fix: a PERMIT2_TRANSFER_FROM before a permit keeps its amount from standing as the Amount figure (closes #655)

A PERMIT2_TRANSFER_FROM that runs before a PERMIT2_PERMIT of the same
token moves it under whatever allowance the router already had, so it
can move more than the permit's amount. The permit's amount still stood
as the Amount figure, which could then read lower than what leaves the
wallet.

decode() now counts such a transfer, into the router or anywhere else,
among the steps before the permit that took its token, so the permit no
longer stands: the Amount line gives the total the steps take, as with
no permit, and the permit gets its own lines. A transfer after the
permit, or of another token before it, still lets it stand; tests cover
both. README Transaction Decoding states the condition.

Model: opus-5-5

tests/uniswap.test.js

@@ -2197,24 +2197,31 @@ describe("uniswap decoder", () => {
         );
 
         // https://git.eeqj.de/sneak/AutistMask/issues/655 — a
-        // PERMIT2_TRANSFER_FROM before the permit moves the permit's token
-        // under whatever allowance the router already had, so it may move
+        // PERMIT2_TRANSFER_FROM of the permit's token before the permit moves
+        // it under whatever allowance the router already had, so it may move
         // more than the permit's amount. The permit's amount then does not
         // stand, and the Amount line gives the total the steps take, as with
-        // no permit. On next at f53b503 the Amount line read the permit's
-        // 600.0000 USDT, and the permit got no lines of its own.
+        // no permit. On next at f53b503 the first two rows read the permit's
+        // 600.0000 USDT as the Amount figure, with no lines for the permit.
+        // One after the permit, as in Uniswap's own V2 routes, moves no more
+        // than the permit's amount, and one of another token before it moves
+        // none of the permit's token, so in the last two rows the permit's
+        // amount stands.
         const USDT_2000 = 2000000000n;
-        // A PERMIT2_TRANSFER_FROM of `amount` of USDT to `recipient`.
-        const permit2TransferFrom = (recipient, amount) =>
+        // The V2 pair of WETH and USDC on Ethereum Mainnet.
+        const WETH_USDC_PAIR = "0xB4e16d0168e52d35CaCD2c6185b44281Ec28C9Dc";
+        // A PERMIT2_TRANSFER_FROM of `amount` of `token` to `recipient`.
+        const permit2TransferFrom = (token, recipient, amount) =>
             coder.encode(
                 ["address", "address", "uint160"],
-                [USDT_ADDR, recipient, amount],
+                [token, recipient, amount],
             );
         test.each([
             {
-                kind: "a PERMIT2_TRANSFER_FROM of 2,000 USDT into the router, then a PERMIT2_PERMIT of 600 USDT and a V3 exact-in of 2,000 USDT paid from the router,",
+                kind: "a PERMIT2_TRANSFER_FROM of 2,000 USDT into the router, then a PERMIT2_PERMIT of 600 USDT and a V3 exact-in of 2,000 USDT paid from the router, shows the total the steps take and the permit's own lines",
+                commands: [0x02, 0x0a, 0x00],
                 inputs: [
-                    permit2TransferFrom(ROUTER_ADDR, USDT_2000),
+                    permit2TransferFrom(USDT_ADDR, ROUTER_ADDR, USDT_2000),
                     encodePermit2(USDT_ADDR, USDT_600, ROUTER_ADDR),
                     coder.encode(
                         ["address", "uint256", "uint256", "bytes", "bool"],
@@ -2230,9 +2237,10 @@ describe("uniswap decoder", () => {
                 lines: [["Amount", "2000.0000 USDT"], ...USDT_600_LINES],
             },
             {
-                kind: "a PERMIT2_TRANSFER_FROM of 2,000 USDT to another address, then a PERMIT2_PERMIT of 600 USDT and a V3 exact-in of 600 USDT,",
+                kind: "a PERMIT2_TRANSFER_FROM of 2,000 USDT to another address, then a PERMIT2_PERMIT of 600 USDT and a V3 exact-in of 600 USDT, shows the total the steps take and the permit's own lines",
+                commands: [0x02, 0x0a, 0x00],
                 inputs: [
-                    permit2TransferFrom(OTHER_ADDR, USDT_2000),
+                    permit2TransferFrom(USDT_ADDR, OTHER_ADDR, USDT_2000),
                     encodePermit2(USDT_ADDR, USDT_600, ROUTER_ADDR),
                     V3_600,
                 ],
@@ -2244,13 +2252,43 @@ describe("uniswap decoder", () => {
                     ...USDT_600_LINES,
                 ],
             },
-        ])(
-            "$kind shows the total the steps take and the permit's own lines",
-            ({ inputs, lines }) => {
-                const result = decodeSteps([0x02, 0x0a, 0x00], inputs);
-                expect(linesBetweenTokens(result)).toEqual(lines);
+            {
+                kind: "a PERMIT2_PERMIT of 1 WETH, then a PERMIT2_TRANSFER_FROM of 1 WETH to the V2 pair and a V2 exact-in of ALREADY_PAID, shows the permit's amount",
+                commands: [0x0a, 0x02, 0x08],
+                inputs: [
+                    encodePermit2(WETH_ADDR, ONE_WETH, ROUTER_ADDR),
+                    permit2TransferFrom(WETH_ADDR, WETH_USDC_PAIR, ONE_WETH),
+                    encodeV2SwapExactIn(USER_ADDR, 0n, 1990000000n, [
+                        WETH_ADDR,
+                        USDC_ADDR,
+                    ]),
+                ],
+                lines: [
+                    ["Amount", "1.0000 WETH"],
+                    ["Moved from the sender", "1.0000 WETH"],
+                    ["Token moved", "WETH (" + WETH_ADDR + ")"],
+                    ["Moved to", "The V2 pair the next swap spends"],
+                ],
             },
-        );
+            {
+                kind: "a PERMIT2_TRANSFER_FROM of 400 DAI to another address, then a PERMIT2_PERMIT of 600 USDT and a V3 exact-in of 600 USDT, shows the permit's amount",
+                commands: [0x02, 0x0a, 0x00],
+                inputs: [
+                    permit2TransferFrom(DAI_ADDR, OTHER_ADDR, DAI_400),
+                    encodePermit2(USDT_ADDR, USDT_600, ROUTER_ADDR),
+                    V3_600,
+                ],
+                lines: [
+                    ["Amount", "600.0000 USDT"],
+                    ["Moved from the sender", "400.0000 DAI"],
+                    ["Token moved", "DAI (" + DAI_ADDR + ")"],
+                    ["Moved to", OTHER_ADDR],
+                ],
+            },
+        ])("$kind", ({ commands, inputs, lines }) => {
+            const result = decodeSteps(commands, inputs);
+            expect(linesBetweenTokens(result)).toEqual(lines);
+        });
 
         // A V3_SWAP_EXACT_OUT the user pays for, of at most `amountInMax`
         // USDT for USDC.