tools/src/lut/lut.go - dawn - Git at Google

 // Copyright 2021 The Tint Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 // Package lut provides a look up table, which compresses indexed data
 package lut

 import (
 	"sort"
 )

 // LUT is a look up table.
 // The table holds a number of items that are stored in a linear list.
 type LUT[T comparable] interface {
 	// Add adds a sequence of items to the table.
 	// The sequence of items stored at [offset, offset+N), where N is the
 	// number of items added will remain equal, even after calling Compact().
 	// Returns a pointer to the start index in the list.
 	Add(items []T) *int
 	// Compact reorders the table items so that the table storage is compacted
 	// by shuffling data around and de-duplicating sequences of common data.
 	// Each originally added sequence is preserved in the resulting table, with
 	// the same contiguous ordering, but with a potentially different offset.
 	// Heuristics are used to shorten the table length, by exploiting common
 	// subsequences, and removing duplicate sequences.
 	// Note that shortest common superstring is NP-hard, so heuristics are used.
 	// Compact updates pointers returned by Add().
 	Compact() []T
 }

 // New returns a new look up table
 func New[T comparable]() LUT[T] {
 	return &lut[T]{storage: []T{}}
 }

 // A sequence represents a span of entries in the table
 type sequence struct {
 	offset *int // Pointer to the start index of the sequence
 	count  int  // Length of the sequence
 }

 // lut implements LUT
 type lut[T comparable] struct {
 	storage   []T        // The storage
 	sequences []sequence // The entries in the LUT
 }

 func (t *lut[T]) Add(items []T) *int {
 	if len(items) == 0 {
 		return nil
 	}

 	offset := len(t.storage)
 	count := len(items)
 	t.storage = append(t.storage, items...)
 	offsetPtr := &offset
 	t.sequences = append(t.sequences, sequence{offsetPtr, count})
 	return offsetPtr
 }

 // match describes a sequence that can be placed.
 type match struct {
 	dst int      // destination offset
 	src sequence // source sequence
 	len int      // number of items that matched
 	idx int      // sequence index
 }

 func (t lut[T]) Compact() []T {
 	// Generate int32 identifiers for each unique item in the table.
 	// We use these to compare items instead of comparing the real data as this
 	// function is comparison-heavy, and integer compares are cheap.
 	srcIDs := t.itemIDs()
 	dstIDs := make([]int32, len(srcIDs))

 	// Make a copy the data held in the table, use the copy as the source, and
 	// t.storage as the destination.
 	srcData := make([]T, len(t.storage))
 	copy(srcData, t.storage)
 	dstData := t.storage

 	// Sort all the sequences by length, with the largest first.
 	// This helps 'seed' the compacted form with the largest items first.
 	// This can improve the compaction as small sequences can pack into larger,
 	// placed items.
 	sort.SliceStable(t.sequences, func(i, j int) bool {
 		return t.sequences[i].count > t.sequences[j].count
 	})

 	// unplaced is the list of sequences that have not yet been placed.
 	// All sequences are initially unplaced.
 	unplaced := make([]sequence, len(t.sequences))
 	copy(unplaced, t.sequences)

 	// placed is the list of sequences that have been placed.
 	// Nothing is initially placed.
 	placed := make([]sequence, 0, len(t.sequences))

 	// remove removes the sequence in unplaced with the index i.
 	remove := func(i int) {
 		placed = append(placed, unplaced[i])
 		if i > 0 {
 			if i < len(unplaced)-1 {
 				copy(unplaced[i:], unplaced[i+1:])
 			}
 			unplaced = unplaced[:len(unplaced)-1]
 		} else {
 			unplaced = unplaced[1:]
 		}
 	}

 	// cp copies data from [srcOffset:srcOffset+count] to [dstOffset:dstOffset+count].
 	cp := func(dstOffset, srcOffset, count int) {
 		copy(dstData[dstOffset:dstOffset+count], srcData[srcOffset:srcOffset+count])
 		copy(dstIDs[dstOffset:dstOffset+count], srcIDs[srcOffset:srcOffset+count])
 	}

 	// number of items that have been placed.
 	newSize := 0

 	// While there's sequences to place...
 	for len(unplaced) > 0 {
 		// Place the next largest, unplaced sequence at the end of the new list
 		cp(newSize, *unplaced[0].offset, unplaced[0].count)
 		*unplaced[0].offset = newSize
 		newSize += unplaced[0].count
 		remove(0)

 		for {
 			// Look for the sequence with the longest match against the
 			// currently placed data. Any mismatches with currently placed data
 			// will nullify the match. The head or tail of this sequence may
 			// extend the currently placed data.
 			best := match{}

 			// For each unplaced sequence...
 			for i := 0; i < len(unplaced); i++ {
 				seq := unplaced[i]

 				if best.len >= seq.count {
 					// The best match is already at least as long as this
 					// sequence and sequences are sorted by size, so best cannot
 					// be beaten. Stop searching.
 					break
 				}

 				// Perform a full sweep from left to right, scoring the match...
 				for shift := -seq.count + 1; shift < newSize; shift++ {
 					dstS := max(shift, 0)
 					dstE := min(shift+seq.count, newSize)
 					count := dstE - dstS
 					srcS := *seq.offset - min(shift, 0)
 					srcE := srcS + count

 					if best.len < count {
 						if equal(srcIDs[srcS:srcE], dstIDs[dstS:dstE]) {
 							best = match{shift, seq, count, i}
 						}
 					}
 				}
 			}

 			if best.src.offset == nil {
 				// Nothing matched. Not even one element.
 				// Resort to placing the next largest sequence at the end.
 				break
 			}

 			if best.dst < 0 {
 				// Best match wants to place the sequence to the left of the
 				// current output. We have to shuffle everything...
 				n := -best.dst
 				copy(dstData[n:n+newSize], dstData)
 				copy(dstIDs[n:n+newSize], dstIDs)
 				newSize += n
 				best.dst = 0
 				for _, p := range placed {
 					*p.offset += n
 				}
 			}

 			// Place the best matching sequence.
 			cp(best.dst, *best.src.offset, best.src.count)
 			newSize = max(newSize, best.dst+best.src.count)
 			*best.src.offset = best.dst
 			remove(best.idx)
 		}
 	}

 	// Shrink the output buffer to the new size.
 	dstData = dstData[:newSize]

 	// All done.
 	return dstData
 }

 // Generate a set of identifiers for all the unique items in storage
 func (t lut[T]) itemIDs() []int32 {
 	storageSize := len(t.storage)
 	keys := make([]int32, storageSize)
 	dataToKey := map[T]int32{}
 	for i := 0; i < storageSize; i++ {
 		data := t.storage[i]
 		key, found := dataToKey[data]
 		if !found {
 			key = int32(len(dataToKey))
 			dataToKey[data] = key
 		}
 		keys[i] = key
 	}
 	return keys
 }

 func max(a, b int) int {
 	if a < b {
 		return b
 	}
 	return a
 }

 func min(a, b int) int {
 	if a > b {
 		return b
 	}
 	return a
 }

 func equal(a, b []int32) bool {
 	for i, v := range a {
 		if b[i] != v {
 			return false
 		}
 	}
 	return true
 }
	// Copyright 2021 The Tint Authors.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	// Package lut provides a look up table, which compresses indexed data
	package lut

	import (
	"sort"
	)

	// LUT is a look up table.
	// The table holds a number of items that are stored in a linear list.
	type LUT[T comparable] interface {
	// Add adds a sequence of items to the table.
	// The sequence of items stored at [offset, offset+N), where N is the
	// number of items added will remain equal, even after calling Compact().
	// Returns a pointer to the start index in the list.
	Add(items []T) *int
	// Compact reorders the table items so that the table storage is compacted
	// by shuffling data around and de-duplicating sequences of common data.
	// Each originally added sequence is preserved in the resulting table, with
	// the same contiguous ordering, but with a potentially different offset.
	// Heuristics are used to shorten the table length, by exploiting common
	// subsequences, and removing duplicate sequences.
	// Note that shortest common superstring is NP-hard, so heuristics are used.
	// Compact updates pointers returned by Add().
	Compact() []T
	}

	// New returns a new look up table
	func New[T comparable]() LUT[T] {
	return &lut[T]{storage: []T{}}
	}

	// A sequence represents a span of entries in the table
	type sequence struct {
	offset *int // Pointer to the start index of the sequence
	count int // Length of the sequence
	}

	// lut implements LUT
	type lut[T comparable] struct {
	storage []T // The storage
	sequences []sequence // The entries in the LUT
	}

	func (t lut[T]) Add(items []T) int {
	if len(items) == 0 {
	return nil
	}

	offset := len(t.storage)
	count := len(items)
	t.storage = append(t.storage, items...)
	offsetPtr := &offset
	t.sequences = append(t.sequences, sequence{offsetPtr, count})
	return offsetPtr
	}

	// match describes a sequence that can be placed.
	type match struct {
	dst int // destination offset
	src sequence // source sequence
	len int // number of items that matched
	idx int // sequence index
	}

	func (t lut[T]) Compact() []T {
	// Generate int32 identifiers for each unique item in the table.
	// We use these to compare items instead of comparing the real data as this
	// function is comparison-heavy, and integer compares are cheap.
	srcIDs := t.itemIDs()
	dstIDs := make([]int32, len(srcIDs))

	// Make a copy the data held in the table, use the copy as the source, and
	// t.storage as the destination.
	srcData := make([]T, len(t.storage))
	copy(srcData, t.storage)
	dstData := t.storage

	// Sort all the sequences by length, with the largest first.
	// This helps 'seed' the compacted form with the largest items first.
	// This can improve the compaction as small sequences can pack into larger,
	// placed items.
	sort.SliceStable(t.sequences, func(i, j int) bool {
	return t.sequences[i].count > t.sequences[j].count
	})

	// unplaced is the list of sequences that have not yet been placed.
	// All sequences are initially unplaced.
	unplaced := make([]sequence, len(t.sequences))
	copy(unplaced, t.sequences)

	// placed is the list of sequences that have been placed.
	// Nothing is initially placed.
	placed := make([]sequence, 0, len(t.sequences))

	// remove removes the sequence in unplaced with the index i.
	remove := func(i int) {
	placed = append(placed, unplaced[i])
	if i > 0 {
	if i < len(unplaced)-1 {
	copy(unplaced[i:], unplaced[i+1:])
	}
	unplaced = unplaced[:len(unplaced)-1]
	} else {
	unplaced = unplaced[1:]
	}
	}

	// cp copies data from [srcOffset:srcOffset+count] to [dstOffset:dstOffset+count].
	cp := func(dstOffset, srcOffset, count int) {
	copy(dstData[dstOffset:dstOffset+count], srcData[srcOffset:srcOffset+count])
	copy(dstIDs[dstOffset:dstOffset+count], srcIDs[srcOffset:srcOffset+count])
	}

	// number of items that have been placed.
	newSize := 0

	// While there's sequences to place...
	for len(unplaced) > 0 {
	// Place the next largest, unplaced sequence at the end of the new list
	cp(newSize, *unplaced[0].offset, unplaced[0].count)
	*unplaced[0].offset = newSize
	newSize += unplaced[0].count
	remove(0)

	for {
	// Look for the sequence with the longest match against the
	// currently placed data. Any mismatches with currently placed data
	// will nullify the match. The head or tail of this sequence may
	// extend the currently placed data.
	best := match{}

	// For each unplaced sequence...
	for i := 0; i < len(unplaced); i++ {
	seq := unplaced[i]

	if best.len >= seq.count {
	// The best match is already at least as long as this
	// sequence and sequences are sorted by size, so best cannot
	// be beaten. Stop searching.
	break
	}

	// Perform a full sweep from left to right, scoring the match...
	for shift := -seq.count + 1; shift < newSize; shift++ {
	dstS := max(shift, 0)
	dstE := min(shift+seq.count, newSize)
	count := dstE - dstS
	srcS := *seq.offset - min(shift, 0)
	srcE := srcS + count

	if best.len < count {
	if equal(srcIDs[srcS:srcE], dstIDs[dstS:dstE]) {
	best = match{shift, seq, count, i}
	}
	}
	}
	}

	if best.src.offset == nil {
	// Nothing matched. Not even one element.
	// Resort to placing the next largest sequence at the end.
	break
	}

	if best.dst < 0 {
	// Best match wants to place the sequence to the left of the
	// current output. We have to shuffle everything...
	n := -best.dst
	copy(dstData[n:n+newSize], dstData)
	copy(dstIDs[n:n+newSize], dstIDs)
	newSize += n
	best.dst = 0
	for _, p := range placed {
	*p.offset += n
	}
	}

	// Place the best matching sequence.
	cp(best.dst, *best.src.offset, best.src.count)
	newSize = max(newSize, best.dst+best.src.count)
	*best.src.offset = best.dst
	remove(best.idx)
	}
	}

	// Shrink the output buffer to the new size.
	dstData = dstData[:newSize]

	// All done.
	return dstData
	}

	// Generate a set of identifiers for all the unique items in storage
	func (t lut[T]) itemIDs() []int32 {
	storageSize := len(t.storage)
	keys := make([]int32, storageSize)
	dataToKey := map[T]int32{}
	for i := 0; i < storageSize; i++ {
	data := t.storage[i]
	key, found := dataToKey[data]
	if !found {
	key = int32(len(dataToKey))
	dataToKey[data] = key
	}
	keys[i] = key
	}
	return keys
	}

	func max(a, b int) int {
	if a < b {
	return b
	}
	return a
	}

	func min(a, b int) int {
	if a > b {
	return b
	}
	return a
	}

	func equal(a, b []int32) bool {
	for i, v := range a {
	if b[i] != v {
	return false
	}
	}
	return true
	}