fix: a swap split across steps the user pays for shows every amount it spends (closes #625)

The Amount line gave only the first swap step's amount. It now gives the
total the steps take of Token In: the amount of the step that sets the
input side plus each later take of it by a step the user pays. A V4
step, the first included, takes what its SETTLEs paid by the user state.
A take of another token, one after an amount that is not a quantity, or
one of unknown amount gets lines of its own. A step that may take what
the decoder did not read gets one line saying so and leaves no figure.
A PERMIT2_PERMIT names Token In; its amount stands only in the plain
shape of Uniswap's own routes.

Also closes #628.

Model: opus-5-5

README.md

@@ -1235,14 +1235,18 @@ only when no earlier step set it. It finds each side by three rules:
    is their sum; in any other case the step gives no `Amount`. The decoder reads
    no amount from an exact-out action, and an open delta is not a stated amount.
    Its `Min. received` is found the same way, from the `amountOutMin` of the
-   actions that buy the output currency. When the user pays the step any of the
-   input currency, its `Amount` is what the user pays instead (see Transaction
-   Decoding).
+   actions that buy the output currency.
 3. If any swap action in the step could not be read, only `SETTLE`,
    `SETTLE_ALL`, `TAKE` and `TAKE_ALL` name its sides, and the step gives no
    `Amount` or `Min. received`. A side they did not name is unknown, and the
    step sets it as unknown with no figure.
 
+Under each of these rules, when the user pays the step any of the input
+currency, its `Amount` is instead what the user pays it of that currency when
+the decoder knows that amount, and otherwise there is none (see Transaction
+Decoding). So a step whose USDT-for-USDC swap action could not be read, with a
+`SETTLE` of 400 USDT paid by the user, reads `Amount 400.0000 USDT`.
+
 A V4 step with no swap action sets only the sides its `SETTLE`, `SETTLE_ALL`,
 `TAKE` or `TAKE_ALL` names.
 

src/shared/uniswap.js

@@ -903,8 +903,10 @@ function totalAmount(amounts) {
 // input side from it only when no earlier step set it. A side it sets that
 // these rules leave unknown is set unknown, with no figure. A step with no
 // swap action sets only the sides SETTLE, SETTLE_ALL, TAKE and TAKE_ALL
-// name. When the user pays the step any of Token In (`paidByUser` below),
-// decode() takes that, not the Amount of rule 2, as the step's amount.
+// name. Under each of the three rules, rule 3 included, when the user pays
+// the step any of Token In (`paidByUser` below), decode() takes what the user
+// pays it of Token In as the step's amount in place of the rules' Amount, or
+// no amount when this decoder cannot tell what that is.
 //
 // `takes` is every TAKE, TAKE_ALL and TAKE_PORTION, as the currency it takes
 // and its recipient as written, MSG_SENDER for a TAKE_ALL. TAKE_PORTION takes

tests/uniswap.test.js

@@ -1645,6 +1645,50 @@ describe("uniswap decoder", () => {
             ]);
         });
 
+        // A step whose swap action cannot be read states no amount of its
+        // own, but what the user pays it of its input currency is still its
+        // amount. On next it had no Amount line.
+        test("a V4 step whose swap action cannot be read, with a SETTLE of 400 USDT by the user, shows the 400 USDT", () => {
+            // USDT for USDC exact-in, in the newer shape the decoder cannot
+            // read (see V4_SWAP_ACTION_SHAPES).
+            const unreadSwap = coder.encode(
+                [`tuple(${POOL_KEY},bool,uint128,uint128,uint256,bytes)`],
+                [
+                    [
+                        [USDC_ADDR, USDT_ADDR, 100, 1, ZERO_ADDR],
+                        false, // currency1 (USDT) for currency0 (USDC)
+                        USDT_400,
+                        390000000n,
+                        0n,
+                        "0x",
+                    ],
+                ],
+            );
+            const result = decodeSteps(
+                [0x10],
+                [
+                    encodeV4Swap(
+                        new Uint8Array([
+                            V4_SWAP_EXACT_IN_SINGLE_ID,
+                            V4_SETTLE_ID,
+                            V4_TAKE_ALL_ID,
+                        ]),
+                        [
+                            unreadSwap,
+                            settle(USDT_ADDR, USDT_400, true),
+                            encodeV4TakeAll(USDC_ADDR, 0n),
+                        ],
+                    ),
+                ],
+            );
+            expect(detail(result, "Steps").value).toBe(
+                "V4 Swap (could not be decoded)",
+            );
+            expect(linesBetweenTokens(result)).toEqual([
+                ["Amount", "400.0000 USDT"],
+            ]);
+        });
+
         // A step that may take from the sender what the decoder did not read
         // leaves no Amount figure, which may be too low, and one line says so.
         const UNREAD_SWAP = V4_SWAP_ACTION_SHAPES[0].newer;