// Package elo implements a practical Elo rating update for head-to-head sports
// such as table soccer. This variant has:
//   - No home/away asymmetry
//   - Margin of victory (MoV) factor using a robust bounded form
//   - Simple burn-in: higher K for the first N games
//   - No draws (inputs must produce a clear winner)
//   - Rating floor
//   - Per-match delta cap
//   - No time weighting and no explicit uncertainty modeling

package main

import (
	"errors"
	"math"
)

var eloCfg = Config{
	Mu0:              1500,
	Scale:            400,
	KNew:             48,
	KStd:             24,
	BurnInGames:      20,
	RatingFloor:      500,
	MaxPerMatchDelta: 60,
	MaxGoals:         10,
}

type Config struct {
	Mu0              float64
	Scale            float64 // KNew applies to players during burn-in (first BurnInGames matches).
	KNew             float64
	KStd             float64
	BurnInGames      int
	RatingFloor      float64
	MaxPerMatchDelta float64
	MaxGoals         int
}

type Team struct {
	Players       []Player
	Score         int
	AverageRating float64
}

// Collect players by side
type Player struct {
	u    *User
	side string
	dElo float64
}

func NewTeam(players []Player, score int) *Team {
	var t1s float64 = 0
	for _, player := range players {
		t1s += float64(player.u.Elo)

	}
	t1a := t1s / float64(len(players))

	return &Team{Players: players, Score: score, AverageRating: t1a}
}

func GetNewElo(teams []*Team) error {
	// Update the teams
	t1 := teams[0]
	t2 := teams[1]
	if t2.Score < 0 || t1.Score < 0 || t2.Score > eloCfg.MaxGoals || t1.Score > eloCfg.MaxGoals {
		return errors.New("goals out of allowed range")
	}
	if t2.Score == t1.Score {
		return errors.New("draws are not supported by this variant")
	}

	// Expected score (Bradley–Terry with logistic base-10)
	for i, t := range teams {
		otherTeam := teams[1-i]
		for i, p := range t.Players {
			newRating, err := CalculateRating(p.u.Elo, otherTeam.AverageRating, t.Score, otherTeam.Score, p.u.GameCount)
			if err != nil {
				return err
			}
			t.Players[i].u.Elo = roundFloat(newRating, 1)
		}
	}

	return nil
}

func CalculateRating(rA, rB float64, goalsA, goalsB, gamesA int) (newrA float64, err error) {
	// Observed score (no draws)
	var sA float64
	if goalsA > goalsB {
		sA = 1.0
	} else {
		sA = 0.0
	}

	// Margin of victory factor with clamped goal difference
	diff := goalsA - goalsB
	if diff < 0 {
		diff = -diff
	}
	if diff < 1 {
		diff = 1
	}
	if diff > eloCfg.MaxGoals {
		diff = eloCfg.MaxGoals
	}

	// Raw delta before caps
	eA := expected(eloCfg.Scale, rA, rB)
	mov := movFactor(rA, rB, diff)

	var Keff float64
	if gamesA <= eloCfg.BurnInGames {
		Keff = eloCfg.KNew
	} else {
		Keff = eloCfg.KStd
	}

	delta := Keff * mov * (sA - eA)

	// Apply symmetric per-match cap
	if delta > eloCfg.MaxPerMatchDelta {
		delta = eloCfg.MaxPerMatchDelta
	} else if delta < -eloCfg.MaxPerMatchDelta {
		delta = -eloCfg.MaxPerMatchDelta
	}

	return rA + delta, nil
}

// expected returns the win probability for player A against B
func expected(scale, rA, rB float64) float64 {
	return 1.0 / (1.0 + math.Pow(10.0, -(rA-rB)/scale))
}

// movFactor returns a bounded margin-of-victory multiplier.
func movFactor(rA, rB float64, diff int) float64 {
	fd := float64(diff)
	return math.Log(fd+1.0) * 2.2 / (math.Abs(rA-rB)*0.001 + 2.2)
}