This is now completely opposite when comparing with cl-pool because for cl-pool when amountSpecified > 0 it means we are specifying amountOut, while for bin-pool we can only specify amountIn and it's always positive.

That's why in the following call i.e. toBalanceDelta we flip the sign for both parts

want to clarify: does the hook for both Bin and CL have the same behavour for their return value? eg. BeforeSwapDelta in beforeSwap and int128 at afterSwap()?

eg.

1/ what does negative BeforeSwapDelta for both value means?
2/ what does negative int128 in afterSwap means?

is this right?

1/ what does negative BeforeSwapDelta for both value means?

1. if beforeSwapDelta is both negative, it means the hook want to give user extra tokenIn and tokenOut
2. if beforeSwapDelta is positive, it means the hook want to take a "fee" in both tokenIn and tokenOut
3. likewise if its (positive , negative) it means hook want to take tokenIn as fee and give extra tokenOut

2/ what does negative int128 in afterSwap means?

1. if int128 is negative, it means the hook want to give amountOut
2. if int128 is positive, it means the hook want to take some amountOut as "fee"

for bin-pool, I think it should be opposite.

1. if beforeSwapDelta is both negative, it means the hook want to take a "fee" in both tokenIn and tokenOut
2. if beforeSwapDelta is positive, it means the hook want to give user extra tokenIn and tokenOut
3. likewise if its (positive , negative) it means hook want to give extra tokenIn and take tokenOut as fee

Let's elaborate case 3 for example:

if BeforeSwapDelta.getSpecifiedDelta() > 0, i.e. amountToSwap is even bigger than trader's specification, so more amountIn needed for this swap, and let's see who will pay for the extra part:

from trader perspective:

The amountIn delta returned from pool.swap is definitely bigger in abs value because amountIn is bigger, but remember the delta is updated through delta - hookDelta in afterSwap

and we know hookDelta = (-hookDeltaSpecified, -hookDeltaUnspecified)
a negative value minus another negative value will result in a bigger negative value i.e. user are actually not paying that part

from hook perspective:

if (hookDelta != BalanceDeltaLibrary.ZERO_DELTA) {
    vault.accountPoolBalanceDelta(key, hookDelta, address(key.hooks));
}

This will count a negative amountIn delta on hook, so inside hook callback there must be a payment to the pool

2/ what does negative int128 in afterSwap means?

Also, should be opposite
if int128 is positive, it means the hook want to give amountOut
if int128 is negative, it means the hook want to take some amountOut as "fee"

In fact
int128 in afterSwap and BeforeSwapDelta.getUnspecifiedDelta() in beforeSwap are actually specifying the same thing..

can see they are combined in afterSwap before generating the final hookDelta:

curious, what does this comment mean. does it mean that it will revert eventually in the transaction if the target has non-zero delta?

eg. the comment might imply that if target has positive delta, this method will revert

when calling settleFor, msg.sender is trying to cover the payment for target right ?

If the target has a positive delta, it means that the target doesn't owe anything to the pool and, in fact, the pool owes tokens to them..(after updating everything flip) In this case, pass in amount=0 (i.e. ettle all outstanding debt for target) is invalid.

when calling settleFor, msg.sender is trying to cover the payment for target right ?

I guess when there's a comment like /// It will revert if target has positive delta -- people might think there's a revert in this settleFor method. But its basically the CurrencyNotSettled() revert that happens at the end right? or is there actually a validation for this target has positive delta specifically?

If the comment is correct, would it mean we can write the same comment in take/settle etc.. it will revert if caller has positive delta as well?

“But its basically the CurrencyNotSettled() revert that happens at the end right? or is there actually a validation for this target has positive delta specifically?”

No, it will revert as in SafeCast.toUint256 because we cant cast a negative number (given delta is positive, minus it results in negative) to uint256

there's also the mint() comment around delta that neeed to flipped

 /// @return delta BalanceDelta, will be positive indicating how much total amt0 and amt1 liquidity added

to

 /// @return delta BalanceDelta, will be negative indicating how much total amt0 and amt1 liquidity added