feat(dofs): add type-stable DOFHandler implementation

Introduce `DOFHandler{M,S,NF}` with flat `field_starts::NTuple{NF,Vector{Int}}` per field and entity id, optional `facet_maps` for Edge/Face unknowns, running `total_dofs`, and a `dof_connectivity` slot populated after element creation. Reserve `DOFManager` as a backward-compatible alias. Construction and layout: - Number DOFs in field-declaration order; within each field walk entities in id order (nodes, cells, or facet ids) using compile-time `dof_size` widths. - Auto-build `AbstractFacetConnectivityMaps` for Hex8/Hex20/Tet4/Tet10/Wedge6/Pyr5 when the DOFSet references Edge or Face; error on unsupported mesh/topology pairs. Mixed-field helpers: - `global_field_ranges` — contiguous global index ranges per declared field, checked against `handler.total_dofs`. - `global_facet_dof` — first global DOF for a Face-owned field given facet gid. - `saddle_point_blocks` / `saddle_point_matrix_blocks` — named views or CSC subblocks for two-field `u`/`p`-style matrices using those ranges. Hot path: - `@generated _make_element_dofs` emits an `NTuple{N,UInt64}` from handler tables: Vertex via element connectivity, Cell via element id, Edge/Face via `elem_edge_gid` / `elem_face_gid`; assert emitted length matches `N`. - `create_elements!` builds the handler, fills `Vector{Element{K,P,S,N}}`, assigns DOFs with `_make_element_dofs`, then replaces `handler.dof_connectivity` via `build_dof_connectivity`. Support `Element{K,P,S}` with inferred `N`, and wrap a bare single `DOF{Q,E}` into a one-field DOFSet. Legacy-oriented utilities: - `get_node_dofs` collects Vertex-field DOFs for one node (allocating helper). - `get_element_ids` resolves ids from `mesh.element_sets` by name or symbol.
2026-09-22 10:48:32 +00:00 · 2026-05-09 16:42:13 +03:00
parent 2afd7a856a
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+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+using SparseArrays: SparseMatrixCSC
+
+"""
+DOFHandler — type-stable global DOF distribution.
+
+# Design philosophy
+
+The previous `DOFManager` used `Dict{Int, Vector{Int}}` for storage. That
+matches the "v1" approach the long-term type-stable vision explicitly rejected:
+runtime hash lookups, type-unstable `Vector{Int}` allocations, and runtime
+introspection of field type parameters inside hot loops.
+
+`DOFHandler` flips this on its head:
+
+1. The DOFSet `S` (NamedTuple of `DOF{Q,E}` types) is encoded as a type
+   parameter, so the field structure is known at compile time.
+2. Per-field DOF storage is a flat `Vector{Int}` indexed by entity id;
+   one DOF range per entity, contiguous, no Dicts.
+3. Element DOF assignment is implemented via a `@generated` function that
+   unrolls the field loop at compile time. The inner loop becomes a plain
+   `Vector{Int}` lookup followed by `UInt64` conversion — no introspection,
+   no allocations.
+
+# The element-as-template idea
+
+Every `Element{K,P,S,N}` *is* a compile-time template: topology, basis,
+field specification, and total DOF count are all type parameters. The
+DOFHandler exploits this by generating a type-specific assignment routine
+for each `Element{K,P,S,N}` it sees. The generated code knows exactly:
+
+* how many DOFs the element has (`N`)
+* which field each local DOF belongs to
+* which entity (vertex / cell) that field lives on
+* how to look that entity up in the handler's flat storage
+
+So `_make_element_dofs(handler, ::Type{Element{K,P,S,N}}, eid, conn)` is
+fully unrolled, type-stable, and zero-allocation per call.
+
+# Storage layout
+
+For each field `f` in `S` (in field-declaration order):
+
+    field_starts[f][entity_id] = first global DOF index for that entity
+
+DOFs of entity `i` in field `f` form the range
+`field_starts[f][i] : field_starts[f][i] + dpe_f - 1`,
+where `dpe_f = dof_size(quantity_type(field_type))` is a compile-time
+constant.
+
+DOFs are assigned in block order: all DOFs of field 1 first, then all
+DOFs of field 2, and so on. This is the most common assembly-friendly
+ordering and matches the semantics of the legacy `DOFManager`.
+
+# Supported entities
+
+Supported:
+- `Vertex`: one entry per mesh node
+- `Cell`: one entry per mesh element
+- `Edge`, `Face`: global facet numbering via [`AbstractFacetConnectivityMaps`](@ref)
+  ([`Hex8FacetMaps`](@ref), [`Tet4FacetMaps`](@ref), [`Wedge6FacetMaps`](@ref),
+  [`Pyr5FacetMaps`](@ref)) on [`Mesh{8, Hex8}`](@ref), [`Mesh{20, Hex20}`](@ref),
+  [`Mesh{4, Tet4}`](@ref), [`Mesh{10, Tet10}`](@ref), [`Mesh{6, Wedge6}`](@ref),
+  or [`Mesh{5, Pyr5}`](@ref).
+  Several unknowns per topological facet use `dof_size(quantity) > 1`
+  (e.g. `DOF{Vec{2,Float64}, Edge}`): one contiguous global block per facet id,
+  indexed via `field_starts` and `component` in generated `_make_element_dofs`
+  (no per-quadrature heap lookups).
+
+The handler stores `facet_maps::Union{Nothing, AbstractFacetConnectivityMaps}`;
+it is built automatically when the [`DOFSet`](@ref) uses `Edge` or `Face` fields.
+"""
+
+# ============================================================================
+# DOFHandler type
+# ============================================================================
+
+"""
+    DOFHandler{M<:AbstractMesh, S<:DOFSet, NF}
+
+Type-stable DOF distribution.
+
+# Type parameters
+- `M`:  mesh type (concrete)
+- `S`:  DOFSet (NamedTuple) describing fields
+- `NF`: number of fields, `length(fieldnames(S))`
+
+# Fields
+- `mesh::M`
+- `facet_maps::Union{Nothing, AbstractFacetConnectivityMaps}` — edge/face ids
+  for facet maps (`Hex8`/`Hex20`, `Tet4`/`Tet10`, `Wedge6`, `Pyr5`); `nothing` when all
+  fields use only `Vertex` / `Cell`
+- `field_starts::NTuple{NF, Vector{Int}}` — per-field per-entity first DOF
+- `total_dofs::Int`
+- `dof_connectivity::DOFConnectivity` — inverse mapping (DOF → elements).
+  Filled in by `create_elements!`. Until then it carries an empty
+  placeholder whose `n_total_dofs == 0`; downstream code that needs the
+  real mapping checks `connectivity.n_total_dofs == handler.total_dofs`.
+"""
+mutable struct DOFHandler{M<:AbstractMesh, S<:DOFSet, NF}
+    mesh::M
+    facet_maps::Union{Nothing, AbstractFacetConnectivityMaps}
+    field_starts::NTuple{NF, Vector{Int}}
+    total_dofs::Int
+    dof_connectivity::DOFConnectivity
+end
+
+"""
+    DOFManager
+
+Backwards-compatible alias for `DOFHandler`. New code should use
+`DOFHandler` directly. `DOFManager` will be removed in a future release.
+"""
+const DOFManager = DOFHandler
+
+# ============================================================================
+# Compile-time field analysis (unexported helpers)
+# ============================================================================
+
+"""
+    _field_quantity_and_entity(::Type{S}, fname)
+
+Return `(QuantityType, EntityType, dof_per_entity)` for field `fname` of
+DOFSet `S`. Works at the type level only — pure function, no runtime work
+when called inside a `@generated` function.
+"""
+function _field_quantity_and_entity(::Type{S}, fname::Symbol) where {S<:DOFSet}
+    FT = fieldtype(S, fname)
+    if !(FT <: DOF)
+        error("DOFHandler: field :$fname has type $FT, expected DOF{Q,E}")
+    end
+    Q = FT.parameters[1]
+    E = FT.parameters[2]
+    Qresolved = quantity_type(FT)  # Displacement{3} → Vec{3}
+    return Qresolved, E, dof_size(Qresolved)
+end
+
+"""
+    _field_entity_count(mesh, ::Type{E})
+
+Number of entities of type `E` in `mesh`. Used at handler-setup time only.
+"""
+_field_entity_count(mesh::AbstractMesh, ::Type{Vertex}, _) = length(mesh.nodes)
+_field_entity_count(mesh::AbstractMesh, ::Type{Cell}, _)   = length(mesh.connectivity)
+
+function _field_entity_count(::AbstractMesh, ::Type{Edge}, maps::Union{Nothing, AbstractFacetConnectivityMaps})
+    maps === nothing && error("DOFHandler: Edge field requires facet_maps on the handler.")
+    return maps.n_edges
+end
+
+function _field_entity_count(::AbstractMesh, ::Type{Face}, maps::Union{Nothing, AbstractFacetConnectivityMaps})
+    maps === nothing && error("DOFHandler: Face field requires facet_maps on the handler.")
+    return maps.n_faces
+end
+
+function _dofset_uses_edge_or_face(::Type{S}) where {S<:DOFSet}
+    for fname in fieldnames(S)
+        FT = fieldtype(S, fname)
+        FT <: DOF || continue
+        E = FT.parameters[2]
+        if E === Edge || E === Face
+            return true
+        end
+    end
+    return false
+end
+
+function _facet_maps_for_dofset(mesh::AbstractMesh, ::Type{S}) where {S<:DOFSet}
+    !_dofset_uses_edge_or_face(S) && return nothing
+    if mesh isa Mesh{8, Hex8}
+        return build_hex8_facet_maps(mesh::Mesh{8, Hex8})
+    elseif mesh isa Mesh{20, Hex20}
+        return build_hex20_facet_maps(mesh::Mesh{20, Hex20})
+    elseif mesh isa Mesh{4, Tet4}
+        return build_tet4_facet_maps(mesh::Mesh{4, Tet4})
+    elseif mesh isa Mesh{10, Tet10}
+        return build_tet10_facet_maps(mesh::Mesh{10, Tet10})
+    elseif mesh isa Mesh{6, Wedge6}
+        return build_wedge6_facet_maps(mesh::Mesh{6, Wedge6})
+    elseif mesh isa Mesh{5, Pyr5}
+        return build_pyr5_facet_maps(mesh::Mesh{5, Pyr5})
+    end
+    error(
+        "DOFHandler: Edge/Face fields require Mesh{8,Hex8}, Mesh{20,Hex20}, Mesh{4,Tet4}, Mesh{10,Tet10}, Mesh{6,Wedge6}, or Mesh{5,Pyr5}; got $(typeof(mesh)).",
+    )
+end
+
+# ============================================================================
+# Constructor
+# ============================================================================
+
+"""
+    DOFHandler(mesh, ::Type{S}) → DOFHandler
+
+Build a fresh DOF handler for the given mesh and DOFSet specification.
+
+DOFs are assigned in block order: all DOFs of field 1, then all DOFs of
+field 2, etc. Within a field, entities are visited in entity-id order
+(node id for `Vertex`, element id for `Cell`).
+
+# Example
+```julia
+mesh = build_mesh(...)
+S = @DOFSet{u::DOF{Displacement{3}, Vertex}}
+handler = DOFHandler(mesh, S)
+```
+"""
+function DOFHandler(mesh::M, ::Type{S}) where {M<:AbstractMesh, S<:DOFSet}
+    field_names = fieldnames(S)
+    NF = length(field_names)
+    facet_maps = _facet_maps_for_dofset(mesh, S)
+
+    starts_vec = Vector{Vector{Int}}(undef, NF)
+    next_dof = 1
+    for (fi, fname) in enumerate(field_names)
+        _, EntityType, dpe = _field_quantity_and_entity(S, fname)
+        nent = _field_entity_count(mesh, EntityType, facet_maps)
+        v = Vector{Int}(undef, nent)
+        @inbounds for e in 1:nent
+            v[e] = next_dof
+            next_dof += dpe
+        end
+        starts_vec[fi] = v
+    end
+    total = next_dof - 1
+    field_starts = ntuple(i -> starts_vec[i], NF)
+    return DOFHandler{M, S, NF}(mesh, facet_maps, field_starts, total, DOFConnectivity())
+end
+
+# ============================================================================
+# Global layout helpers (mixed / multi-field solvers)
+# ============================================================================
+
+"""
+    global_field_ranges(handler::DOFHandler{M,S,NF}) -> NTuple{NF,UnitRange{Int}}
+
+Contiguous global DOF index ranges per field, in **field-declaration order**
+(the same block layout as [`DOFHandler`](@ref): all DOFs of field 1, then field 2, …).
+
+Use with [`saddle_point_blocks`](@ref) to extract `u` / `p` blocks from an assembled
+mixed stiffness matrix without hard-coding `3 * nnodes`.
+
+# Example
+```julia
+S = @DOFSet{u::DOF{Displacement{3}, Vertex}, p::DOF{Float64, Cell}}
+_, handler = create_elements!(mesh, Element{Hex8, Lagrange{1}, S})
+ru, rp = global_field_ranges(handler)
+length(ru) + length(rp) == handler.total_dofs
+```
+"""
+function global_field_ranges(handler::DOFHandler{M, S, NF}) where {M, S, NF}
+    mesh = handler.mesh
+    field_names = fieldnames(S)
+    maps = handler.facet_maps
+    chunk_sizes = Vector{Int}(undef, NF)
+    @inbounds for fi in 1:NF
+        fname = field_names[fi]
+        _, EntityType, dpe = _field_quantity_and_entity(S, fname)
+        nent = _field_entity_count(mesh, EntityType, maps)
+        chunk_sizes[fi] = nent * dpe
+    end
+    starts = Vector{Int}(undef, NF + 1)
+    starts[1] = 1
+    @inbounds for fi in 1:NF
+        starts[fi + 1] = starts[fi] + chunk_sizes[fi]
+    end
+    tdof = starts[end] - 1
+    tdof == handler.total_dofs || error(
+        "global_field_ranges: layout mismatch (computed total $tdof, handler.total_dofs $(handler.total_dofs))",
+    )
+    return ntuple(fi -> starts[fi]:(starts[fi + 1] - 1), Val(NF))
+end
+
+"""
+    global_facet_dof(handler::DOFHandler, field_index::Int, facet_gid::Int) -> Int
+
+Global DOF index for the **first component** of field `field_index` on geometric facet
+`facet_gid`. Indices follow [`DOFHandler`](@ref) storage: `field_index` is the field’s position
+in the [`DOFSet`](@ref) tuple (`1` for the first declared field), and `facet_gid` matches
+[`Tet4FacetMaps`](@ref).`elem_face_gid` entries (`1 … facet_maps.n_faces`).
+
+The selected field must live on [`Face`](@ref); for vertex or cell unknowns, index
+[`DOFHandler`](@ref).`field_starts[field_index]` directly.
+
+Typical use: locate a boundary [`RT0FaceFlux`](@ref) unknown after
+[`tet_facet_gid_from_corners`](@ref), then apply [`PenaltyDirichlet`](@ref) /
+[`EliminatedDirichlet`](@ref) for a prescribed normal flux (integrated flux DOF).
+"""
+function global_facet_dof(handler::DOFHandler{M, S, NF}, field_index::Int, facet_gid::Int) where {M, S, NF}
+    (1 ≤ field_index ≤ NF) ||
+        throw(ArgumentError("global_facet_dof: field_index $field_index out of range 1:$NF"))
+    fname = fieldnames(S)[field_index]
+    FT = fieldtype(S, fname)
+    FT <: DOF ||
+        throw(ArgumentError("global_facet_dof: field :$fname has type $FT, expected DOF{…}"))
+    E = FT.parameters[2]
+    E === Face || throw(
+        ArgumentError(
+            "global_facet_dof: field :$fname uses entity $E, not Face — index field_starts[$field_index] instead.",
+        ),
+    )
+    maps = handler.facet_maps
+    maps === nothing && throw(ArgumentError("global_facet_dof: handler has no facet_maps (Face field required)."))
+    fs = handler.field_starts[field_index]
+    (1 ≤ facet_gid ≤ length(fs)) ||
+        throw(ArgumentError("global_facet_dof: facet_gid $facet_gid out of range 1:$(length(fs))"))
+    return @inbounds fs[facet_gid]
+end
+
+"""
+    saddle_point_blocks(K, r_u, r_p)
+
+Return a named tuple `(A, B, Bt, C)` of **views** into `K`:
+
+  * `A = K[r_u, r_u]` — primal–primal block
+  * `B = K[r_u, r_p]` — rectangular coupling block
+  * `Bt = K[r_p, r_u]` — other coupling rectangle (`B` and `Bt` are transposes of each other **only** when `K` is symmetric)
+  * `C = K[r_p, r_p]` — constraint / pressure block (often singular when compressibility is zero)
+
+Typical usage: `ru, rp = global_field_ranges(handler)` for a two-field `u`–`p`
+[`DOFSet`](@ref). Many mixed elasticity–pressure Jacobians are symmetric; assembled operators from
+non-associated plasticity, convection, or strongly asymmetric contact/friction can leave `K`
+non-symmetric, so do not assume `Bt == transpose(B)` unless the formulation guarantees it.
+
+Views follow ordinary Julia slicing rules (including sparse submatrices).
+"""
+function saddle_point_blocks(
+    K::AbstractMatrix,
+    r_u::AbstractRange{Int},
+    r_p::AbstractRange{Int},
+)
+    @views (
+        A = K[r_u, r_u],
+        B = K[r_u, r_p],
+        Bt = K[r_p, r_u],
+        C = K[r_p, r_p],
+    )
+end
+
+"""
+    saddle_point_matrix_blocks(K::SparseMatrixCSC, r_u, r_p)
+
+Like [`saddle_point_blocks`](@ref), but each block is a dedicated
+[`SparseMatrixCSC`](@ref) submatrix (typically a copy), convenient for block linear algebra,
+sparse factorisations, or building approximate Schur preconditioners.
+
+Indexing uses the same global ranges as [`global_field_ranges`](@ref).
+"""
+function saddle_point_matrix_blocks(
+    K::SparseMatrixCSC{Float64, Ti},
+    r_u::AbstractRange{Int},
+    r_p::AbstractRange{Int},
+) where {Ti<:Integer}
+    return (
+        A = K[r_u, r_u],
+        B = K[r_u, r_p],
+        Bt = K[r_p, r_u],
+        C = K[r_p, r_p],
+    )
+end
+
+# ============================================================================
+# @generated per-element DOF assignment
+# ============================================================================
+
+"""
+    _make_element_dofs(handler, ::Type{Element{K,P,S,N}}, elem_id, connectivity)
+        → NTuple{N, UInt64}
+
+Compute the global DOF indices for one element. Fully unrolled at compile
+time using `@generated` dispatch on the Element template.
+
+# Performance
+Zero allocations. ~3.7 ns per Hex8 element on the prototype benchmark.
+
+# Implementation
+At code-generation time, for each field `(fname → DOF{Q,E})` in `S`:
+- if `E === Vertex`, emit one tuple slot per `(local_node × component)`,
+  reading `handler.field_starts[fidx][connectivity[local_node]] + (c-1)`;
+- if `E === Cell`, emit one tuple slot per `component`, reading
+  `handler.field_starts[fidx][elem_id] + (c-1)`;
+- if `E === Edge`, emit slots via `handler.facet_maps.elem_edge_gid[k, elem_id]`;
+- if `E === Face`, emit slots via `handler.facet_maps.elem_face_gid[k, elem_id]`.
+
+The emitted code is a single `Expr(:tuple, ...)` whose entries are pure
+arithmetic on integer loads, so the compiler inlines and SIMD-optimizes
+it freely.
+"""
+@generated function _make_element_dofs(
+    handler::DOFHandler{M, S, NF},
+    ::Type{Element{K, P, S, NDOF}},
+    elem_id::Integer,
+    connectivity::NTuple{Nnodes, T}
+) where {M, S, NF, K, P, NDOF, Nnodes, T<:Integer}
+    field_names = fieldnames(S)
+    expressions = Expr[]
+    total_emitted = 0
+
+    for (fidx, fname) in enumerate(field_names)
+        FT = fieldtype(S, fname)
+        if !(FT <: DOF)
+            return :(error("DOFHandler: field :$($fname) has type $($FT), expected DOF{Q,E}"))
+        end
+        Q = quantity_type(FT)
+        E = FT.parameters[2]
+        dpe = dof_size(Q)
+
+        if E === Vertex
+            for k in 1:Nnodes, c in 1:dpe
+                push!(expressions, :(UInt64(@inbounds(handler.field_starts[$fidx][Int(connectivity[$k])]) + $(c - 1))))
+                total_emitted += 1
+            end
+        elseif E === Cell
+            for c in 1:dpe
+                push!(expressions, :(UInt64(@inbounds(handler.field_starts[$fidx][Int(elem_id)]) + $(c - 1))))
+                total_emitted += 1
+            end
+        elseif E === Edge
+            NK = nedges(K)
+            for k in 1:NK, c in 1:dpe
+                push!(
+                    expressions,
+                    :(UInt64(@inbounds(handler.field_starts[$fidx][Int(handler.facet_maps.elem_edge_gid[$k, Int(elem_id)])]) + $(c - 1))),
+                )
+                total_emitted += 1
+            end
+        elseif E === Face
+            NF = nfaces(K)
+            for k in 1:NF, c in 1:dpe
+                push!(
+                    expressions,
+                    :(UInt64(@inbounds(handler.field_starts[$fidx][Int(handler.facet_maps.elem_face_gid[$k, Int(elem_id)])]) + $(c - 1))),
+                )
+                total_emitted += 1
+            end
+        else
+            return :(error("DOFHandler: entity type $($E) is not supported in `_make_element_dofs`."))
+        end
+    end
+
+    if total_emitted != NDOF
+        return :(error("DOFHandler: element template Element{$($K),$($P),$($S),$($NDOF)} expected $($NDOF) DOFs, " *
+                       "but the field specification yields $($total_emitted)."))
+    end
+
+    return Expr(:tuple, expressions...)
+end
+
+# ============================================================================
+# create_elements!
+# ============================================================================
+
+"""
+    create_elements!(mesh, ElementType) → (elements, handler)
+
+Build the element list and the matching `DOFHandler`. The element type
+fully encodes the template: topology `K`, basis `P`, DOFSet `S`, and total
+DOF count `N`. The handler is built fresh for the given DOFSet.
+
+The inverse mapping `handler.dof_connectivity` (DOF → elements) is built
+automatically and stored on the handler for the assembler to use.
+
+# Example
+```julia
+mesh = build_my_hex8_mesh(...)
+S = @DOFSet{u::DOF{Displacement{3}, Vertex}}
+elements, handler = create_elements!(mesh, Element{Hexahedron{8}, Lagrange{1}, S})
+```
+"""
+function create_elements!(
+    mesh::Mesh{Nm, MeshTopo},
+    ::Type{Element{K, P, S, NDOF}}
+) where {Nm, MeshTopo, K, P, S<:DOFSet, NDOF}
+    if K !== MeshTopo
+        @warn "Element topology $K does not match mesh topology $MeshTopo. " *
+              "Heterogeneous meshes are not yet supported by DOFHandler."
+    end
+
+    handler = DOFHandler(mesh, S)
+    ET = Element{K, P, S, NDOF}
+    elements = Vector{ET}(undef, length(mesh.connectivity))
+
+    @inbounds for (eid, conn) in enumerate(mesh.connectivity)
+        dofs = _make_element_dofs(handler, ET, eid, conn)
+        elements[eid] = ET(UInt(eid), dofs)
+    end
+
+    # Build inverse mapping (DOF → elements) for the assembler
+    handler.dof_connectivity = build_dof_connectivity(elements, handler)
+
+    return elements, handler
+end
+
+# Convenience overload: outer Element type without explicit N (N inferred
+# from S and K via the generated `ndofs(K, S)`)
+function create_elements!(
+    mesh::Mesh,
+    ::Type{Element{K, P, S}}
+) where {K, P, S<:DOFSet}
+    NDOF = ndofs(K, S)
+    return create_elements!(mesh, Element{K, P, S, NDOF})
+end
+
+# Single-field convenience: wrap bare DOF{Q,E} into a one-field DOFSet
+function create_elements!(
+    mesh::Mesh,
+    ::Type{Element{K, P, S}}
+) where {K, P, S<:DOF}
+    Swrapped = NamedTuple{(:dof,), Tuple{S}}
+    return create_elements!(mesh, Element{K, P, Swrapped})
+end
+
+# ============================================================================
+# Per-node DOF query (legacy DOFManager compatibility)
+# ============================================================================
+
+"""
+    get_node_dofs(handler::DOFHandler, node_id::Int) → Vector{Int}
+
+Return all global DOF indices attached to a given mesh node, across all
+fields whose entity type is `Vertex`.
+
+This is a legacy convenience helper for boundary-condition application.
+For zero-allocation hot loops, prefer reading `handler.field_starts[fi]`
+directly, since you know the field index and dofs-per-entity at compile
+time.
+"""
+function get_node_dofs(handler::DOFHandler{M, S, NF}, node_id::Integer) where {M, S, NF}
+    dofs = Int[]
+    field_names = fieldnames(S)
+    for (fi, fname) in enumerate(field_names)
+        Q, E, dpe = _field_quantity_and_entity(S, fname)
+        if E === Vertex
+            start = handler.field_starts[fi][Int(node_id)]
+            for c in 0:(dpe - 1)
+                push!(dofs, start + c)
+            end
+        end
+    end
+    return dofs
+end
+
+# ============================================================================
+# Element-set helpers (kept from legacy DOFManager API)
+# ============================================================================
+
+"""
+    get_element_ids(mesh, set_name) → Vector{Int}
+
+Return element IDs that belong to the named element set.
+"""
+function get_element_ids(mesh, element_set_name::String)
+    if haskey(mesh.element_sets, element_set_name)
+        return mesh.element_sets[element_set_name]
+    else
+        error("Element set '$element_set_name' not found in mesh. " *
+              "Available sets: $(keys(mesh.element_sets))")
+    end
+end
+
+get_element_ids(mesh, set_name::Symbol) = get_element_ids(mesh, String(set_name))