// Copyright (C) 2022 Mya Pitzeruse
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.
//

package okit

import (
	"context"
	"runtime"
	"strings"
	"time"

	"go.pitz.tech/okit/pb"
)

// Client provides a default implementation.
type Client struct {
	now  func() time.Time
	uuid func() string
	tags []Tag
}

func (o Client) withCaller() Client {
	pkg, fn, line := infer(caller(3))

	o.tags = append(o.tags,
		String("package", pkg),
		String("func", fn),
		Int("line", line),
	)

	return o
}

// With adds additional tags to the observer instance and returns a new copy for reference.
func (o Client) With(tags ...Tag) Client {
	o.tags = append(o.tags, tags...)

	return o
}

// Emit allows for the emission of an event which has now value and only associated tags.
func (o Client) Emit(event string, tags ...Tag) {
	o.withCaller().With(tags...).emit(&pb.Entry{
		Kind: pb.Kind_Event,
		Name: event,
	})
}

// Observe reports a metric and it's associated value.
func (o Client) Observe(metric string, value float64, tags ...Tag) {
	o.withCaller().With(tags...).emit(&pb.Entry{
		Value: &pb.Entry_Double{Double: value},
		Kind:  pb.Kind_Metric,
		Name:  metric,
	})
}

// Trace allows a method to be traced, and it's execution time recorded and reported for viewing.
func (o Client) Trace(ctx context.Context, tags ...Tag) (context.Context, DoneFunc) {
	pkg, fn, _ := infer(caller(2))
	name := pkg + "." + fn

	start := o.now()

	span := Span{
		TraceID: o.uuid(),
		ID:      o.uuid(),
	}

	v := ctx.Value(spanKey)
	if v != nil {
		parent, hasParent := v.(Span)
		if hasParent {
			span.TraceID = parent.TraceID
			span.ParentID = parent.ID
		}
	}

	return context.WithValue(ctx, spanKey, span), func() {
		duration := o.now().Sub(start)

		tags = append(
			[]Tag{
				String("trace-id", span.TraceID),
				String("trace-span-id", span.ID),
				String("trace-parent-id", span.ParentID),
			},
			tags...,
		)

		o.withCaller().With(tags...).emit(&pb.Entry{
			Kind:  pb.Kind_Trace,
			Name:  name,
			Value: &pb.Entry_Duration{Duration: pb.DurationPB(duration)},
		})
	}
}

// Debug produces a debug log event.
func (o Client) Debug(msg string, tags ...Tag) {
	// todo: is debug enabled?

	o.withCaller().With(tags...).emit(&pb.Entry{
		Kind:  pb.Kind_Log,
		Name:  msg,
		Value: &pb.Entry_String_{String_: "debug"},
	})
}

// Info produces an information log event.
func (o Client) Info(msg string, tags ...Tag) {
	// todo: is info enabled?

	o.withCaller().With(tags...).emit(&pb.Entry{
		Kind:  pb.Kind_Log,
		Name:  msg,
		Value: &pb.Entry_String_{String_: "info"},
	})
}

// Warn produces a warning that is surfaced to operators.
func (o Client) Warn(msg string, tags ...Tag) {
	o.withCaller().With(tags...).emit(&pb.Entry{
		Kind:  pb.Kind_Log,
		Name:  msg,
		Value: &pb.Entry_String_{String_: "warn"},
	})
}

// Error produces a message that communicates an error has occurred.
func (o Client) Error(msg string, tags ...Tag) {
	o.withCaller().With(tags...).emit(&pb.Entry{
		Kind:  pb.Kind_Log,
		Name:  msg,
		Value: &pb.Entry_String_{String_: "error"},
	})
}

func (o Client) emit(entry *pb.Entry) {
	for _, tag := range o.tags {
		if tagpb := tag.AsTagPB(); tagpb != nil {
			entry.Tags = append(entry.Tags, tagpb)
		}
	}

	// todo: send to sinks
}

// max returns the largest number in a series.
func max(series ...int) int {
	largest := 0

	for _, v := range series {
		if v > largest {
			largest = v
		}
	}

	return largest
}

// caller attempts to get method caller information. This information is used for tracing information across an
// applications source code.
func caller(skip int) (name string, line int) {
	pctr, _, line, ok := runtime.Caller(skip)
	if !ok {
		return "", 0
	}

	return runtime.FuncForPC(pctr).Name(), line
}

// infer attempts to determine the calling package and function using runtime information.
func infer(name string, line int) (pkg, fn string, l int) {
	lastSlash := max(0, strings.LastIndexByte(name, '/'))
	lastDot := lastSlash + strings.LastIndexByte(name[lastSlash:], '.')

	return name[:lastDot], name[lastDot+1:], line
}