🐞🦋 fix generation of classical LiftedCode and code_n, code_s paras, define code_k

Author: Fe-r-oz

CHANGELOG.md

@@ -7,6 +7,7 @@
 
 ## v0.10.0 - 2025-06-25
 
+- **(fix)**  The `parity_checks`, `code_n`, and `code_s` methods were throwing errors and `code_k` was not defined for the classical `LiftedCode`s.
 - **(fix)** The gates `SQRTY`, `CXYZ`, `CZYX` were computing phases incorrectly when acting on `I` stabilizers.
 - **(fix)** Paulis with imaginary phases had their phases incorrectly tracked.
 - **(fix)** `rowdecompose` was not accounting for the phase of the input Pauli string, leading to potential errors in non-Clifford functionality.

ext/QuantumCliffordHeckeExt/QuantumCliffordHeckeExt.jl

@@ -4,12 +4,14 @@ using QECCore
 import QECCore: code_n, code_s, code_k, rate, distance
 using DocStringExtensions
 
-import QuantumClifford, LinearAlgebra
+import QuantumClifford
+import LinearAlgebra
+import LinearAlgebra: rank
 import Hecke: Group, GroupElem, AdditiveGroupElem,
     GroupAlgebra, GroupAlgebraElem, FqFieldElem, representation_matrix, dim, base_ring,
-    multiplication_table, coefficients, abelian_group, group_algebra, rand
+    multiplication_table, coefficients, abelian_group, group_algebra, rand, order, group
 import Nemo
-import Nemo: characteristic, matrix_repr, GF, ZZ, lift
+import Nemo: characteristic, matrix_repr, GF, ZZ, lift, matrix
 
 import QuantumClifford.ECC: iscss, parity_checks,
     two_block_group_algebra_codes, generalized_bicycle_codes, bicycle_codes, check_repr_commutation_relation,

ext/QuantumCliffordHeckeExt/lifted.jl

@@ -83,7 +83,35 @@ function LiftedCode(group_elem_array::Matrix{<: GroupOrAdditiveGroupElem}; GA::G
     end
 end
 
-# TODO document and doctest example
+"""
+Constructs a group algebra code over `GF(2)` by lifting a matrix of cyclic shifts
+(entries modulo `l`)  to the group algebra of the abelian group `ℤ/lℤ` of order `l`.
+
+# Example
+
+```jldoctest
+julia> import Hecke; import QuantumClifford.ECC
+
+julia> base_matrix = [0 0 0 0; 0 1 2 5; 0 6 3 1]; l = 3;
+
+julia> c = LiftedCode(base_matrix, l);
+
+julia> parity_checks(c)
+9×12 Matrix{Bool}:
+ 1  0  0  1  0  0  1  0  0  1  0  0
+ 0  1  0  0  1  0  0  1  0  0  1  0
+ 0  0  1  0  0  1  0  0  1  0  0  1
+ 1  0  0  0  0  1  0  1  0  0  1  0
+ 0  1  0  1  0  0  0  0  1  0  0  1
+ 0  0  1  0  1  0  1  0  0  1  0  0
+ 1  0  0  1  0  0  1  0  0  0  0  1
+ 0  1  0  0  1  0  0  1  0  1  0  0
+ 0  0  1  0  0  1  0  0  1  0  1  0
+
+julia> code_n(c), code_k(c), code_s(c)
+(12, 5, 9)
+```
+"""
 function LiftedCode(shift_array::Matrix{Int}, l::Int; GA::GroupAlgebra=group_algebra(GF(2), abelian_group(l)))
     A = zeros(GA, size(shift_array)...)
     for i in 1:size(shift_array, 1)
@@ -103,10 +131,12 @@ function concat_lift_repr(repr, mat)
     return z
 end
 
-function parity_checks(c::LiftedCode)
-    return lift(c.repr, c.A)
+function parity_matrix(c::LiftedCode)
+    return concat_lift_repr(c.repr, c.A)
 end
 
-code_n(c::LiftedCode) = size(c.A, 2) * size(zero(c.GA), 2)
+code_n(c::LiftedCode) = size(c.A, 2) * order(group(c.GA))
+
+code_s(c::LiftedCode) = size(c.A, 1) * order(group(c.GA))
 
-code_s(c::LiftedCode) = size(c.A, 1) * size(zero(c.GA), 1)
+code_k(c::LiftedCode) = code_n(c) - rank(matrix(GF(2), parity_checks(c)))

src/ecc/ECC.jl

@@ -1,7 +1,7 @@
 module ECC
 
 using QECCore
-import QECCore: code_n, code_s, code_k, rate, distance, parity_matrix_x, parity_matrix_z,parity_matrix
+import QECCore: code_n, code_s, code_k, rate, distance, parity_matrix_x, parity_matrix_z, parity_matrix
 using LinearAlgebra: LinearAlgebra, I, rank, tr
 using QuantumClifford: QuantumClifford, AbstractOperation, AbstractStabilizer,
     AbstractTwoQubitOperator, Stabilizer, PauliOperator,