CosmicClash/server/domain/teams.go

package domain

import (
	"fmt"
	"sort"
)

type Teams struct {
	Team0 []Candidate
	Team1 []Candidate
}

// PartitionTeams exhaustively evaluates balanced two-team assignments. The
// first team is anchored to the lexically smallest player to remove the
// equivalent team-0/team-1 mirror; this makes the result stable across worker
// order and database row order.
func PartitionTeams(players []Candidate) (Teams, error) {
	if len(players) < 2 || len(players) > 6 || len(players)%2 != 0 {
		return Teams{}, fmt.Errorf("team partition requires an even player count from 2 through 6")
	}
	ordered := append([]Candidate(nil), players...)
	sort.Slice(ordered, func(i, j int) bool { return ordered[i].PlayerID < ordered[j].PlayerID })
	teamSize := len(ordered) / 2
	anchor := ordered[0].PlayerID
	best := Teams{}
	found := false
	chosen := make([]Candidate, 0, teamSize)
	var visit func(int)
	visit = func(start int) {
		if len(chosen) == teamSize {
			if !containsPlayer(chosen, anchor) {
				return
			}
			team1 := make([]Candidate, 0, teamSize)
			for _, player := range ordered {
				if !containsPlayer(chosen, player.PlayerID) {
					team1 = append(team1, player)
				}
			}
			if !found || betterTeams(chosen, team1, best) {
				best = Teams{Team0: append([]Candidate(nil), chosen...), Team1: team1}
				found = true
			}
			return
		}
		for i := start; i < len(ordered); i++ {
			chosen = append(chosen, ordered[i])
			visit(i + 1)
			chosen = chosen[:len(chosen)-1]
		}
	}
	visit(0)
	if !found {
		return Teams{}, fmt.Errorf("no balanced team partition")
	}
	return best, nil
}

func betterTeams(team0, team1 []Candidate, best Teams) bool {
	if best.Team0 == nil {
		return true
	}
	meanDelta, maxOpposing := teamScore(team0, team1)
	bestMean, bestMaxOpposing := teamScore(best.Team0, best.Team1)
	if meanDelta != bestMean {
		return meanDelta < bestMean
	}
	if maxOpposing != bestMaxOpposing {
		return maxOpposing < bestMaxOpposing
	}
	return playerIDs(team0) < playerIDs(best.Team0)
}

func teamScore(team0, team1 []Candidate) (float64, float64) {
	mean0, mean1 := meanRating(team0), meanRating(team1)
	maxOpposing := 0.0
	for _, left := range team0 {
		for _, right := range team1 {
			delta := abs(left.Rating - right.Rating)
			if delta > maxOpposing {
				maxOpposing = delta
			}
		}
	}
	return abs(mean0 - mean1), maxOpposing
}

func meanRating(players []Candidate) float64 {
	total := 0.0
	for _, player := range players {
		total += player.Rating
	}
	return total / float64(len(players))
}

func containsPlayer(players []Candidate, playerID string) bool {
	for _, player := range players {
		if player.PlayerID == playerID {
			return true
		}
	}
	return false
}

func playerIDs(players []Candidate) string {
	ids := make([]string, 0, len(players))
	for _, player := range players {
		ids = append(ids, player.PlayerID)
	}
	sort.Strings(ids)
	result := ""
	for _, id := range ids {
		result += id + "\x00"
	}
	return result
}