stonks/internal/output/discord.go

package output

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"os"
	"strings"
	"time"

	"gitea.konchin.com/service/stonks/internal/config"
	"gitea.konchin.com/service/stonks/internal/report"
)

func init() {
	Register("discord", newDiscordOutput)
}

// discordMessageLimit is Discord's max character count for a single
// webhook message's `content` field.
const discordMessageLimit = 2000

// discordParams is the plugin-specific config for the discord output.
type discordParams struct {
	// WebhookEnvVar names the environment variable holding the Discord
	// webhook URL. Defaults to DISCORD_WEBHOOK_URL.
	WebhookEnvVar string `yaml:"webhook_env_var"`
}

// discordOutput posts the report to a Discord incoming webhook, splitting it
// into multiple messages if it exceeds Discord's 2000-character limit.
type discordOutput struct {
	webhookURL string
	client     *http.Client
}

func newDiscordOutput(cfg config.PluginConfig) (Output, error) {
	params := discordParams{WebhookEnvVar: "DISCORD_WEBHOOK_URL"}
	if err := cfg.DecodeParams(&params); err != nil {
		return nil, err
	}

	url := os.Getenv(params.WebhookEnvVar)
	if url == "" {
		return nil, fmt.Errorf("discord: environment variable %q is not set", params.WebhookEnvVar)
	}

	return &discordOutput{
		webhookURL: url,
		client:     &http.Client{Timeout: 15 * time.Second},
	}, nil
}

func (d *discordOutput) Name() string { return "discord" }

func (d *discordOutput) Send(ctx context.Context, rep report.Report) error {
	for i, chunk := range splitForDiscord(rep.Text, discordMessageLimit) {
		if err := d.post(ctx, chunk); err != nil {
			return fmt.Errorf("discord: send chunk %d: %w", i+1, err)
		}
	}

	if embeds := buildRecommendationEmbeds(rep.Recommendations); len(embeds) > 0 {
		if err := d.postEmbeds(ctx, embeds); err != nil {
			return fmt.Errorf("discord: send recommendation embeds: %w", err)
		}
	}
	return nil
}

func (d *discordOutput) post(ctx context.Context, content string) error {
	return d.postJSON(ctx, map[string]any{"content": content})
}

func (d *discordOutput) postEmbeds(ctx context.Context, embeds []map[string]any) error {
	return d.postJSON(ctx, map[string]any{"embeds": embeds})
}

func (d *discordOutput) postJSON(ctx context.Context, payload map[string]any) error {
	body, err := json.Marshal(payload)
	if err != nil {
		return err
	}

	req, err := http.NewRequestWithContext(ctx, http.MethodPost, d.webhookURL, bytes.NewReader(body))
	if err != nil {
		return err
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := d.client.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode >= 300 {
		respBody, _ := io.ReadAll(resp.Body)
		return fmt.Errorf("unexpected status %d: %s", resp.StatusCode, string(respBody))
	}
	return nil
}

// Discord embed colors (decimal RGB).
const (
	colorBuy  = 0x57F287 // green
	colorSell = 0xED4245 // red
)

// discordDescriptionLimit is Discord's max character count for a single
// embed's `description`.
const discordDescriptionLimit = 4096

// buildRecommendationEmbeds renders the buy and sell recommendations as
// separate Discord embeds, one per non-empty list. If there are no
// recommendations at all, it returns nil and no message is sent — an empty
// buy/sell call isn't worth a Discord notification.
func buildRecommendationEmbeds(recs []report.Recommendation) []map[string]any {
	var buyLines, sellLines []string
	for _, r := range recs {
		line := fmt.Sprintf("**%s** — %s", r.Symbol, r.Reason)
		if r.Action == "sell" {
			sellLines = append(sellLines, line)
		} else {
			buyLines = append(buyLines, line)
		}
	}

	var embeds []map[string]any
	if len(buyLines) > 0 {
		embeds = append(embeds, recommendationEmbed("🟢 Buy", colorBuy, buyLines))
	}
	if len(sellLines) > 0 {
		embeds = append(embeds, recommendationEmbed("🔴 Sell", colorSell, sellLines))
	}
	return embeds
}

func recommendationEmbed(title string, color int, lines []string) map[string]any {
	return map[string]any{
		"title":       title,
		"color":       color,
		"description": truncateForDescription(strings.Join(lines, "\n")),
	}
}

func truncateForDescription(s string) string {
	if len(s) <= discordDescriptionLimit {
		return s
	}
	return s[:discordDescriptionLimit-3] + "..."
}

// splitForDiscord breaks text into chunks of at most limit characters,
// preferring to split on paragraph/line boundaries so Markdown formatting
// stays intact across messages.
func splitForDiscord(text string, limit int) []string {
	if len(text) <= limit {
		return []string{text}
	}

	var chunks []string
	var current strings.Builder

	flush := func() {
		if current.Len() > 0 {
			chunks = append(chunks, strings.TrimSpace(current.String()))
			current.Reset()
		}
	}

	for _, line := range strings.Split(text, "\n") {
		// A single line longer than the limit must be hard-split.
		for len(line) > limit {
			if current.Len() > 0 {
				flush()
			}
			chunks = append(chunks, line[:limit])
			line = line[limit:]
		}
		if current.Len()+len(line)+1 > limit {
			flush()
		}
		current.WriteString(line)
		current.WriteString("\n")
	}
	flush()

	return chunks
}