feat: joke & dice command

internal/commands/commands.go

@@ -10,6 +10,9 @@ import (
 
 var AllCommands = []*discordgo.ApplicationCommand{
 	fun.Ping,
+	fun.Joke,
+	fun.Coinflip,
+	fun.Dice,
 	help.Help,
 	cmdlevel.Slvl,
 	cmdlevel.Rank,
@@ -18,8 +21,11 @@ var AllCommands = []*discordgo.ApplicationCommand{
 }
 
 var handlers = map[string]func(s *discordgo.Session, i *discordgo.InteractionCreate){
-	"ping": fun.PingHandler,
+	"ping":        fun.PingHandler,
-	"help": help.HelpHandler,
+	"joke":        fun.JokeHandler,
+	"coinflip":    fun.CoinflipHandler,
+	"dice":        fun.DiceHandler,
+	"help":        help.HelpHandler,
 	"slvl":        cmdlevel.SlvlHandler,
 	"rank":        cmdlevel.RankHandler,
 	"leaderboard": cmdlevel.LeaderboardHandler,

internal/commands/fun/public/coinflip.go

@@ -0,0 +1,22 @@
+package public
+
+import (
+	"math/rand"
+	"velox-bot/internal/commands/fun/shared"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+var Coinflip = &discordgo.ApplicationCommand{
+	Name:        "coinflip",
+	Description: "Flip a coin",
+}
+
+func CoinflipHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	num := rand.Intn(2)
+	if num == 0 {
+		shared.Respond(s, i, "Heads!")
+	} else {
+		shared.Respond(s, i, "Tails!")
+	}
+}

internal/commands/fun/public/dice.go

@@ -0,0 +1,62 @@
+package public
+
+import (
+	"strings"
+
+	"velox-bot/internal/commands/fun/shared"
+	"velox-bot/internal/diceexpr"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+var Dice = &discordgo.ApplicationCommand{
+	Name:        "dice",
+	Description: "Roll dice expressions (e.g. d20, 2d20+1, 2#d20+1)",
+	Options: []*discordgo.ApplicationCommandOption{
+		{
+			Type:        discordgo.ApplicationCommandOptionString,
+			Name:        "expr",
+			Description: "Dice expression to roll",
+			Required:    true,
+		},
+	},
+	Contexts: &[]discordgo.InteractionContextType{
+		discordgo.InteractionContextGuild,
+		discordgo.InteractionContextBotDM,
+		discordgo.InteractionContextPrivateChannel,
+	},
+}
+
+func DiceHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	expr := ""
+	if opts := i.ApplicationCommandData().Options; len(opts) > 0 {
+		expr = opts[0].StringValue()
+	}
+	expr = strings.TrimSpace(expr)
+	if expr == "" {
+		respondDiceFailure(s, i)
+		return
+	}
+
+	res, err := diceexpr.EvalMany(expr, diceexpr.DefaultLimits())
+	if err != nil {
+		respondDiceFailure(s, i)
+		return
+	}
+
+	out := diceexpr.FormatResult(res, diceexpr.DefaultFormatOptions())
+	if len(out) > 1900 {
+		// Keep within Discord message limits; trim safely.
+		out = out[:1900] + "…"
+	}
+	shared.Respond(s, i, out)
+}
+
+func respondDiceFailure(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	if i.GuildID != "" {
+		shared.RespondEphemeral(s, i, "Failure!")
+		return
+	}
+	shared.Respond(s, i, "Failure!")
+}
+

internal/commands/fun/public/joke.go

@@ -0,0 +1,153 @@
+package public
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"time"
+
+	"velox-bot/internal/commands/fun/shared"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+var Joke = &discordgo.ApplicationCommand{
+	Name:        "joke",
+	Description: "Get a random joke",
+	Contexts: &[]discordgo.InteractionContextType{
+		discordgo.InteractionContextGuild,
+		discordgo.InteractionContextBotDM,
+		discordgo.InteractionContextPrivateChannel,
+	},
+	Options: []*discordgo.ApplicationCommandOption{
+		{
+			Type:        discordgo.ApplicationCommandOptionString,
+			Name:        "type",
+			Description: "The type of joke to get",
+			Required:    false,
+			Choices: []*discordgo.ApplicationCommandOptionChoice{
+				{
+					Name:  "Programming",
+					Value: "Programming",
+				},
+				{
+					Name:  "Misc",
+					Value: "Miscellaneous",
+				},
+				{
+					Name:  "Dark",
+					Value: "Dark",
+				},
+				{
+					Name:  "Pun",
+					Value: "Pun",
+				},
+				{
+					Name:  "Spooky",
+					Value: "Spooky",
+				},
+				{
+					Name:  "Christmas",
+					Value: "Christmas",
+				},
+			},
+		},
+	},
+}
+
+func JokeHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	jokeType := "Any"
+	if opts := i.ApplicationCommandData().Options; len(opts) > 0 {
+		if v := opts[0].StringValue(); v != "" {
+			jokeType = v
+		}
+	}
+	if !isValidJokeType(jokeType) {
+		jokeType = "Any"
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 6*time.Second)
+	defer cancel()
+
+	content, err := fetchJoke(ctx, jokeType)
+	if err != nil {
+		if i.GuildID != "" {
+			shared.RespondEphemeral(s, i, "Failed to get joke.")
+		} else {
+			shared.Respond(s, i, "Failed to get joke.")
+		}
+		return
+	}
+
+	shared.Respond(s, i, content)
+}
+
+func isValidJokeType(t string) bool {
+	switch t {
+	case "Any", "Programming", "Misc", "Dark", "Pun", "Spooky", "Christmas":
+		return true
+	default:
+		return false
+	}
+}
+
+type jokeAPIResponse struct {
+	Error    bool   `json:"error"`
+	Message  string `json:"message"`
+	Type     string `json:"type"`
+	Joke     string `json:"joke"`
+	Setup    string `json:"setup"`
+	Delivery string `json:"delivery"`
+}
+
+func fetchJoke(ctx context.Context, jokeType string) (string, error) {
+	const baseURL = "https://v2.jokeapi.dev/joke"
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, fmt.Sprintf("%s/%s", baseURL, jokeType), nil)
+	if err != nil {
+		return "", err
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return "", fmt.Errorf("joke api status: %s", resp.Status)
+	}
+
+	var data jokeAPIResponse
+	if err := json.NewDecoder(resp.Body).Decode(&data); err != nil {
+		return "", err
+	}
+	if data.Error {
+		if data.Message != "" {
+			return "", errors.New(data.Message)
+		}
+		return "", errors.New("joke api returned error")
+	}
+
+	switch data.Type {
+	case "single":
+		if data.Joke == "" {
+			return "", errors.New("empty joke")
+		}
+		return data.Joke, nil
+	case "twopart":
+		if data.Setup == "" && data.Delivery == "" {
+			return "", errors.New("empty joke")
+		}
+		if data.Setup == "" {
+			return data.Delivery, nil
+		}
+		if data.Delivery == "" {
+			return data.Setup, nil
+		}
+		return data.Setup + "\n" + data.Delivery, nil
+	default:
+		return "", fmt.Errorf("unknown joke type: %q", data.Type)
+	}
+}

internal/commands/fun/public/ping.go

@@ -1,4 +1,4 @@
-package fun
+package public
 
 import "github.com/bwmarrin/discordgo"
 

internal/commands/fun/registry.go

@@ -0,0 +1,23 @@
+package fun
+
+import (
+	"velox-bot/internal/commands/fun/public"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+// Commands
+var (
+	Ping     *discordgo.ApplicationCommand = public.Ping
+	Joke     *discordgo.ApplicationCommand = public.Joke
+	Coinflip *discordgo.ApplicationCommand = public.Coinflip
+	Dice     *discordgo.ApplicationCommand = public.Dice
+)
+
+// Handlers
+func PingHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.PingHandler(s, i) }
+func JokeHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.JokeHandler(s, i) }
+func CoinflipHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	public.CoinflipHandler(s, i)
+}
+func DiceHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.DiceHandler(s, i) }

internal/commands/fun/shared/shared.go

@@ -0,0 +1,24 @@
+package shared
+
+import (
+	"github.com/bwmarrin/discordgo"
+)
+
+func Respond(s *discordgo.Session, i *discordgo.InteractionCreate, msg string) {
+	_ = s.InteractionRespond(i.Interaction, &discordgo.InteractionResponse{
+		Type: discordgo.InteractionResponseChannelMessageWithSource,
+		Data: &discordgo.InteractionResponseData{
+			Content: msg,
+		},
+	})
+}
+
+func RespondEphemeral(s *discordgo.Session, i *discordgo.InteractionCreate, msg string) {
+	_ = s.InteractionRespond(i.Interaction, &discordgo.InteractionResponse{
+		Type: discordgo.InteractionResponseChannelMessageWithSource,
+		Data: &discordgo.InteractionResponseData{
+			Content: msg,
+			Flags:   discordgo.MessageFlagsEphemeral,
+		},
+	})
+}

internal/commands/level/registry.go

@@ -16,8 +16,11 @@ var (
 )
 
 // Handlers
-func RankHandler(s *discordgo.Session, i *discordgo.InteractionCreate)        { public.RankHandler(s, i) }
+func RankHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.RankHandler(s, i) }
-func LeaderboardHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { public.LeaderboardHandler(s, i) }
+func LeaderboardHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
-func RewardsHandler(s *discordgo.Session, i *discordgo.InteractionCreate)     { public.RewardsHandler(s, i) }
+	public.LeaderboardHandler(s, i)
-func SlvlHandler(s *discordgo.Session, i *discordgo.InteractionCreate)        { config.SlvlHandler(s, i) }
+}
-
+func RewardsHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	public.RewardsHandler(s, i)
+}
+func SlvlHandler(s *discordgo.Session, i *discordgo.InteractionCreate) { config.SlvlHandler(s, i) }

internal/diceexpr/diceexpr.go

@@ -0,0 +1,744 @@
+package diceexpr
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"math/big"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+type Limits struct {
+	MaxExprLen        int
+	MaxRepeat         int
+	MaxDice           int
+	MaxSides          int
+	MaxAstDepth       int
+	MaxExplosionDepth int
+}
+
+func DefaultLimits() Limits {
+	return Limits{
+		MaxExprLen:        200,
+		MaxRepeat:         25,
+		MaxDice:           300,
+		MaxSides:          1000000,
+		MaxAstDepth:       64,
+		MaxExplosionDepth: 100,
+	}
+}
+
+type EvalResult struct {
+	Total int
+	Trace *Trace
+}
+
+type Trace struct {
+	// Text is a normalized display of the (non-repeat) expression.
+	Text string
+	// Rolls includes all dice rolls (including dropped ones) in encounter order.
+	Rolls []RollGroup
+}
+
+type RollGroup struct {
+	Count int
+	Sides int
+	// Dice is the per-die roll chain; each die may have multiple faces due to explosions.
+	Dice []Die
+	// KeepDrop is the modifier used (kh/kl/dh/dl). Empty if none.
+	KeepDrop string
+	KeepDropN int
+}
+
+type Die struct {
+	Faces []int
+	Kept  bool
+}
+
+type MultiResult struct {
+	Repeat int
+	Items  []EvalResult
+}
+
+func EvalMany(input string, limits Limits) (MultiResult, error) {
+	input = strings.TrimSpace(input)
+	if input == "" {
+		return MultiResult{}, errors.New("empty")
+	}
+	if limits.MaxExprLen > 0 && len(input) > limits.MaxExprLen {
+		return MultiResult{}, errors.New("too long")
+	}
+
+	repeat, expr := splitRepeat(input)
+	if repeat < 1 {
+		repeat = 1
+	}
+	if limits.MaxRepeat > 0 && repeat > limits.MaxRepeat {
+		return MultiResult{}, fmt.Errorf("repeat too large")
+	}
+
+	// Parse once, evaluate many times.
+	p := newParser(expr, limits)
+	ast, err := p.parse()
+	if err != nil {
+		return MultiResult{}, err
+	}
+	norm := formatExpr(ast)
+
+	out := MultiResult{Repeat: repeat, Items: make([]EvalResult, 0, repeat)}
+	for range repeat {
+		e := evaluator{limits: limits}
+		total, trace, err := e.eval(ast)
+		if err != nil {
+			return MultiResult{}, err
+		}
+		trace.Text = norm
+		out.Items = append(out.Items, EvalResult{Total: total, Trace: trace})
+	}
+	return out, nil
+}
+
+func splitRepeat(s string) (int, string) {
+	// Accept "R#expr" where R is positive integer.
+	// We only treat the first '#' as repeat separator if it occurs before any dice operator.
+	// This keeps "d#" or other oddities from being interpreted as repeat.
+	i := strings.IndexByte(s, '#')
+	if i <= 0 {
+		return 1, s
+	}
+	prefix := strings.TrimSpace(s[:i])
+	if prefix == "" {
+		return 1, s
+	}
+	for _, r := range prefix {
+		if !unicode.IsDigit(r) {
+			return 1, s
+		}
+	}
+	n, err := strconv.Atoi(prefix)
+	if err != nil || n <= 0 {
+		return 1, s
+	}
+	return n, strings.TrimSpace(s[i+1:])
+}
+
+// ---- Formatting ----
+
+type FormatOptions struct {
+	BoldMinMax bool
+}
+
+func DefaultFormatOptions() FormatOptions {
+	return FormatOptions{BoldMinMax: true}
+}
+
+func FormatResult(r MultiResult, opts FormatOptions) string {
+	lines := make([]string, 0, len(r.Items))
+	for _, item := range r.Items {
+		lines = append(lines, formatOne(item, opts))
+	}
+	return strings.Join(lines, "\n")
+}
+
+func formatOne(item EvalResult, opts FormatOptions) string {
+	// If there were multiple roll groups, we still show them in sequence, e.g. "[...][...] expr".
+	parts := make([]string, 0, len(item.Trace.Rolls))
+	for _, g := range item.Trace.Rolls {
+		parts = append(parts, formatRollGroup(g, opts))
+	}
+	breakdown := strings.Join(parts, " ")
+	if breakdown == "" {
+		breakdown = "[]"
+	}
+	return fmt.Sprintf("%d \u2190 %s %s", item.Total, breakdown, item.Trace.Text)
+}
+
+func formatRollGroup(g RollGroup, opts FormatOptions) string {
+	items := make([]string, 0, len(g.Dice))
+	for _, d := range g.Dice {
+		items = append(items, formatDie(d, g.Sides, opts))
+	}
+	return "[" + strings.Join(items, ", ") + "]"
+}
+
+func formatDie(d Die, sides int, opts FormatOptions) string {
+	// Chain faces with '+' if exploded.
+	faceParts := make([]string, 0, len(d.Faces))
+	for _, v := range d.Faces {
+		faceParts = append(faceParts, formatFace(v, sides, opts))
+	}
+	txt := strings.Join(faceParts, "+")
+	if len(d.Faces) > 1 {
+		// denote explosion occurred (kept compact)
+		txt = txt + "!"
+	}
+	if !d.Kept {
+		return "~~" + txt + "~~"
+	}
+	return txt
+}
+
+func formatFace(v, sides int, opts FormatOptions) string {
+	if !opts.BoldMinMax {
+		return strconv.Itoa(v)
+	}
+	if v == 1 || v == sides {
+		return "**" + strconv.Itoa(v) + "**"
+	}
+	return strconv.Itoa(v)
+}
+
+// ---- AST / Parser / Evaluator ----
+
+type nodeKind int
+
+const (
+	kindNumber nodeKind = iota
+	kindUnary
+	kindBinary
+	kindDice
+)
+
+type node struct {
+	kind nodeKind
+
+	// number
+	num int
+
+	// unary
+	unOp byte
+	unA  *node
+
+	// binary
+	binOp byte
+	binL  *node
+	binR  *node
+
+	// dice
+	diceCount *node
+	diceSides *node
+	explode   bool
+	keepDrop  string // kh/kl/dh/dl
+	keepN     *node
+}
+
+type parser struct {
+	s      string
+	i      int
+	limits Limits
+	depth  int
+}
+
+func newParser(s string, limits Limits) *parser {
+	return &parser{s: strings.TrimSpace(s), limits: limits}
+}
+
+func (p *parser) parse() (*node, error) {
+	p.skipSpaces()
+	n, err := p.parseExpr()
+	if err != nil {
+		return nil, err
+	}
+	p.skipSpaces()
+	if p.i != len(p.s) {
+		return nil, errors.New("trailing input")
+	}
+	return n, nil
+}
+
+func (p *parser) parseExpr() (*node, error) {
+	// expr = term {( + | - ) term}
+	left, err := p.parseTerm()
+	if err != nil {
+		return nil, err
+	}
+	for {
+		p.skipSpaces()
+		if p.peek() != '+' && p.peek() != '-' {
+			return left, nil
+		}
+		op := p.next()
+		right, err := p.parseTerm()
+		if err != nil {
+			return nil, err
+		}
+		left = &node{kind: kindBinary, binOp: op, binL: left, binR: right}
+	}
+}
+
+func (p *parser) parseTerm() (*node, error) {
+	// term = factor {( * | / ) factor}
+	left, err := p.parseFactor()
+	if err != nil {
+		return nil, err
+	}
+	for {
+		p.skipSpaces()
+		if p.peek() != '*' && p.peek() != '/' {
+			return left, nil
+		}
+		op := p.next()
+		right, err := p.parseFactor()
+		if err != nil {
+			return nil, err
+		}
+		left = &node{kind: kindBinary, binOp: op, binL: left, binR: right}
+	}
+}
+
+func (p *parser) parseFactor() (*node, error) {
+	// factor = unary
+	return p.parseUnary()
+}
+
+func (p *parser) parseUnary() (*node, error) {
+	p.skipSpaces()
+	if p.peek() == '+' || p.peek() == '-' {
+		op := p.next()
+		a, err := p.parseUnary()
+		if err != nil {
+			return nil, err
+		}
+		return &node{kind: kindUnary, unOp: op, unA: a}, nil
+	}
+	return p.parsePrimaryOrDice()
+}
+
+func (p *parser) parsePrimaryOrDice() (*node, error) {
+	p.skipSpaces()
+	if p.peek() == '(' {
+		p.next()
+		p.depth++
+		if p.limits.MaxAstDepth > 0 && p.depth > p.limits.MaxAstDepth {
+			return nil, errors.New("expression too deep")
+		}
+		n, err := p.parseExpr()
+		if err != nil {
+			return nil, err
+		}
+		p.skipSpaces()
+		if p.peek() != ')' {
+			return nil, errors.New("missing )")
+		}
+		p.next()
+		p.depth--
+		return n, nil
+	}
+
+	// Try parse leading number or dice with omitted count.
+	start := p.i
+	num, hasNum, err := p.tryParseInt()
+	if err != nil {
+		return nil, err
+	}
+	p.skipSpaces()
+	if p.peekLower() == 'd' {
+		// dice: [count] d sides [keep/drop] [explode]
+		var countNode *node
+		if hasNum {
+			countNode = &node{kind: kindNumber, num: num}
+		} else {
+			countNode = &node{kind: kindNumber, num: 1}
+		}
+		p.next() // d
+		sidesNode, err := p.parseSides()
+		if err != nil {
+			return nil, err
+		}
+		keepDrop, keepN, err := p.parseKeepDrop()
+		if err != nil {
+			return nil, err
+		}
+		explode := false
+		p.skipSpaces()
+		if p.peek() == '!' {
+			explode = true
+			p.next()
+		}
+		return &node{
+			kind:      kindDice,
+			diceCount: countNode,
+			diceSides: sidesNode,
+			explode:   explode,
+			keepDrop:  keepDrop,
+			keepN:     keepN,
+		}, nil
+	}
+
+	// Not a dice; if we had a number, return number; else error.
+	if hasNum {
+		return &node{kind: kindNumber, num: num}, nil
+	}
+	p.i = start
+	return nil, errors.New("expected number, dice, or (")
+}
+
+func (p *parser) parseSides() (*node, error) {
+	p.skipSpaces()
+	if p.peek() == '%' {
+		p.next()
+		return &node{kind: kindNumber, num: 100}, nil
+	}
+	n, ok, err := p.tryParseInt()
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, errors.New("expected sides")
+	}
+	return &node{kind: kindNumber, num: n}, nil
+}
+
+func (p *parser) parseKeepDrop() (string, *node, error) {
+	p.skipSpaces()
+	// khN klN dhN dlN
+	if p.peekLower() != 'k' && p.peekLower() != 'd' {
+		return "", nil, nil
+	}
+	c1 := p.peekLower()
+	if c1 != 'k' && c1 != 'd' {
+		return "", nil, nil
+	}
+	if p.i+1 >= len(p.s) {
+		return "", nil, nil
+	}
+	c2 := byte(unicode.ToLower(rune(p.s[p.i+1])))
+	if c2 != 'h' && c2 != 'l' {
+		return "", nil, nil
+	}
+	op := string([]byte{c1, c2})
+	p.i += 2
+	n, ok, err := p.tryParseInt()
+	if err != nil {
+		return "", nil, err
+	}
+	if !ok || n <= 0 {
+		return "", nil, errors.New("expected keep/drop count")
+	}
+	return op, &node{kind: kindNumber, num: n}, nil
+}
+
+func (p *parser) tryParseInt() (int, bool, error) {
+	p.skipSpaces()
+	if p.i >= len(p.s) || !unicode.IsDigit(rune(p.s[p.i])) {
+		return 0, false, nil
+	}
+	start := p.i
+	for p.i < len(p.s) && unicode.IsDigit(rune(p.s[p.i])) {
+		p.i++
+	}
+	v, err := strconv.Atoi(p.s[start:p.i])
+	if err != nil {
+		return 0, false, err
+	}
+	return v, true, nil
+}
+
+func (p *parser) skipSpaces() {
+	for p.i < len(p.s) && unicode.IsSpace(rune(p.s[p.i])) {
+		p.i++
+	}
+}
+
+func (p *parser) peek() byte {
+	if p.i >= len(p.s) {
+		return 0
+	}
+	return p.s[p.i]
+}
+
+func (p *parser) peekLower() byte {
+	if p.i >= len(p.s) {
+		return 0
+	}
+	return byte(unicode.ToLower(rune(p.s[p.i])))
+}
+
+func (p *parser) next() byte {
+	if p.i >= len(p.s) {
+		return 0
+	}
+	b := p.s[p.i]
+	p.i++
+	return b
+}
+
+func normalizeSpaces(s string) string {
+	// Keep compact: remove spaces entirely, but preserve leading repeat already stripped outside.
+	var b strings.Builder
+	b.Grow(len(s))
+	for _, r := range s {
+		if !unicode.IsSpace(r) {
+			b.WriteRune(r)
+		}
+	}
+	return b.String()
+}
+
+// formatExpr renders a canonical expression with explicit dice counts and
+// spaces around binary operators.
+func formatExpr(n *node) string {
+	return formatExprPrec(n, 0)
+}
+
+func formatExprPrec(n *node, parentPrec int) string {
+	switch n.kind {
+	case kindNumber:
+		return strconv.Itoa(n.num)
+	case kindUnary:
+		inner := formatExprPrec(n.unA, 3)
+		if n.unOp == '+' {
+			return inner
+		}
+		return string(n.unOp) + inner
+	case kindDice:
+		// count and sides are currently numbers, but we keep it generic.
+		count := formatExprPrec(n.diceCount, 4)
+		sides := formatExprPrec(n.diceSides, 4)
+		out := count + "d" + sides
+		if n.keepDrop != "" && n.keepN != nil {
+			out += n.keepDrop + formatExprPrec(n.keepN, 4)
+		}
+		if n.explode {
+			out += "!"
+		}
+		return out
+	case kindBinary:
+		prec := binPrec(n.binOp)
+		l := formatExprPrec(n.binL, prec)
+		r := formatExprPrec(n.binR, prec+1)
+		out := l + " " + string(n.binOp) + " " + r
+		if prec < parentPrec {
+			return "(" + out + ")"
+		}
+		return out
+	default:
+		return ""
+	}
+}
+
+func binPrec(op byte) int {
+	switch op {
+	case '+', '-':
+		return 1
+	case '*', '/':
+		return 2
+	default:
+		return 0
+	}
+}
+
+type evaluator struct {
+	limits Limits
+	dice   int
+}
+
+func (e *evaluator) eval(n *node) (int, *Trace, error) {
+	t := &Trace{}
+	v, err := e.evalNode(n, t)
+	if err != nil {
+		return 0, nil, err
+	}
+	return v, t, nil
+}
+
+func (e *evaluator) evalNode(n *node, t *Trace) (int, error) {
+	switch n.kind {
+	case kindNumber:
+		return n.num, nil
+	case kindUnary:
+		v, err := e.evalNode(n.unA, t)
+		if err != nil {
+			return 0, err
+		}
+		switch n.unOp {
+		case '+':
+			return v, nil
+		case '-':
+			return -v, nil
+		default:
+			return 0, errors.New("bad unary op")
+		}
+	case kindBinary:
+		l, err := e.evalNode(n.binL, t)
+		if err != nil {
+			return 0, err
+		}
+		r, err := e.evalNode(n.binR, t)
+		if err != nil {
+			return 0, err
+		}
+		switch n.binOp {
+		case '+':
+			return l + r, nil
+		case '-':
+			return l - r, nil
+		case '*':
+			return l * r, nil
+		case '/':
+			if r == 0 {
+				return 0, errors.New("division by zero")
+			}
+			return l / r, nil
+		default:
+			return 0, errors.New("bad binary op")
+		}
+	case kindDice:
+		return e.evalDice(n, t)
+	default:
+		return 0, errors.New("unknown node")
+	}
+}
+
+func (e *evaluator) evalDice(n *node, t *Trace) (int, error) {
+	count, err := e.evalNode(n.diceCount, t)
+	if err != nil {
+		return 0, err
+	}
+	sides, err := e.evalNode(n.diceSides, t)
+	if err != nil {
+		return 0, err
+	}
+	if count <= 0 || sides <= 0 {
+		return 0, errors.New("invalid dice")
+	}
+	if e.limits.MaxDice > 0 && e.dice+count > e.limits.MaxDice {
+		return 0, errors.New("too many dice")
+	}
+	if e.limits.MaxSides > 0 && sides > e.limits.MaxSides {
+		return 0, errors.New("sides too large")
+	}
+	e.dice += count
+
+	keepDrop := n.keepDrop
+	keepN := 0
+	if n.keepN != nil {
+		keepN, err = e.evalNode(n.keepN, t)
+		if err != nil {
+			return 0, err
+		}
+	}
+	if keepDrop != "" && keepN <= 0 {
+		return 0, errors.New("invalid keep/drop")
+	}
+	if keepDrop != "" && keepN > count {
+		keepN = count
+	}
+
+	g := RollGroup{
+		Count:     count,
+		Sides:     sides,
+		Dice:      make([]Die, 0, count),
+		KeepDrop:  keepDrop,
+		KeepDropN: keepN,
+	}
+
+	type dieScore struct {
+		idx   int
+		score int
+	}
+	scores := make([]dieScore, 0, count)
+
+	// Roll base dice.
+	for i := 0; i < count; i++ {
+		faces, err := rollExploding(sides, n.explode, e.limits.MaxExplosionDepth)
+		if err != nil {
+			return 0, err
+		}
+		sum := 0
+		for _, v := range faces {
+			sum += v
+		}
+		g.Dice = append(g.Dice, Die{Faces: faces, Kept: true})
+		scores = append(scores, dieScore{idx: i, score: sum})
+	}
+
+	// Apply keep/drop to base dice by comparing chain sums.
+	if keepDrop != "" {
+		sort.SliceStable(scores, func(i, j int) bool { return scores[i].score < scores[j].score })
+
+		kept := make(map[int]bool, count)
+		switch keepDrop {
+		case "kh":
+			for i := len(scores) - keepN; i < len(scores); i++ {
+				if i >= 0 && i < len(scores) {
+					kept[scores[i].idx] = true
+				}
+			}
+		case "kl":
+			for i := 0; i < keepN && i < len(scores); i++ {
+				kept[scores[i].idx] = true
+			}
+		case "dh":
+			// drop highest N => keep all except highest N
+			for i := 0; i < len(scores)-keepN; i++ {
+				if i >= 0 && i < len(scores) {
+					kept[scores[i].idx] = true
+				}
+			}
+		case "dl":
+			// drop lowest N => keep highest count-N
+			for i := keepN; i < len(scores); i++ {
+				kept[scores[i].idx] = true
+			}
+		default:
+			return 0, errors.New("unknown keep/drop")
+		}
+		for i := range g.Dice {
+			g.Dice[i].Kept = kept[i]
+		}
+	}
+
+	// Sum kept dice.
+	total := 0
+	for _, d := range g.Dice {
+		if !d.Kept {
+			continue
+		}
+		for _, v := range d.Faces {
+			total += v
+		}
+	}
+
+	t.Rolls = append(t.Rolls, g)
+	return total, nil
+}
+
+func rollExploding(sides int, explode bool, maxDepth int) ([]int, error) {
+	v, err := roll1(sides)
+	if err != nil {
+		return nil, err
+	}
+	out := []int{v}
+	if !explode {
+		return out, nil
+	}
+	depth := 0
+	for v == sides {
+		depth++
+		if maxDepth > 0 && depth > maxDepth {
+			return nil, errors.New("explosion depth limit")
+		}
+		v, err = roll1(sides)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, v)
+	}
+	return out, nil
+}
+
+func roll1(sides int) (int, error) {
+	if sides <= 0 {
+		return 0, errors.New("bad sides")
+	}
+	n, err := rand.Int(rand.Reader, big.NewInt(int64(sides)))
+	if err != nil {
+		return 0, err
+	}
+	return int(n.Int64()) + 1, nil
+}
+