ok> init(JSContext* cx, size_t numberOfSymbols,
|
||||
uint8_t largestBitLength);
|
||||
|
||||
#ifdef DEBUG
|
||||
void selfCheck();
|
||||
#endif // DEBUG
|
||||
|
||||
// Add a `(bit, bits_length) => value` mapping.
|
||||
JS::Result<Ok> addSymbol(uint32_t bits, uint8_t bits_length, T&& value);
|
||||
|
||||
HuffmanTableImplementationSaturated() = delete;
|
||||
HuffmanTableImplementationSaturated(HuffmanTableImplementationSaturated&) =
|
||||
delete;
|
||||
|
||||
// Lookup a value in the table.
|
||||
//
|
||||
// Return an entry with a value of `nullptr` if the value is not in the table.
|
||||
//
|
||||
// The lookup may advance `key` by `[0, key.bitLength]` bits. Typically, in a
|
||||
// table with a single instance, or if the value is not in the table, it
|
||||
// will advance by 0 bits. The caller is responsible for advancing its
|
||||
// bitstream by `result.key.bitLength` bits.
|
||||
HuffmanEntry<const T*> lookup(HuffmanLookup key) const;
|
||||
|
||||
// The number of values in the table.
|
||||
size_t length() const { return values.length(); }
|
||||
|
||||
// Iterating in the order of insertion.
|
||||
struct Iterator {
|
||||
explicit Iterator(const HuffmanEntry<T>* position);
|
||||
void operator++();
|
||||
const T* operator*() const;
|
||||
bool operator==(const Iterator& other) const;
|
||||
bool operator!=(const Iterator& other) const;
|
||||
|
||||
private:
|
||||
const HuffmanEntry<T>* position;
|
||||
};
|
||||
Iterator begin() const { return Iterator(values.begin()); }
|
||||
Iterator end() const { return Iterator(values.end()); }
|
||||
|
||||
private:
|
||||
// The entries in this Huffman Table, sorted in the order of insertion.
|
||||
//
|
||||
// Invariant (once `init*` has been called):
|
||||
// - Length is the number of values inserted in the table.
|
||||
// - for all i, `values[i].bitLength <= maxBitLength`.
|
||||
Vector<HuffmanEntry<T>> values;
|
||||
|
||||
// The entries in this Huffman table, prepared for lookup.
|
||||
// The `size_t` argument is an index into `values`.
|
||||
// FIXME: We could make this a `uint8_t` to save space.
|
||||
//
|
||||
// Invariant (once `init*` has been called):
|
||||
// - Length is `1 << maxBitLength`.
|
||||
// - for all i, `saturated[i] < values.length()`
|
||||
Vector<size_t> saturated;
|
||||
|
||||
// The maximal bitlength of a value in this table.
|
||||
//
|
||||
// Invariant (once `init*` has been called):
|
||||
// - `maxBitLength <= MAX_CODE_BIT_LENGTH`
|
||||
uint8_t maxBitLength;
|
||||
|
||||
friend class HuffmanPreludeReader;
|
||||
};
|
||||
|
||||
// An empty Huffman table. Attempting to get a value from this table is a syntax
|
||||
// error. This is the default value for `HuffmanTableValue` and represents all
|
||||
// states that may not be reached.
|
||||
|
|
@ -360,6 +512,7 @@ struct HuffmanTableImplementationGeneric {
|
|||
size_t length() const;
|
||||
|
||||
struct Iterator {
|
||||
Iterator(typename HuffmanTableImplementationSaturated<T>::Iterator&&);
|
||||
Iterator(typename HuffmanTableImplementationMap<T>::Iterator&&);
|
||||
void operator++();
|
||||
const T* operator*() const;
|
||||
|
|
@ -367,7 +520,8 @@ struct HuffmanTableImplementationGeneric {
|
|||
bool operator!=(const Iterator& other) const;
|
||||
|
||||
private:
|
||||
mozilla::Variant<typename HuffmanTableImplementationMap<T>::Iterator>
|
||||
mozilla::Variant<typename HuffmanTableImplementationSaturated<T>::Iterator,
|
||||
typename HuffmanTableImplementationMap<T>::Iterator>
|
||||
implementation;
|
||||
};
|
||||
|
||||
|
|
@ -386,7 +540,8 @@ struct HuffmanTableImplementationGeneric {
|
|||
HuffmanEntry<const T*> lookup(HuffmanLookup key) const;
|
||||
|
||||
private:
|
||||
mozilla::Variant<HuffmanTableImplementationMap<T>, HuffmanTableUnreachable>
|
||||
mozilla::Variant<HuffmanTableImplementationSaturated<T>,
|
||||
HuffmanTableImplementationMap<T>, HuffmanTableUnreachable>
|
||||
implementation;
|
||||
};
|
||||
|
||||
|
|
|
|||
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