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# 5 Manipulating Codes

### Sections

1. Functions that Generate a New Code from a Given Code
2. Functions that Generate a New Code from Two Given Codes

In this chapter we describe several functions GUAVA uses to manipulate codes. Some of the best codes are obtained by starting with for example a BCH code, and manipulating it.

In some cases, it is faster to perform calculations with a manipulated code than to use the original code. For example, if the dimension of the code is larger than half the word length, it is generally faster to compute the weight distribution by first calculating the weight distribution of the dual code than by directly calculating the weight distribution of the original code. The size of the dual code is smaller in these cases.

Because GUAVA keeps all information in a code record, in some cases the information can be preserved after manipulations. Therefore, computations do not always have to start from scratch.

In Section Functions that Generate a New Code from a Given Code, we describe functions that take a code with certain parameters, modify it in some way and return a different code (see ExtendedCode, PuncturedCode, EvenWeightSubcode, PermutedCode, ExpurgatedCode, AugmentedCode, RemovedElementsCode, AddedElementsCode, ShortenedCode, LengthenedCode, ResidueCode, ConstructionBCode, DualCode, ConversionFieldCode, ConstantWeightSubcode, StandardFormCode and CosetCode).

In Section Functions that Generate a New Code from Two Given Codes, we describe functions that generate a new code out of two codes (see DirectSumCode, UUVCode, DirectProductCode, IntersectionCode and UnionCode).

## 5.1 Functions that Generate a New Code from a Given Code

- `ExtendedCode(` C`[,` i`] )`

  `ExtendedCode` extends the code C i times and returns the result. i is equal to 1 by default. Extending is done by adding a parity check bit after the last coordinate. The coordinates of all codewords now add up to zero. In the binary case, each codeword has even weight.

  The word length increases by i. The size of the code remains the same. In the binary case, the minimum distance increases by one if it was odd. In other cases, that is not always true.

  A cyclic code in general is no longer cyclic after extending.

      gap> C1 := HammingCode( 3, GF(2) );
      a linear [7,4,3]1 Hamming (3,2) code over GF(2)
      gap> C2 := ExtendedCode( C1 );
      a linear [8,4,4]2 extended code
      gap> IsEquivalent( C2, ReedMullerCode( 1, 3 ) );
      true
      gap> List( AsSSortedList( C2 ), WeightCodeword );
      [ 0, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 8 ]
      gap> C3 := EvenWeightSubcode( C1 );
      a linear [7,3,4]2..3 even weight subcode

  To undo extending, call `PuncturedCode` (see PuncturedCode). The function `EvenWeightSubcode` (see EvenWeightSubcode) also returns a related code with only even weights, but without changing its word length.

- `PuncturedCode(` C`)`

  `PuncturedCode` punctures C in the last column, and returns the result. Puncturing is done simply by cutting off the last column from each codeword. This means the word length decreases by one. The minimum distance in general also decrease by one.

- `PuncturedCode(` C`,` L`)`

  `PuncturedCode` punctures C in the columns specified by L, a list of integers. All columns specified by L are omitted from each codeword. If l is the length of L (so the number of removed columns), the word length decreases by l. The minimum distance can also decrease by l or less.

  Puncturing a cyclic code in general results in a non-cyclic code. If the code is punctured in all the columns where a word of minimal weight is unequal to zero, the dimension of the resulting code decreases.

      gap> C1 := BCHCode( 15, 5, GF(2) );
      a cyclic [15,7,5]3..5 BCH code, delta=5, b=1 over GF(2)
      gap> C2 := PuncturedCode( C1 );
      a linear [14,7,4]3..5 punctured code
      gap> ExtendedCode( C2 ) = C1;
      false
      gap> PuncturedCode( C1, [1,2,3,4,5,6,7] );
      a linear [8,7,1]1 punctured code
      gap> PuncturedCode( WholeSpaceCode( 4, GF(5) ) );
      a linear [3,3,1]0 punctured code  # The dimension decreased from 4 to 3

  `ExtendedCode` extends the code again (see ExtendedCode) although in general this does not result in the old code.

- `EvenWeightSubcode(` C`)`

  `EvenWeightSubcode` returns the even weight subcode of C, consisting of all codewords of C with even weight. If C is a linear code and contains words of odd weight, the resulting code has a dimension of one less. The minimum distance always increases with one if it was odd. If C is a binary cyclic code, and g(x) is its generator polynomial, the even weight subcode either has generator polynomial g(x) (if g(x) is divisible by x-1) or g(x)*(x-1) (if no factor x-1 was present in g(x)). So the even weight subcode is again cyclic.

  Of course, if all codewords of C are already of even weight, the returned code is equal to C.

      gap> C1 := EvenWeightSubcode( BCHCode( 8, 4, GF(3) ) );
      an (8,33,4..8)3..8 even weight subcode
      gap> List( AsSSortedList( C1 ), WeightCodeword );
      [ 0, 4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 6, 4, 4, 6, 4, 4, 8, 6, 4, 6, 8, 4, 4,
        4, 6, 4, 6, 8, 4, 6, 8 ]
      gap> EvenWeightSubcode( ReedMullerCode( 1, 3 ) );
      a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2)

  `ExtendedCode` also returns a related code of only even weights, but without reducing its dimension (see ExtendedCode).

- `PermutedCode(` C`,` L`)`

  `PermutedCode` returns C after column permutations. L is the permutation to be executed on the columns of C. If C is cyclic, the result in general is no longer cyclic. If a permutation results in the same code as C, this permutation belongs to the automorphism group of C (see AutomorphismGroup). In any case, the returned code is equivalent to C (see IsEquivalent).

      gap> C1 := PuncturedCode( ReedMullerCode( 1, 4 ) );
      a linear [15,5,7]5 punctured code
      gap> C2 := BCHCode( 15, 7, GF(2) );
      a cyclic [15,5,7]5 BCH code, delta=7, b=1 over GF(2)
      gap> C2 = C1;
      false
      gap> p := CodeIsomorphism( C1, C2 );
      ( 2, 4,14, 9,13, 7,11,10, 6, 8,12, 5)
      gap> C3 := PermutedCode( C1, p );
      a linear [15,5,7]5 permuted code
      gap> C2 = C3;
      true

- `ExpurgatedCode(` C`,` L`)`

  `ExpurgatedCode` expurgates code C by throwing away codewords in list L. C must be a linear code. L must be a list of codeword input. The generator matrix of the new code no longer is a basis for the codewords specified by L. Since the returned code is still linear, it is very likely that, besides the words of L, more codewords of C are no longer in the new code.

      gap> C1 := HammingCode( 4 );; WeightDistribution( C1 );
      [ 1, 0, 0, 35, 105, 168, 280, 435, 435, 280, 168, 105, 35, 0, 0, 1 ]
      gap> L := Filtered( AsSSortedList(C1), i -> WeightCodeword(i) = 3 );;
      gap> C2 := ExpurgatedCode( C1, L );
      a linear [15,4,3..4]5..11 code, expurgated with 7 word(s)
      gap> WeightDistribution( C2 );
      [ 1, 0, 0, 0, 14, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0 ]

  This function does not work on non-linear codes. For removing words from a non-linear code, use `RemovedElementsCode` (see RemovedElementsCode). For expurgating a code of all words of odd weight, use `EvenWeightSubcode` (see EvenWeightSubcode).

- `AugmentedCode(` C`,` L`)`

  `AugmentedCode` returns C after augmenting. C must be a linear code, L must be a list of codeword input. The generator matrix of the new code is a basis for the codewords specified by L as well as the words that were already in code C. Note that the new code in general will consist of more words than only the codewords of C and the words L. The returned code is also a linear code.

      gap> C31 := ReedMullerCode( 1, 3 );
      a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2)
      gap> C32 := AugmentedCode(C31,["00000011","00000101","00010001"]);
      a linear [8,7,1..2]1 code, augmented with 3 word(s)
      gap> C32 = ReedMullerCode( 2, 3 );
      true

- `AugmentedCode(` C`)`

  When called without a list of codewords, `AugmentedCode` returns C after adding the all-ones vector to the generator matrix. C must be a linear code. If the all-ones vector was already in the code, nothing happens and a copy of the argument is returned. If C is a binary code which does not contain the all-ones vector, the complement of all codewords is added.

      gap> C1 := CordaroWagnerCode(6);
      a linear [6,2,4]2..3 Cordaro-Wagner code over GF(2)
      gap> Codeword( [0,0,1,1,1,1] ) in C1;
      true
      gap> C2 := AugmentedCode( C1 );
      a linear [6,3,1..2]2..3 code, augmented with 1 word(s)
      gap> Codeword( [1,1,0,0,0,0] ) in C2;
      true

  The function `AddedElementsCode` adds elements to the codewords instead of adding them to the basis (see AddedElementsCode).

- `RemovedElementsCode(` C`,` L`)`

  `RemovedElementsCode` returns code C after removing a list of codewords L from its elements. L must be a list of codeword input. The result is an unrestricted code.

      gap> C1 := HammingCode( 4 );; WeightDistribution( C1 );
      [ 1, 0, 0, 35, 105, 168, 280, 435, 435, 280, 168, 105, 35, 0, 0, 1 ]
      gap> L := Filtered( AsSSortedList(C1), i -> WeightCodeword(i) = 3 );;
      gap> C2 := RemovedElementsCode( C1, L );
      a (15,2013,3..15)2..15 code with 35 word(s) removed
      gap> WeightDistribution( C2 );
      [ 1, 0, 0, 0, 105, 168, 280, 435, 435, 280, 168, 105, 35, 0, 0, 1 ]
      gap> MinimumDistance( C2 );
      3        # C2 is not linear, so the minimum weight does not have to
               # be equal to the minimum distance

  Adding elements to a code is done by the function `AddedElementsCode` (see AddedElementsCode). To remove codewords from the base of a linear code, use `ExpurgatedCode` (see ExpurgatedCode).

- `AddedElementsCode(` C`,` L`)`

  `AddedElementsCode` returns code C after adding a list of codewords L to its elements. L must be a list of codeword input. The result is an unrestricted code.

      gap> C1 := NullCode( 6, GF(2) );
      a cyclic [6,0,6]6 nullcode over GF(2)
      gap> C2 := AddedElementsCode( C1, [ "111111" ] );
      a (6,2,1..6)3 code with 1 word(s) added
      gap> IsCyclicCode( C2 );
      true
      gap> C3 := AddedElementsCode( C2, [ "101010", "010101" ] );
      a (6,4,1..6)2 code with 2 word(s) added
      gap> IsCyclicCode( C3 );
      true

  To remove elements from a code, use `RemovedElementsCode` (see RemovedElementsCode). To add elements to the base of a linear code, use `AugmentedCode` (see AugmentedCode).

- `ShortenedCode(` C`)`

  `ShortenedCode` returns code C shortened by taking a cross section. If C is a linear code, this is done by removing all codewords that start with a non-zero entry, after which the first column is cut off. If C was a [n,k,d] code, the shortened code generally is a [n-1,k-1,d] code. It is possible that the dimension remains the same; it is also possible that the minimum distance increases.

      gap> C1 := HammingCode( 4 );
      a linear [15,11,3]1 Hamming (4,2) code over GF(2)
      gap> C2 := ShortenedCode( C1 );
      a linear [14,10,3]2 shortened code

  If C is a non-linear code, `ShortenedCode` first checks which finite field element occurs most often in the first column of the codewords. The codewords not starting with this element are removed from the code, after which the first column is cut off. The resulting shortened code has at least the same minimum distance as C.

      gap> C1 := ElementsCode( ["1000", "1101", "0011" ], GF(2) );
      a (4,3,1..4)2 user defined unrestricted code over GF(2)
      gap> MinimumDistance( C1 );
      2
      gap> C2 := ShortenedCode( C1 );
      a (3,2,2..3)1..2 shortened code
      gap> AsSSortedList( C2 );
      [ [ 0 0 0 ], [ 1 0 1 ] ]

- `ShortenedCode(` C`,` L`)`

  When called in this format, `ShortenedCode` repeats the shortening process on each of the columns specified by L. L therefore is a list of integers. The column numbers in L are the numbers as they are before the shortening process. If L has l entries, the returned code has a word length of l positions shorter than C.

      gap> C1 := HammingCode( 5, GF(2) );
      a linear [31,26,3]1 Hamming (5,2) code over GF(2)
      gap> C2 := ShortenedCode( C1, [ 1, 2, 3 ] );
      a linear [28,23,3]2 shortened code
      gap> OptimalityLinearCode( C2 );
      0

  The function `LengthenedCode` lengthens the code again (only for linear codes), see LengthenedCode. In general, this is not exactly the inverse function.

- `LengthenedCode(` C`[,` i`] )`

  `LengthenedCode` returns code C lengthened. C must be a linear code. First, the all-ones vector is added to the generator matrix (see AugmentedCode). If the all-ones vector was already a codeword, nothing happens to the code. Then, the code is extended i times (see ExtendedCode). i is equal to 1 by default. If C was an [n,k] code, the new code generally is a [n+i,k+1] code.

      gap> C1 := CordaroWagnerCode( 5 );
      a linear [5,2,3]2 Cordaro-Wagner code over GF(2)
      gap> C2 := LengthenedCode( C1 );
      a linear [6,3,2]2..3 code, lengthened with 1 column(s)

  `ShortenedCode` shortens the code, see ShortenedCode. In general, this is not exactly the inverse function.

- `ResidueCode(` C`[,` w`] )`

  The function `ResidueCode` takes a codeword c of C of weight w (if w is omitted, a codeword of minimal weight is used). C must be a linear code and w must be greater than zero. It removes this word and all its linear combinations from the code and then punctures the code in the coordinates where c is unequal to zero. The resulting code is an [n-w, k-1, d-ë w*(q-1)/q û] code.

      gap> C1 := BCHCode( 15, 7 );
      a cyclic [15,5,7]5 BCH code, delta=7, b=1 over GF(2)
      gap> C2 := ResidueCode( C1 );
      a linear [8,4,4]2 residue code
      gap> c := Codeword( [ 0,0,0,1,0,0,1,1,0,1,0,1,1,1,1 ], C1);;
      gap> C3 := ResidueCode( C1, c );
      a linear [7,4,3]1 residue code

- `ConstructionBCode(` C`)`

  The function `ConstructionBCode` takes a binary linear code C and calculates the minimum distance of the dual of C (see DualCode). It then removes the columns of the parity check matrix of C where a codeword of the dual code of minimal weight has coordinates unequal to zero. the resulting matrix is a parity check matrix for an [n-dd, k-dd+1, ³ d] code, where dd is the minimum distance of the dual of C.

      gap> C1 := ReedMullerCode( 2, 5 );
      a linear [32,16,8]6 Reed-Muller (2,5) code over GF(2)
      gap> C2 := ConstructionBCode( C1 );
      a linear [24,9,8]5..10 Construction B (8 coordinates)
      gap> BoundsMinimumDistance( 24, 9, GF(2) );
      rec( n := 24, k := 9, q := 2, references := rec(  ),
        construction := [ <Operation "UUVCode">,
            [ [ <Operation "UUVCode">, [ [ <Operation "DualCode">,
                            [ [ <Operation "RepetitionCode">, [ 6, 2 ] ] ] ],
                        [ <Operation "CordaroWagnerCode">, [ 6 ] ] ] ],
                [ <Operation "CordaroWagnerCode">, [ 12 ] ] ] ], lowerBound := 8,
        lowerBoundExplanation := [ "Lb(24,9)=8, u u+v construction of C1 and C2:",
            "Lb(12,7)=4, u u+v construction of C1 and C2:",
            "Lb(6,5)=2, dual of the repetition code",
            "Lb(6,2)=4, Cordaro-Wagner code", "Lb(12,2)=8, Cordaro-Wagner code" ],
        upperBound := 8,
        upperBoundExplanation := [ "Ub(24,9)=8, otherwise construction B would contr\
      adict:", "Ub(18,4)=8, Griesmer bound" ] )
      # so C2 is optimal

- `DualCode(` C`)`

  `DualCode` returns the dual code of C. The dual code consists of all codewords that are orthogonal to the codewords of C. If C is a linear code with generator matrix G, the dual code has parity check matrix G (or if C has parity check matrix H, the dual code has generator matrix H). So if C is a linear [n, k] code, the dual code of C is a linear [n, n-k] code. If C is a cyclic code with generator polynomial g(x), the dual code has the reciprocal polynomial of g(x) as check polynomial.

  The dual code is always a linear code, even if C is non-linear.

  If a code C is equal to its dual code, it is called self-dual.

      gap> R := ReedMullerCode( 1, 3 );
      a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2)
      gap> RD := DualCode( R );
      a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2)
      gap> R = RD;
      true
      gap> N := WholeSpaceCode( 7, GF(4) );
      a cyclic [7,7,1]0 whole space code over GF(4)
      gap> DualCode( N ) = NullCode( 7, GF(4) );
      true

- `ConversionFieldCode(` C`)`

  `ConversionFieldCode` returns code C after converting its field. If the field of C is `GF(qm)`, the returned code has field `GF(q)`. Each symbol of every codeword is replaced by a concatenation of m symbols from `GF(q)`. If C is an (n, M, d1) code, the returned code is a (n*m, M, d2) code, where d2 > d1.

  See also HorizontalConversionFieldMat.

      gap> R := RepetitionCode( 4, GF(4) );
      a cyclic [4,1,4]3 repetition code over GF(4)
      gap> R2 := ConversionFieldCode( R );
      a linear [8,2,4]3..4 code, converted to basefield GF(2)
      gap> Size( R ) = Size( R2 );
      true
      gap> GeneratorMat( R );
      [ [ Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0 ] ]
      gap> GeneratorMat( R2 );
      [ [ Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2) ],
        [ 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0 ] ]

- `CosetCode(` C`,` w`)`

  `CosetCode` returns the coset of a code C with respect to word w. w must be of the codeword type. Then, w is added to each codeword of C, yielding the elements of the new code. If C is linear and w is an element of C, the new code is equal to C, otherwise the new code is an unrestricted code.

  Generating a coset is also possible by simply adding the word w to C. See Operations for Codes.

      gap> H := HammingCode(3, GF(2));
      a linear [7,4,3]1 Hamming (3,2) code over GF(2)
      gap> c := Codeword("1011011");; c in H;
      false
      gap> C := CosetCode(H, c);
      a (7,16,3)1 coset code
      gap> List(AsSSortedList(C), el-> Syndrome(H, el));
      [ [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ],
        [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ],
        [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ] ]
      # All elements of the coset have the same syndrome in H

- `ConstantWeightSubcode(` C`,` w`)`

  `ConstantWeightSubcode` returns the subcode of C that only has codewords of weight w. The resulting code is a non-linear code, because it does not contain the all-zero vector.

      gap> N := NordstromRobinsonCode();; WeightDistribution(N);
      [ 1, 0, 0, 0, 0, 0, 112, 0, 30, 0, 112, 0, 0, 0, 0, 0, 1 ]
      gap> C := ConstantWeightSubcode(N, 8);
      a (16,30,6..16)5..8 code with codewords of weight 8
      gap> WeightDistribution(C);
      [ 0, 0, 0, 0, 0, 0, 0, 0, 30, 0, 0, 0, 0, 0, 0, 0, 0 ]

- `ConstantWeightSubcode(` C`)`

  In this format, `ConstantWeightSubcode` returns the subcode of C consisting of all minimum weight codewords of C.

      gap> eg := ExtendedTernaryGolayCode();; WeightDistribution(eg);
      [ 1, 0, 0, 0, 0, 0, 264, 0, 0, 440, 0, 0, 24 ]
      gap> C := ConstantWeightSubcode(eg);
      a (12,264,6..12)3..6 code with codewords of weight 6
      gap> WeightDistribution(C);
      [ 0, 0, 0, 0, 0, 0, 264, 0, 0, 0, 0, 0, 0 ]

- `StandardFormCode(` C`)`

  `StandardFormCode` returns C after putting it in standard form. If C is a non-linear code, this means the elements are organized using lexicographical order. This means they form a legal GAP `Set`.

  If C is a linear code, the generator matrix and parity check matrix are put in standard form. The generator matrix then has an identity matrix in its left part, the parity check matrix has an identity matrix in its right part. Although GUAVA always puts both matrices in a standard form using `BaseMat`, this never alters the code. `StandardFormCode` even applies column permutations if unavoidable, and thereby changes the code. The column permutations are recorded in the construction history of the new code (see Display). C and the new code are of course equivalent.

  If C is a cyclic code, its generator matrix cannot be put in the usual upper triangular form, because then it would be inconsistent with the generator polynomial. The reason is that generating the elements from the generator matrix would result in a different order than generating the elements from the generator polynomial. This is an unwanted effect, and therefore `StandardFormCode` just returns a copy of C for cyclic codes.

      gap> G := GeneratorMatCode( Z(2) * [ [0,1,1,0], [0,1,0,1], [0,0,1,1] ],
      > "random form code", GF(2) );
      a linear [4,2,1..2]1..2 random form code over GF(2)
      gap> Codeword( GeneratorMat( G ) );
      [ [ 0 1 0 1 ], [ 0 0 1 1 ] ]
      gap> Codeword( GeneratorMat( StandardFormCode( G ) ) );
      [ [ 1 0 0 1 ], [ 0 1 0 1 ] ]

  ## 5.2 Functions that Generate a New Code from Two Given Codes

- `DirectSumCode(` C1`,` C2`)`

  `DirectSumCode` returns the direct sum of codes C1 and C2. The direct sum code consists of every codeword of C1 concatenated by every codeword of C2. Therefore, if Ci was a (ni,Mi,di) code, the result is a (n1+n2,M1*M2,min(d1,d2)) code.

  If both C1 and C2 are linear codes, the result is also a linear code. If one of them is non-linear, the direct sum is non-linear too. In general, a direct sum code is not cyclic.

  Performing a direct sum can also be done by adding two codes (see Section Operations for Codes). Another often used method is the ``u, u+v''-construction, described in UUVCode.

      gap> C1 := ElementsCode( [ [1,0], [4,5] ], GF(7) );;
      gap> C2 := ElementsCode( [ [0,0,0], [3,3,3] ], GF(7) );;
      gap> D := DirectSumCode(C1, C2);;
      gap> AsSSortedList(D);
      [ [ 1 0 0 0 0 ], [ 1 0 3 3 3 ], [ 4 5 0 0 0 ], [ 4 5 3 3 3 ] ]
      gap> D = C1 + C2;   # addition = direct sum
      true

- `UUVCode(` C1`,` C2`)`

  `UUVCode` returns the so-called (u||u+v) construction applied to C1 and C2. The resulting code consists of every codeword u of C1 concatenated by the sum of u and every codeword v of C2. If C1 and C2 have different word lengths, sufficient zeros are added to the shorter code to make this sum possible. If Ci is a (ni,Mi,di) code, the result is a (n1+max(n1,n2),M1*M2,min(2*d1,d2)) code.

  If both C1 and C2 are linear codes, the result is also a linear code. If one of them is non-linear, the UUV sum is non-linear too. In general, a UUV sum code is not cyclic.

  The function `DirectSumCode` returns another sum of codes (see DirectSumCode).

      gap> C1 := EvenWeightSubcode(WholeSpaceCode(4, GF(2)));
      a cyclic [4,3,2]1 even weight subcode
      gap> C2 := RepetitionCode(4, GF(2));
      a cyclic [4,1,4]2 repetition code over GF(2)
      gap> R := UUVCode(C1, C2);
      a linear [8,4,4]2 U U+V construction code
      gap> R = ReedMullerCode(1,3);
      true

- `DirectProductCode(` C1`,` C2`)`

  `DirectProductCode` returns the direct product of codes C1 and C2. Both must be linear codes. Suppose Ci has generator matrix Gi. The direct product of C1 and C2 then has the Kronecker product of G1 and G2 as the generator matrix (see `KroneckerProduct`).

  If Ci is a [ni, ki, di] code, the direct product then is a [n1*n2,k1*k2,d1*d2] code.

      gap> L1 := LexiCode(10, 4, GF(2));
      a linear [10,5,4]2..4 lexicode over GF(2)
      gap> L2 := LexiCode(8, 3, GF(2));
      a linear [8,4,3]2..3 lexicode over GF(2)
      gap> D := DirectProductCode(L1, L2);
      a linear [80,20,12]20..45 direct product code

- `IntersectionCode(` C1`,` C2`)`

  `IntersectionCode` returns the intersection of codes C1 and C2. This code consists of all codewords that are both in C1 and C2. If both codes are linear, the result is also linear. If both are cyclic, the result is also cyclic.

      gap> C := CyclicCodes(7, GF(2));
      [ a cyclic [7,7,1]0 enumerated code over GF(2),
        a cyclic [7,6,1..2]1 enumerated code over GF(2),
        a cyclic [7,3,1..4]2..3 enumerated code over GF(2),
        a cyclic [7,0,7]7 enumerated code over GF(2),
        a cyclic [7,3,1..4]2..3 enumerated code over GF(2),
        a cyclic [7,4,1..3]1 enumerated code over GF(2),
        a cyclic [7,1,7]3 enumerated code over GF(2),
        a cyclic [7,4,1..3]1 enumerated code over GF(2) ]
      gap> IntersectionCode(C[6], C[8]) = C[7];
      true

- `UnionCode(` C1`,` C2`)`

  `UnionCode` returns the union of codes C1 and C2. This code consists of the union of all codewords of C1 and C2 and all linear combinations. Therefore this function works only for linear codes. The function `AddedElementsCode` can be used for non-linear codes, or if the resulting code should not include linear combinations. See AddedElementsCode. If both arguments are cyclic, the result is also cyclic.

      gap> G := GeneratorMatCode([[1,0,1],[0,1,1]]*Z(2)^0, GF(2));
      a linear [3,2,1..2]1 code defined by generator matrix over GF(2)
      gap> H := GeneratorMatCode([[1,1,1]]*Z(2)^0, GF(2));
      a linear [3,1,3]1 code defined by generator matrix over GF(2)
      gap> U := UnionCode(G, H);
      a linear [3,3,1]0 union code
      gap> c := Codeword("010");; c in G;
      false
      gap> c in H;
      false
      gap> c in U;
      true

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