// Copyright 2022 The Dawn 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 result

import (
	"sort"

	"dawn.googlesource.com/dawn/tools/src/cts/query"
)

// MinimalVariantTags accepts a list of tag-sets (e.g GPU tags, OS tags, etc),
// and returns an optimized list of variants, folding together variants that
// have identical result query-to-status mappings, and removing redundant tags.
//
// MinimalVariantTags will attempt to remove variant tags starting with the
// first set of tags in tagSets, then second, and so on. If a tag-set cannot
// be removed, then the tags of the set are left alone, and the algorithm will
// progress to the next tag-set.
//
// MinimalVariantTags assumes that there are no duplicate results (same query,
// same tags) in l.
func (l List) MinimalVariantTags(tagSets []Tags) []Variant {
	type VariantData struct {
		// The variant tags
		tags Variant
		// The query -> status for all results in l that have this variant's
		// tags.
		queryToStatus map[query.Query]Status
	}

	variants := []VariantData{}

	// Build the initial list of variants from l.
	// Bin result [query -> status] to the variant.
	{
		variantIndices := map[string]int{}
		for _, r := range l {
			key := TagsToString(r.Tags)
			if idx, found := variantIndices[key]; !found {
				variantIndices[key] = len(variants)
				variants = append(variants, VariantData{
					tags: Variant(r.Tags.Clone()),
					queryToStatus: map[query.Query]Status{
						r.Query: r.Status,
					},
				})
			} else {
				variants[idx].queryToStatus[r.Query] = r.Status
			}
		}
	}

	// canReduce checks that the variant would match the same results if the
	// tags were reduced to 'tags'. Returns true if the variant's tags could
	// be reduced, otherwise false.
	canReduce := func(variant VariantData, tags Tags) bool {
		for _, r := range l.FilterByTags(tags) {
			existing, found := variant.queryToStatus[r.Query]
			if !found {
				// Removing the tag has expanded the set of queries.
				return false
			}
			if existing != r.Status {
				// Removing the tag has resulted in two queries with different
				// results.
				return false
			}
		}
		return true
	}

	// tryToRemoveTags will remove all the tags in 'tags' from all variants
	// iff doing so does not affect the set of results filtered by each variant.
	// If it was possible to remove the tags, then variants that now have the
	// same tags may be folded together, reducing the total number of variants.
	tryToRemoveTags := func(tags Tags) {
		newVariants := make([]VariantData, 0, len(variants))

		for _, v := range variants {
			// Does the variant even contain these tags?
			if !v.tags.ContainsAny(tags) {
				// Nope. Skip the canReduce() call, and keep the variant.
				newVariants = append(newVariants, v)
				continue
			}

			// Build the new set of tags with 'tags' removed.
			newTags := v.tags.Clone()
			newTags.RemoveAll(tags)

			// Check wether removal of these tags affected the outcome.
			if !canReduce(v, newTags) {
				// Removing these tags resulted in differences.
				return // Abort
			}
			newVariants = append(newVariants, VariantData{newTags, v.queryToStatus})
		}

		// Remove variants that are now subsets of others.
		// Start by sorting the variants by number of tags.
		// This ensures that the variants with fewer tags (fewer constraints)
		// come first.
		sort.Slice(newVariants, func(i, j int) bool {
			return len(newVariants[i].tags) < len(newVariants[j].tags)
		})

		// Now check each variant's tags against the previous variant tags.
		// As we've sorted, we know that supersets (fewer-tags) come before
		// subsets (more-tags).
		variants = []VariantData{}

	nextVariant:
		for i, v1 := range newVariants { // for variants 0..N
			for _, v2 := range newVariants[:i] { // for variants 0..i
				if v1.tags.ContainsAll(v2.tags) {
					continue nextVariant // v1 is a subset of v2. Omit.
				}
			}
			variants = append(variants, v1)
		}
	}

	// Attempt to remove the tag sets from the variants, one by one.
	for _, tags := range tagSets {
		tryToRemoveTags(tags)
	}

	// Return the final set of unique variants
	out := make([]Variant, len(variants))
	for i, v := range variants {
		out[i] = v.tags
	}
	return out
}