// Copyright 2019, OpenTelemetry 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 trace

import (
	"math/rand"
	"sync"
	"sync/atomic"

	"go.opentelemetry.io/api/core"
	"go.opentelemetry.io/sdk/trace/internal"
)

type defaultIDGenerator struct {
	sync.Mutex

	// Please keep these as the first fields
	// so that these 8 byte fields will be aligned on addresses
	// divisible by 8, on both 32-bit and 64-bit machines when
	// performing atomic increments and accesses.
	// See:
	// * https://github.com/census-instrumentation/opencensus-go/issues/587
	// * https://github.com/census-instrumentation/opencensus-go/issues/865
	// * https://golang.org/pkg/sync/atomic/#pkg-note-BUG
	nextSpanID uint64
	spanIDInc  uint64

	traceIDAdd  [2]uint64
	traceIDRand *rand.Rand
}

var _ internal.IDGenerator = &defaultIDGenerator{}

// NewSpanID returns a non-zero span ID from a randomly-chosen sequence.
func (gen *defaultIDGenerator) NewSpanID() uint64 {
	var id uint64
	for id == 0 {
		id = atomic.AddUint64(&gen.nextSpanID, gen.spanIDInc)
	}
	return id
}

// NewTraceID returns a non-zero trace ID from a randomly-chosen sequence.
// mu should be held while this function is called.
func (gen *defaultIDGenerator) NewTraceID() core.TraceID {
	gen.Lock()
	defer gen.Unlock()
	// Construct the trace ID from two outputs of traceIDRand, with a constant
	// added to each half for additional entropy.
	tid := core.TraceID{
		High: gen.traceIDRand.Uint64() + gen.traceIDAdd[0],
		Low:  gen.traceIDRand.Uint64() + gen.traceIDAdd[1],
	}
	return tid
}