// Copyright 2020 Google Inc. All rights reserved.
//
// 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 depset

import (
	"fmt"
	"iter"
	"slices"

	"github.com/google/blueprint/gobtools"
)

// DepSet is designed to be conceptually compatible with Bazel's depsets:
// https://docs.bazel.build/versions/master/skylark/depsets.html

type Order int

const (
	PREORDER Order = iota
	POSTORDER
	TOPOLOGICAL
)

func (o Order) String() string {
	switch o {
	case PREORDER:
		return "PREORDER"
	case POSTORDER:
		return "POSTORDER"
	case TOPOLOGICAL:
		return "TOPOLOGICAL"
	default:
		panic(fmt.Errorf("Invalid Order %d", o))
	}
}

type depSettableType comparable

// A DepSet efficiently stores a slice of an arbitrary type from transitive dependencies without
// copying. It is stored as a DAG of DepSet nodes, each of which has some direct contents and a list
// of dependency DepSet nodes.
//
// A DepSet has an order that will be used to walk the DAG when ToList() is called.  The order
// can be POSTORDER, PREORDER, or TOPOLOGICAL.  POSTORDER and PREORDER orders return a postordered
// or preordered left to right flattened list.  TOPOLOGICAL returns a list that guarantees that
// elements of children are listed after all of their parents (unless there are duplicate direct
// elements in the DepSet or any of its transitive dependencies, in which case the ordering of the
// duplicated element is not guaranteed).
//
// A DepSet is created by New or NewBuilder.Build from the slice for direct contents
// and the *DepSets of dependencies. A DepSet is immutable once created.
type DepSet[T depSettableType] struct {
	handle *depSet[T]
}

type depSet[T depSettableType] struct {
	preorder   bool
	reverse    bool
	order      Order
	direct     []T
	transitive []DepSet[T]
}

func (d DepSet[T]) impl() *depSet[T] {
	return d.handle
}

func (d DepSet[T]) order() Order {
	impl := d.impl()
	return impl.order
}

type depSetGob[T depSettableType] struct {
	Preorder   bool
	Reverse    bool
	Order      Order
	Direct     []T
	Transitive []DepSet[T]
}

func (d *DepSet[T]) ToGob() *depSetGob[T] {
	impl := d.impl()
	return &depSetGob[T]{
		Preorder:   impl.preorder,
		Reverse:    impl.reverse,
		Order:      impl.order,
		Direct:     impl.direct,
		Transitive: impl.transitive,
	}
}

func (d *DepSet[T]) FromGob(data *depSetGob[T]) {
	d.handle = &depSet[T]{
		preorder:   data.Preorder,
		reverse:    data.Reverse,
		order:      data.Order,
		direct:     data.Direct,
		transitive: data.Transitive,
	}
}

func (d DepSet[T]) GobEncode() ([]byte, error) {
	return gobtools.CustomGobEncode[depSetGob[T]](&d)
}

func (d *DepSet[T]) GobDecode(data []byte) error {
	return gobtools.CustomGobDecode[depSetGob[T]](data, d)
}

// New returns an immutable DepSet with the given order, direct and transitive contents.
func New[T depSettableType](order Order, direct []T, transitive []DepSet[T]) DepSet[T] {
	var directCopy []T
	var transitiveCopy []DepSet[T]
	nonEmptyTransitiveCount := 0
	for _, t := range transitive {
		if t.handle != nil {
			if t.order() != order {
				panic(fmt.Errorf("incompatible order, new DepSet is %s but transitive DepSet is %s",
					order, t.order()))
			}
			nonEmptyTransitiveCount++
		}
	}

	directCopy = slices.Clone(direct)
	if nonEmptyTransitiveCount > 0 {
		transitiveCopy = make([]DepSet[T], 0, nonEmptyTransitiveCount)
	}
	var transitiveIter iter.Seq2[int, DepSet[T]]
	if order == TOPOLOGICAL {
		// TOPOLOGICAL is implemented as a postorder traversal followed by reversing the output.
		// Pre-reverse the inputs here so their order is maintained in the output.
		slices.Reverse(directCopy)
		transitiveIter = slices.Backward(transitive)
	} else {
		transitiveIter = slices.All(transitive)
	}
	for _, t := range transitiveIter {
		if t.handle != nil {
			transitiveCopy = append(transitiveCopy, t)
		}
	}

	if len(directCopy) == 0 && len(transitive) == 0 {
		return DepSet[T]{nil}
	}

	depSet := &depSet[T]{
		preorder:   order == PREORDER,
		reverse:    order == TOPOLOGICAL,
		order:      order,
		direct:     directCopy,
		transitive: transitiveCopy,
	}

	return DepSet[T]{depSet}
}

// Builder is used to create an immutable DepSet.
type Builder[T depSettableType] struct {
	order      Order
	direct     []T
	transitive []DepSet[T]
}

// NewBuilder returns a Builder to create an immutable DepSet with the given order and
// type, represented by a slice of type that will be in the DepSet.
func NewBuilder[T depSettableType](order Order) *Builder[T] {
	return &Builder[T]{
		order: order,
	}
}

// DirectSlice adds direct contents to the DepSet being built by a Builder. Newly added direct
// contents are to the right of any existing direct contents.
func (b *Builder[T]) DirectSlice(direct []T) *Builder[T] {
	b.direct = append(b.direct, direct...)
	return b
}

// Direct adds direct contents to the DepSet being built by a Builder. Newly added direct
// contents are to the right of any existing direct contents.
func (b *Builder[T]) Direct(direct ...T) *Builder[T] {
	b.direct = append(b.direct, direct...)
	return b
}

// Transitive adds transitive contents to the DepSet being built by a Builder. Newly added
// transitive contents are to the right of any existing transitive contents.
func (b *Builder[T]) Transitive(transitive ...DepSet[T]) *Builder[T] {
	for _, t := range transitive {
		if t.handle != nil && t.order() != b.order {
			panic(fmt.Errorf("incompatible order, new DepSet is %s but transitive DepSet is %s",
				b.order, t.order()))
		}
	}
	b.transitive = append(b.transitive, transitive...)
	return b
}

// Build returns the DepSet being built by this Builder.  The Builder retains its contents
// for creating more depSets.
func (b *Builder[T]) Build() DepSet[T] {
	return New(b.order, b.direct, b.transitive)
}

// walk calls the visit method in depth-first order on a DepSet, preordered if d.preorder is set,
// otherwise postordered.
func (d DepSet[T]) walk(visit func([]T)) {
	visited := make(map[DepSet[T]]bool)

	var dfs func(d DepSet[T])
	dfs = func(d DepSet[T]) {
		impl := d.impl()
		visited[d] = true
		if impl.preorder {
			visit(impl.direct)
		}
		for _, dep := range impl.transitive {
			if !visited[dep] {
				dfs(dep)
			}
		}

		if !impl.preorder {
			visit(impl.direct)
		}
	}

	dfs(d)
}

// ToList returns the DepSet flattened to a list.  The order in the list is based on the order
// of the DepSet.  POSTORDER and PREORDER orders return a postordered or preordered left to right
// flattened list.  TOPOLOGICAL returns a list that guarantees that elements of children are listed
// after all of their parents (unless there are duplicate direct elements in the DepSet or any of
// its transitive dependencies, in which case the ordering of the duplicated element is not
// guaranteed).
func (d DepSet[T]) ToList() []T {
	if d.handle == nil {
		return nil
	}
	impl := d.impl()
	var list []T
	d.walk(func(paths []T) {
		list = append(list, paths...)
	})
	list = firstUniqueInPlace(list)
	if impl.reverse {
		slices.Reverse(list)
	}
	return list
}

// firstUniqueInPlace returns all unique elements of a slice, keeping the first copy of
// each.  It modifies the slice contents in place, and returns a subslice of the original
// slice.
func firstUniqueInPlace[T comparable](slice []T) []T {
	// 128 was chosen based on BenchmarkFirstUniqueStrings results.
	if len(slice) > 128 {
		return firstUniqueMap(slice)
	}
	return firstUniqueList(slice)
}

// firstUniqueList is an implementation of firstUnique using an O(N^2) list comparison to look for
// duplicates.
func firstUniqueList[T any](in []T) []T {
	writeIndex := 0
outer:
	for readIndex := 0; readIndex < len(in); readIndex++ {
		for compareIndex := 0; compareIndex < writeIndex; compareIndex++ {
			if interface{}(in[readIndex]) == interface{}(in[compareIndex]) {
				// The value at readIndex already exists somewhere in the output region
				// of the slice before writeIndex, skip it.
				continue outer
			}
		}
		if readIndex != writeIndex {
			in[writeIndex] = in[readIndex]
		}
		writeIndex++
	}
	return in[0:writeIndex]
}

// firstUniqueMap is an implementation of firstUnique using an O(N) hash set lookup to look for
// duplicates.
func firstUniqueMap[T comparable](in []T) []T {
	writeIndex := 0
	seen := make(map[T]bool, len(in))
	for readIndex := 0; readIndex < len(in); readIndex++ {
		if _, exists := seen[in[readIndex]]; exists {
			continue
		}
		seen[in[readIndex]] = true
		if readIndex != writeIndex {
			in[writeIndex] = in[readIndex]
		}
		writeIndex++
	}
	return in[0:writeIndex]
}