fix: a swap's unknown first take shows beside a PERMIT2_TRANSFER_FROM, only when the user pays it (closes #634)

decode() now records whether the user pays the V4 step that sets the
input side any of its input token, and decides once, after the loop,
whether that step counts as a take from the sender. A stated amount
counts whoever pays it; with none stated, the step counts only when the
user pays it. Paid from the router, it gets no Moved from the sender
lines, with a PERMIT2_PERMIT of another token or without. When it counts
and states no amount, its Unknown (could not be decoded) line also shows
beside a PERMIT2_TRANSFER_FROM of its token, whose figure is on that
move's own lines or, into the router, on its Paid to line. README
Transaction Decoding says the same.

Model: opus-5-5

README.md

@@ -3185,7 +3185,8 @@ Currently supported:
   step whose input it cannot read, in whole or in part, and a step it fails on.
   In any other case the line gives the total, as with no permit. When the step
   that sets the input side spends a token other than the permit's, or one the
-  decoder does not know, its amount gets lines of its own and the total has no
+  decoder does not know, its amount gets lines of its own, unless it states no
+  amount and the user pays it none of its input token, and the total has no
   figure to start from: unless the permit's amount stands there is no `Amount`
   line, and each take of the permit's token gets lines of its own. Each take the
   total does not count gives three lines after the `Amount` line (after
@@ -3197,11 +3198,13 @@ Currently supported:
   it some of its input token, and there is no `Amount` line, that step gets them
   too whenever another take's lines give a figure, or a `PERMIT2_TRANSFER_FROM`
   of its token gives one, on the lines below or, into the router, on its
-  `Paid to` line, so that figure cannot read as all that leaves. Paid from the
-  router, that step takes nothing from the user's wallet and never gets them.
-  They come in step order, the step that sets the input side first when it has
-  them: `Moved from the sender`, its amount, worded as the `Amount` line words
-  one, or `Unknown (could not be decoded)` when the decoder does not know it;
+  `Paid to` line, so that figure cannot read as all that leaves. When it states
+  no amount and the user pays it none of its input token, as when the router
+  pays it, that step takes none of that token from the user's wallet and never
+  gets them, with a permit or without. They come in step order, the step that
+  sets the input side first when it has them: `Moved from the sender`, its
+  amount, worded as the `Amount` line words one, or
+  `Unknown (could not be decoded)` when the decoder does not know it;
   `Token moved`, giving the token's address as the `Token In` line does; and
   `Moved to`, which reads `The swap step that spends it`. A token the decoder
   does not know gives the `Moved from the sender` line alone. A step may also

src/shared/uniswap.js

@@ -1849,17 +1849,22 @@ function decode(data, toAddress, chainId, sources, fromAddress) {
         // (see spentUnread()), or a token it does not know.
         const spentFromSender = [];
         let total = null;
-        // The take of the step that set the input side, when the user pays
-        // that step, the total starts from it and it stated no amount (see
-        // below), or null. Paid from the router, the step takes nothing from
-        // the sender.
+        // Whether the step that set the input side counts as a take from the
+        // sender, which may get lines of its own. The amount it stated counts
+        // whoever pays it. Stating none, it counts only when the user pays it
+        // some of its input token: paid from the router, it takes nothing
+        // from the sender.
+        const hasFirstTake =
+            inputEstablished && (present(inputAmount) || inputPaidByUser);
+        // The take of the step that set the input side, when the total
+        // starts from it and it stated no amount (see below), or null.
         let firstTakeUnstated = null;
         if (permit === null || sameAddress(inputToken, permit.token)) {
             total = inputAmount;
-            if (inputEstablished && inputPaidByUser && !present(inputAmount)) {
+            if (hasFirstTake && !present(inputAmount)) {
                 firstTakeUnstated = { token: inputToken, amount: null };
             }
-        } else if (inputEstablished) {
+        } else if (hasFirstTake) {
             // The step that set the input side spent a token other than the
             // permit's, or one not known, so the total starts with no figure.
             spentFromSender.push({ token: inputToken, amount: inputAmount });

tests/uniswap.test.js

@@ -2395,6 +2395,36 @@ describe("uniswap decoder", () => {
             },
         );
 
+        // https://git.eeqj.de/sneak/AutistMask/issues/634 — after a
+        // PERMIT2_PERMIT of another token, the V4 exact-out step paid from
+        // the router still takes nothing from the wallet, so it gets no lines
+        // of its own. On next it got lines saying its amount is not known.
+        const ONE_DAI_PERMIT = encodePermit2(DAI_ADDR, 10n ** 18n, ROUTER_ADDR);
+        test.each([
+            {
+                kind: "a PERMIT2_PERMIT of 1 DAI, then a V4 exact-out step of at most 600 USDT paid from the router",
+                commands: [0x0a, 0x10],
+                inputs: [ONE_DAI_PERMIT, V4_EXACT_OUT_600_FROM_ROUTER],
+                lines: [["Amount", "1.0000 DAI"]],
+            },
+            {
+                kind: "a PERMIT2_PERMIT of 1 DAI, a V4 exact-out step of at most 600 USDT paid from the router, then a V2 exact-out of at most 400 USDT the user pays for",
+                commands: [0x0a, 0x10, 0x09],
+                inputs: [
+                    ONE_DAI_PERMIT,
+                    V4_EXACT_OUT_600_FROM_ROUTER,
+                    V2_EXACT_OUT_400,
+                ],
+                lines: [["Amount", "1.0000 DAI"], ...UP_TO_400_USDT],
+            },
+        ])(
+            "$kind, gives the V4 step no lines of its own",
+            ({ commands, inputs, lines }) => {
+                const result = decodeSteps(commands, inputs);
+                expect(linesBetweenTokens(result)).toEqual(lines);
+            },
+        );
+
         // The V4 exact-out route above the other way round, where the later
         // take's unknown amount leaves no figure at all, and other ways the
         // step that sets the input side states no amount.