Counting the position of ones in a binary vector

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This will not be straightforward to realise in a combinatorial circuit because what you describe is a rather sequential process: traverse the input vector from right to left, if you encounter a bit set to '1' add its position index into the next available output slot.

This doesn't mean it can't be done combinatorially, but the result will be huge as it has to cater for all combinations (all 65356 of them ...).

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Actually it can be done in a combinatorial way but you dont take a case statement approach... i think there is a way...

Actually i have managed to get a partial output.

eg if input is 16'b0000_0000_0001_0101 then i have managed to get the output 64'h0000_0000_0004_0200... i dont know how to remove those extra zeros. In this case the expected output is 64'h0000_0000_0000_0420.