リアルタイム金価格でジュエリー価格計算ツールを作る

地金の価値 = 重量（g）× 1グラムあたりの金価格（24K）× 純度
加工賃     = 1グラムあたりの金額、地金の価値に対する割合、または定額
税抜価格   = 地金の価値 + 加工賃
VAT        = 税抜価格 × VAT税率
合計       = 税抜価格 + VAT

curl "https://metalapi.com/api/v1/latest?base=EUR&currencies=XAU&unit=gram" \
  -H "X-API-Key: YOUR_API_KEY"

{
  "success": true,
  "timestamp": 1792834892,
  "date": "2026-10-24",
  "base": "EUR",
  "unit": "gram",
  "rates": {
    "XAU": 0.0090909091,
    "EURXAU": 110
  },
  "rate_limit": { "limit": 10000, "remaining": 9874, "...": "..." }
}

// gold-quote.ts — Node 18以降。APIキーはサーバー上にとどまります。
const API_KEY = process.env.METALAPI_KEY!;
const CURRENCY = "EUR";
const PLAN_INTERVAL_S = 600; // Standardプラン：価格は10分ごとに更新
const MIN_TTL_S = 0;         // Liteプラン：月1,000リクエスト以内に収めるには3600に設定

interface GoldPrice {
  perGram: number;   // 純金（24K）、1グラムあたりのCURRENCY建て価格
  timestamp: number; // 価格データの時刻（Unix秒）
  fetchedAt: number;
  expiresAt: number;
}

let cached: GoldPrice | null = null;
let inflight: Promise<GoldPrice> | null = null;

// 価格はプランの更新間隔の倍数（00:00 UTCから起算）でのみ変わります。
function nextBoundary(ms: number): number {
  const step = PLAN_INTERVAL_S * 1000;
  return Math.floor(ms / step) * step + step;
}

async function fetchGold(): Promise<GoldPrice> {
  const url = `https://metalapi.com/api/v1/latest?base=${CURRENCY}&currencies=XAU&unit=gram`;
  const res = await fetch(url, { headers: { "X-API-Key": API_KEY }, signal: AbortSignal.timeout(10_000) });
  const body = await res.json();
  if (!body.success) throw new Error(`MetalAPI ${body.error.code}: ${body.error.info}`);
  const now = Date.now();
  return {
    perGram: body.rates[`${CURRENCY}XAU`],
    timestamp: body.timestamp,
    fetchedAt: now,
    expiresAt: nextBoundary(now + MIN_TTL_S * 1000) + 5_000, // 数秒の余裕を持たせる
  };
}

export function goldPerGram(): Promise<GoldPrice> {
  if (cached && Date.now() < cached.expiresAt) return Promise.resolve(cached);
  // シングルフライト：同時リクエストで1回のAPI呼び出しを共有します。
  inflight ??= fetchGold()
    .then((fresh) => (cached = fresh))
    .catch((err) => {
      // 最後に取得できた価格を最大6時間使い続け、1分後に再試行します。
      if (cached && Date.now() - cached.fetchedAt < 6 * 3600_000) {
        cached.expiresAt = Date.now() + 60_000;
        return cached;
      }
      throw err;
    })
    .finally(() => (inflight = null));
  return inflight;
}

export type MakingCharge =
  | { type: "per_gram"; amount: number }
  | { type: "percent"; rate: number } // 0.08 = 地金代の8%
  | { type: "flat"; amount: number };

const money = (n: number) => Math.round(n * 100 + Number.EPSILON) / 100;

export async function quote(grams: number, purity: number, making: MakingCharge, vatRate: number) {
  const gold = await goldPerGram();
  const metal = money(grams * gold.perGram * purity);
  const labour = money(
    making.type === "per_gram" ? grams * making.amount
      : making.type === "percent" ? metal * making.rate
      : making.amount,
  );
  const net = money(metal + labour);
  const vat = money(net * vatRate);
  return {
    currency: CURRENCY,
    grams,
    purity,
    gold_per_gram_24k: gold.perGram,
    price_time: new Date(gold.timestamp * 1000).toISOString(),
    metal,
    making: labour,
    net,
    vat,
    total: money(net + vat),
  };
}

// server.ts — npm install expressの後、METALAPI_KEY=mk_live_... npx tsx server.ts で実行
import express from "express";
import { quote } from "./gold-quote";

// 製品に刻印されている純度係数（上のカラット表を参照）。
const PURITY: Record<string, number> = { "24": 0.999, "22": 0.916, "21": 0.875, "18": 0.75, "14": 0.585, "9": 0.375 };
const app = express();

app.get("/api/jewelry-quote", async (req, res) => {
  const grams = Number(req.query.grams);
  const purity = PURITY[String(req.query.karat)];
  if (!purity || !(grams > 0 && grams <= 2000)) {
    res.status(400).json({ error: "Use karat 24, 22, 21, 18, 14 or 9 and grams between 0 and 2000." });
    return;
  }
  try {
    // 加工賃とVATはサーバー側の設定値で、クライアントの入力は受け付けません。
    res.json(await quote(grams, purity, { type: "per_gram", amount: 12 }, 0.2));
  } catch (err) {
    console.error(err);
    res.status(503).json({ error: "Gold price temporarily unavailable." });
  }
});

app.listen(3000, () => console.log("Try http://localhost:3000/api/jewelry-quote?karat=18&grams=5.2"));

{
  "currency": "EUR",
  "grams": 5.2,
  "purity": 0.75,
  "gold_per_gram_24k": 110,
  "price_time": "2026-10-24T09:41:32.000Z",
  "metal": 429,
  "making": 62.4,
  "net": 491.4,
  "vat": 98.28,
  "total": 589.68
}

# gold_quote.py — Python 3.9以降、pip install requests
import math
import os
import threading
import time
from decimal import ROUND_HALF_UP, Decimal

import requests

API_KEY = os.environ["METALAPI_KEY"]
CURRENCY = "EUR"
PLAN_INTERVAL_S = 600  # Standardプラン：価格は10分ごとに更新
MIN_TTL_S = 0          # Liteプラン：月1,000リクエスト以内に収めるには3600に設定

_lock = threading.Lock()
_cache = {"per_gram": None, "timestamp": 0, "fetched": 0.0, "expires": 0.0}


def _next_boundary(t: float) -> float:
    return (math.floor(t / PLAN_INTERVAL_S) + 1) * PLAN_INTERVAL_S


def gold_per_gram() -> tuple[Decimal, int]:
    """純金（24K）の1グラムあたりのCURRENCY建て価格と、その価格のUnix時刻。"""
    with _lock:  # API呼び出しは一度に1回。待機中のスレッドはその結果を再利用します
        now = time.time()
        if _cache["per_gram"] is not None and now < _cache["expires"]:
            return _cache["per_gram"], _cache["timestamp"]
        try:
            r = requests.get(
                "https://metalapi.com/api/v1/latest",
                params={"base": CURRENCY, "currencies": "XAU", "unit": "gram"},
                headers={"X-API-Key": API_KEY},
                timeout=10,
            )
            body = r.json()
            if not body.get("success"):
                raise RuntimeError(f"MetalAPI {body['error']['code']}: {body['error']['info']}")
        except Exception:
            if _cache["per_gram"] is not None and now - _cache["fetched"] < 6 * 3600:
                _cache["expires"] = now + 60  # 最後に取得できた価格を返し、1分後に再試行
                return _cache["per_gram"], _cache["timestamp"]
            raise
        _cache.update(
            per_gram=Decimal(str(body["rates"][f"{CURRENCY}XAU"])),
            timestamp=body["timestamp"],
            fetched=now,
            expires=_next_boundary(now + MIN_TTL_S) + 5,
        )
        return _cache["per_gram"], _cache["timestamp"]


def money(x: Decimal) -> Decimal:
    return x.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)


def quote(grams: Decimal, purity: Decimal, making_per_gram: Decimal, vat_rate: Decimal) -> dict:
    per_gram, ts = gold_per_gram()
    metal = money(grams * per_gram * purity)
    making = money(grams * making_per_gram)
    net = metal + making
    vat = money(net * vat_rate)
    return {
        "currency": CURRENCY,
        "gold_per_gram_24k": str(per_gram),
        "price_time": time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime(ts)),
        "metal": str(metal),
        "making": str(making),
        "net": str(net),
        "vat": str(vat),
        "total": str(net + vat),
    }

# app.py — pip install fastapi uvicorn requestsの後、METALAPI_KEY=mk_live_... uvicorn app:app で実行
from decimal import Decimal

from fastapi import FastAPI, HTTPException, Query

from gold_quote import quote

# 製品に刻印されている純度係数。
PURITY = {24: Decimal("0.999"), 22: Decimal("0.916"), 21: Decimal("0.875"),
          18: Decimal("0.750"), 14: Decimal("0.585"), 9: Decimal("0.375")}
app = FastAPI()


@app.get("/api/jewelry-quote")
def jewelry_quote(karat: int, grams: Decimal = Query(gt=0, le=2000)):
    if karat not in PURITY:
        raise HTTPException(400, "Use karat 24, 22, 21, 18, 14 or 9.")
    try:
        return quote(grams, PURITY[karat], making_per_gram=Decimal("12.00"), vat_rate=Decimal("0.20"))
    except Exception:
        raise HTTPException(503, "Gold price temporarily unavailable.")