Include mining fees in the Spending deposit preview

2 files changed, 80 insertions, 9 deletions

diff --git a/src/features/ark.js b/src/features/ark.js
index 6398537..730cccd 100644
--- a/src/features/ark.js
+++ b/src/features/ark.js
@@ -25,6 +25,20 @@ import { shortAddr, shortTxid, timeAgo, ARK_ICON, ARK_MARK, BITCOIN_ICON } from
 // t?ark1… bech32m — an Ark address for this or another ASP.
 export function isArkAddress(a) { return /^t?ark1[a-z0-9]{20,}$/i.test((a || '').trim()); }
 
+// Use the funding transaction's actual input selection and P2TR output size
+// for both Max and the fee preview. Mining fees are paid from Savings; only
+// the ASP service fee comes out of the amount deposited into Spending.
+export function previewBoardFunding(wallet, ark, amountSat = 0, sendMax = false) {
+  const feeRate = (wallet.feeRates && wallet.feeRates.halfHourFee) || 5;
+  const hrp = { bitcoin: 'bc', regtest: 'bcrt' }[ark?.info?.network] || 'tb';
+  const dest = ark ? p2trAddress(ark._key(0).pubkey.slice(1), hrp) : wallet.freshChange().address;
+  const draft = wallet.buildTx({ recipients: [{ address: dest, amount: amountSat }], feeRate, sendMax, noSort: true });
+  const fundingSat = draft.outputs[0].amount;
+  const serviceFeeSat = boardFee(fundingSat, ark?.info?.boardFees);
+  return { fundingSat, chainFeeSat: draft.fee, serviceFeeSat,
+    feeSat: draft.fee + serviceFeeSat, netSat: fundingSat - serviceFeeSat };
+}
+
 // Ark movement types that actually touch the chain — each is a real bitcoin
 // tx with a txid, so it wears the Bitcoin mark, not the Ark one: board pulls
 // coins in from on-chain, offboard and exit push them back out.
@@ -2743,12 +2757,7 @@ export function arkFeature(ctx) {
   // key stands in for the funding key — same size, always a valid point.
   function maxBoardSat() {
     try {
-      const feeRate = (wallet.feeRates && wallet.feeRates.halfHourFee) || 5;
-      const hrp = { bitcoin: 'bc', regtest: 'bcrt' }[ark && ark.info && ark.info.network] || 'tb';
-      // _key() hands back a compressed point; the witness program is x-only
-      const dest = ark ? p2trAddress(ark._key(0).pubkey.slice(1), hrp) : wallet.freshChange().address;
-      const draft = wallet.buildTx({ recipients: [{ address: dest, amount: 0 }], feeRate, sendMax: true });
-      return Math.max(0, (draft.outputs[0]?.amount || 0));
+      return Math.max(0, previewBoardFunding(wallet, ark, 0, true).fundingSat);
     } catch {
       return 0;
     }
@@ -2778,9 +2787,16 @@ export function arkFeature(ctx) {
       (() => {
         const sats = parseInt((ui.arkBoardAmt || '').trim(), 10);
         if (!sats || sats < minBoard || !ark) return null;
-        const fee = boardFee(sats, ark.info.boardFees);
-        return h('div', { class: 'small muted', style: 'text-align:center' },
-          t('arkBoardFeeNote', { fee: fmtAmount(fee), net: fmtAmount(sats - fee) }));
+        try {
+          const quote = previewBoardFunding(wallet, ark, sats);
+          return h('div', { class: 'col small muted', style: 'text-align:center;gap:4px' },
+            t('arkBoardFeeNote', { fee: quote.feeSat.toLocaleString(), net: quote.netSat.toLocaleString() }),
+            h('div', { class: 'small faint' },
+              t('arkFeeChain') + ': ' + fmtAmount(quote.chainFeeSat) + ' ' + unitLabel() + ' · '
+              + t('arkFeeService') + ': ' + fmtAmount(quote.serviceFeeSat) + ' ' + unitLabel()));
+        } catch (e) {
+          return h('div', { class: 'notice err small' }, e.message);
+        }
       })(),
       canBoard
         ? h('div', { class: 'small faint', style: 'text-align:center' },
diff --git a/tools/ark/board-fee-preview-test.js b/tools/ark/board-fee-preview-test.js
new file mode 100644
index 0000000..0ff96cd
--- /dev/null
+++ b/tools/ark/board-fee-preview-test.js
@@ -0,0 +1,55 @@
+// Fee previews use the real wallet transaction builder, without signing or
+// broadcasting anything. Run: bun tools/ark/board-fee-preview-test.js
+import assert from 'node:assert/strict';
+import { generateMnemonic } from '@scure/bip39';
+import { wordlist } from '@scure/bip39/wordlists/english';
+import { Wallet } from '../../src/wallet.js';
+import { previewBoardFunding } from '../../src/features/ark.js';
+import { p2trAddress } from '../../src/ark/board.js';
+
+const wallet = new Wallet();
+wallet.load({ mnemonic: generateMnemonic(wordlist), netName: 'mainnet', offline: true });
+wallet.utxos = [{ txid: 'a'.repeat(64), vout: 0, value: 300000, chain: 0, index: 0, confirmed: true }];
+wallet.feeRates = { halfHourFee: 1 };
+const ark = { info: { network: 'bitcoin', boardFees: {} }, _key: () => ({ pubkey: wallet.derive(0, 0).pubkey }) };
+// A different P2TR program stands in for the final funding address. Its
+// output size matches the preview while its key is unrelated to the probe.
+const fundingAddress = p2trAddress(wallet.derive(0, 1).pubkey.slice(1), 'bc');
+const actualDraft = sats => wallet.buildTx({ recipients: [{ address: fundingAddress, amount: sats }],
+  feeRate: wallet.feeRates.halfHourFee, noSort: true });
+
+const max = previewBoardFunding(wallet, ark, 0, true);
+assert.deepEqual(max, { fundingSat: 299878, chainFeeSat: 122, serviceFeeSat: 0, feeSat: 122, netSat: 299878 });
+const maxPreview = previewBoardFunding(wallet, ark, max.fundingSat);
+assert.deepEqual(maxPreview, max);
+assert.equal(maxPreview.chainFeeSat, actualDraft(max.fundingSat).fee);
+assert.equal(maxPreview.netSat + maxPreview.feeSat, 300000);
+console.log('✓ 300,000-sat Max shows a 122-sat fee and 299,878 sats arriving');
+
+const partial = previewBoardFunding(wallet, ark, 100000);
+assert.equal(partial.netSat, 100000, 'the mining fee is paid from Savings, not deducted again from the deposit');
+assert.equal(partial.chainFeeSat, actualDraft(100000).fee);
+assert.ok(partial.chainFeeSat > max.chainFeeSat, 'partial deposits include the change output fee');
+console.log('✓ partial deposits include the change output and preserve the correct received amount');
+
+ark.info.boardFees = { baseFeeSat: 25, ppm: 1000 };
+const service = previewBoardFunding(wallet, ark, max.fundingSat);
+assert.equal(service.serviceFeeSat, 325);
+assert.equal(service.feeSat, 447);
+assert.equal(service.netSat, 299553);
+assert.equal(service.netSat + service.feeSat, 300000);
+console.log('✓ nonzero service fees are included in the total and deducted exactly once');
+
+ark.info.boardFees = {};
+wallet.utxos = [
+  { txid: 'a'.repeat(64), vout: 0, value: 200000, chain: 0, index: 0, confirmed: true },
+  { txid: 'b'.repeat(64), vout: 0, value: 100000, chain: 0, index: 1, confirmed: true },
+];
+wallet.feeRates.halfHourFee = 1.6;
+const multi = previewBoardFunding(wallet, ark, 0, true);
+assert.equal(multi.chainFeeSat, actualDraft(multi.fundingSat).fee);
+assert.equal(multi.netSat + multi.feeSat, 300000);
+assert.ok(multi.feeSat > max.feeSat);
+assert.throws(() => previewBoardFunding(wallet, ark, 300000), /Insufficient funds/);
+console.log('✓ multiple inputs, fee-rate rounding, and insufficient funds follow the actual funding transaction');
+process.exit(0);