Refactor bitcoin exchange rate fetching and wallet

backend/spec/billing.md

@@ -2,20 +2,20 @@
 
 Billing encapsulates logic related to synchronizing state with Stripe, processing payments via NWC, and managing subscription lifecycle.
 
-It owns the domain logic only: every raw Stripe REST call goes through the `Stripe` wrapper (see `spec/stripe.md`), and every Lightning (NWC) wallet operation and the fiat-minor-units → millisatoshi conversion go through the helpers in `spec/bitcoin.md`.
+It owns the domain logic only: Stripe REST calls go through `Stripe` (see `spec/stripe.md`), NWC wallet operations through `Wallet` (see `spec/wallet.md`), and fiat → msats conversion through `bitcoin` (see `spec/bitcoin.md`).
 
 Members:
 
-- `stripe: Stripe` - thin wrapper around the Stripe REST API (see `spec/stripe.md`), built from `STRIPE_SECRET_KEY` / `STRIPE_WEBHOOK_SECRET`
+- `stripe: Stripe` - Stripe REST wrapper (see `spec/stripe.md`)
-- `bitcoin: Bitcoin` - the Bitcoin-facing config: system NWC wallet (`NWC_URL`) plus the BTC price-feed HTTP client (see `spec/bitcoin.md`)
+- `wallet: Wallet` - the system NWC wallet, used to issue and look up bolt11 invoices (see `spec/wallet.md`)
 - `query: Query`
 - `command: Command`
 - `robot: Robot`
 
 ## `pub fn new(query: Query, command: Command, robot: Robot) -> Self`
 
-- Builds `stripe` via `Stripe::from_env()` and `bitcoin` via `Bitcoin::from_env()`
+- Builds `stripe` via `Stripe::from_env()` and `wallet` from `NWC_URL`
-- Panics if `STRIPE_SECRET_KEY` or `STRIPE_WEBHOOK_SECRET` is missing/empty (`NWC_URL` is optional)
+- Panics if `STRIPE_SECRET_KEY`, `STRIPE_WEBHOOK_SECRET`, or `NWC_URL` is missing or malformed
 
 ## `pub fn start(&self)`
 
@@ -72,7 +72,7 @@ Stripe uses **pay-in-advance** by default: when a subscription is first created,
 
 ## `pub async fn reconcile_manual_lightning_invoice(&self, invoice_id: &str, invoice: &Value) -> Result<Value, InvoiceLookupError>`
 
-If `invoice.status == "open"` and a manual-Lightning bolt11 was previously issued for it (`query.get_invoice_manual_lightning_bolt11`), check whether that bolt11 has settled (`bitcoin.system_wallet().invoice_settled(...)`). If it has, mark the Stripe invoice paid out of band (`stripe.pay_invoice_out_of_band`) and return the refreshed invoice. On any lookup/settlement failure, log and return the invoice unchanged.
+If `invoice.status == "open"` and a manual-Lightning bolt11 was previously issued for it (`query.get_invoice_manual_lightning_bolt11`), check whether that bolt11 has settled (`self.wallet.is_settled(...)`). If it has, mark the Stripe invoice paid out of band (`stripe.pay_invoice_out_of_band`) and return the refreshed invoice. On any lookup/settlement failure, log and return the invoice unchanged.
 
 ## `pub async fn get_or_create_manual_lightning_bolt11(&self, invoice_id: &str, tenant_pubkey: &str, amount_due_minor: i64, currency: &str) -> Result<String>`
 
@@ -81,8 +81,8 @@ If `invoice.status == "open"` and a manual-Lightning bolt11 was previously issue
 
 ## `pub async fn create_bolt11(&self, amount_due_minor: i64, currency: &str) -> Result<String>`
 
-- Converts the fiat amount to msats via `bitcoin.fiat_minor_to_msats` (fetches the live BTC spot price — see `spec/bitcoin.md`)
+- Converts the fiat amount to msats via `bitcoin::fiat_to_msats` (fetches the live BTC spot price — see `spec/bitcoin.md`)
-- Issues a bolt11 invoice for that amount on the system NWC wallet (`bitcoin.system_wallet().make_invoice(...)`)
+- Issues a bolt11 invoice for that amount on the system NWC wallet (`self.wallet.make_invoice(...)`)
 - Returns the bolt11 invoice string
 
 ## `pub async fn pay_outstanding_nwc_invoices(&self, tenant: &Tenant) -> Result<()>`
@@ -111,7 +111,7 @@ Attempts Stripe-side collection for open invoices when the tenant has a card on
 Attempts to pay a new subscription invoice. Because Stripe defaults to pay-in-advance, this webhook fires immediately when a subscription is created (i.e. when a paid relay is added or a plan is upgraded). Payment priority:
 
 1. **NWC auto-pay**: If the tenant has a `nwc_url`, run `nwc_pay_invoice` (decrypting the tenant's stored `nwc_url` first):
-   - The system wallet (`bitcoin.system_wallet()`) issues a bolt11 invoice for the fiat amount; the tenant's wallet (`Wallet::parse` of the decrypted URL) pays it. A `pending` row in `invoice_nwc_payment` guards against double-charging across retries.
+   - The system wallet (`self.wallet`) issues a bolt11 invoice for the fiat amount; the tenant's wallet (`Wallet::from_url` of the decrypted URL) pays it. A `pending` row in `invoice_nwc_payment` guards against double-charging across retries.
    - If payment succeeds: mark the Stripe invoice paid out of band (`stripe.pay_invoice_out_of_band`) and clear `nwc_error` via `command.clear_tenant_nwc_error`. Done.
    - If it fails before any charge could have gone out: set `nwc_error` on the tenant via `command.set_tenant_nwc_error`, and fall through to the next option (carrying a short summary of the error into the eventual DM).
    - If it fails after a charge may have gone out (needs reconciliation): set `nwc_error` and return the error without falling through — a human must reconcile before any retry.

backend/spec/bitcoin.md

@@ -1,62 +1,11 @@
-# `bitcoin` — Bitcoin / Lightning helpers
+# `bitcoin` — fiat ↔ Bitcoin conversion
 
-Small wrappers around the Bitcoin-facing services the app talks to: Nostr Wallet Connect (NWC) wallets for Lightning invoices/payments, and a fiat↔BTC spot price feed, plus the fiat-minor-units → millisatoshi conversion that ties them together. The billing-specific orchestration (which wallet pays which invoice, double-charge guards, DMs, etc.) lives in `spec/billing.md`.
+Free async helpers for pricing fiat amounts in Lightning units against a live BTC spot price. The NWC wallet lives in `spec/wallet.md`; billing orchestration lives in `spec/billing.md`.
 
-## `pub struct Bitcoin`
+## `pub async fn fiat_to_msats(amount_fiat_minor: i64, currency: &str) -> Result<u64>`
 
-Owns the app's Bitcoin-facing configuration: the system NWC wallet URL (the wallet used to *receive* payments — issue and look up bolt11 invoices) and the HTTP client used for the fiat↔BTC spot price feed.
+Converts a Stripe-style minor-unit fiat amount to millisatoshis using the live BTC spot price for `currency` and Stripe's per-currency decimal exponent (most currencies 2; `JPY`/`KRW`/… 0; `BHD`/`KWD`/… 3).
 
-Members:
+## `pub async fn get_bitcoin_price(currency: &str) -> Result<f64>`
 
-- `system_nwc_url: String` - from `NWC_URL`
+Returns the current BTC spot price in `currency`, fetched from Coinbase's public spot-price endpoint.
-- `http: reqwest::Client`
-
-### `pub fn from_env() -> Self`
-
-Reads `NWC_URL` (the system / receiving wallet) and constructs a fresh `reqwest::Client`. Unlike the Stripe keys, `NWC_URL` is **optional**: if it's unset, Lightning operations fail at use time with a clear error rather than panicking at startup. This is what `Billing::new` calls.
-
-### `pub fn system_wallet(&self) -> Result<Wallet>`
-
-Returns the system `Wallet` (parsed from `system_nwc_url` with label `"system"`). Errors if `NWC_URL` is unset or malformed.
-
-### `pub async fn fiat_minor_to_msats(&self, amount_due_minor: i64, currency: &str) -> Result<u64>`
-
-Fetches the live BTC spot price (via `btc_spot_price_from_base` against Coinbase) for `currency` and converts `amount_due_minor` to millisatoshis via `fiat_minor_to_msats_from_quote`. `currency` is upper-cased before use.
-
-## `pub struct Wallet`
-
-A handle to a single NWC wallet. Each operation opens a fresh NWC connection and tears it down (`shutdown`) afterwards.
-
-Member:
-
-- `uri: NostrWalletConnectURI` - the parsed `nostr+walletconnect://…` URI
-
-### `pub fn parse(uri: &str, label: &str) -> Result<Self>`
-
-Parses an `nostr+walletconnect://` URI. `label` (e.g. `"system"` / `"tenant"`) only flavours the error message — `invalid {label} NWC URL` — so callers can tell which wallet was misconfigured.
-
-### `pub async fn make_invoice(&self, amount_msats: u64, description: &str) -> Result<String>`
-
-Issues a bolt11 invoice for `amount_msats` with the given description. Returns the bolt11 string.
-
-### `pub async fn pay_invoice(&self, bolt11: String) -> Result<()>`
-
-Pays a bolt11 invoice.
-
-### `pub async fn invoice_settled(&self, bolt11: &str) -> Result<bool>`
-
-Looks up a bolt11 invoice (previously issued by this wallet) and returns whether it has settled — true if the transaction state is `Settled` or `settled_at` is present.
-
-## `pub async fn btc_spot_price_from_base(http: &reqwest::Client, api_base: &str, currency: &str) -> Result<f64>`
-
-Fetches the BTC spot price denominated in `currency` (an ISO-4217 code) from a Coinbase-shaped API at `api_base` (`{api_base}/BTC-{currency}/spot`); production callers reach it via `Bitcoin::fiat_minor_to_msats` with the real Coinbase base, while tests can point it at a stub. Errors if the quote is missing, unparseable, or non-positive.
-
-## `pub fn fiat_minor_to_msats_from_quote(amount_due_minor: i64, currency: &str, btc_price_in_fiat: f64) -> Result<u64>`
-
-Converts a fiat amount expressed in minor units (cents, etc.) to millisatoshis, given a BTC price quote in that currency.
-
-- Errors if `amount_due_minor <= 0` or `btc_price_in_fiat <= 0`
-- Converts minor units to a major-unit amount using the currency's decimal exponent (most currencies 2; `JPY`/`KRW`/… 0; `BHD`/`KWD`/… 3 — following Stripe's currency conventions; an unrecognized or malformed code is an error)
-- `msats = (amount_fiat / btc_price_in_fiat) * 100_000_000_000`
-- Rounds **up** so we never under-charge, but snaps to the nearest integer when within `1e-6` of one to avoid floating-point artifacts at integer boundaries
-- Errors if the result is non-finite, non-positive, or exceeds `u64::MAX`

backend/spec/wallet.md

@@ -0,0 +1,23 @@
+# `pub struct Wallet`
+
+A handle to a single Nostr Wallet Connect (NWC) wallet. `Billing` holds one as its system wallet (receives — issues and looks up invoices); tenant wallets (pay invoices) are constructed ad-hoc from the decrypted `tenant.nwc_url` at the call site.
+
+Member:
+
+- `url: NostrWalletConnectURI` — the parsed `nostr+walletconnect://…` URI
+
+## `pub fn from_url(url: &str) -> Result<Self>`
+
+Parses an `nostr+walletconnect://` URI.
+
+## `pub async fn make_invoice(&self, amount_msats: u64, description: &str) -> Result<String>`
+
+Issues a bolt11 invoice for `amount_msats` and returns it.
+
+## `pub async fn pay_invoice(&self, bolt11: String) -> Result<()>`
+
+Pays a bolt11 invoice.
+
+## `pub async fn is_settled(&self, bolt11: &str) -> Result<bool>`
+
+Returns whether a bolt11 invoice (previously issued by this wallet) has settled.

backend/src/billing.rs

@@ -1,12 +1,13 @@
 use anyhow::{Result, anyhow};
 use std::collections::BTreeMap;
 
-use crate::bitcoin::{Bitcoin, Wallet};
+use crate::bitcoin;
 use crate::command::Command;
 use crate::models::{Activity, RELAY_STATUS_ACTIVE, RELAY_STATUS_DELINQUENT, Relay};
 use crate::query::Query;
 use crate::robot::Robot;
 use crate::stripe::{InvoiceLookupError, Stripe};
+use crate::wallet::Wallet;
 
 const MANUAL_LIGHTNING_PAYMENT_DM: &str = "Payment is due for your relay subscription. Please visit the application to complete a manual Lightning payment.";
 const NWC_ERROR_DM_PREFIX: &str = "NWC auto-payment failed:";
@@ -22,7 +23,7 @@ enum NwcInvoicePaymentOutcome {
 #[derive(Clone)]
 pub struct Billing {
     stripe: Stripe,
-    bitcoin: Bitcoin,
+    wallet: Wallet,
     query: Query,
     command: Command,
     robot: Robot,
@@ -32,7 +33,7 @@ impl Billing {
     pub fn new(query: Query, command: Command, robot: Robot) -> Self {
         Self {
             stripe: Stripe::from_env(),
-            bitcoin: Bitcoin::from_env(),
+            wallet: Wallet::from_url(&std::env::var("NWC_URL").unwrap_or_default()).expect("invalid NWC_URL"),
             query,
             command,
             robot,
@@ -713,12 +714,8 @@ impl Billing {
     }
 
     pub async fn create_bolt11(&self, amount_due_minor: i64, currency: &str) -> Result<String> {
-        let amount_msats = self
+        let amount_msats = bitcoin::fiat_to_msats(amount_due_minor, currency).await?;
-            .bitcoin
+        self.wallet
-            .fiat_minor_to_msats(amount_due_minor, currency)
-            .await?;
-        self.bitcoin
-            .system_wallet()?
             .make_invoice(amount_msats, LIGHTNING_INVOICE_DESCRIPTION)
             .await
     }
@@ -891,7 +888,7 @@ impl Billing {
     }
 
     async fn is_manual_lightning_invoice_settled(&self, bolt11: &str) -> Result<bool> {
-        self.bitcoin.system_wallet()?.invoice_settled(bolt11).await
+        self.wallet.is_settled(bolt11).await
     }
 
     /// Charges a Stripe invoice over Lightning: the system wallet issues a bolt11
@@ -912,20 +909,13 @@ impl Billing {
             return Ok(existing_outcome);
         }
 
-        let amount_msats = match self
+        let amount_msats = match bitcoin::fiat_to_msats(amount_due_minor, currency).await {
-            .bitcoin
+            Ok(msats) => msats,
-            .fiat_minor_to_msats(amount_due_minor, currency)
-            .await
-        {
-            Ok(amount_msats) => amount_msats,
             Err(error) => return Ok(NwcInvoicePaymentOutcome::Fallback(error)),
         };
 
-        let system_wallet = match self.bitcoin.system_wallet() {
+        let bolt11 = match self
-            Ok(wallet) => wallet,
+            .wallet
-            Err(error) => return Ok(NwcInvoicePaymentOutcome::Fallback(error)),
-        };
-        let bolt11 = match system_wallet
             .make_invoice(amount_msats, LIGHTNING_INVOICE_DESCRIPTION)
             .await
         {
@@ -933,7 +923,7 @@ impl Billing {
             Err(error) => return Ok(NwcInvoicePaymentOutcome::Fallback(error)),
         };
 
-        let tenant_wallet = match Wallet::parse(tenant_nwc_url, "tenant") {
+        let tenant_wallet = match Wallet::from_url(tenant_nwc_url) {
             Ok(wallet) => wallet,
             Err(error) => return Ok(NwcInvoicePaymentOutcome::Fallback(error)),
         };
@@ -1078,6 +1068,18 @@ mod tests {
         }
     }
 
+    #[allow(unused_unsafe)]
+    fn set_nwc_url(value: Option<&str>) {
+        match value {
+            Some(v) => unsafe { std::env::set_var("NWC_URL", v) },
+            None => unsafe { std::env::remove_var("NWC_URL") },
+        }
+    }
+
+    /// A syntactically valid NWC URI usable for `Wallet::from_env()` in tests.
+    /// The keys are random fixtures — the wallet is never actually contacted.
+    const TEST_NWC_URL: &str = "nostr+walletconnect://ef9824790df75f1f71d3fb9ffe9d8350f169df5cdd56a7a38592b407c61f4be7?relay=wss://relay.example.com&secret=baee312da88dcc52e9315e3962c2ea1bc8fdb5682a7fd6e6559084a41387e797";
+
     struct StripeSecretKeyGuard {
         previous: Option<String>,
     }
@@ -1114,6 +1116,24 @@ mod tests {
         }
     }
 
+    struct NwcUrlGuard {
+        previous: Option<String>,
+    }
+
+    impl NwcUrlGuard {
+        fn set(value: Option<&str>) -> Self {
+            let previous = std::env::var("NWC_URL").ok();
+            set_nwc_url(value);
+            Self { previous }
+        }
+    }
+
+    impl Drop for NwcUrlGuard {
+        fn drop(&mut self) {
+            set_nwc_url(self.previous.as_deref());
+        }
+    }
+
     async fn test_pool() -> SqlitePool {
         let connect_options = SqliteConnectOptions::from_str("sqlite::memory:")
             .expect("valid sqlite memory url")
@@ -1138,6 +1158,7 @@ mod tests {
         let _lock = env_lock().lock().await;
         let _secret_env = StripeSecretKeyGuard::set(None);
         let _webhook_env = StripeWebhookSecretGuard::set(Some("whsec_test_dummy"));
+        let _nwc_env = NwcUrlGuard::set(None);
 
         let pool = test_pool().await;
         let query = Query::new(pool.clone());
@@ -1171,6 +1192,7 @@ mod tests {
         let _lock = env_lock().lock().await;
         let _secret_env = StripeSecretKeyGuard::set(Some("sk_test_dummy"));
         let _webhook_env = StripeWebhookSecretGuard::set(None);
+        let _nwc_env = NwcUrlGuard::set(None);
 
         let pool = test_pool().await;
         let query = Query::new(pool.clone());
@@ -1204,6 +1226,7 @@ mod tests {
         let _lock = env_lock().lock().await;
         let _secret_env = StripeSecretKeyGuard::set(Some("sk_test_dummy"));
         let _webhook_env = StripeWebhookSecretGuard::set(Some("   "));
+        let _nwc_env = NwcUrlGuard::set(None);
 
         let pool = test_pool().await;
         let query = Query::new(pool.clone());
@@ -1237,6 +1260,7 @@ mod tests {
         let _lock = env_lock().lock().await;
         let _secret_env = StripeSecretKeyGuard::set(Some("sk_test_dummy"));
         let _webhook_env = StripeWebhookSecretGuard::set(Some("whsec_test_dummy"));
+        let _nwc_env = NwcUrlGuard::set(Some(TEST_NWC_URL));
 
         let pool = test_pool().await;
         let billing = Billing::new(

backend/src/bitcoin.rs

@@ -1,51 +1,11 @@
-//! Small wrappers around the Bitcoin-facing services this app talks to: Nostr
-//! Wallet Connect wallets (for Lightning invoices/payments) and a fiat↔BTC spot
-//! price feed, plus the fiat-minor-units → millisatoshi conversion that ties them
-//! together. The billing-specific orchestration lives in [`crate::billing`].
-
 use anyhow::{Result, anyhow};
-use nwc::prelude::{
-    LookupInvoiceRequest, LookupInvoiceResponse, MakeInvoiceRequest, NWC, NostrWalletConnectURI,
-    PayInvoiceRequest, TransactionState,
-};
 
-const COINBASE_SPOT_API: &str = "https://api.coinbase.com/v2/prices";
+pub async fn fiat_to_msats(amount_fiat_minor: i64, currency: &str) -> Result<u64> {
-/// Millisatoshis per bitcoin.
+    let price = get_bitcoin_price(&currency.to_uppercase()).await?;
-const MSATS_PER_BTC: f64 = 100_000_000_000.0;
+    let divisor = 10_f64.powi(currency_minor_exponent(currency)? as i32);
-
+    let amount_fiat = (amount_fiat_minor as f64) / divisor;
-/// Owns the app's Bitcoin-facing configuration: the system NWC wallet URL (the
+    let amount_msats = (amount_fiat / price * 100_000_000_000.0).round();
-/// wallet used to *receive* payments — issue and look up bolt11 invoices) and the
+    Ok(amount_msats as u64)
-/// HTTP client used for the fiat↔BTC spot price feed.
-#[derive(Clone)]
-pub struct Bitcoin {
-    system_nwc_url: String,
-    http: reqwest::Client,
-}
-
-impl Bitcoin {
-    /// Reads `NWC_URL` (the system / receiving wallet). Unlike the Stripe keys this
-    /// is optional: if it's unset, Lightning operations fail at use time with a
-    /// clear error rather than at startup.
-    pub fn from_env() -> Self {
-        Self {
-            system_nwc_url: std::env::var("NWC_URL").unwrap_or_default(),
-            http: reqwest::Client::new(),
-        }
-    }
-
-    /// The system wallet — issues and looks up the bolt11 invoices we want paid to
-    /// us. Errors if `NWC_URL` is unset or malformed.
-    pub fn system_wallet(&self) -> Result<Wallet> {
-        Wallet::parse(&self.system_nwc_url, "system")
-    }
-
-    /// Fetches the live BTC spot price and converts a fiat amount in minor units
-    /// (cents, etc.) to millisatoshis.
-    pub async fn fiat_minor_to_msats(&self, amount_due_minor: i64, currency: &str) -> Result<u64> {
-        let currency = currency.to_uppercase();
-        let btc_price = btc_spot_price_from_base(&self.http, COINBASE_SPOT_API, &currency).await?;
-        fiat_minor_to_msats_from_quote(amount_due_minor, &currency, btc_price)
-    }
 }
 
 #[derive(serde::Deserialize)]
@@ -58,123 +18,17 @@ struct CoinbaseSpotPriceData {
     amount: String,
 }
 
-/// A handle to a single Nostr Wallet Connect wallet. Each operation opens a fresh
+pub async fn get_bitcoin_price(currency: &str) -> Result<f64> {
-/// connection and tears it down afterwards.
+    let http = reqwest::Client::new();
-pub struct Wallet {
+    let url = format!("https://api.coinbase.com/v2/prices/BTC-{currency}/spot");
-    uri: NostrWalletConnectURI,
-}
-
-impl Wallet {
-    /// Parses an `nostr+walletconnect://` URI. `label` only flavours the error
-    /// message so callers can tell which wallet was misconfigured.
-    pub fn parse(uri: &str, label: &str) -> Result<Self> {
-        let uri = uri
-            .parse::<NostrWalletConnectURI>()
-            .map_err(|_| anyhow!("invalid {label} NWC URL"))?;
-        Ok(Self { uri })
-    }
-
-    /// Issues a bolt11 invoice for `amount_msats`.
-    pub async fn make_invoice(&self, amount_msats: u64, description: &str) -> Result<String> {
-        let nwc = NWC::new(self.uri.clone());
-        let result = nwc
-            .make_invoice(MakeInvoiceRequest {
-                amount: amount_msats,
-                description: Some(description.to_string()),
-                description_hash: None,
-                expiry: None,
-            })
-            .await;
-        nwc.shutdown().await;
-        Ok(result
-            .map_err(|e| anyhow!("failed to create invoice: {e}"))?
-            .invoice)
-    }
-
-    /// Pays a bolt11 invoice.
-    pub async fn pay_invoice(&self, bolt11: String) -> Result<()> {
-        let nwc = NWC::new(self.uri.clone());
-        let result = nwc.pay_invoice(PayInvoiceRequest::new(bolt11)).await;
-        nwc.shutdown().await;
-        result.map(|_| ()).map_err(|e| anyhow!("{e}"))
-    }
-
-    /// Returns whether a bolt11 invoice (previously issued by this wallet) has been
-    /// settled.
-    pub async fn invoice_settled(&self, bolt11: &str) -> Result<bool> {
-        let nwc = NWC::new(self.uri.clone());
-        let result = nwc
-            .lookup_invoice(LookupInvoiceRequest {
-                payment_hash: None,
-                invoice: Some(bolt11.to_string()),
-            })
-            .await;
-        nwc.shutdown().await;
-        let response = result.map_err(|e| anyhow!("failed to lookup invoice: {e}"))?;
-        Ok(lookup_invoice_response_is_settled(&response))
-    }
-}
-
-fn lookup_invoice_response_is_settled(response: &LookupInvoiceResponse) -> bool {
-    response.state == Some(TransactionState::Settled) || response.settled_at.is_some()
-}
-
-/// Fetches the BTC spot price denominated in `currency` (an ISO-4217 code) from a
-/// Coinbase-shaped API at `api_base`. Exposed so tests can stub the price feed;
-/// production callers go through [`Bitcoin::fiat_minor_to_msats`].
-pub async fn btc_spot_price_from_base(
-    http: &reqwest::Client,
-    api_base: &str,
-    currency: &str,
-) -> Result<f64> {
-    let pair = format!("BTC-{currency}");
-    let url = format!("{}/{pair}/spot", api_base.trim_end_matches('/'));
     let resp = http.get(url).send().await?;
     let body: CoinbaseSpotPriceResponse = resp.error_for_status()?.json().await?;
 
-    let amount = body
+    Ok(body
         .data
         .amount
         .parse::<f64>()
-        .map_err(|e| anyhow!("invalid BTC spot quote for {currency}: {e}"))?;
+        .map_err(|e| anyhow!("invalid BTC spot quote for {currency}: {e}"))?)
-    if amount <= 0.0 {
-        return Err(anyhow!(
-            "invalid non-positive BTC spot quote for {currency}"
-        ));
-    }
-    Ok(amount)
-}
-
-/// Converts a fiat amount expressed in minor units (cents, etc.) to millisatoshis,
-/// given a BTC price quote in that currency. Rounds up so we never under-charge,
-/// but snaps to the nearest integer when within a hair of one to avoid floating
-/// point artifacts at integer boundaries.
-pub fn fiat_minor_to_msats_from_quote(
-    amount_due_minor: i64,
-    currency: &str,
-    btc_price_in_fiat: f64,
-) -> Result<u64> {
-    if amount_due_minor <= 0 {
-        return Err(anyhow!("amount_due must be positive"));
-    }
-    if btc_price_in_fiat <= 0.0 {
-        return Err(anyhow!("btc_price_in_fiat must be positive"));
-    }
-
-    let divisor = 10_f64.powi(currency_minor_exponent(currency)? as i32);
-    let amount_fiat = (amount_due_minor as f64) / divisor;
-    let amount_btc = amount_fiat / btc_price_in_fiat;
-    let raw_msats = amount_btc * MSATS_PER_BTC;
-    let amount_msats = if (raw_msats - raw_msats.round()).abs() < 1e-6 {
-        raw_msats.round()
-    } else {
-        raw_msats.ceil()
-    };
-
-    if !amount_msats.is_finite() || amount_msats <= 0.0 || amount_msats > u64::MAX as f64 {
-        return Err(anyhow!("calculated msat amount is out of bounds"));
-    }
-    Ok(amount_msats as u64)
 }
 
 /// Number of decimal places in `currency`'s minor unit, following Stripe's
@@ -192,29 +46,3 @@ fn currency_minor_exponent(currency: &str) -> Result<u8> {
     };
     Ok(exponent)
 }
-
-#[cfg(test)]
-mod tests {
-    use super::fiat_minor_to_msats_from_quote;
-
-    #[test]
-    fn converts_usd_minor_units_with_quote() {
-        let msats = fiat_minor_to_msats_from_quote(100, "usd", 100_000.0)
-            .expect("conversion should succeed");
-        assert_eq!(msats, 1_000_000);
-    }
-
-    #[test]
-    fn converts_zero_decimal_currency_with_quote() {
-        let msats = fiat_minor_to_msats_from_quote(100, "jpy", 10_000_000.0)
-            .expect("conversion should succeed");
-        assert_eq!(msats, 1_000_000);
-    }
-
-    #[test]
-    fn rejects_malformed_currency_code() {
-        // Not three ASCII letters: rejected outright.
-        assert!(fiat_minor_to_msats_from_quote(100, "usdd", 100_000.0).is_err());
-        assert!(fiat_minor_to_msats_from_quote(100, "us1", 100_000.0).is_err());
-    }
-}

backend/src/lib.rs

@@ -9,3 +9,4 @@ pub mod pool;
 pub mod query;
 pub mod robot;
 pub mod stripe;
+pub mod wallet;

backend/src/main.rs

@@ -9,6 +9,7 @@ mod pool;
 mod query;
 mod robot;
 mod stripe;
+mod wallet;
 
 use anyhow::Result;
 use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};

backend/src/wallet.rs

@@ -0,0 +1,55 @@
+use anyhow::{Result, anyhow};
+use nwc::prelude::{
+    LookupInvoiceRequest, MakeInvoiceRequest, NWC, NostrWalletConnectURI, PayInvoiceRequest,
+    TransactionState,
+};
+
+#[derive(Clone)]
+pub struct Wallet {
+    url: NostrWalletConnectURI,
+}
+
+impl Wallet {
+    pub fn from_url(url: &str) -> Result<Self> {
+        let url = url
+            .parse::<NostrWalletConnectURI>()
+            .map_err(|_| anyhow!("invalid NWC URL"))?;
+        Ok(Self { url })
+    }
+
+    pub async fn make_invoice(&self, amount_msats: u64, description: &str) -> Result<String> {
+        let nwc = NWC::new(self.url.clone());
+        let result = nwc
+            .make_invoice(MakeInvoiceRequest {
+                amount: amount_msats,
+                description: Some(description.to_string()),
+                description_hash: None,
+                expiry: None,
+            })
+            .await;
+        nwc.shutdown().await;
+        Ok(result
+            .map_err(|e| anyhow!("failed to create invoice: {e}"))?
+            .invoice)
+    }
+
+    pub async fn pay_invoice(&self, bolt11: String) -> Result<()> {
+        let nwc = NWC::new(self.url.clone());
+        let result = nwc.pay_invoice(PayInvoiceRequest::new(bolt11)).await;
+        nwc.shutdown().await;
+        result.map(|_| ()).map_err(|e| anyhow!("{e}"))
+    }
+
+    pub async fn is_settled(&self, bolt11: &str) -> Result<bool> {
+        let nwc = NWC::new(self.url.clone());
+        let result = nwc
+            .lookup_invoice(LookupInvoiceRequest {
+                payment_hash: None,
+                invoice: Some(bolt11.to_string()),
+            })
+            .await;
+        nwc.shutdown().await;
+        let response = result.map_err(|e| anyhow!("failed to lookup invoice: {e}"))?;
+        Ok(response.state == Some(TransactionState::Settled) || response.settled_at.is_some())
+    }
+}