Boot: price the Ark exit advisory by size only — no more 5s of secp256k1 on load.

The exit-depth advisory rebuilt every spendable coin's exit chain to learn
its fee, and each hop's input taproot key ran a musig key aggregation
(several secp256k1 multiplies on uncached points). With 27 coins of depth
up to 86 (1685 hops) that was ~5.3s of solid main-thread work on every
boot, laptop and phone alike — requestIdleCallback only delayed it, and
the Android wrapper hits a full boot whenever the OS evicts the app, so
"reopening the app" froze for seconds.

A fee only needs vsizes, and a taproot output script is 34 bytes whatever
key it holds: vtxoTransactions/transitionInputTxout gain a sizeOnly mode
with stand-in keys, exit.js exposes exitTxVsizes (same chain checks, no
EC, nothing broadcastable), and the advisory + coins page use it. Verified
against the real signed chain on the live coin set: identical vsizes on
all 27 coins, 9s → 26ms.

Also: the sync-slot replay at boot merged the ark snapshot and rewrote
~1MB of state (plus re-adopting it into the manager) twice per boot even
when the union brought nothing. Compare the serialized union to what's
stored and skip the write/adopt when unchanged.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Nw1Dijik1tLVKRM1WkU7xE

2 files changed, 41 insertions, 11 deletions

diff --git a/src/ark/exit.js b/src/ark/exit.js
index e2b253f..616c6a1 100644
--- a/src/ark/exit.js
+++ b/src/ark/exit.js
@@ -18,7 +18,7 @@ import { schnorr, secp256k1 } from '@noble/curves/secp256k1';
 
 import { concatBytes } from './proto.js';
 import { pubkeyPolicyTaproot, txid } from './send.js';
-import { vtxoTransactions, harkLeafTaproot, tapLeafHash, taprootScriptSighash } from './refresh.js';
+import { vtxoTransactions, harkLeafTaproot, harkUnlockWitnessShape, tapLeafHash, taprootScriptSighash } from './refresh.js';
 
 const u16le = (n) => Uint8Array.of(n & 0xff, (n >> 8) & 0xff);
 const u32le = (n) => Uint8Array.of(n & 0xff, (n >> 8) & 0xff, (n >> 16) & 0xff, (n >>> 24) & 0xff);
@@ -57,8 +57,16 @@ export function serializeTxW(tx, witnesses) {
 
 // The vtxo's chain as broadcastable transactions, oldest first.
 // Throws if any transition lacks its signature (an unsigned chain can't exit).
-export function signedExitTxs(vtxo, serverPub) {
-  const items = vtxoTransactions(vtxo, serverPub);
+export const signedExitTxs = (vtxo, serverPub) => exitChain(vtxo, serverPub, false);
+
+// Just the vsize of each hop's transaction, with the same "can this chain
+// exit at all" checks — but no elliptic-curve work: the output keys are
+// stand-ins of the right length. For fee estimates only; nothing here is
+// broadcastable.
+export const exitTxVsizes = (vtxo, serverPub) => exitChain(vtxo, serverPub, true).map((t) => t.vsize);
+
+function exitChain(vtxo, serverPub, sizeOnly) {
+  const items = vtxoTransactions(vtxo, serverPub, sizeOnly);
   return items.map((it, i) => {
     const t = vtxo.genesis[i].transition;
     let witness;
@@ -70,14 +78,20 @@ export function signedExitTxs(vtxo, serverPub) {
         throw new Error('vtxo chain is missing its unlock preimage — cannot exit');
       }
       const preimage = hex.decode(t.unlock.preimage);
-      const tp = harkLeafTaproot(hex.decode(t.userPubkey), serverPub, vtxo.expiryHeight, sha256(preimage));
-      const cb = concatBytes(Uint8Array.of(0xc0 | tp.outputParity), tp.internalXOnly, tp.expiryLeaf);
-      witness = [hex.decode(t.signature), preimage, tp.unlockScript, cb];
+      if (sizeOnly) {
+        const shape = harkUnlockWitnessShape(sha256(preimage));
+        witness = [hex.decode(t.signature), preimage, shape.unlockScript, shape.controlBlock];
+      } else {
+        const tp = harkLeafTaproot(hex.decode(t.userPubkey), serverPub, vtxo.expiryHeight, sha256(preimage));
+        const cb = concatBytes(Uint8Array.of(0xc0 | tp.outputParity), tp.internalXOnly, tp.expiryLeaf);
+        witness = [hex.decode(t.signature), preimage, tp.unlockScript, cb];
+      }
     } else {
       throw new Error('unsupported genesis transition: ' + t.type);
     }
-    const idInternal = txid(it.tx);
     const { raw, vsize } = serializeTxW(it.tx, [witness]);
+    if (sizeOnly) return { vsize };
+    const idInternal = txid(it.tx);
     return {
       txid: hex.encode(idInternal.slice().reverse()),
       txidInternal: idInternal,
diff --git a/src/ark/refresh.js b/src/ark/refresh.js
index 2894977..4bd74f2 100644
--- a/src/ark/refresh.js
+++ b/src/ark/refresh.js
@@ -114,7 +114,23 @@ function forfeitClaimTaproot(userPub, serverPub, exitDelta, unlockHash) {
 const otherSum = (item) =>
   item.feeSat + item.otherOutputs.reduce((n, o) => n + o.valueSat, 0);
 
-export function transitionInputTxout(transition, amountSat, serverPub, expiryHeight) {
+// A taproot output script is 34 bytes whatever key sits inside it. Callers
+// that only need transaction SIZES (fee estimates) pass sizeOnly and skip
+// the musig key aggregation — several secp256k1 multiplies per hop, which
+// across a wallet's coins was seconds of main-thread work at boot.
+const DUMMY_P2TR = concatBytes(hex.decode('5120'), new Uint8Array(32));
+// The hark unlock witness's script + control block, shaped for size only.
+export function harkUnlockWitnessShape(unlockHash, gen = 1) {
+  const unlockScript = (gen === 1 ? hashSignScriptV1 : hashSignScript)(unlockHash, new Uint8Array(32));
+  return { unlockScript, controlBlock: new Uint8Array(65) };
+}
+
+export function transitionInputTxout(transition, amountSat, serverPub, expiryHeight, sizeOnly = false) {
+  if (sizeOnly) {
+    const known = ['cosigned', 'hashLockedCosigned', 'hashLockedCosigned_v0', 'arkoor'];
+    if (!known.includes(transition.type)) throw new Error('input_txout unsupported for transition ' + transition.type);
+    return { valueSat: amountSat, scriptPubKey: DUMMY_P2TR };
+  }
   if (transition.type === 'cosigned') {
     const pubkeys = transition.pubkeys.map((p) => hex.decode(p));
     return { valueSat: amountSat, scriptPubKey: cosignedInputTaproot(pubkeys, serverPub, expiryHeight).scriptPubKey };
@@ -140,7 +156,7 @@ export const vtxoAnchorAmount = (vtxo) =>
   vtxo.amountSat + vtxo.genesis.reduce((n, it) => n + otherSum(it), 0);
 
 // Walk the genesis chain, returning [{tx, outputIdx}] like Vtxo::transactions()
-export function vtxoTransactions(vtxo, serverPub) {
+export function vtxoTransactions(vtxo, serverPub, sizeOnly = false) {
   const anchorAmount = vtxoAnchorAmount(vtxo);
   let prev = vtxo.anchorPoint.raw;
   let currentAmount = anchorAmount;
@@ -149,9 +165,9 @@ export function vtxoTransactions(vtxo, serverPub) {
     const item = vtxo.genesis[i];
     const nextAmount = currentAmount - otherSum(item);
     const nextOutput = i + 1 < vtxo.genesis.length
-      ? transitionInputTxout(vtxo.genesis[i + 1].transition, nextAmount, serverPub, vtxo.expiryHeight)
+      ? transitionInputTxout(vtxo.genesis[i + 1].transition, nextAmount, serverPub, vtxo.expiryHeight, sizeOnly)
       : { valueSat: vtxo.amountSat,
-          scriptPubKey: policyTaproot(vtxo.policy, serverPub, vtxo.exitDelta).scriptPubKey };
+          scriptPubKey: sizeOnly ? DUMMY_P2TR : policyTaproot(vtxo.policy, serverPub, vtxo.exitDelta).scriptPubKey };
     // decoded txouts carry hex-string scripts — normalize to bytes
     const others = item.otherOutputs.map((o) => ({ valueSat: o.valueSat, scriptPubKey: hex.decode(o.scriptPubKey) }));
     const outs = [