package apollofederatedtracingv1

import (
	"context"
	"fmt"
	"sync"
	"time"

	"google.golang.org/protobuf/types/known/timestamppb"

	"github.com/99designs/gqlgen/graphql"
	"github.com/99designs/gqlgen/graphql/handler/apollofederatedtracingv1/generated"
)

type TreeBuilder struct {
	Trace    *generated.Trace
	rootNode generated.Trace_Node
	nodes    map[string]NodeMap // nodes is used to store a pointer map using the node path (e.g. todo[0].id) to itself as well as it's parent

	startTime *time.Time
	stopped   bool
	mu        sync.Mutex
}

type NodeMap struct {
	self   *generated.Trace_Node
	parent *generated.Trace_Node
}

// NewTreeBuilder is used to start the node tree with a default root node, along with the related tree nodes map entry
func NewTreeBuilder() *TreeBuilder {
	tb := TreeBuilder{
		rootNode: generated.Trace_Node{},
	}

	t := generated.Trace{
		Root: &tb.rootNode,
	}
	tb.nodes = make(map[string]NodeMap)
	tb.nodes[""] = NodeMap{self: &tb.rootNode, parent: nil}

	tb.Trace = &t

	return &tb
}

// StartTimer marks the time using protobuf timestamp format for use in timing calculations
func (tb *TreeBuilder) StartTimer(ctx context.Context) {
	if tb.startTime != nil {
		fmt.Println(fmt.Errorf("StartTimer called twice"))
	}
	if tb.stopped {
		fmt.Println(fmt.Errorf("StartTimer called after StopTimer"))
	}

	rc := graphql.GetOperationContext(ctx)
	start := rc.Stats.OperationStart

	tb.Trace.StartTime = timestamppb.New(start)
	tb.startTime = &start
}

// StopTimer marks the end of the timer, along with setting the related fields in the protobuf representation
func (tb *TreeBuilder) StopTimer(ctx context.Context) {
	if tb.startTime == nil {
		fmt.Println(fmt.Errorf("StopTimer called before StartTimer"))
	}
	if tb.stopped {
		fmt.Println(fmt.Errorf("StopTimer called twice"))
	}

	ts := graphql.Now().UTC()
	tb.Trace.DurationNs = uint64(ts.Sub(*tb.startTime).Nanoseconds())
	tb.Trace.EndTime = timestamppb.New(ts)
	tb.stopped = true
}

// On each field, it calculates the time started at as now - tree.StartTime, as well as a deferred function upon full resolution of the
// field as now - tree.StartTime; these are used by Apollo to calculate how fields are being resolved in the AST
func (tb *TreeBuilder) WillResolveField(ctx context.Context) {
	if tb.startTime == nil {
		fmt.Println(fmt.Errorf("WillResolveField called before StartTimer"))
		return
	}
	if tb.stopped {
		fmt.Println(fmt.Errorf("WillResolveField called after StopTimer"))
		return
	}
	fc := graphql.GetFieldContext(ctx)

	node := tb.newNode(fc)
	node.StartTime = uint64(graphql.Now().Sub(*tb.startTime).Nanoseconds())
	defer func() {
		node.EndTime = uint64(graphql.Now().Sub(*tb.startTime).Nanoseconds())
	}()

	node.Type = fc.Field.Definition.Type.String()
	node.ParentType = fc.Object
}

// newNode is called on each new node within the AST and sets related values such as the entry in the tree.node map and ID attribute
func (tb *TreeBuilder) newNode(path *graphql.FieldContext) *generated.Trace_Node {
	// if the path is empty, it is the root node of the operation
	if path.Path().String() == "" {
		return &tb.rootNode
	}

	self := &generated.Trace_Node{}
	pn := tb.ensureParentNode(path)

	if path.Index != nil {
		self.Id = &generated.Trace_Node_Index{Index: uint32(*path.Index)}
	} else {
		self.Id = &generated.Trace_Node_ResponseName{ResponseName: path.Field.Name}
	}

	// lock the map from being read/written concurrently to avoid panics
	tb.mu.Lock()
	nodeRef := tb.nodes[path.Path().String()]
	// set the values for the node references to help build the tree
	nodeRef.parent = pn
	nodeRef.self = self

	// since they are references, we point the parent to it's children nodes
	nodeRef.parent.Child = append(nodeRef.parent.Child, self)
	nodeRef.self = self
	tb.nodes[path.Path().String()] = nodeRef
	tb.mu.Unlock()

	return self
}

// ensureParentNode ensures the node isn't orphaned
func (tb *TreeBuilder) ensureParentNode(path *graphql.FieldContext) *generated.Trace_Node {
	// lock to read briefly, then unlock to avoid r/w issues
	tb.mu.Lock()
	nodeRef := tb.nodes[path.Parent.Path().String()]
	tb.mu.Unlock()

	if nodeRef.self != nil {
		return nodeRef.self
	}

	return tb.newNode(path.Parent)
}