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Sketch backtracking Newton's method

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This code is a mess, but I'm committing it to record a working state
before I start trying to clean up.
This commit is contained in:
Aaron Fenyes 2024-07-15 11:32:04 -07:00
parent 3910b9f740
commit 25b09ebf92
4 changed files with 254 additions and 8 deletions
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237
engine-proto/gram-test/Engine.jl
View File

@ -1,8 +1,10 @@
module Engine
using LinearAlgebra
using GenericLinearAlgebra
using SparseArrays
using Random
using Optim
export rand_on_shell, Q, DescentHistory, realize_gram
@ -76,8 +78,11 @@ end
struct DescentHistory{T}
scaled_loss::Array{T}
neg_grad::Array{Matrix{T}}
base_step::Array{Matrix{T}}
hess::Array{Hermitian{T, Matrix{T}}}
slope::Array{T}
stepsize::Array{T}
used_grad::Array{Bool}
backoff_steps::Array{Int64}
last_line_L::Array{Matrix{T}}
last_line_loss::Array{T}
@ -85,13 +90,16 @@ struct DescentHistory{T}
function DescentHistory{T}(
scaled_loss = Array{T}(undef, 0),
neg_grad = Array{Matrix{T}}(undef, 0),
hess = Array{Hermitian{T, Matrix{T}}}(undef, 0),
base_step = Array{Matrix{T}}(undef, 0),
slope = Array{T}(undef, 0),
stepsize = Array{T}(undef, 0),
used_grad = Bool[],
backoff_steps = Int64[],
last_line_L = Array{Matrix{T}}(undef, 0),
last_line_loss = Array{T}(undef, 0)
) where T
new(scaled_loss, neg_grad, slope, stepsize, backoff_steps, last_line_L, last_line_loss)
new(scaled_loss, neg_grad, hess, base_step, slope, stepsize, used_grad, backoff_steps, last_line_L, last_line_loss)
end
end
@ -101,7 +109,7 @@ function realize_gram_gradient(
gram::SparseMatrixCSC{T, <:Any},
guess::Matrix{T};
scaled_tol = 1e-30,
target_improvement = 0.5,
min_efficiency = 0.5,
init_stepsize = 1.0,
backoff = 0.9,
max_descent_steps = 600,
@ -152,7 +160,7 @@ function realize_gram_gradient(
improvement = loss_last - loss
push!(history.last_line_L, L)
push!(history.last_line_loss, loss / scale_adjustment)
if improvement >= target_improvement * stepsize * slope
if improvement >= min_efficiency * stepsize * slope
history.backoff_steps[end] = backoff_steps
break
end
@ -201,7 +209,7 @@ function realize_gram_newton(
scale_adjustment = sqrt(T(length(constrained)))
tol = scale_adjustment * scaled_tol
# use newton's method
# use Newton's method
L = copy(guess)
for step in 0:max_steps
# evaluate the loss function
@ -229,8 +237,10 @@ function realize_gram_newton(
deriv_grad = 4*Q*(-basis_mat*Δ_proj + L*neg_dΔ_proj)
hess[:, (k-1)*element_dim + j] = reshape(deriv_grad, total_dim)
end
hess = Hermitian(hess)
push!(history.hess, hess)
# compute the newton step
# compute the Newton step
step = hess \ reshape(neg_grad, total_dim)
L += rate * reshape(step, dims)
end
@ -239,4 +249,221 @@ function realize_gram_newton(
L, history
end
LinearAlgebra.eigen!(A::Symmetric{BigFloat, Matrix{BigFloat}}; sortby::Nothing) =
eigen!(Hermitian(A))
function realize_gram_optim(
gram::SparseMatrixCSC{T, <:Any},
guess::Matrix{T}
) where T <: Number
# find the dimension of the search space
dims = size(guess)
element_dim, construction_dim = dims
total_dim = element_dim * construction_dim
# list the constrained entries of the gram matrix
J, K, _ = findnz(gram)
constrained = zip(J, K)
# scale the loss function
scale_adjustment = length(constrained)
function loss(L_vec)
L = reshape(L_vec, dims)
Δ_proj = proj_diff(gram, L'*Q*L)
dot(Δ_proj, Δ_proj) / scale_adjustment
end
function loss_grad!(storage, L_vec)
L = reshape(L_vec, dims)
Δ_proj = proj_diff(gram, L'*Q*L)
storage .= reshape(-4*Q*L*Δ_proj, total_dim) / scale_adjustment
end
function loss_hess!(storage, L_vec)
L = reshape(L_vec, dims)
Δ_proj = proj_diff(gram, L'*Q*L)
indices = [(j, k) for k in 1:construction_dim for j in 1:element_dim]
for (j, k) in indices
basis_mat = basis_matrix(T, j, k, dims)
neg_dΔ = basis_mat'*Q*L + L'*Q*basis_mat
neg_dΔ_proj = proj_to_entries(neg_dΔ, constrained)
deriv_grad = 4*Q*(-basis_mat*Δ_proj + L*neg_dΔ_proj) / scale_adjustment
storage[:, (k-1)*element_dim + j] = reshape(deriv_grad, total_dim)
end
end
optimize(
loss, loss_grad!, loss_hess!,
reshape(guess, total_dim),
NewtonTrustRegion()
)
end
# seek a matrix `L` for which `L'QL` matches the sparse matrix `gram` at every
# explicit entry of `gram`. use gradient descent starting from `guess`
function realize_gram(
gram::SparseMatrixCSC{T, <:Any},
guess::Matrix{T};
scaled_tol = 1e-30,
min_efficiency = 0.5,
init_rate = 1.0,
backoff = 0.9,
reg_scale = 1.1,
max_descent_steps = 200,
max_backoff_steps = 110
) where T <: Number
# start history
history = DescentHistory{T}()
# find the dimension of the search space
dims = size(guess)
element_dim, construction_dim = dims
total_dim = element_dim * construction_dim
# list the constrained entries of the gram matrix
J, K, _ = findnz(gram)
constrained = zip(J, K)
# scale the tolerance
scale_adjustment = sqrt(T(length(constrained)))
tol = scale_adjustment * scaled_tol
# initialize variables
grad_rate = init_rate
L = copy(guess)
# use Newton's method with backtracking and gradient descent backup
Δ_proj = proj_diff(gram, L'*Q*L)
loss = dot(Δ_proj, Δ_proj)
for step in 1:max_descent_steps
# stop if the loss is tolerably low
if loss < tol
break
end
# find the negative gradient of loss function
neg_grad = 4*Q*L*Δ_proj
# find the negative Hessian of the loss function
hess = Matrix{T}(undef, total_dim, total_dim)
indices = [(j, k) for k in 1:construction_dim for j in 1:element_dim]
for (j, k) in indices
basis_mat = basis_matrix(T, j, k, dims)
neg_dΔ = basis_mat'*Q*L + L'*Q*basis_mat
neg_dΔ_proj = proj_to_entries(neg_dΔ, constrained)
deriv_grad = 4*Q*(-basis_mat*Δ_proj + L*neg_dΔ_proj)
hess[:, (k-1)*element_dim + j] = reshape(deriv_grad, total_dim)
end
hess = Hermitian(hess)
push!(history.hess, hess)
# choose a base step: the Newton step if the Hessian is non-singular, and
# the gradient descent direction otherwise
#=
sing = false
base_step = try
reshape(hess \ reshape(neg_grad, total_dim), dims)
catch ex
if isa(ex, SingularException)
sing = true
normalize(neg_grad)
else
throw(ex)
end
end
=#
#=
if !sing
rate = one(T)
end
=#
#=
if cond(Float64.(hess)) < 1e5
sing = false
base_step = reshape(hess \ reshape(neg_grad, total_dim), dims)
else
sing = true
base_step = normalize(neg_grad)
end
=#
#=
if cond(Float64.(hess)) > 1e3
sing = true
hess += big"1e-5"*I
else
sing = false
end
base_step = reshape(hess \ reshape(neg_grad, total_dim), dims)
=#
min_eigval = minimum(eigvals(hess))
if min_eigval < 0
hess -= reg_scale * min_eigval * I
end
push!(history.used_grad, false)
base_step = reshape(hess \ reshape(neg_grad, total_dim), dims)
push!(history.base_step, base_step)
#=
push!(history.used_grad, sing)
=#
# store the current position, loss, and slope
L_last = L
loss_last = loss
push!(history.scaled_loss, loss / scale_adjustment)
push!(history.neg_grad, neg_grad)
push!(history.slope, norm(neg_grad))
# find a good step size using backtracking line search
push!(history.stepsize, 0)
push!(history.backoff_steps, max_backoff_steps)
empty!(history.last_line_L)
empty!(history.last_line_loss)
rate = one(T)
for backoff_steps in 0:max_backoff_steps
history.stepsize[end] = rate
# try Newton step, but not on the first step. doing at least one step of
# gradient descent seems to help prevent getting stuck, for some reason?
if step > 0
L = L_last + rate * base_step
Δ_proj = proj_diff(gram, L'*Q*L)
loss = dot(Δ_proj, Δ_proj)
improvement = loss_last - loss
push!(history.last_line_L, L)
push!(history.last_line_loss, loss / scale_adjustment)
if improvement >= min_efficiency * rate * dot(neg_grad, base_step)
history.backoff_steps[end] = backoff_steps
break
end
end
# try gradient descent step
slope = norm(neg_grad)
dir = neg_grad / slope
L = L_last + rate * grad_rate * dir
Δ_proj = proj_diff(gram, L'*Q*L)
loss = dot(Δ_proj, Δ_proj)
improvement = loss_last - loss
if improvement >= min_efficiency * rate * grad_rate * slope
grad_rate *= rate
history.used_grad[end] = true
history.backoff_steps[end] = backoff_steps
break
end
rate *= backoff
end
# [DEBUG] if we've hit a wall, quit
if history.backoff_steps[end] == max_backoff_steps
return L_last, history
end
end
# return the factorization and its history
push!(history.scaled_loss, loss / scale_adjustment)
L, history
end
end

7
engine-proto/gram-test/circles-in-triangle.jl
View File

@ -81,12 +81,16 @@ guess = hcat(
=#
# complete the gram matrix using gradient descent followed by Newton's method
#=
L, history = Engine.realize_gram_gradient(gram, guess, scaled_tol = 0.01)
L_pol, history_pol = Engine.realize_gram_newton(gram, L, rate = 0.3, scaled_tol = 1e-9)
L_pol2, history_pol2 = Engine.realize_gram_newton(gram, L_pol)
=#
L, history = Engine.realize_gram(Float64.(gram), Float64.(guess))
completed_gram = L'*Engine.Q*L
println("Completed Gram matrix:\n")
display(completed_gram)
#=
println(
"\nSteps: ",
size(history.scaled_loss, 1),
@ -94,3 +98,6 @@ println(
" + ", size(history_pol2.scaled_loss, 1)
)
println("Loss: ", history_pol2.scaled_loss[end], "\n")
=#
println("\nSteps: ", size(history.scaled_loss, 1))
println("Loss: ", history.scaled_loss[end], "\n")

11
engine-proto/gram-test/overlapping-pyramids.jl
View File

@ -47,17 +47,25 @@ guess = hcat(
Engine.rand_on_shell(fill(BigFloat(-1), 2))
)
# complete the gram matrix using gradient descent followed by Newton's method
# complete the gram matrix
#=
L, history = Engine.realize_gram_gradient(gram, guess, scaled_tol = 0.01)
L_pol, history_pol = Engine.realize_gram_newton(gram, L)
=#
L, history = Engine.realize_gram(Float64.(gram), Float64.(guess))
completed_gram = L'*Engine.Q*L
println("Completed Gram matrix:\n")
display(completed_gram)
#=
println("\nSteps: ", size(history.scaled_loss, 1), " + ", size(history_pol.scaled_loss, 1))
println("Loss: ", history_pol.scaled_loss[end], "\n")
=#
println("\nSteps: ", size(history.scaled_loss, 1))
println("Loss: ", history.scaled_loss[end], "\n")
# === algebraic check ===
#=
R, gens = polynomial_ring(AbstractAlgebra.Rationals{BigInt}(), ["x", "t₁", "t₂", "t₃"])
x = gens[1]
t = gens[2:4]
@ -85,3 +93,4 @@ x_constraint = 25//16 * to_univariate(S, evaluate(rank_constraints[1], [2], [ind
t₂_constraint = 25//16 * to_univariate(S, evaluate(rank_constraints[3], [2], [indep_val]))
x_vals = PolynomialRoots.roots(x_constraint.coeffs)
t₂_vals = PolynomialRoots.roots(t₂_constraint.coeffs)
=#

5
engine-proto/gram-test/sphere-in-tetrahedron.jl
View File

@ -33,8 +33,11 @@ guess = sqrt(1/BigFloat(3)) * BigFloat[
1 1 1 1 1
] + 0.2*Engine.rand_on_shell(fill(BigFloat(-1), 5))
# complete the gram matrix using Newton's method
# complete the gram matrix
#=
L, history = Engine.realize_gram_newton(gram, guess)
=#
L, history = Engine.realize_gram(gram, guess, max_descent_steps = 50)
completed_gram = L'*Engine.Q*L
println("Completed Gram matrix:\n")
display(completed_gram)