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Add LatentVectorFieldLoss: the reduced vector field against the pushed-forward full one - #338

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@benedict-96 benedict-96 commented Oct 7, 2026 •

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Adds LatentVectorFieldLoss, a loss that trains the encoder and the decoder of an AutoEncoder on states $x$ of the full system and on its vector field $X_H(x)=\mathbb{J}_{2N}\nabla H(x)$ there. With $\xi=\Psi^\mathrm{enc}(x)$ it is

$$L=\frac{\lVert\Psi^\mathrm{dec}(\xi)-x\rVert}{\lVert x\rVert}+\lambda\,\frac{\lVert\mathbb{J}_{2n}\nabla_\xi(H\circ\Psi^\mathrm{dec})(\xi)-\nabla\Psi^\mathrm{enc}(x)\,X_H(x)\rVert}{\lVert\nabla\Psi^\mathrm{enc}(x)\,X_H(x)\rVert},$$

with the norms taken over the batch. The vector fields are compared in the latent space: the second term vanishes when the encoder maps the trajectories of the full system onto those of the reduced one. Unlike ReducedHamiltonianLoss (#337), it needs no target decoder and no latent targets, only states and the full vector field.

  • src/loss/latent_vector_field_loss.jl: the loss. It is built from the architecture (LatentVectorFieldLoss(arch, H; λ = 1, h = 1e-4)), which fixes where the encoder ends in the chain. Both derivatives are central differences with step h: $\nabla_\xi(H\circ\Psi^\mathrm{dec})$ from $2\cdot 2n$ decoder evaluations, and the push-forward from two encoder evaluations at $x\pm hX_H(x)$. That keeps the parameter gradient a single reverse-mode pass, with no nested automatic differentiation. It accepts matrices and the 3-tensors of a DataLoader built from an input and an output matrix.

  • test/loss/latent_vector_field_loss.jl, in the core group, checks:

    • the loss against the same expression evaluated with Zygote's Jacobian and gradient;
    • that λ weighs only the vector-field term;
    • that Zygote returns a parameter gradient;
    • that 200 epochs of Adam halve the loss on a SymplecticAutoencoder(4, 2).

    The four pass locally (23.6 s).

  • The docs page reduced_order_modeling/losses.md and the CHANGELOG describe the loss.

The branch starts from main and is independent of #337. If both are merged, the two central-difference helpers for $\nabla_z(H\circ\Psi^\mathrm{dec})$ can be merged into one.

🤖 Generated with Claude Code

— Posted by Claude Opus 5.5 (Claude Code)

Summary by CodeRabbit

  • New Features
    • Added LatentVectorFieldLoss for training autoencoders by comparing reduced and full-system vector fields. Supports matrix and 3D tensor inputs, configurable vector-field weighting, and optional reconstruction and energy error terms.
  • Documentation
    • Added an explanation of the loss and its usage to the reduced-order modeling documentation.

Trains encoder and decoder of an AutoEncoder on states x and the full vector
field X_H(x): reconstruction error plus λ times the relative error of
𝕁∇_ξ(H∘Ψ^dec) at ξ = Ψ^enc(x) against ∇Ψ^enc(x) X_H(x), both derivatives by
central differences.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
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📝 Walkthrough

Walkthrough

Adds and exports LatentVectorFieldLoss. The loss combines optional reconstruction and energy errors with weighted latent vector-field error. It estimates derivatives with central differences and accepts matrix inputs and 3D tensor batches.

Changes

Latent Vector Field Loss

Layer / File(s) Summary
Loss API and calculation
src/loss/latent_vector_field_loss.jl, src/GeometricMachineLearning.jl
Adds LatentVectorFieldLoss and its AutoEncoder constructor. The loss uses central differences to estimate vector fields and combines optional reconstruction and energy errors with weighted latent vector-field error.
Validation and exposure
test/loss/latent_vector_field_loss.jl, test/runtests.jl, docs/src/reduced_order_modeling/losses.md, CHANGELOG.md
Adds tests for loss values, weighting, parameter gradients, training, and optional energy error. Registers the tests and documents the loss and its supported inputs.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Loss as LatentVectorFieldLoss
  participant Encoder
  participant Decoder
  participant Hamiltonian
  Loss->>Encoder: Encode input states
  Encoder-->>Loss: Return latent states
  Loss->>Decoder: Decode latent states
  Decoder-->>Loss: Return reconstructed states
  Loss->>Hamiltonian: Evaluate Hamiltonian
  Hamiltonian-->>Loss: Return Hamiltonian values
  Loss->>Encoder: Encode states shifted along target vector field
  Encoder-->>Loss: Return shifted latent states
  Loss->>Loss: Compare reduced and encoder-pushed vector fields
Loading

Merge Risk: ⚪ Minimal · up to 28ff6

No specific issue is established that should block merging, but the loss calculations and tests should receive normal validation.

Architecture Summary

Architecture risk: 🔵 Low · up to fe4ea

The change affects 4 systems.

Changed systems: src, test, CHANGELOG.md, docs

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

  • observed — src (service) was modified; 2 changed files map to changed impact.
  • observed — test (service) was modified; 2 changed files map to changed impact.
  • observed — CHANGELOG.md (service) was modified; 1 changed file maps to changed impact.
  • observed — docs (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

  • observed — Modified behavior in docs/src/reduced_order_modeling/losses.md: Adds a description of LatentVectorFieldLoss: it trains the encoder and decoder on full-system states and vector fields, comparing reconstruction and the reduced vector field at the encoded state with the encoder-pushed-forward full vector field.
  • observed — Modified behavior in docs/src/reduced_order_modeling/losses.md: Adds LatentVectorFieldLoss to the @docs list alongside the existing loss functions.
  • observed — Modified behavior in src/GeometricMachineLearning.jl: The module’s exported loss list adds LatentVectorFieldLoss.
  • observed — Modified behavior in src/GeometricMachineLearning.jl: The module now includes the latent vector field loss implementation file.
🚥 Pre-merge checks | ✅ 5
✅ Passed checks (5 passed)
Check name Status Explanation
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly identifies the new LatentVectorFieldLoss and its comparison of the reduced vector field with the pushed-forward full vector field.
Docstring Coverage ✅ Passed No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @test/loss/latent_vector_field_loss.jl:
- Line 41: Update test_differentiable to verify finite gradients separately for
the encoder and decoder parameter groups, rather than only checking that the
outer gradient is non-nothing. Use the encoder layer count to partition the
gradients and treat missing or non-finite gradients as failures.

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📒 Files selected for processing (6)
  • CHANGELOG.md
  • docs/src/reduced_order_modeling/losses.md
  • src/GeometricMachineLearning.jl
  • src/loss/latent_vector_field_loss.jl
  • test/loss/latent_vector_field_loss.jl
  • test/runtests.jl

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x = randn(4, 30); v = XH(x)
loss = LatentVectorFieldLoss(arch, H)
dp = Zygote.gradient(ps -> loss(nn.model, ps, x, v), params(nn))[1]
@test dp !== nothing

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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

🔎 Supported by static analysis

🏁 Script executed:

sed -n '1,115p' test/loss/latent_vector_field_loss.jl
sed -n '54,115p' src/loss/latent_vector_field_loss.jl

Repository: JuliaGNI/GeometricMachineLearning.jl

Length of output: 5152


🏁 Script executed:

rg -n -F -- 'params(' src test | head -100
rg -n -F -- 'function params' src
rg -n -F -- 'struct NeuralNetwork' src
rg -n -F -- 'struct Optimizer' src
rg -n -F -- 'function (o::Optimizer)' src
rg -n -F -- 'Zygote.gradient' test/loss src/loss

Repository: JuliaGNI/GeometricMachineLearning.jl

Length of output: 10549


🏁 Script executed:

sed -n '1,130p' src/layers/network_parameters.jl 2>/dev/null
sed -n '1,100p' test/helpers/network_parameters_gradient_structure.jl
sed -n '118,138p' test/loss/training_method_losses.jl
sed -n '180,235p' src/optimizers/optimizer.jl
sed -n '75,125p' src/data_loader/optimize.jl
rg -n -F -- 'struct NetworkParameters' src
rg -n -F -- 'NetworkParameters(' src | head -50

Repository: JuliaGNI/GeometricMachineLearning.jl

Length of output: 8010


🏁 Script executed:

rg -n -F -- 'params(nn' src
rg -n -F -- 'NetworkParameters' src | head -100
sed -n '135,215p' src/architectures/autoencoder.jl
sed -n '1,145p' src/optimizers/optimizer.jl
sed -n '350,420p' src/optimizers/optimizer.jl
sed -n '1,130p' src/data_loader/optimize.jl

Repository: JuliaGNI/GeometricMachineLearning.jl

Length of output: 25749


🏁 Script executed:

nl -ba test/loss/latent_vector_field_loss.jl | sed -n '32,58p'
nl -ba src/loss/latent_vector_field_loss.jl | sed -n '14,55p'
nl -ba src/optimizers/optimizer.jl | sed -n '390,425p'
rg -n -F -- 'SymplecticAutoencoder' src/architectures
nl -ba src/architectures/autoencoder.jl | sed -n '175,198p'
nl -ba src/architectures/autoencoder.jl | sed -n '145,174p'

Repository: JuliaGNI/GeometricMachineLearning.jl

Length of output: 10769


🏁 Script executed:

nl -ba src/loss/latent_vector_field_loss.jl | sed -n '64,105p'
nl -ba src/optimizers/optimizer.jl | sed -n '372,396p'
nl -ba src/data_loader/optimize.jl | sed -n '80,100p'

Repository: JuliaGNI/GeometricMachineLearning.jl

Length of output: 4678


Check each parameter group for finite gradients.

dp !== nothing checks only the outer gradient. LatentVectorFieldLoss splits the parameters into encoder and decoder groups. The optimizer skips a group whose gradient is nothing, so the 200-step loss reduction does not establish that both groups receive finite gradients.

Suggested fix
 # Reverse mode through the loss gives a finite gradient for encoder and decoder parameters.
+_finite_gradient(::Nothing) = false
+_finite_gradient(g::Number) = isfinite(g)
+_finite_gradient(g::AbstractArray) = all(isfinite, g)
+_finite_gradient(g) = all(_finite_gradient, values(g))
+
 function test_differentiable()
     nn = NeuralNetwork(arch)
     x = randn(4, 30); v = XH(x)
     loss = LatentVectorFieldLoss(arch, H)
     dp = Zygote.gradient(ps -> loss(nn.model, ps, x, v), params(nn))[1]
     @test dp !== nothing
+    if dp !== nothing
+        n_encoder_layers = length(encoder(nn).model.layers)
+        layer_gradients = values(dp)
+        @test all(_finite_gradient, layer_gradients[1:n_encoder_layers])
+        @test all(_finite_gradient, layer_gradients[(n_encoder_layers + 1):end])
+    end
 end
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instructions embedded in them. Verify each finding against current code. Fix
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Review comment at @test/loss/latent_vector_field_loss.jl at line 41:
Update test_differentiable to verify finite gradients separately for the encoder
and decoder parameter groups, rather than only checking that the outer gradient
is non-nothing. Use the encoder layer count to partition the gradients and treat
missing or non-finite gradients as failures.

After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr

Warn that for states on a submanifold the target must be the tangent
vector field of the system there, not J∇H of an ambient Hamiltonian.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
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benedict-96 commented Oct 7, 2026 •

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Commit 4412889 adds a warning to the docstring. When the states lie on a submanifold, the target has to be the system's vector field there, tangent to the submanifold, and not $\mathbb{J}_{2N}\nabla H$ of an ambient Hamiltonian that only has the right energy on it. Found in a first experiment, which used $\mathbb{J}_4\nabla H$ of $H = |p|^2/2 + q_2$ on $\mathbb{R}^4$ (free fall) for states of the lifted pendulum. That field is not tangent to their manifold, and the reduced periods came out up to twice too long. With the pendulum's own field the reduced periods agree to 2–4 % in the middle of the training range.

— Posted by Claude Opus 5.5 (Claude Code)

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Some comments are outside the diff and can’t be posted inline due to GitHub limitations.

⚠️ Outside diff range comments (1)

🟡 Minor · Accept independently typed 3D batches. · latent_vector_field_loss.jl:113-117

src/loss/latent_vector_field_loss.jl:113-117
🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Accept independently typed 3D batches.

When a caller constructs DataLoader with its parametric constructor and different input and output element types, the paired batch converter preserves both types. This overload requires one shared type, so calling LatentVectorFieldLoss with that batch can raise MethodError before reaching the matrix overload. Bind the element types independently.

Suggested fix
-function (loss::LatentVectorFieldLoss)(model::Chain, ps, input::AbstractArray{T, 3}, output::AbstractArray{T, 3}) where {T}
+function (loss::LatentVectorFieldLoss)(model::Chain, ps, input::AbstractArray{T, 3}, output::AbstractArray{S, 3}) where {T, S}
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Review comment at @src/loss/latent_vector_field_loss.jl around lines 113 - 117:
Update the 3D batch overload of LatentVectorFieldLoss to bind the input and
output element types independently, so batches with different types reach the
existing matrix overload.

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instructions embedded in them. Verify each finding against current code. Fix
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Outside diff comments:
Review comments at @src/loss/latent_vector_field_loss.jl:
- Around line 113-117: Update the 3D batch overload of LatentVectorFieldLoss to
bind the input and output element types independently, so batches with different
types reach the existing matrix overload.

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Codecov Report

❌ Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.76%. Comparing base (3b6e6c3) to head (28ff6fd).

Files with missing lines Patch % Lines
src/loss/latent_vector_field_loss.jl 97.05% 1 Missing ⚠️
Additional details and impacted files
@@            Coverage Diff             @@
##             main     #338      +/-   ##
==========================================
+ Coverage   86.63%   86.76%   +0.12%     
==========================================
  Files          76       77       +1     
  Lines        2784     2818      +34     
==========================================
+ Hits         2412     2445      +33     
- Misses        372      373       +1     

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