package review

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

	"gonum.org/v1/gonum/optimize"
)

// Estimator encapsulates a fitted model and exposes estimation methods.
type Estimator interface {
	Estimate1RM() float64
	EstimateReps(targetWeight float64) float64
	EstimateMaxWeight(nReps float64) float64
	ModelType() string
	Params() []float64
}

// Supported model types
const (
	ModelPowerLaw   = "powerlaw"
	ModelLinear     = "linear"
	ModelExponential = "exponential"
)

// FitOption is a functional option for configuring the Fit process.
type FitOption func(*fitConfig)

// fitConfig holds configuration for fitting.
type fitConfig struct {
	modelTypes   []string
	halfLifeDays float64
}

// WithModel specifies which model(s) to fit. If multiple, Fit selects the best.
func WithModel(models ...string) FitOption {
	return func(cfg *fitConfig) {
		cfg.modelTypes = models
	}
}

// WithAllModels configures Fit to try all built-in model types.
func WithAllModels() FitOption {
	return func(cfg *fitConfig) {
		cfg.modelTypes = []string{ModelPowerLaw, ModelLinear, ModelExponential}
	}
}

// WithHalfLife sets the half-life (in days) for time weighting.
func WithHalfLife(days float64) FitOption {
	return func(cfg *fitConfig) {
		cfg.halfLifeDays = days
	}
}

// WithModelSelection is an alias for WithModel, for clarity.
func WithModelSelection(models []string) FitOption {
	return WithModel(models...)
}

// Default settings
const defaultHalfLife = 30.0
var defaultModelTypes = []string{ModelPowerLaw}

// Fit fits the specified model(s) to the data and returns an Estimator.
// If multiple models are specified, Fit selects the best based on residual sum of squares.
func Fit(weight, reps []float64, dates []time.Time, opts ...FitOption) (Estimator, error) {
	if len(weight) != len(reps) || len(weight) != len(dates) {
		return nil, errors.New("weight, reps, and dates must have the same length")
	}
	if len(weight) < 2 {
		return nil, errors.New("at least two data points are required")
	}

	// Apply options
	cfg := &fitConfig{
		modelTypes:   defaultModelTypes,
		halfLifeDays: defaultHalfLife,
	}
	for _, opt := range opts {
		opt(cfg)
	}
	if len(cfg.modelTypes) == 0 {
		cfg.modelTypes = defaultModelTypes
	}

	// Fit each model and select the best (lowest residual)
	var best Estimator
	var bestResidual float64 = math.Inf(1)
	now := time.Now()

	for _, model := range cfg.modelTypes {
		var est Estimator
		var residual float64
		var err error

		switch model {
		case ModelPowerLaw:
			est, residual, err = fitPowerLaw(weight, reps, dates, now, cfg.halfLifeDays)
		case ModelLinear:
			est, residual, err = fitLinear(weight, reps, dates, now, cfg.halfLifeDays)
		case ModelExponential:
			est, residual, err = fitExponential(weight, reps, dates, now, cfg.halfLifeDays)
		default:
			return nil, fmt.Errorf("unknown model type: %s", model)
		}
		if err != nil {
			continue // Skip models that fail to fit
		}
		if residual < bestResidual {
			best = est
			bestResidual = residual
		}
	}
	if best == nil {
		return nil, errors.New("no model could be fitted to the data")
	}
	return best, nil
}

// --- Model Implementations ---

// PowerLawEstimator: w = a * reps^b
type PowerLawEstimator struct {
	a, b        float64
	halfLife    float64
	modelType   string
	residualSum float64
}

func (e *PowerLawEstimator) Estimate1RM() float64 {
	return e.a * math.Pow(1, e.b)
}
func (e *PowerLawEstimator) EstimateReps(targetWeight float64) float64 {
	if e.a == 0 || e.b == 0 {
		return 0
	}
	return math.Pow(targetWeight/e.a, 1/e.b)
}
func (e *PowerLawEstimator) EstimateMaxWeight(nReps float64) float64 {
	return e.a * math.Pow(nReps, e.b)
}
func (e *PowerLawEstimator) ModelType() string { return e.modelType }
func (e *PowerLawEstimator) Params() []float64 { return []float64{e.a, e.b} }

// LinearEstimator: w = a + b*reps
type LinearEstimator struct {
	a, b        float64
	halfLife    float64
	modelType   string
	residualSum float64
}

func (e *LinearEstimator) Estimate1RM() float64 {
	return e.a + e.b*1
}
func (e *LinearEstimator) EstimateReps(targetWeight float64) float64 {
	if e.b == 0 {
		return 0
	}
	return (targetWeight - e.a) / e.b
}
func (e *LinearEstimator) EstimateMaxWeight(nReps float64) float64 {
	return e.a + e.b*nReps
}
func (e *LinearEstimator) ModelType() string { return e.modelType }
func (e *LinearEstimator) Params() []float64 { return []float64{e.a, e.b} }

// ExponentialEstimator: w = a * exp(b * reps)
type ExponentialEstimator struct {
	a, b        float64
	halfLife    float64
	modelType   string
	residualSum float64
}

func (e *ExponentialEstimator) Estimate1RM() float64 {
	return e.a * math.Exp(e.b*1)
}
func (e *ExponentialEstimator) EstimateReps(targetWeight float64) float64 {
	if e.a == 0 || e.b == 0 {
		return 0
	}
	return math.Log(targetWeight/e.a) / e.b
}
func (e *ExponentialEstimator) EstimateMaxWeight(nReps float64) float64 {
	return e.a * math.Exp(e.b*nReps)
}
func (e *ExponentialEstimator) ModelType() string { return e.modelType }
func (e *ExponentialEstimator) Params() []float64 { return []float64{e.a, e.b} }

// --- Fitting Functions ---

// fitPowerLaw fits w = a * reps^b
func fitPowerLaw(weight, reps []float64, dates []time.Time, now time.Time, halfLifeDays float64) (Estimator, float64, error) {
	params := []float64{max(weight), -0.1}
	problem := optimize.Problem{
		Func: func(x []float64) float64 {
			return weightedResidualsPowerLaw(x, weight, reps, dates, now, halfLifeDays)
		},
	}
	result, err := optimize.Minimize(problem, params, nil, nil)
	if err != nil {
		return nil, 0, err
	}
	residual := weightedResidualsPowerLaw(result.X, weight, reps, dates, now, halfLifeDays)
	return &PowerLawEstimator{
		a:          result.X[0],
		b:          result.X[1],
		halfLife:   halfLifeDays,
		modelType:  ModelPowerLaw,
		residualSum: residual,
	}, residual, nil
}

func weightedResidualsPowerLaw(params, weight, reps []float64, dates []time.Time, now time.Time, halfLifeDays float64) float64 {
	a, b := params[0], params[1]
	var sum float64
	for i := range weight {
		daysAgo := now.Sub(dates[i]).Hours() / 24
		weightDecay := math.Exp(-math.Ln2 * daysAgo / halfLifeDays)
		predicted := a * math.Pow(reps[i], b)
		residual := weight[i] - predicted
		sum += weightDecay * residual * residual
	}
	return sum
}

// fitLinear fits w = a + b*reps
func fitLinear(weight, reps []float64, dates []time.Time, now time.Time, halfLifeDays float64) (Estimator, float64, error) {
	params := []float64{weight[0], 0.0}
	problem := optimize.Problem{
		Func: func(x []float64) float64 {
			return weightedResidualsLinear(x, weight, reps, dates, now, halfLifeDays)
		},
	}
	result, err := optimize.Minimize(problem, params, nil, nil)
	if err != nil {
		return nil, 0, err
	}
	residual := weightedResidualsLinear(result.X, weight, reps, dates, now, halfLifeDays)
	return &LinearEstimator{
		a:          result.X[0],
		b:          result.X[1],
		halfLife:   halfLifeDays,
		modelType:  ModelLinear,
		residualSum: residual,
	}, residual, nil
}

func weightedResidualsLinear(params, weight, reps []float64, dates []time.Time, now time.Time, halfLifeDays float64) float64 {
	a, b := params[0], params[1]
	var sum float64
	for i := range weight {
		daysAgo := now.Sub(dates[i]).Hours() / 24
		weightDecay := math.Exp(-math.Ln2 * daysAgo / halfLifeDays)
		predicted := a + b*reps[i]
		residual := weight[i] - predicted
		sum += weightDecay * residual * residual
	}
	return sum
}

// fitExponential fits w = a * exp(b*reps)
func fitExponential(weight, reps []float64, dates []time.Time, now time.Time, halfLifeDays float64) (Estimator, float64, error) {
	params := []float64{max(weight), -0.01}
	problem := optimize.Problem{
		Func: func(x []float64) float64 {
			return weightedResidualsExponential(x, weight, reps, dates, now, halfLifeDays)
		},
	}
	result, err := optimize.Minimize(problem, params, nil, nil)
	if err != nil {
		return nil, 0, err
	}
	residual := weightedResidualsExponential(result.X, weight, reps, dates, now, halfLifeDays)
	return &ExponentialEstimator{
		a:          result.X[0],
		b:          result.X[1],
		halfLife:   halfLifeDays,
		modelType:  ModelExponential,
		residualSum: residual,
	}, residual, nil
}

func weightedResidualsExponential(params, weight, reps []float64, dates []time.Time, now time.Time, halfLifeDays float64) float64 {
	a, b := params[0], params[1]
	var sum float64
	for i := range weight {
		daysAgo := now.Sub(dates[i]).Hours() / 24
		weightDecay := math.Exp(-math.Ln2 * daysAgo / halfLifeDays)
		predicted := a * math.Exp(b*reps[i])
		residual := weight[i] - predicted
		sum += weightDecay * residual * residual
	}
	return sum
}

// --- Utility Functions ---

// max returns the maximum value in a slice
func max(slice []float64) float64 {
	m := slice[0]
	for _, v := range slice {
		if v > m {
			m = v
		}
	}
	return m
}