evcc-io/core/site_optimizer.go

package core

import (
	"cmp"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"os"
	"slices"
	"strings"
	"sync"
	"time"

	"github.com/evcc-io/evcc/api"
	"github.com/evcc-io/evcc/core/keys"
	"github.com/evcc-io/evcc/core/loadpoint"
	"github.com/evcc-io/evcc/core/metrics"
	"github.com/evcc-io/evcc/core/types"
	"github.com/evcc-io/evcc/tariff"
	"github.com/evcc-io/evcc/util/config"
	"github.com/evcc-io/evcc/util/request"
	"github.com/evcc-io/evcc/util/sponsor"
	optimizer "github.com/evcc-io/optimizer/client"
	"github.com/jinzhu/now"
	"github.com/samber/lo"
	"golang.org/x/exp/constraints"
)

var (
	eta          = float32(0.9)  // efficiency of the battery charging/discharging
	batteryPower = float32(6000) // default power of the battery in W

	mu               sync.Mutex
	optimizerUpdated time.Time
)

// optimizerChargingStrategies are the valid grid charging strategies; the first
// entry is the default and preserves the previous hard-coded behavior.
var optimizerChargingStrategies = []string{
	string(optimizer.OptimizerStrategyChargingStrategyChargeBeforeExport),
	string(optimizer.OptimizerStrategyChargingStrategyAttenuateGridPeaks),
	string(optimizer.OptimizerStrategyChargingStrategyNone),
}

const defaultOptimizerChargingStrategy = string(optimizer.OptimizerStrategyChargingStrategyChargeBeforeExport)

// triggerOptimizer re-runs the optimizer immediately so a changed setting takes
// effect without waiting for the next slot. It is a no-op when the optimizer is
// not active or a run is already in progress; the running update reflects the
// change on its next slot.
func (site *Site) triggerOptimizer() {
	if !sponsor.IsAuthorized() || !optimizerEnabled() {
		return
	}
	if !mu.TryLock() {
		return
	}
	optimizerUpdated = time.Time{} // bypass the slot/debounce gate
	mu.Unlock()

	go site.optimizerUpdateAsync()
}

// optimizerResult wraps the optimizer publish payload to implement BytesMarshaler.
// This ensures publishComplex serializes it as a single JSON message instead of
// recursively decomposing each struct field and array element into individual MQTT
// topics (~1,500 messages per optimizer run).
type optimizerResult struct {
	Updated time.Time                    `json:"updated"`
	Req     optimizer.OptimizationInput  `json:"req"`
	Res     optimizer.OptimizationResult `json:"res"`
	Details requestDetails               `json:"details"`
}

var _ api.BytesMarshaler = (*optimizerResult)(nil)

func (r optimizerResult) MarshalBytes() ([]byte, error) {
	return json.Marshal(r)
}

type batteryType string

const (
	OPTIMIZER_URI = "https://optimizer.evcc.io"

	batteryTypeLoadpoint batteryType = "loadpoint"
	batteryTypeVehicle   batteryType = "vehicle"
	batteryTypeBattery   batteryType = "battery"
)

type batteryDetail struct {
	Type     batteryType `json:"type"`
	Title    string      `json:"title,omitempty"`
	Name     string      `json:"name,omitempty"`
	Capacity float64     `json:"capacity,omitempty"`
}

type batteryResult struct {
	batteryDetail
	Full  time.Time `json:"full,omitzero"`
	Empty time.Time `json:"empty,omitzero"`
}

type requestDetails struct {
	Timestamps     []time.Time     `json:"timestamp"`
	BatteryDetails []batteryDetail `json:"batteryDetails"`
}

const slotsPerHour = float64(time.Hour / tariff.SlotDuration)

func (site *Site) optimizerUpdateAsync() {
	if !mu.TryLock() {
		return
	}
	defer mu.Unlock()

	if time.Since(optimizerUpdated) < 2*time.Minute {
		return
	}

	var err error

	defer func() {
		optimizerUpdated = time.Now()

		if r := recover(); r != nil {
			err = fmt.Errorf("panic %v", r)
		}

		if err != nil {
			site.log.ERROR.Println("optimizer:", err)
		}
	}()

	err = site.optimizerUpdate(site.battery.Devices)
}

func (site *Site) optimizerUpdate(battery []types.Measurement) error {
	solarTariff := site.GetTariff(api.TariffUsageSolar)
	solar := currentRates(solarTariff)

	grid := currentRates(site.GetTariff(api.TariffUsageGrid))
	feedIn := currentRates(site.GetTariff(api.TariffUsageFeedIn))

	minLen := lo.Min([]int{len(grid), len(feedIn)})
	// exclude empty solar forecast from minLen
	if solarTariff != nil && len(solar) > 0 {
		minLen = min(minLen, len(solar))
	}

	uri := cmp.Or(os.Getenv("OPTIMIZER_URI"), OPTIMIZER_URI)
	if uri == OPTIMIZER_URI {
		// limit to 2 days for sake of performance
		minLen = min(2*96, minLen)
	}

	if expectedSlots := 8; minLen < expectedSlots {
		if solarTariff != nil {
			return fmt.Errorf("not enough forecast slots for meaningful optimization: %d < %d (grid=%d, feedIn=%d, solar=%d)", minLen, expectedSlots, len(grid), len(feedIn), len(solar))
		}
		return fmt.Errorf("not enough forecast slots for meaningful optimization: %d < %d (grid=%d, feedIn=%d)", minLen, expectedSlots, len(grid), len(feedIn))
	}

	now := time.Now()
	dt := timeSteps(minLen, now)
	firstSlotDuration := time.Duration(dt[0]) * time.Second

	site.log.DEBUG.Printf("optimizer: optimizing %d slots until %v: grid=%d, feedIn=%d, solar=%d, first slot: %v",
		minLen,
		grid[minLen-1].End.Local(),
		len(grid), len(feedIn), len(solar),
		firstSlotDuration,
	)

	gt, err := site.homeProfile(minLen)
	if err != nil {
		return err
	}

	// allow empty solar forecast
	ft := lo.RepeatBy(minLen, func(i int) float32 { return float32(0) })
	if solarTariff != nil && len(solar) > 0 {
		solarEnergy, err := solarRatesToEnergy(solar)
		if err != nil {
			return err
		}

		ft = prorate(scaleAndPrune(solarEnergy, site.solarScale(), minLen), firstSlotDuration)
	}

	req := optimizer.OptimizationInput{
		Strategy: optimizer.OptimizerStrategy{
			ChargingStrategy:    optimizer.OptimizerStrategyChargingStrategy(site.GetOptimizerChargingStrategy()),
			DischargingStrategy: optimizer.OptimizerStrategyDischargingStrategyDischargeBeforeImport,
		},
		EtaC: eta,
		EtaD: eta,
		TimeSeries: optimizer.TimeSeries{
			Dt: dt,
			Gt: prorate(gt, firstSlotDuration),
			Ft: ft,
			PN: scaleAndPrune(grid, 0.001, minLen),
			PE: scaleAndPrune(feedIn, 0.001, minLen),
		},
	}

	// end of horizon Wh value
	pa := lo.Min(req.TimeSeries.PN) * eta * 0.99

	details := requestDetails{
		Timestamps: asTimestamps(dt, now),
	}

	if site.circuit != nil {
		if pMaxImp := site.circuit.GetMaxPower(); pMaxImp > 0 {
			req.Grid = optimizer.GridConfig{
				// hard grid import limit if no price penalty is set by PrcPExcImp
				PMaxImp: float32(pMaxImp),
			}
		}
	}

	add := func(battery optimizer.BatteryConfig, detail batteryDetail) {
		battery.PA = pa
		req.Batteries = append(req.Batteries, battery)
		details.BatteryDetails = append(details.BatteryDetails, detail)
	}

	for _, lp := range site.Loadpoints() {
		// ignore disconnected loadpoints, including StatusNone
		if s := lp.GetStatus(); s != api.StatusB && s != api.StatusC {
			continue
		}

		if v := lp.GetVehicle(); v == nil || v.Capacity() == 0 {
			continue
		}

		// skip disabled loadpoints
		if req, detail := site.loadpointRequest(lp, minLen, firstSlotDuration, grid); req.CMax > 0 {
			add(req, detail)
		}
	}

	for i, dev := range site.batteryMeters {
		// measurements may lag the configured meters on an off-cycle trigger
		if i >= len(battery) {
			break
		}
		b := battery[i]

		if b.Capacity == nil || *b.Capacity == 0 || b.Soc == nil {
			continue
		}

		add(site.batteryRequest(dev, b, grid, minLen, firstSlotDuration))
	}

	// empty request- all loadpoints disabled
	if len(req.Batteries) == 0 {
		return nil
	}

	httpClient := request.NewClient(site.log)
	httpClient.Timeout = 90 * time.Second

	apiClient, err := optimizer.NewClientWithResponses(uri, optimizer.WithHTTPClient(httpClient))
	if err != nil {
		return err
	}

	resp, err := apiClient.PostOptimizeChargeScheduleWithResponse(context.TODO(), req, func(_ context.Context, req *http.Request) error {
		if sponsor.IsAuthorized() {
			req.Header.Set("Authorization", "Bearer "+sponsor.Token)
		}
		return nil
	})
	if err != nil {
		return err
	}

	if resp.StatusCode() != http.StatusOK {
		return apiError(resp)
	}

	if resp.JSON200.Status != optimizer.Optimal {
		return errors.New(string(resp.JSON200.Status))
	}

	site.publish("evopt", optimizerResult{
		Updated: time.Now(),
		Req:     req,
		Res:     *resp.JSON200,
		Details: details,
	})

	var batteries []batteryResult
	for i, batReq := range req.Batteries {
		batResp := resp.JSON200.Batteries[i]

		batResult := batteryResult{
			batteryDetail: details.BatteryDetails[i],
			Full: matchSoc(batResp.StateOfCharge, func(soc float32) bool {
				return soc >= batReq.SMax
			}),
			Empty: matchSoc(batResp.StateOfCharge, func(soc float32) bool {
				return soc <= batReq.SMin
			}),
		}

		batteries = append(batteries, batResult)
	}

	site.publish("evopt-batteries", batteries)

	site.battery.Forecast = site.addBatteryForecastTotals(req.Batteries, resp.JSON200.Batteries)

	site.publish(keys.Battery, site.battery)

	return nil
}

func (site *Site) addBatteryForecastTotals(req []optimizer.BatteryConfig, resp []optimizer.BatteryResult) *types.BatteryForecast {
	if len(resp) == 0 || len(resp[0].StateOfCharge) == 0 {
		return nil
	}

	high, low := batteryForecastSocExtremes(req, resp)
	if high == nil && low == nil {
		return nil
	}

	cutoff := time.Now()
	now := cutoff.Round(tariff.SlotDuration)
	point := func(p *batteryForecastSlot) *types.BatteryForecastPoint {
		if p == nil {
			return nil
		}
		ts := now.Add(time.Duration(p.slot) * tariff.SlotDuration)
		if !ts.After(cutoff) {
			return nil
		}
		return &types.BatteryForecastPoint{Soc: p.soc, Time: ts, Limit: p.limit}
	}

	res := types.BatteryForecast{
		Highest: point(high),
		Lowest:  point(low),
	}
	if res.Highest == nil && res.Lowest == nil {
		return nil
	}
	return &res
}

type batteryForecastSlot struct {
	slot  int
	soc   float64 // percent
	limit bool    // true when SMax (highest) or SMin (lowest) boundary reached
}

// batteryForecastSocExtremes returns the highest and lowest aggregate SOC
// points across home batteries (SCapacity > 0) over the forecast horizon.
// The Limit flag indicates whether the SOC reached the configured SMax (for
// the highest point) or SMin (for the lowest point) boundary - in which case
// the battery is forecasted to become fully charged or empty.
// Returns nil for either point when no home battery is present.
func batteryForecastSocExtremes(req []optimizer.BatteryConfig, resp []optimizer.BatteryResult) (*batteryForecastSlot, *batteryForecastSlot) {
	homeIndices := lo.FilterMap(req, func(b optimizer.BatteryConfig, i int) (int, bool) {
		return i, b.SCapacity > 0
	})
	if len(homeIndices) == 0 || len(resp) == 0 {
		return nil, nil
	}

	totalCapacity := lo.SumBy(homeIndices, func(i int) float32 { return req[i].SCapacity })
	totalSMax := lo.SumBy(homeIndices, func(i int) float32 { return req[i].SMax })
	totalSMin := lo.SumBy(homeIndices, func(i int) float32 { return req[i].SMin })

	var high, low *batteryForecastSlot
	for i := range resp[homeIndices[0]].StateOfCharge {
		sum := lo.SumBy(homeIndices, func(idx int) float32 { return resp[idx].StateOfCharge[i] })
		soc := float64(sum/totalCapacity) * 100
		fullReached := totalSMax > 0 && sum >= totalSMax
		emptyReached := sum <= totalSMin

		// first slot at SMax wins for highest
		if high == nil || (!high.limit && (soc > high.soc || fullReached)) {
			high = &batteryForecastSlot{slot: i, soc: soc, limit: fullReached}
		}
		// first slot at SMin wins for lowest
		if low == nil || (!low.limit && (soc < low.soc || emptyReached)) {
			low = &batteryForecastSlot{slot: i, soc: soc, limit: emptyReached}
		}
	}

	return high, low
}

func (site *Site) loadpointRequest(lp loadpoint.API, minLen int, firstSlotDuration time.Duration, grid api.Rates) (optimizer.BatteryConfig, batteryDetail) {
	bat := optimizer.BatteryConfig{
		ChargeFromGrid: true,
		CMin:           float32(lp.EffectiveMinPower()),
		CMax:           float32(lp.EffectiveMaxPower()),
		DMax:           0,
		SMin:           0,
		// PA:             pa,
	}

	if profile := loadpointProfile(lp, minLen); profile != nil {
		bat.PDemand = prorate(profile, firstSlotDuration)
	}

	detail := batteryDetail{
		Type:  batteryTypeLoadpoint,
		Title: lp.GetTitle(),
	}

	// vehicle
	v := lp.GetVehicle()

	maxSoc := v.Capacity() * 1e3 // Wh
	if v := lp.EffectiveLimitSoc(); v > 0 {
		maxSoc *= float64(v) / 100
	} else if v := lp.GetLimitEnergy(); v > 0 {
		maxSoc = v * 1e3
	}

	bat.SInitial = float32(v.Capacity() * lp.GetSoc() * 10) // Wh
	bat.SMax = max(bat.SInitial, float32(maxSoc))           // prevent infeasible if current soc above maximum

	detail.Type = batteryTypeVehicle
	detail.Capacity = v.Capacity()

	if vt := v.GetTitle(); vt != "" {
		if detail.Title != "" {
			detail.Title += " – "
		}
		detail.Title += vt
	}

	// find vehicle name/id
	for _, dev := range config.Vehicles().Devices() {
		if dev.Instance() == v {
			detail.Name = dev.Config().Name
		}
	}

	var demand []float32

	switch lp.GetMode() {
	case api.ModeOff:
		// disable charging
		bat.CMax = 0

	case api.ModeNow:
		// forced max charging
		demand = continuousDemand(lp, minLen)

	case api.ModeMinPV:
		// forced min charging
		demand = continuousDemand(lp, minLen)
		// add smartcost limit and plan goal, if configured
		demand = applySmartCostLimit(lp, demand, grid, minLen)
		site.applyPlanGoal(lp, &bat, minLen)

	case api.ModePV:
		// add smartcost limit and plan goal, if configured
		demand = applySmartCostLimit(lp, nil, grid, minLen)
		site.applyPlanGoal(lp, &bat, minLen)
	}

	if demand != nil {
		bat.PDemand = prorate(demand, firstSlotDuration)
	}

	return bat, detail
}

func (site *Site) batteryRequest(dev config.Device[api.Meter], b types.Measurement, grid api.Rates, minLen int, firstSlotDuration time.Duration) (optimizer.BatteryConfig, batteryDetail) {
	bat := optimizer.BatteryConfig{
		CMax:      batteryPower,
		DMax:      batteryPower,
		SCapacity: float32(*b.Capacity * 1e3),         // Wh
		SInitial:  float32(*b.Capacity * *b.Soc * 10), // Wh
		// PA:       pa,
	}

	instance := dev.Instance()

	if api.HasCap[api.BatteryController](instance) {
		bat.ChargeFromGrid = true
	}

	if m, ok := api.Cap[api.BatteryPowerLimiter](instance); ok {
		charge, discharge := m.GetPowerLimits()
		bat.CMax = float32(charge)
		bat.DMax = float32(discharge)
	}

	if m, ok := api.Cap[api.BatterySocLimiter](instance); ok {
		minSoc, maxSoc := m.GetSocLimits()
		bat.SMin = float32(*b.Capacity * minSoc * 10) // Wh
		bat.SMax = float32(*b.Capacity * maxSoc * 10) // Wh
	}

	detail := batteryDetail{
		Type:     batteryTypeBattery,
		Name:     dev.Config().Name,
		Title:    deviceProperties(dev).Title,
		Capacity: *b.Capacity,
	}

	// tariff forecast-based grid charging demand
	if bat.ChargeFromGrid {
		if demand := site.applyBatteryGridChargeLimit(bat.CMax, grid, minLen); demand != nil {
			bat.PDemand = prorate(demand, firstSlotDuration)
		}
	}

	return bat, detail
}

func matchSoc(ts []float32, fun func(float32) bool) time.Time {
	for i, soc := range ts {
		if fun(soc) {
			// TODO first slot
			return time.Now().Add(time.Duration(i+1) * tariff.SlotDuration)
		}
	}

	return time.Time{}
}

// continuousDemand creates a slice of power demands depending on loadpoint mode
func continuousDemand(lp loadpoint.API, minLen int) []float32 {
	if lp.GetStatus() != api.StatusC {
		return nil
	}

	pwr := lp.EffectiveMaxPower()
	if lp.GetMode() == api.ModeMinPV {
		pwr = lp.EffectiveMinPower()
	}

	return lo.RepeatBy(minLen, func(i int) float32 {
		return float32(pwr / slotsPerHour)
	})
}

// loadpointProfile returns the loadpoint's charging profile in Wh
// TODO consider charging efficiency
func loadpointProfile(lp loadpoint.API, minLen int) []float64 {
	mode := lp.GetMode()
	status := lp.GetStatus()

	if status != api.StatusC || (mode != api.ModeMinPV && mode != api.ModeNow) {
		return nil
	}

	power := lp.GetChargePower()
	if minP := lp.EffectiveMinPower(); mode == api.ModeMinPV && minP < power {
		power = minP
	}

	energy := lp.GetRemainingEnergy() * 1e3 // Wh
	energyKnown := energy > 0

	res := make([]float64, 0, minLen)
	for range minLen {
		deltaEnergy := power * float64(tariff.SlotDuration) / float64(time.Hour) // Wh
		if energyKnown && deltaEnergy >= energy {
			deltaEnergy = energy
		}
		energy -= deltaEnergy

		res = append(res, deltaEnergy)
	}

	return res
}

// homeProfile returns the home base load in Wh
func (site *Site) homeProfile(minLen int) ([]float64, error) {
	// kWh over last 30 days
	profile, err := site.collectors[metrics.Home].EnergyProfile(now.BeginningOfDay().AddDate(0, 0, -30))
	if err != nil {
		return nil, err
	}

	// max 4 days
	slots := make([]float64, 0, minLen+1)
	for len(slots) <= minLen+24*4 { // allow for prorating first day
		slots = append(slots, profile[:]...)
	}

	res := profileSlotsFromNow(slots)
	if len(res) < minLen {
		return nil, fmt.Errorf("minimum home profile length %d is less than required %d", len(res), minLen)
	}
	if len(res) > minLen {
		res = res[:minLen]
	}

	// convert to Wh
	return lo.Map(res, func(v float64, i int) float64 {
		return v * 1e3
	}), nil
}

// profileSlotsFromNow strips away any slots before "now".
// The profile contains 48 15min slots (00:00-23:45) that repeat for multiple days.
func profileSlotsFromNow(profile []float64) []float64 {
	firstSlot := int(time.Now().Truncate(tariff.SlotDuration).Sub(now.BeginningOfDay()) / tariff.SlotDuration)
	return profile[firstSlot:]
}

// prorate adjusts the first slot's energy amount according to remaining duration
func prorate[T constraints.Float](slots []T, firstSlotDuration time.Duration) []float32 {
	// return empty slice instead of nil to make api happy
	if len(slots) == 0 {
		return []float32{}
	}

	res := slices.Clone(slots)
	res[0] = res[0] * T(firstSlotDuration) / T(tariff.SlotDuration)
	return lo.Map(res, func(f T, _ int) float32 {
		return float32(f)
	})
}

func solarRatesToEnergy(rr api.Rates) (api.Rates, error) {
	res := make(api.Rates, 0, len(rr))

	for _, r := range rr {
		energy := solarEnergy(rr, r.Start, r.End)
		if energy < 0 {
			return nil, fmt.Errorf("negative solar energy from %v to %v: %.3f", r.Start, r.End, energy)
		}

		res = append(res, api.Rate{
			Start: r.Start,
			End:   r.End,
			Value: energy,
		})
	}

	return res, nil
}

func currentRates(tariff api.Tariff) api.Rates {
	if tariff == nil {
		return nil
	}

	rates, err := tariff.Rates()
	if err != nil {
		return nil
	}

	// filter past slots
	now := time.Now()
	return lo.Filter(rates, func(slot api.Rate, _ int) bool {
		return slot.End.After(now)
	})
}

func timeSteps(minLen int, now time.Time) []int {
	res := make([]int, 0, minLen)

	eos := now.Truncate(tariff.SlotDuration).Add(tariff.SlotDuration)
	if d := eos.Sub(now); d > time.Second && d < tariff.SlotDuration {
		res = append(res, int(d.Seconds()))
	}

	for i := len(res); i < minLen; i++ {
		res = append(res, int(tariff.SlotDuration.Seconds())) // 15min slots
	}

	return res
}

func asTimestamps(dt []int, now time.Time) []time.Time {
	res := make([]time.Time, 0, len(dt))

	eos := now.Truncate(tariff.SlotDuration).Add(tariff.SlotDuration)
	res = append(res, eos.Add(-time.Duration(dt[0])*time.Second))

	for i := range len(dt) - 1 {
		res = append(res, res[i].Add(time.Duration(dt[i])*time.Second))
	}

	return res
}

func scaleAndPrune(rates api.Rates, scale float64, maxLen int) []float32 {
	res := make([]float32, 0, maxLen)

	for _, slot := range rates {
		res = append(res, float32(slot.Value*scale))
		if len(res) >= maxLen {
			break
		}
	}

	return res
}

func (site *Site) applyPlanGoal(lp loadpoint.API, bat *optimizer.BatteryConfig, minLen int) {
	goal, socBased := lp.GetPlanGoal()
	if goal <= 0 {
		return
	}

	// Convert to Wh
	if vehicle := lp.GetVehicle(); socBased && vehicle != nil {
		goal *= vehicle.Capacity() * 10
	} else {
		goal *= 1000 // Wh
	}

	ts := lp.EffectivePlanTime()
	if ts.IsZero() {
		return
	}

	// TODO precise slot placement
	slot := int(time.Until(ts) / tariff.SlotDuration)
	if slot >= 0 && slot < minLen {
		bat.SGoal = make([]float32, minLen)
		bat.SGoal[slot] = float32(goal)
		bat.SMax = max(bat.SMax, float32(goal))
	} else {
		site.log.DEBUG.Printf("plan beyond forecast range or overrun: %.1f at %v slot %d", goal, ts.Round(time.Minute), slot)
	}
}

// TODO remove once smart cost limit usage becomes obsolete
func applySmartCostLimit(lp loadpoint.API, demand []float32, grid api.Rates, minLen int) []float32 {
	costLimit := lp.GetSmartCostLimit()
	if costLimit == nil {
		return demand
	}

	maxLen := min(minLen, len(grid))

	// Check if any slots meet the cost limit
	if hasAffordableSlots := slices.ContainsFunc(grid[:maxLen], func(r api.Rate) bool {
		return r.Value <= *costLimit
	}); !hasAffordableSlots {
		return demand
	}

	maxPower := lp.EffectiveMaxPower()

	if demand == nil {
		demand = make([]float32, minLen)
	}

	for i := range maxLen {
		if grid[i].Value <= *costLimit {
			demand[i] = float32(maxPower / slotsPerHour)
		}
		// else: keep existing demand (either 0 or minPower from ModeMinPV)
	}

	return demand
}

func (site *Site) applyBatteryGridChargeLimit(cMax float32, grid api.Rates, minLen int) []float32 {
	limit := site.GetBatteryGridChargeLimit()
	if limit == nil {
		return nil
	}

	maxLen := min(minLen, len(grid))

	if hasAffordableSlots := slices.ContainsFunc(grid[:maxLen], func(r api.Rate) bool {
		return r.Value <= *limit
	}); !hasAffordableSlots {
		return nil
	}

	demand := make([]float32, minLen)
	for i := range maxLen {
		if grid[i].Value <= *limit {
			demand[i] = float32(float64(cMax) / slotsPerHour)
		}
	}

	return demand
}

// apiError extracts error message from optimizer API response
func apiError(resp *optimizer.PostOptimizeChargeScheduleResponse) error {
	var errObj *optimizer.Error
	switch resp.StatusCode() {
	case http.StatusBadRequest:
		errObj = resp.JSON400
	case http.StatusInternalServerError:
		errObj = resp.JSON500
	}

	if errObj == nil {
		return fmt.Errorf("invalid status: %d: %s", resp.StatusCode(), resp.Body)
	}

	if len(errObj.Details) > 0 {
		var details []string
		for field, msg := range errObj.Details {
			details = append(details, fmt.Sprintf("%s: %s", field, msg))
		}
		slices.Sort(details)
		return fmt.Errorf("%s (%s)", errObj.Message, strings.Join(details, ", "))
	}

	return errors.New(errObj.Message)
}