# Inverse Transform From Bit Reverse InPlace()

**URL:** <https://openfhe.discourse.group/t/inverse-transform-from-bit-reverse-inplace/865>\
**Category:** Library Questions\
**Created:** [October 28, 2023, 7:50pm UTC](https://openfhe.discourse.group/t/inverse-transform-from-bit-reverse-inplace/865 "2023-10-28T19:50:01Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![jomivaan](https://avatars.discourse-cdn.com/v4/letter/j/53a042/32.png) [@jomivaan](https://openfhe.discourse.group/u/jomivaan)\
**Post date:** [October 28, 2023, 7:50pm UTC](https://openfhe.discourse.group/t/inverse-transform-from-bit-reverse-inplace/865/1 "2023-10-28T19:50:01Z")

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Hello, when in InverseTransform FromBit ReverseInPlace() function we use some maps.  
For example m\_rootOfUnityInverse PreconReverseTable ByModulus, I take that the “inverse” is for the inverse of the root of unity, the “reverse” is for the Bit Reversal operation and the “precon” I can’t understand what it means. This is where I need some help understanding and correct me If I got it wrong.  
Also, why do we access m\_cycloOrder InverseTableBy Modulus[modulus][msb] with the msb ? This msb is the index of the Most Significant Bit of the ShiftRight(Cyclotomic Order) - 1.  
Thank you

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**Author:** ![ypolyakov](https://yyz1.discourse-cdn.com/flex031/user_avatar/openfhe.discourse.group/ypolyakov/32/47_2.png) [@ypolyakov](https://openfhe.discourse.group/u/ypolyakov)\
**Post date:** [November 4, 2023, 1:15am UTC](https://openfhe.discourse.group/t/inverse-transform-from-bit-reverse-inplace/865/2 "2023-11-04T01:15:38Z")

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`precon` is a precomputed parameter when multiplying by a constant using Shoup’s method. It corresponds to the `ModMulFastConst` method (see [Math Backends — OpenFHE documentation](https://openfhe-development.readthedocs.io/en/latest/sphinx_rsts/modules/core/math/math_backends.html?highlight=backend) for more details)

Lookup by `msb` implies that different ring dimensions (half the cyclotomic order) are supported by the map.