Grades and indices

nonzero_grades(::Type{<:AbstractCliffordNumber})
nonzero_grades(::AbstractCliffordNumber)

A function returning an indexable object representing all nonzero grades of a Clifford number representation.

This function is used to define the indexing of RepresentedGrades, and should be defined for any subtypes of AbstractCliffordNumber.

Examples

julia> CliffordNumbers.nonzero_grades(CliffordNumber{APS})
0:3

julia> CliffordNumbers.nonzero_grades(KVector{2,APS})
2:2

has_grades_of(S::Type{<:AbstractCliffordNumber}, T::Type{<:AbstractCliffordNumber}) -> Bool
has_grades_of(x::AbstractCliffordNumber, y::AbstractCliffordNumber) -> Bool

Returns true if the grades represented in S are also represented in T; false otherwise.

BitIndex{Q}

A representation of an index corresponding to a basis blade of the geometric algebra with quadratic form Q.

CliffordNumbers.is_same_blade(a::BitIndex{Q}, b::BitIndex{Q})

Checks if a and b perform identical indexing up to sign.

scalar_index(x::AbstractCliffordNumber{Q}) -> BitIndex{Q}

Constructs the BitIndex used to obtain the scalar (grade zero) portion of x.

pseudoscalar_index(x::AbstractCliffordNumber{Q}) -> BitIndex{Q}

Constructs the BitIndex used to obtain the pseudoscalar (highest grade) portion of x.

adjoint(i::BitIndex) = reverse(i::BitIndex) = i' -> BitIndex
adjoint(x::AbstractCliffordNumber) = reverse(x::AbstractCliffordNumber) = x' -> typeof(x)

Performs the reverse operation on the basis blade indexed by b or the Clifford number x. The sign of the reverse depends on the grade of the basis blade g, and is positive for g % 4 in 0:1 and negative for g % 4 in 2:3.

grade_involution(i::BitIndex) -> BitIndex
grade_involution(x::AbstractCliffordNumber) -> typeof(x)

Calculates the grade involution of the basis blade indexed by b or the Clifford number x. This effectively reflects all of the basis vectors of the space along their own mirror operation, which makes elements of odd grade flip sign.

conj(i::BitIndex) -> BitIndex
conj(x::AbstractCliffordNumber) -> typeof(x)

Calculates the Clifford conjugate of the basis blade indexed by b or the Clifford number x. This is equal to grade_involution(reverse(x)).

left_complement(b::BitIndex{Q}) -> BitIndex{Q}

Returns the left complement of b, define so that left_complement(b) * b generates the pseudoscalar index of elements of the algebra Q.

When the left complement is applied twice, the original BitIndex object is returned up to a change of sign, given by (-1)^(grade(b) * (dimension(Q) - grade(b))). This implies that in algebras of odd dimension, the left complement and [right complement](@ref right_complement) are identical because eithergrade(b)ordimension(Q) - grade(b)must be even. The complement is independent of the signature ofQ`, depending only on the dimension.

Lengyel's convention for the left complement is an underbar.

right_complement(b::BitIndex{Q}) -> BitIndex{Q}

Returns the right complement of b, define so that b * right_complement(b) generates the pseudoscalar index of elements of the algebra Q.

When the right complement is applied twice, the original BitIndex object is returned up to a change of sign, given by (-1)^(grade(b) * (dimension(Q) - grade(b))). This implies that in algebras of odd dimension, the [left complement](@ref left_complement) and right complement are identical because eithergrade(b)ordimension(Q) - grade(b)must be even. The complement is independent of the signature ofQ`, depending only on the dimension.

Lengyel's convention for the right complement is an overbar.

grade(i::BitIndex) -> Int

Returns the grade of the basis blade represented by i, which ranges from 0 to the dimension of the space represented by i (equal to dimension(signature(i))).

CliffordNumbers.sign_of_square(b::BitIndex) -> Int8

Returns the sign associated with squaring the basis blade indexed by b using an Int8 as proxy: positive signs return Int8(1), negative signs return Int8(-1), and zeros from degenerate components return Int8(0).

CliffordNumbers.signbit_of_square(b::BitIndex) -> Bool

Returns the signbit associated with squaring the basis blade indexed by b.

For basis blades squaring to zero, the result is not meaningful.

CliffordNumbers.nondegenerate_square(b::BitIndex) -> Bool

Returns false if squaring the basis blade b is zero due to a degenerate component, true otherwise. For a nondegenerate metric, this is always true.

CliffordNumbers.sign_of_mult(a::T, b::T) where T<:BitIndex -> Int8

Returns an Int8 that carries the sign associated with the multiplication of two basis blades of Clifford/geometric algebras of the same quadratic form.

CliffordNumbers.signbit_of_mult(a::Integer, [b::Integer]) -> Bool
CliffordNumbers.signbit_of_mult(a::BitIndex, [b::BitIndex]) -> Bool

Calculates the sign bit associated with multiplying basis elements indexed with bit indices supplied as either integers or BitIndex instances. The sign bit flips when the order of a and b are reversed, unless a === b.

As with Base.signbit(), true represents a negative sign and false a positive sign. However, in degenerate metrics (such as those of projective geometric algebras) the sign bit may be irrelevant as the multiplication of those basis blades would result in zero.

CliffordNumbers.nondegenerate_mult(a::T, b::T) where T<:BitIndex -> Bool

Returns false if the product of a and b is zero due to the squaring of a degenerate component, true otherwise. This function always returns true if R === 0.

*(a::BitIndex{Q}, b::BitIndex{Q}) -> BitIndex{Q}

Returns the BitIndex corresponding to the basis blade resulting from the geometric product of the basis blades indexed by a and b.

CliffordNumbers.has_wedge(a::BitIndex{Q}, b::BitIndex{Q}, [c::BitIndex{Q}...]) -> Bool

Returns true if the basis blades indexed by a, b, or any other blades c... have a nonzero wedge product; false otherwise. This is determined by comparing all bits of the arguments (except the sign bit) to identify any matching basis blades using bitwise AND.

AbstractBitIndices{Q,C<:AbstractCliffordNumber{Q}} <: AbstractVector{BitIndex{Q}}

Supertype for vectors containing all valid BitIndex{Q} objects for the basis elements represented by C.

BitIndices{Q,C<:AbstractCliffordNumber{Q,<:Any}} <: AbstractVector{BitIndex{Q}}

Represents a range of valid BitIndex objects for the nonzero components of a given multivector of algebra Q.

For a generic AbstractCliffordNumber{Q}, this returns BitIndices{Q,CliffordNumber{Q}}, which contains all possible indices for a multivector associated with the algebra parameter Q. For sparser representations, such as KVector{K,Q}, the object only contains the indices of the nonzero elements of the multivector.

Construction

BitIndices can be constructed by calling the type constructor with the Clifford number or its type. This will automatically strip some type parameters so that identical BitIndices objects are constructed regardless of the scalar type.

For this reason, you should not use BitIndices{Q,C}(); instead use BitIndices(C).

Indexing

BitIndices always uses one-based indexing like most Julia arrays. Although it is more natural in the dense case to use zero-based indexing, as the basis blades are naturally encoded in the indices for the dense representation of CliffordNumber, one-based indexing is used by the tuples which contain the data associated with this package's implementations of Clifford numbers.

Broadcasting

Because BitIndices(x) only lazily references the indices of x, we define a new type, TransformedBitIndices, which allows for a function f to be lazily associated with the BitIndices object, and this type is constructed when a f.(BitIndices(x)) is called.

Interfaces for new subtypes of AbstractCliffordNumber

When defining the behavior of BitIndices for new subtypes T of AbstractCliffordNumber, Base.getindex(::BitIndices{Q,T}, i::Integer) should be defined so that all indices of T that are not constrained to be zero are returned.

You should also define CliffordNumbers.bitindices_type(::Type{T}) so that type parameters that do not affect the construction of the BitIndices object are stripped.

TransformedBitIndices{Q,C,F} <: AbstractBitIndices{Q,C}

Lazy representation of BitIndices{Q,C} with some function of type f applied to each element. These objects can be used to perform common operations which act on basis blades or grades, such as the reverse or grade involution.