apskel-pos-backend/internal/processor/reward_calculator.go

package processor

import (
	"bytes"
	"crypto/rand"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"math"
	"math/big"
	"strings"

	"apskel-pos-be/internal/constants"
)

var (
	// ErrInvalidRewardRules means a reward configuration's rules do not fit its type
	// (docs/rfc-enakgame.md §8). Checked when the configuration is created.
	ErrInvalidRewardRules = errors.New("invalid reward rules")
	// ErrRewardResultUnusable means a result lacks what the rules need to price it: no
	// score for SCORE_BASED, an outcome the rules do not list, a score outside every
	// band. The session earns nothing and is flagged.
	ErrRewardResultUnusable = errors.New("result cannot be rewarded")
)

// rewardWeightLimit bounds the total weight of a PROBABILITY table, far below where
// the sum could overflow.
const rewardWeightLimit = int64(1_000_000_000_000)

// SessionResult is what the client reports about a play: data only, never a reward
// amount (P1).
type SessionResult struct {
	Score   *int64  `json:"score,omitempty"`
	Outcome *string `json:"outcome,omitempty"`
}

// RewardRNG draws the random number of a PROBABILITY reward.
type RewardRNG interface {
	// Int63n returns a uniform number in [0, n).
	Int63n(n int64) (int64, error)
}

// CryptoRewardRNG draws from crypto/rand, so the outcome cannot be predicted from the
// time the way a reseeded math/rand can (§8).
type CryptoRewardRNG struct{}

func (CryptoRewardRNG) Int63n(n int64) (int64, error) {
	v, err := rand.Int(rand.Reader, big.NewInt(n))
	if err != nil {
		return 0, err
	}
	return v.Int64(), nil
}

// RewardCalculator computes the base reward of one reward type (§8). Base rewards are
// whole EnakCoin; event modifiers and the max_reward cap come after.
type RewardCalculator interface {
	// Validate checks rules when a configuration is created.
	Validate(rules json.RawMessage) error
	// Calculate prices a result under rules that passed Validate. detail says how the
	// amount was reached, for reward_breakdown.
	Calculate(rules json.RawMessage, result SessionResult, rng RewardRNG) (base int64, detail map[string]any, err error)
	// Scale multiplies every amount in rules by multiplier/10000, rounded down, for
	// the Budget Controller (§10). Everything else, such as score bands and weights,
	// stays.
	Scale(rules json.RawMessage, multiplier int64) (json.RawMessage, error)
}

// rewardMultiplierOne is a multiplier of 1 in the ten-thousandths Scale takes.
const rewardMultiplierOne = int64(10_000)

// scaleRewardAmount is amount × multiplier/10000, rounded down (RFC §19.2 #1).
func scaleRewardAmount(amount, multiplier int64) (int64, error) {
	if multiplier < 0 {
		return 0, invalidRules("multiplier cannot be negative")
	}
	if amount != 0 && multiplier > math.MaxInt64/amount {
		return 0, invalidRules("amount %d is too large to scale", amount)
	}
	return amount * multiplier / rewardMultiplierOne, nil
}

func scaleRewardAmounts(multiplier int64, amounts ...*int64) error {
	for _, a := range amounts {
		scaled, err := scaleRewardAmount(*a, multiplier)
		if err != nil {
			return err
		}
		*a = scaled
	}
	return nil
}

func marshalRules(v any) (json.RawMessage, error) {
	b, err := json.Marshal(v)
	if err != nil {
		return nil, err
	}
	return b, nil
}

var rewardCalculators = map[string]RewardCalculator{
	constants.GameRewardTypeFixed:        fixedReward{},
	constants.GameRewardTypeScoreBased:   scoreBasedReward{},
	constants.GameRewardTypeOutcomeBased: outcomeBasedReward{},
	constants.GameRewardTypeProbability:  probabilityReward{},
}

// RewardCalculatorFor returns the calculator of a reward type.
func RewardCalculatorFor(rewardType string) (RewardCalculator, error) {
	c, ok := rewardCalculators[rewardType]
	if !ok {
		return nil, fmt.Errorf("%w: unknown reward type %q", ErrInvalidRewardRules, rewardType)
	}
	return c, nil
}

func invalidRules(format string, args ...any) error {
	return fmt.Errorf("%w: %s", ErrInvalidRewardRules, fmt.Sprintf(format, args...))
}

func unusableResult(format string, args ...any) error {
	return fmt.Errorf("%w: %s", ErrRewardResultUnusable, fmt.Sprintf(format, args...))
}

// decodeRules reads rules strictly: an unknown field is a typo, not something to
// ignore.
func decodeRules(rules json.RawMessage, v any) error {
	dec := json.NewDecoder(bytes.NewReader(rules))
	dec.DisallowUnknownFields()
	if err := dec.Decode(v); err != nil {
		return invalidRules("%v", err)
	}
	if _, err := dec.Token(); err != io.EOF {
		return invalidRules("unexpected data after the rules")
	}
	return nil
}

// FIXED: {"amount": 5}.
type fixedReward struct{}

type fixedRules struct {
	Amount *int64 `json:"amount"`
}

func (fixedReward) parse(rules json.RawMessage) (fixedRules, error) {
	var r fixedRules
	if err := decodeRules(rules, &r); err != nil {
		return r, err
	}
	if r.Amount == nil || *r.Amount < 0 {
		return r, invalidRules("amount must be 0 or more")
	}
	return r, nil
}

func (f fixedReward) Validate(rules json.RawMessage) error {
	_, err := f.parse(rules)
	return err
}

func (f fixedReward) Calculate(rules json.RawMessage, _ SessionResult, _ RewardRNG) (int64, map[string]any, error) {
	r, err := f.parse(rules)
	if err != nil {
		return 0, nil, err
	}
	return *r.Amount, map[string]any{"reward_type": constants.GameRewardTypeFixed}, nil
}

func (f fixedReward) Scale(rules json.RawMessage, multiplier int64) (json.RawMessage, error) {
	r, err := f.parse(rules)
	if err != nil {
		return nil, err
	}
	if err := scaleRewardAmounts(multiplier, r.Amount); err != nil {
		return nil, err
	}
	return marshalRules(r)
}

// SCORE_BASED: {"bands": [{"min": 0, "max": 100, "amount": 1}, {"min": 101, "amount": 20}]}.
// Bands run in order from 0, each starting right after the previous one ends; only the
// last may leave max out, to cover every higher score.
type scoreBasedReward struct{}

type scoreBand struct {
	Min *int64 `json:"min"`
	// Left out of the last band, also when scaled rules are written back.
	Max    *int64 `json:"max,omitempty"`
	Amount *int64 `json:"amount"`
}

type scoreRules struct {
	Bands []scoreBand `json:"bands"`
}

func (scoreBasedReward) parse(rules json.RawMessage) (scoreRules, error) {
	var r scoreRules
	if err := decodeRules(rules, &r); err != nil {
		return r, err
	}
	if len(r.Bands) == 0 {
		return r, invalidRules("at least one band is required")
	}
	for i, b := range r.Bands {
		if b.Min == nil || b.Amount == nil {
			return r, invalidRules("band %d needs min and amount", i+1)
		}
		if *b.Amount < 0 {
			return r, invalidRules("band %d amount must be 0 or more", i+1)
		}
		if b.Max != nil && *b.Max < *b.Min {
			return r, invalidRules("band %d max is below its min", i+1)
		}
		if i == 0 {
			if *b.Min != 0 {
				return r, invalidRules("the first band must start at 0")
			}
			continue
		}
		prev := r.Bands[i-1]
		if prev.Max == nil {
			return r, invalidRules("only the last band may leave max out")
		}
		switch {
		case *b.Min <= *prev.Max:
			return r, invalidRules("band %d overlaps band %d", i+1, i)
		case *b.Min > *prev.Max+1:
			return r, invalidRules("scores %d to %d are in no band", *prev.Max+1, *b.Min-1)
		}
	}
	return r, nil
}

func (s scoreBasedReward) Validate(rules json.RawMessage) error {
	_, err := s.parse(rules)
	return err
}

func (s scoreBasedReward) Calculate(rules json.RawMessage, result SessionResult, _ RewardRNG) (int64, map[string]any, error) {
	r, err := s.parse(rules)
	if err != nil {
		return 0, nil, err
	}
	if result.Score == nil {
		return 0, nil, unusableResult("a score is required")
	}
	score := *result.Score
	for i, b := range r.Bands {
		if score >= *b.Min && (b.Max == nil || score <= *b.Max) {
			return *b.Amount, map[string]any{"reward_type": constants.GameRewardTypeScoreBased, "score": score, "band": i + 1}, nil
		}
	}
	return 0, nil, unusableResult("score %d is in no band", score)
}

func (s scoreBasedReward) Scale(rules json.RawMessage, multiplier int64) (json.RawMessage, error) {
	r, err := s.parse(rules)
	if err != nil {
		return nil, err
	}
	for _, b := range r.Bands {
		if err := scaleRewardAmounts(multiplier, b.Amount); err != nil {
			return nil, err
		}
	}
	return marshalRules(r)
}

// OUTCOME_BASED: {"outcomes": {"PERFECT": 20, "GOOD": 10, "FAIL": 0}}.
type outcomeBasedReward struct{}

type outcomeRules struct {
	Outcomes map[string]int64 `json:"outcomes"`
}

func (outcomeBasedReward) parse(rules json.RawMessage) (outcomeRules, error) {
	var r outcomeRules
	if err := decodeRules(rules, &r); err != nil {
		return r, err
	}
	if len(r.Outcomes) == 0 {
		return r, invalidRules("at least one outcome is required")
	}
	for outcome, amount := range r.Outcomes {
		if strings.TrimSpace(outcome) == "" {
			return r, invalidRules("an outcome needs a name")
		}
		if amount < 0 {
			return r, invalidRules("outcome %s amount must be 0 or more", outcome)
		}
	}
	return r, nil
}

func (o outcomeBasedReward) Validate(rules json.RawMessage) error {
	_, err := o.parse(rules)
	return err
}

func (o outcomeBasedReward) Calculate(rules json.RawMessage, result SessionResult, _ RewardRNG) (int64, map[string]any, error) {
	r, err := o.parse(rules)
	if err != nil {
		return 0, nil, err
	}
	if result.Outcome == nil {
		return 0, nil, unusableResult("an outcome is required")
	}
	amount, ok := r.Outcomes[*result.Outcome]
	if !ok {
		return 0, nil, unusableResult("unknown outcome %q", *result.Outcome)
	}
	return amount, map[string]any{"reward_type": constants.GameRewardTypeOutcomeBased, "outcome": *result.Outcome}, nil
}

func (o outcomeBasedReward) Scale(rules json.RawMessage, multiplier int64) (json.RawMessage, error) {
	r, err := o.parse(rules)
	if err != nil {
		return nil, err
	}
	for outcome, amount := range r.Outcomes {
		scaled, err := scaleRewardAmount(amount, multiplier)
		if err != nil {
			return nil, err
		}
		r.Outcomes[outcome] = scaled
	}
	return marshalRules(r)
}

// PROBABILITY: {"table": [{"weight": 1, "amount": 1000}, {"weight": 999, "amount": 0}]}.
// Weights are whole numbers, so no check depends on floating point (§8). The draw is
// kept in the detail for audit.
type probabilityReward struct{}

type probabilityEntry struct {
	Weight *int64 `json:"weight"`
	Amount *int64 `json:"amount"`
}

type probabilityRules struct {
	Table []probabilityEntry `json:"table"`
}

func (probabilityReward) parse(rules json.RawMessage) (probabilityRules, int64, error) {
	var r probabilityRules
	if err := decodeRules(rules, &r); err != nil {
		return r, 0, err
	}
	if len(r.Table) == 0 {
		return r, 0, invalidRules("at least one entry is required")
	}
	var total int64
	for i, e := range r.Table {
		if e.Weight == nil || e.Amount == nil {
			return r, 0, invalidRules("entry %d needs weight and amount", i+1)
		}
		if *e.Weight < 1 {
			return r, 0, invalidRules("entry %d weight must be at least 1", i+1)
		}
		if *e.Amount < 0 {
			return r, 0, invalidRules("entry %d amount must be 0 or more", i+1)
		}
		total += *e.Weight
		if total > rewardWeightLimit {
			return r, 0, invalidRules("the weights add up to more than %d", rewardWeightLimit)
		}
	}
	return r, total, nil
}

func (p probabilityReward) Validate(rules json.RawMessage) error {
	_, _, err := p.parse(rules)
	return err
}

func (p probabilityReward) Calculate(rules json.RawMessage, _ SessionResult, rng RewardRNG) (int64, map[string]any, error) {
	r, total, err := p.parse(rules)
	if err != nil {
		return 0, nil, err
	}
	roll, err := rng.Int63n(total)
	if err != nil {
		return 0, nil, fmt.Errorf("failed to draw a reward: %w", err)
	}
	cumulative := int64(0)
	for i, e := range r.Table {
		cumulative += *e.Weight
		if roll < cumulative {
			return *e.Amount, map[string]any{
				"reward_type": constants.GameRewardTypeProbability, "roll": roll, "total_weight": total, "entry": i + 1,
			}, nil
		}
	}
	return 0, nil, fmt.Errorf("draw %d is outside the total weight %d", roll, total)
}

// Scale changes the prizes, not their odds.
func (p probabilityReward) Scale(rules json.RawMessage, multiplier int64) (json.RawMessage, error) {
	r, _, err := p.parse(rules)
	if err != nil {
		return nil, err
	}
	for _, e := range r.Table {
		if err := scaleRewardAmounts(multiplier, e.Amount); err != nil {
			return nil, err
		}
	}
	return marshalRules(r)
}