refactor(core): simplify PlanMap

[mayeul-zama]

entries of the HashMap are always initialized so we don't need them to be wrapped in OnceLock

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This change is 

[IceTDrinker]

Could be worth checking there is no deadlock possible, I seem to recall the nasty type was required to avoid them, also this PR only updates the NTT one

[mayeul-zama]

The 2 commits should be reviewed separately

We may only keep the first one

[IceTDrinker]

just as an FYI, if we go ahead with this, I would prefer it for Q1 26 (because it's a change at the bottom of the stack with potential weird bugs due to threading) so not for 1.5

Reviewable status: 0 of 5 files reviewed, all discussions resolved

[mayeul-zama]

just as an FYI, if we go ahead with this, I would prefer it for Q1 26 (because it's a change at the bottom of the stack with potential weird bugs due to threading) so not for 1.5

Let's only keep the first commit for now then

[IceTDrinker]

@mayeul-zama if you can rebase on main and launch tests with the approved label to see if we hit deadlocks

[IceTDrinker]

Now that I had time to read the PR, it looks like it should work for our usage and is indeed a nice simplification

a few small comments

@IceTDrinker reviewed 5 files and all commit messages, and made 6 comments.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @mayeul-zama).

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tfhe/src/core_crypto/commons/math/ntt/ntt64.rs line 27 at r3 (raw file):

// Key is (polynomial size, modulus).
type PlanMap = crate::core_crypto::commons::plan::PlanMap<(usize, u64), Plan>;

import PlanMap directly from the same location of new_from_plan_map, or even create your own wrapper type and put new_from_plan_map on it (and rename it perhaps as indicated in another file)

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tfhe/src/core_crypto/fft_impl/fft128/math/fft/mod.rs line 13 at r3 (raw file):

#[derive(Clone)]
pub(crate) struct PlanWrapper(Plan);

is that PlanWrapper doing anything ? Now that I'm looking seems a bit odd, I don't see it for the other plans

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tfhe/src/core_crypto/fft_impl/fft128/math/fft/mod.rs line 49 at r3 (raw file):

}

type PlanMap = crate::core_crypto::commons::plan::PlanMap<usize, PlanWrapper>;

import PlanMap directly

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tfhe/src/core_crypto/commons/plan.rs line 7 at r2 (raw file):

pub type PlanMap<Key, Value> = RwLock<HashMap<Key, Arc<Value>>>;

pub fn new_from_plan_map<Key: Eq + Hash + Copy, Value>(

get_or_init ? looks very much like the OnceLock function with the same name

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tfhe/src/core_crypto/fft_impl/fft64/math/fft/mod.rs line 105 at r3 (raw file):

}

type PlanMap = crate::core_crypto::commons::plan::PlanMap<usize, (Twisties, Plan)>;

import directly

[IceTDrinker]

not sure which PR between this one and #3116 should be the one to review ?

this one looked ok but the other one has more changed files but conflicts