# This file is a part of Groebner.jl. License is GNU GPL v2.

# Parts of this file were adapted from msolve:
# https://github.com/algebraic-solving/msolve
# msolve is distributed under GNU GPL v2+:
# https://github.com/algebraic-solving/msolve/blob/master/COPYING

# y, s.t. y >= x and n | y.
align_up(x::Integer, n::Integer) = (x + (n - 1)) & (~(n - 1))

# y, s.t y <= x and n | y.
align_down(x::Integer, n::Integer) = x âŠ» (n - 1)

### 
# Sorting monomials, polynomials, and other things.

_default_sorting_alg() = Base.Sort.DEFAULT_UNSTABLE

# Sorts arr at the range of indices from..to. 
# This function is perhaps type unstable
function sort_part!(
    arr::AbstractVector{T},
    from::Int,
    to::Int;
    lt=isless,
    alg=_default_sorting_alg(),
    by=identity,
    scratch::Union{Vector{T}, Nothing}=nothing
) where {T}
    from > to && return nothing
    ordr = Base.Sort.ord(lt, by, nothing)
    sort!(arr, from, to, alg, ordr, scratch)
    nothing
end

# Permute a part of the array from the given index according to the permutation.
function permute_array!(
    arr::AbstractVector{T},
    perm::Vector{I},
    buf::Vector{T},
    from::Int,
    sz::Int
) where {T, I}
    @invariant length(buf) >= sz && sz <= length(perm)
    @invariant from + sz - 1 <= length(arr)
    @inbounds for i in 1:sz
        buf[i] = arr[perm[i]]
    end
    @inbounds for i in 1:sz
        arr[from + i - 1] = buf[i]
    end
    nothing
end

function sort_polys_by_lead_increasing!(
    basis::Basis,
    hashtable::MonomialHashtable,
    changematrix::Bool;
    ord::Ord=hashtable.ord
) where {Ord <: AbstractMonomialOrdering}
    b_monoms = basis.monoms
    h_monoms = hashtable.monoms
    permutation = collect(1:(basis.n_filled))
    cmps =
        (x, y) -> monom_isless(
            @inbounds(h_monoms[b_monoms[x][1]]),
            @inbounds(h_monoms[b_monoms[y][1]]),
            ord
        )

    # stable sort to preserve the order of polynomials with the same lead
    sort!(permutation, lt=cmps, alg=Base.Sort.DEFAULT_STABLE)

    # use array assignment insted of elemewise assignment
    # (seems to compile to better code)
    basis.monoms[1:(basis.n_filled)] = basis.monoms[permutation]
    basis.coeffs[1:(basis.n_filled)] = basis.coeffs[permutation]

    if changematrix
        @invariant length(basis.changematrix) >= basis.n_filled
        @inbounds basis.changematrix[1:(basis.n_filled)] = basis.changematrix[permutation]
    end

    permutation
end

function is_sorted_by_lead_increasing(
    basis::Basis,
    hashtable::MonomialHashtable,
    ord::Ord=hashtable.ord
) where {Ord <: AbstractMonomialOrdering}
    b_monoms = basis.monoms
    h_monoms = hashtable.monoms
    permutation = collect(1:(basis.n_filled))
    cmps =
        (x, y) -> monom_isless(
            @inbounds(h_monoms[b_monoms[x][1]]),
            @inbounds(h_monoms[b_monoms[y][1]]),
            ord
        )
    issorted(permutation, lt=cmps)
end

function sort_pairset_by_degree!(pairset::Pairset, from::Int, sz::Int)
    pairs = pairset.pairs
    degs = pairset.degrees
    if length(pairset.scratch4) < sz
        resize!(pairset.scratch4, nextpow(2, sz + 1))
    end
    permutation = pairset.scratch4
    for i in 1:sz
        permutation[i] = from + i - 1
    end
    sort_part!(permutation, 1, sz, by=i -> degs[i], scratch=pairset.scratch1)
    if length(pairset.scratch2) < length(permutation)
        resize!(pairset.scratch2, nextpow(2, length(permutation) + 1))
    end
    if length(pairset.scratch3) < length(permutation)
        resize!(pairset.scratch3, nextpow(2, length(permutation) + 1))
    end
    permute_array!(pairs, permutation, pairset.scratch2, from, sz)
    permute_array!(degs, permutation, pairset.scratch3, from, sz)
    nothing
end

function sort_pairset_by_lcm!(pairset::Pairset, npairs::Int, hashtable::MonomialHashtable)
    monoms = hashtable.monoms
    pairs = pairset.pairs
    cmps =
        (i, j) -> monom_isless(
            @inbounds(monoms[pairs[i].lcm]),
            @inbounds(monoms[pairs[j].lcm]),
            hashtable.ord
        )
    permutation = collect(1:npairs)
    sort_part!(permutation, 1, npairs, lt=cmps, scratch=pairset.scratch1)
    if length(pairset.scratch2) < length(permutation)
        resize!(pairset.scratch2, nextpow(2, length(permutation) + 1))
    end
    if length(pairset.scratch3) < length(permutation)
        resize!(pairset.scratch3, nextpow(2, length(permutation) + 1))
    end
    permute_array!(pairs, permutation, pairset.scratch2, 1, npairs)
    permute_array!(pairset.degrees, permutation, pairset.scratch3, 1, npairs)
    nothing
end

function sort_generators_by_position!(polys::Vector{Int}, load::Int)
    sort_part!(polys, 1, load)
end

###
# Sorting matrix rows and columns.

function matrix_row_decreasing_cmp(a::Vector{T}, b::Vector{T}) where {T <: ColumnLabel}
    # Compare the indices of the leading columns
    @inbounds va = a[1]
    @inbounds vb = b[1]
    if va > vb
        return false
    end
    va < vb
end

function matrix_row_increasing_cmp(a::Vector{T}, b::Vector{T}) where {T <: ColumnLabel}
    # Compare the indices of the leading columns
    @inbounds va = a[1]
    @inbounds vb = b[1]
    if va > vb
        return true
    end
    if va < vb
        return false
    end
    # Compare the density of rows
    va = length(a)
    vb = length(b)
    if va > vb
        return false
    end
    if va < vb
        return true
    end
    return false
end

function sort_matrix_upper_rows!(matrix::MacaulayMatrix)
    #= smaller means pivot being more to the left  =#
    #= and density being smaller                   =#
    permutation = collect(1:(matrix.nrows_filled_upper))
    # TODO: use "let" here!
    cmp =
        (x, y) -> matrix_row_decreasing_cmp(
            @inbounds(matrix.upper_rows[x]),
            @inbounds(matrix.upper_rows[y])
        )
    sort!(permutation, lt=cmp, alg=_default_sorting_alg())
    matrix.upper_rows[1:(matrix.nrows_filled_upper)] = matrix.upper_rows[permutation]
    matrix.upper_to_coeffs[1:(matrix.nrows_filled_upper)] = matrix.upper_to_coeffs[permutation]
    # TODO: this is a bit hacky
    if !isempty(matrix.upper_to_mult)
        matrix.upper_to_mult[1:(matrix.nrows_filled_upper)] = matrix.upper_to_mult[permutation]
    end
    matrix
end

function sort_matrix_lower_rows!(matrix::MacaulayMatrix)
    #= smaller means pivot being more to the right =#
    #= and density being larger                    =#
    permutation = collect(1:(matrix.nrows_filled_lower))
    cmp =
        (x, y) -> matrix_row_increasing_cmp(
            @inbounds(matrix.lower_rows[x]),
            @inbounds(matrix.lower_rows[y])
        )
    sort!(permutation, lt=cmp, alg=_default_sorting_alg())
    matrix.lower_rows[1:(matrix.nrows_filled_lower)] = matrix.lower_rows[permutation]
    matrix.lower_to_coeffs[1:(matrix.nrows_filled_lower)] = matrix.lower_to_coeffs[permutation]
    # TODO: this is a bit hacky
    if !isempty(matrix.lower_to_mult)
        matrix.lower_to_mult[1:(matrix.nrows_filled_lower)] = matrix.lower_to_mult[permutation]
    end
    matrix
end

function sort_partition_columns_by_labels!(
    column_to_monom::Vector{T},
    symbol_ht::MonomialHashtable
) where {T}
    labels = symbol_ht.labels
    m = length(column_to_monom)
    i, j = 0, m + 1
    @inbounds while true
        i += 1
        j -= 1
        while i < m && labels[i + 1] == PIVOT_COLUMN
            i += 1
        end
        while j > 1 && (labels[j + 1] == NON_PIVOT_COLUMN || labels[j + 1] == UNKNOWN_PIVOT_COLUMN)
            j -= 1
        end
        i >= j && break
        column_to_monom[i], column_to_monom[j] = column_to_monom[j], column_to_monom[i]
    end
    nothing
end

function sort_input_terms_to_change_ordering!(
    exps::Vector{Vector{M}},
    coeffs::Vector{Vector{C}},
    ord::AbstractMonomialOrdering
) where {M <: Monom, C <: Coeff}
    permutations = Vector{Vector{Int}}(undef, length(exps))
    @inbounds for polyidx in 1:length(exps)
        comparator =
            (x, y) -> monom_isless(@inbounds(exps[polyidx][y]), @inbounds(exps[polyidx][x]), ord)

        permutation = collect(1:length(exps[polyidx]))
        sort!(permutation, lt=comparator, alg=_default_sorting_alg())

        exps[polyidx][1:end] = exps[polyidx][permutation]
        coeffs[polyidx][1:end] = coeffs[polyidx][permutation]

        permutations[polyidx] = permutation
    end
    permutations
end
