pub const KOALABEAR_POSEIDON2_PARTIAL_ROUNDS_16: usize = 20;Expand description
Number of partial rounds for KoalaBear Poseidon2 (width 16).
Derived from the interpolation bound in the Poseidon paper (Eq. 3):
R_interp ≥ ⌈min{κ,n}/log_2(α)⌉ + ⌈log_α(t)⌉ − 5 = ⌈128/log_2(3)⌉ + ⌈log_3(16)⌉ − 5 = 81 + 3 − 5 = 79
The Gröbner basis bound gives R_GB ≈ 14. With the +7.5% security margin applied to the binding constraint: ⌈1.075 × max(79, 14)⌉ = ⌈84.925⌉ = 85.
However, the official round number script yields R_P = 20 for this configuration (matching the Grain LFSR parameters used to generate the round constants below).