mixmaker/backend/internal/domain/balancer.go

package domain

import (
	"fmt"
	"math"
	"slices"
	"strings"
)

type BalanceCandidate struct {
	Teams       []Team   `json:"teams"`
	Reserve     []string `json:"reserve"`
	Score       int      `json:"score"`
	Explanation []string `json:"explanation"`
}

// Balance deterministically creates three alternatives. Players are assigned to
// their strongest role while preserving the 1/2/2 composition of every team.
func Balance(eventID string, players []Player) ([]BalanceCandidate, error) {
	if len(players) < 10 {
		return nil, fmt.Errorf("%w: at least ten players are required", ErrInvalid)
	}
	base := slices.Clone(players)
	slices.SortFunc(base, func(a, b Player) int {
		if a.ID < b.ID {
			return -1
		}
		if a.ID > b.ID {
			return 1
		}
		return 0
	})
	roleOrders := [][]Role{
		{Tank, Damage, Support},
		{Tank, Support, Damage},
		{Damage, Tank, Support},
		{Damage, Support, Tank},
		{Support, Tank, Damage},
		{Support, Damage, Tank},
	}
	all := make([]BalanceCandidate, 0, len(roleOrders)*2)
	for orderIndex, order := range roleOrders {
		for offset := 0; offset < 2; offset++ {
			all = append(all, buildCandidate(eventID, base, orderIndex*2+offset, order, offset))
		}
	}
	slices.SortStableFunc(all, func(a, b BalanceCandidate) int { return a.Score - b.Score })
	result := make([]BalanceCandidate, 0, 3)
	seen := map[string]bool{}
	for _, candidate := range all {
		signature := candidateSignature(candidate)
		if seen[signature] {
			continue
		}
		seen[signature] = true
		result = append(result, candidate)
		if len(result) == 3 {
			break
		}
	}
	return result, nil
}

func buildCandidate(eventID string, players []Player, variant int, roleOrder []Role, offset int) BalanceCandidate {
	teamCount := len(players) / 5
	used := teamCount * 5
	c := BalanceCandidate{Reserve: make([]string, 0, len(players)-used)}
	for _, p := range players[used:] {
		c.Reserve = append(c.Reserve, p.ID)
	}
	active := slices.Clone(players[:used])
	for i := 0; i < teamCount; i++ {
		c.Teams = append(c.Teams, Team{
			ID:      fmt.Sprintf("%s-v%d-team-%d", eventID, variant+1, i+1),
			EventID: eventID,
			Name:    fmt.Sprintf("Team %d", i+1),
		})
	}
	for _, role := range roleOrder {
		slices.SortStableFunc(active, func(a, b Player) int {
			ar, br := rating(a, role), rating(b, role)
			if ar != br {
				return br - ar
			}
			if a.ID < b.ID {
				return -1
			}
			return 1
		})
		perTeam := 2
		if role == Tank {
			perTeam = 1
		}
		count := teamCount * perTeam
		selected := slices.Clone(active[:count])
		active = active[count:]
		for i, p := range selected {
			teamIndex := (i + offset) % teamCount
			if (i/teamCount)%2 == 1 {
				teamIndex = teamCount - 1 - teamIndex
				if teamIndex < 0 {
					teamIndex += teamCount
				}
			}
			c.Teams[teamIndex].Slots = append(c.Teams[teamIndex].Slots, Slot{PlayerID: p.ID, Role: role, Rating: rating(p, role)})
		}
	}
	optimizeCandidate(c.Teams, players)
	c.Score = balanceScore(c.Teams, players)
	preferredRoles, preferredTeammates := preferenceStats(c.Teams, players)
	c.Explanation = []string{
		fmt.Sprintf("%d preferred role assignments satisfied", preferredRoles),
		fmt.Sprintf("%d preferred teammate choices satisfied", preferredTeammates),
	}
	return c
}

func balanceScore(teams []Team, players []Player) int {
	playerByID := make(map[string]Player, len(players))
	for _, player := range players {
		playerByID[player.ID] = player
	}
	minTotal, maxTotal := math.MaxInt, 0
	roleTotals := map[Role][]int{Tank: {}, Damage: {}, Support: {}}
	weakRolePenalty := 0
	preferredRolePenalty := 0
	teamByPlayer := make(map[string]int)
	for teamIndex, team := range teams {
		total := 0
		perRole := map[Role]int{}
		for _, slot := range team.Slots {
			total += slot.Rating
			perRole[slot.Role] += slot.Rating
			player := playerByID[slot.PlayerID]
			teamByPlayer[slot.PlayerID] = teamIndex
			strongest := max(player.Ratings.Tank, player.Ratings.Damage, player.Ratings.Support)
			weakRolePenalty += strongest - slot.Rating
			if len(player.PreferredRoles) > 0 && !slices.Contains(player.PreferredRoles, slot.Role) {
				preferredRolePenalty += 12
			}
		}
		minTotal = min(minTotal, total)
		maxTotal = max(maxTotal, total)
		for role := range roleTotals {
			roleTotals[role] = append(roleTotals[role], perRole[role])
		}
	}
	preferredTeammatePenalty := 0
	for _, player := range players {
		teamIndex, active := teamByPlayer[player.ID]
		if !active {
			continue
		}
		for _, preferredID := range player.PreferredPlayerIDs {
			if preferredTeam, preferredActive := teamByPlayer[preferredID]; preferredActive && preferredTeam != teamIndex {
				preferredTeammatePenalty += 8
			}
		}
	}
	score := (maxTotal-minTotal)*3 + weakRolePenalty + preferredRolePenalty + preferredTeammatePenalty
	for _, totals := range roleTotals {
		minimum, maximum := math.MaxInt, 0
		for _, total := range totals {
			minimum = min(minimum, total)
			maximum = max(maximum, total)
		}
		score += (maximum - minimum) * 2
	}
	return score
}

func optimizeCandidate(teams []Team, players []Player) {
	for iteration := 0; iteration < 20; iteration++ {
		bestScore := balanceScore(teams, players)
		bestA, bestB, bestSlotA, bestSlotB := -1, -1, -1, -1
		for teamA := 0; teamA < len(teams); teamA++ {
			for teamB := teamA + 1; teamB < len(teams); teamB++ {
				for slotA := range teams[teamA].Slots {
					for slotB := range teams[teamB].Slots {
						if teams[teamA].Slots[slotA].Role != teams[teamB].Slots[slotB].Role {
							continue
						}
						teams[teamA].Slots[slotA], teams[teamB].Slots[slotB] = teams[teamB].Slots[slotB], teams[teamA].Slots[slotA]
						score := balanceScore(teams, players)
						teams[teamA].Slots[slotA], teams[teamB].Slots[slotB] = teams[teamB].Slots[slotB], teams[teamA].Slots[slotA]
						if score < bestScore {
							bestScore = score
							bestA, bestB, bestSlotA, bestSlotB = teamA, teamB, slotA, slotB
						}
					}
				}
			}
		}
		if bestA < 0 {
			return
		}
		teams[bestA].Slots[bestSlotA], teams[bestB].Slots[bestSlotB] = teams[bestB].Slots[bestSlotB], teams[bestA].Slots[bestSlotA]
	}
}

func preferenceStats(teams []Team, players []Player) (int, int) {
	playerByID := make(map[string]Player, len(players))
	teamByPlayer := make(map[string]int)
	roleByPlayer := make(map[string]Role)
	for _, player := range players {
		playerByID[player.ID] = player
	}
	for teamIndex, team := range teams {
		for _, slot := range team.Slots {
			teamByPlayer[slot.PlayerID] = teamIndex
			roleByPlayer[slot.PlayerID] = slot.Role
		}
	}
	roleMatches, teammateMatches := 0, 0
	for playerID, teamIndex := range teamByPlayer {
		player := playerByID[playerID]
		if len(player.PreferredRoles) > 0 && slices.Contains(player.PreferredRoles, roleByPlayer[playerID]) {
			roleMatches++
		}
		for _, preferredID := range player.PreferredPlayerIDs {
			if preferredTeam, ok := teamByPlayer[preferredID]; ok && preferredTeam == teamIndex {
				teammateMatches++
			}
		}
	}
	return roleMatches, teammateMatches
}

func candidateSignature(candidate BalanceCandidate) string {
	parts := make([]string, 0, len(candidate.Teams))
	for _, team := range candidate.Teams {
		slots := slices.Clone(team.Slots)
		slices.SortFunc(slots, func(a, b Slot) int {
			return strings.Compare(string(a.Role)+a.PlayerID, string(b.Role)+b.PlayerID)
		})
		var values []string
		for _, slot := range slots {
			values = append(values, string(slot.Role)+":"+slot.PlayerID)
		}
		parts = append(parts, strings.Join(values, ","))
	}
	slices.Sort(parts)
	return strings.Join(parts, "|")
}

func rating(p Player, role Role) int {
	switch role {
	case Tank:
		return p.Ratings.Tank
	case Damage:
		return p.Ratings.Damage
	default:
		return p.Ratings.Support
	}
}