package moon

import (
	"context"
	"errors"
	"fmt"
	"math"
	"time"

	"code.humancabbage.net/sam/moonmath/coindesk"
	"github.com/sourcegraph/conc/pool"
)

type Math struct {
	Asset        coindesk.Asset
	CurrentPrice coindesk.Price
	Columns      []Column
	Goals        []Goal
	Labels       []string
}

func NewMath(asset coindesk.Asset, goals []Goal, bases []Base) (m Math) {
	if goals == nil || bases == nil {
		panic("goals and bases must be set")
	}
	m.Asset = asset
	m.Goals = goals
	m.Labels = []string{"Starting", "CDPR"}
	m.Columns = make([]Column, len(bases))
	for i := range bases {
		m.Columns[i].Base = bases[i]
	}
	for i := range goals {
		m.Labels = append(m.Labels, goals[i].Name)
	}
	return
}

func CDPR(days, gain float64) float64 {
	if gain <= 0 {
		return 0
	}
	cdpr := math.Pow(gain, 1/days)
	return cdpr
}

type Projection struct {
	Dates []time.Time
}

func ProjectDates(
	from time.Time, currentPrice float64, cdpr float64, goals []Goal,
) (p Projection) {
	if cdpr <= 0 {
		return
	}
	logP := math.Log(currentPrice)
	logR := math.Log(cdpr)
	for _, goal := range goals {
		daysToGo := (math.Log(goal.Value) - logP) / logR
		date := from.Add(time.Hour * 24 * time.Duration(daysToGo))
		p.Dates = append(p.Dates, date)
	}

	return
}

func (m *Math) Refresh(ctx context.Context) (err error) {
	resp, err := coindesk.GetAssetTickers(ctx, m.Asset)
	if err != nil {
		return
	}
	m.CurrentPrice = resp.Data[m.Asset].OHLC.Closing

	tasks := pool.New().WithErrors()
	tasks.WithMaxGoroutines(len(m.Columns))
	//tasks.WithMaxGoroutines(1)
	now := time.Now()
	for i := range m.Columns {
		c := &m.Columns[i]
		tasks.Go(func() error {
			return c.project(ctx, m, now)
		})
	}
	err = tasks.Wait()

	return
}

type Column struct {
	Base          Base
	StartingDate  time.Time
	StartingPrice coindesk.Price
	Gain          float64
	CDPR          float64
	Projections   Projection
}

func (c *Column) project(ctx context.Context, m *Math, now time.Time) (err error) {
	err = c.fillStartingPrice(ctx, m.Asset, now)
	if err != nil {
		return
	}

	c.Gain = float64(m.CurrentPrice) / float64(c.StartingPrice)
	days := now.Sub(c.StartingDate).Hours() / 24
	c.CDPR = CDPR(days, c.Gain)
	c.Projections = ProjectDates(
		now, float64(m.CurrentPrice),
		c.CDPR, m.Goals,
	)
	return
}

func (c *Column) fillStartingPrice(
	ctx context.Context, asset coindesk.Asset, now time.Time,
) error {
	// if base provides a hardcoded starting price, use it
	c.StartingDate = c.Base.From(now)
	c.StartingPrice = coindesk.Price(c.Base.GetStartingPrice())
	if c.StartingPrice != 0 {
		return nil
	}

	// otherwise, look up the starting price via Coindesk
	nextDay := c.StartingDate.Add(time.Hour * 24)
	resp, err := coindesk.GetPriceValues(ctx, asset, c.StartingDate, nextDay)
	if err != nil {
		err = fmt.Errorf("getting price for %s on %v: %w",
			asset, c.StartingDate, err)
		return err
	}
	if len(resp.Data.Entries) == 0 {
		c.Projections.Dates = nil
		return errEmptyPriceEntries
	}
	c.StartingPrice = resp.Data.Entries[0].Price
	return nil
}

var errEmptyPriceEntries = errors.New("price values response has no entries")

type Goal struct {
	Name  string  `koanf:"name"`
	Value float64 `koanf:"value"`
}

// Base is a temporal point of comparison used for price projection.
type Base interface {
	From(now time.Time) time.Time
	Label() string
	GetStartingPrice() float64
}

// ConstantBase is a base that is a constant time, e.g. 2020-01-01.
type ConstantBase struct {
	Name          string    `koanf:"name"`
	Time          time.Time `koanf:"time"`
	StartingPrice float64   `koanf:"startingPrice"`
}

func (cb ConstantBase) From(_ time.Time) time.Time {
	return cb.Time
}

func (cb ConstantBase) Label() string {
	return cb.Name
}

func (cb ConstantBase) GetStartingPrice() float64 {
	return cb.StartingPrice
}

// RelativeBase is a base that is relative, e.g. "90 days ago".
type RelativeBase struct {
	Name   string        `koanf:"name"`
	Offset time.Duration `koanf:"offset"`
}

func (rb RelativeBase) From(now time.Time) time.Time {
	then := now.Add(time.Duration(rb.Offset))
	return then
}

func (rb RelativeBase) Label() string {
	return rb.Name
}

func (rb RelativeBase) GetStartingPrice() float64 {
	return 0
}