Linear code constructors that do not preserve the structural information¶
This file contains a variety of constructions which builds the generator matrix
of special (or random) linear codes and wraps them in a
sage.coding.linear_code.LinearCode
object. These constructions are
therefore not rich objects such as
sage.coding.grs_code.GeneralizedReedSolomonCode
.
All codes available here can be accessed through the codes
object:
sage: codes.random_linear_code(GF(2), 5, 2)
[5, 2] linear code over GF(2)
REFERENCES:
AUTHORS:
David Joyner (2007-05): initial version
David Joyner (2008-02): added cyclic codes, Hamming codes
David Joyner (2008-03): added BCH code, LinearCodeFromCheckmatrix, ReedSolomonCode, WalshCode, DuadicCodeEvenPair, DuadicCodeOddPair, QR codes (even and odd)
David Joyner (2008-09) fix for bug in BCHCode reported by F. Voloch
David Joyner (2008-10) small docstring changes to WalshCode and walsh_matrix
-
sage.coding.code_constructions.
DuadicCodeEvenPair
(F, S1, S2)¶ Constructs the “even pair” of duadic codes associated to the “splitting” (see the docstring for
_is_a_splitting
for the definition) S1, S2 of n.Warning
Maybe the splitting should be associated to a sum of q-cyclotomic cosets mod n, where q is a prime.
EXAMPLES:
sage: from sage.coding.code_constructions import _is_a_splitting sage: n = 11; q = 3 sage: C = Zmod(n).cyclotomic_cosets(q); C [[0], [1, 3, 4, 5, 9], [2, 6, 7, 8, 10]] sage: S1 = C[1] sage: S2 = C[2] sage: _is_a_splitting(S1,S2,11) True sage: codes.DuadicCodeEvenPair(GF(q),S1,S2) ([11, 5] Cyclic Code over GF(3), [11, 5] Cyclic Code over GF(3))
-
sage.coding.code_constructions.
DuadicCodeOddPair
(F, S1, S2)¶ Constructs the “odd pair” of duadic codes associated to the “splitting” S1, S2 of n.
Warning
Maybe the splitting should be associated to a sum of q-cyclotomic cosets mod n, where q is a prime.
EXAMPLES:
sage: from sage.coding.code_constructions import _is_a_splitting sage: n = 11; q = 3 sage: C = Zmod(n).cyclotomic_cosets(q); C [[0], [1, 3, 4, 5, 9], [2, 6, 7, 8, 10]] sage: S1 = C[1] sage: S2 = C[2] sage: _is_a_splitting(S1,S2,11) True sage: codes.DuadicCodeOddPair(GF(q),S1,S2) ([11, 6] Cyclic Code over GF(3), [11, 6] Cyclic Code over GF(3))
This is consistent with Theorem 6.1.3 in [HP2003].
-
sage.coding.code_constructions.
ExtendedQuadraticResidueCode
(n, F)¶ The extended quadratic residue code (or XQR code) is obtained from a QR code by adding a check bit to the last coordinate. (These codes have very remarkable properties such as large automorphism groups and duality properties - see [HP2003], Section 6.6.3-6.6.4.)
INPUT:
n
- an odd primeF
- a finite prime field F whose order must be a quadratic residue modulo n.
OUTPUT: Returns an extended quadratic residue code.
EXAMPLES:
sage: C1 = codes.QuadraticResidueCode(7,GF(2)) sage: C2 = C1.extended_code() sage: C3 = codes.ExtendedQuadraticResidueCode(7,GF(2)); C3 Extension of [7, 4] Cyclic Code over GF(2) sage: C2 == C3 True sage: C = codes.ExtendedQuadraticResidueCode(17,GF(2)) sage: C Extension of [17, 9] Cyclic Code over GF(2) sage: C3 = codes.QuadraticResidueCodeOddPair(7,GF(2))[0] sage: C3x = C3.extended_code() sage: C4 = codes.ExtendedQuadraticResidueCode(7,GF(2)) sage: C3x == C4 True
AUTHORS:
David Joyner (07-2006)
-
sage.coding.code_constructions.
QuadraticResidueCode
(n, F)¶ A quadratic residue code (or QR code) is a cyclic code whose generator polynomial is the product of the polynomials \(x-\alpha^i\) (\(\alpha\) is a primitive \(n^{th}\) root of unity; \(i\) ranges over the set of quadratic residues modulo \(n\)).
See QuadraticResidueCodeEvenPair and QuadraticResidueCodeOddPair for a more general construction.
INPUT:
n
- an odd primeF
- a finite prime field F whose order must be a quadratic residue modulo n.
OUTPUT: Returns a quadratic residue code.
EXAMPLES:
sage: C = codes.QuadraticResidueCode(7,GF(2)) sage: C [7, 4] Cyclic Code over GF(2) sage: C = codes.QuadraticResidueCode(17,GF(2)) sage: C [17, 9] Cyclic Code over GF(2) sage: C1 = codes.QuadraticResidueCodeOddPair(7,GF(2))[0] sage: C2 = codes.QuadraticResidueCode(7,GF(2)) sage: C1 == C2 True sage: C1 = codes.QuadraticResidueCodeOddPair(17,GF(2))[0] sage: C2 = codes.QuadraticResidueCode(17,GF(2)) sage: C1 == C2 True
AUTHORS:
David Joyner (11-2005)
-
sage.coding.code_constructions.
QuadraticResidueCodeEvenPair
(n, F)¶ Quadratic residue codes of a given odd prime length and base ring either don’t exist at all or occur as 4-tuples - a pair of “odd-like” codes and a pair of “even-like” codes. If \(n > 2\) is prime then (Theorem 6.6.2 in [HP2003]) a QR code exists over \(GF(q)\) iff q is a quadratic residue mod \(n\).
They are constructed as “even-like” duadic codes associated the splitting (Q,N) mod n, where Q is the set of non-zero quadratic residues and N is the non-residues.
EXAMPLES:
sage: codes.QuadraticResidueCodeEvenPair(17, GF(13)) # known bug (#25896) ([17, 8] Cyclic Code over GF(13), [17, 8] Cyclic Code over GF(13)) sage: codes.QuadraticResidueCodeEvenPair(17, GF(2)) ([17, 8] Cyclic Code over GF(2), [17, 8] Cyclic Code over GF(2)) sage: codes.QuadraticResidueCodeEvenPair(13,GF(9,"z")) # known bug (#25896) ([13, 6] Cyclic Code over GF(9), [13, 6] Cyclic Code over GF(9)) sage: C1,C2 = codes.QuadraticResidueCodeEvenPair(7,GF(2)) sage: C1.is_self_orthogonal() True sage: C2.is_self_orthogonal() True sage: C3 = codes.QuadraticResidueCodeOddPair(17,GF(2))[0] sage: C4 = codes.QuadraticResidueCodeEvenPair(17,GF(2))[1] sage: C3.systematic_generator_matrix() == C4.dual_code().systematic_generator_matrix() True
This is consistent with Theorem 6.6.9 and Exercise 365 in [HP2003].
-
sage.coding.code_constructions.
QuadraticResidueCodeOddPair
(n, F)¶ Quadratic residue codes of a given odd prime length and base ring either don’t exist at all or occur as 4-tuples - a pair of “odd-like” codes and a pair of “even-like” codes. If n 2 is prime then (Theorem 6.6.2 in [HP2003]) a QR code exists over GF(q) iff q is a quadratic residue mod n.
They are constructed as “odd-like” duadic codes associated the splitting (Q,N) mod n, where Q is the set of non-zero quadratic residues and N is the non-residues.
EXAMPLES:
sage: codes.QuadraticResidueCodeOddPair(17, GF(13)) # known bug (#25896) ([17, 9] Cyclic Code over GF(13), [17, 9] Cyclic Code over GF(13)) sage: codes.QuadraticResidueCodeOddPair(17, GF(2)) ([17, 9] Cyclic Code over GF(2), [17, 9] Cyclic Code over GF(2)) sage: codes.QuadraticResidueCodeOddPair(13, GF(9,"z")) # known bug (#25896) ([13, 7] Cyclic Code over GF(9), [13, 7] Cyclic Code over GF(9)) sage: C1 = codes.QuadraticResidueCodeOddPair(17, GF(2))[1] sage: C1x = C1.extended_code() sage: C2 = codes.QuadraticResidueCodeOddPair(17, GF(2))[0] sage: C2x = C2.extended_code() sage: C2x.spectrum(); C1x.spectrum() [1, 0, 0, 0, 0, 0, 102, 0, 153, 0, 153, 0, 102, 0, 0, 0, 0, 0, 1] [1, 0, 0, 0, 0, 0, 102, 0, 153, 0, 153, 0, 102, 0, 0, 0, 0, 0, 1] sage: C3 = codes.QuadraticResidueCodeOddPair(7, GF(2))[0] sage: C3x = C3.extended_code() sage: C3x.spectrum() [1, 0, 0, 0, 14, 0, 0, 0, 1]
This is consistent with Theorem 6.6.14 in [HP2003].
-
sage.coding.code_constructions.
ToricCode
(P, F)¶ Let \(P\) denote a list of lattice points in \(\ZZ^d\) and let \(T\) denote the set of all points in \((F^x)^d\) (ordered in some fixed way). Put \(n=|T|\) and let \(k\) denote the dimension of the vector space of functions \(V = \mathrm{Span}\{x^e \ |\ e \in P\}\). The associated toric code \(C\) is the evaluation code which is the image of the evaluation map
\[\mathrm{eval_T} : V \rightarrow F^n,\]where \(x^e\) is the multi-index notation (\(x=(x_1,...,x_d)\), \(e=(e_1,...,e_d)\), and \(x^e = x_1^{e_1}...x_d^{e_d}\)), where \(eval_T (f(x)) = (f(t_1),...,f(t_n))\), and where \(T=\{t_1,...,t_n\}\). This function returns the toric codes discussed in [Joy2004].
INPUT:
P
- all the integer lattice points in a polytope defining the toric variety.F
- a finite field.
OUTPUT: Returns toric code with length n = , dimension k over field F.
EXAMPLES:
sage: C = codes.ToricCode([[0,0],[1,0],[2,0],[0,1],[1,1]],GF(7)) sage: C [36, 5] linear code over GF(7) sage: C.minimum_distance() 24 sage: C = codes.ToricCode([[-2,-2],[-1,-2],[-1,-1],[-1,0],[0,-1],[0,0],[0,1],[1,-1],[1,0]],GF(5)) sage: C [16, 9] linear code over GF(5) sage: C.minimum_distance() 6 sage: C = codes.ToricCode([ [0,0],[1,1],[1,2],[1,3],[1,4],[2,1],[2,2],[2,3],[3,1],[3,2],[4,1]],GF(8,"a")) sage: C [49, 11] linear code over GF(8)
This is in fact a [49,11,28] code over GF(8). If you type next
C.minimum_distance()
and wait overnight (!), you should get 28.AUTHOR:
David Joyner (07-2006)
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sage.coding.code_constructions.
WalshCode
(m)¶ Return the binary Walsh code of length \(2^m\).
The matrix of codewords correspond to a Hadamard matrix. This is a (constant rate) binary linear \([2^m,m,2^{m-1}]\) code.
EXAMPLES:
sage: C = codes.WalshCode(4); C [16, 4] linear code over GF(2) sage: C = codes.WalshCode(3); C [8, 3] linear code over GF(2) sage: C.spectrum() [1, 0, 0, 0, 7, 0, 0, 0, 0] sage: C.minimum_distance() 4 sage: C.minimum_distance(algorithm='gap') # check d=2^(m-1) 4
REFERENCES:
-
sage.coding.code_constructions.
from_parity_check_matrix
(H)¶ Return the linear code that has
H
as a parity check matrix.If
H
has dimensions \(h \times n\) then the linear code will have dimension \(n-h\) and length \(n\).EXAMPLES:
sage: C = codes.HammingCode(GF(2), 3); C [7, 4] Hamming Code over GF(2) sage: H = C.parity_check_matrix(); H [1 0 1 0 1 0 1] [0 1 1 0 0 1 1] [0 0 0 1 1 1 1] sage: C2 = codes.from_parity_check_matrix(H); C2 [7, 4] linear code over GF(2) sage: C2.systematic_generator_matrix() == C.systematic_generator_matrix() True
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sage.coding.code_constructions.
permutation_action
(g, v)¶ Returns permutation of rows g*v. Works on lists, matrices, sequences and vectors (by permuting coordinates). The code requires switching from i to i+1 (and back again) since the SymmetricGroup is, by convention, the symmetric group on the “letters” 1, 2, …, n (not 0, 1, …, n-1).
EXAMPLES:
sage: V = VectorSpace(GF(3),5) sage: v = V([0,1,2,0,1]) sage: G = SymmetricGroup(5) sage: g = G([(1,2,3)]) sage: permutation_action(g,v) (1, 2, 0, 0, 1) sage: g = G([()]) sage: permutation_action(g,v) (0, 1, 2, 0, 1) sage: g = G([(1,2,3,4,5)]) sage: permutation_action(g,v) (1, 2, 0, 1, 0) sage: L = Sequence([1,2,3,4,5]) sage: permutation_action(g,L) [2, 3, 4, 5, 1] sage: MS = MatrixSpace(GF(3),3,7) sage: A = MS([[1,0,0,0,1,1,0],[0,1,0,1,0,1,0],[0,0,0,0,0,0,1]]) sage: S5 = SymmetricGroup(5) sage: g = S5([(1,2,3)]) sage: A [1 0 0 0 1 1 0] [0 1 0 1 0 1 0] [0 0 0 0 0 0 1] sage: permutation_action(g,A) [0 1 0 1 0 1 0] [0 0 0 0 0 0 1] [1 0 0 0 1 1 0]
It also works on lists and is a “left action”:
sage: v = [0,1,2,0,1] sage: G = SymmetricGroup(5) sage: g = G([(1,2,3)]) sage: gv = permutation_action(g,v); gv [1, 2, 0, 0, 1] sage: permutation_action(g,v) == g(v) True sage: h = G([(3,4)]) sage: gv = permutation_action(g,v) sage: hgv = permutation_action(h,gv) sage: hgv == permutation_action(h*g,v) True
AUTHORS:
David Joyner, licensed under the GPL v2 or greater.
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sage.coding.code_constructions.
random_linear_code
(F, length, dimension)¶ Generate a random linear code of length
length
, dimensiondimension
and over the fieldF
.This function is Las Vegas probabilistic: always correct, usually fast. Random matrices over the
F
are drawn until one with full rank is hit.If
F
is infinite, the distribution of the elements in the random generator matrix will be random according to the distribution ofF.random_element()
.EXAMPLES:
sage: C = codes.random_linear_code(GF(2), 10, 3) sage: C [10, 3] linear code over GF(2) sage: C.generator_matrix().rank() 3
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sage.coding.code_constructions.
walsh_matrix
(m0)¶ This is the generator matrix of a Walsh code. The matrix of codewords correspond to a Hadamard matrix.
EXAMPLES:
sage: walsh_matrix(2) [0 0 1 1] [0 1 0 1] sage: walsh_matrix(3) [0 0 0 0 1 1 1 1] [0 0 1 1 0 0 1 1] [0 1 0 1 0 1 0 1] sage: C = LinearCode(walsh_matrix(4)); C [16, 4] linear code over GF(2) sage: C.spectrum() [1, 0, 0, 0, 0, 0, 0, 0, 15, 0, 0, 0, 0, 0, 0, 0, 0]
This last code has minimum distance 8.
REFERENCES: