JuliaFEM.jl/src/assemblers/nodal_based.jl

# This file is a part of JuliaFEM.
# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md

"""
Nodal-based assembly (future implementation).

Node-by-node assembly using inverse connectivity (node-to-elements map).
Natural for GPU parallelization (one thread per node).

**Performance**: Expected 2-10x speedup on GPU for large problems (> 100k nodes).
**Best for**: GPU acceleration, very large problems.
**Status**: Planned for future implementation.
"""

using SparseArrays

"""
    assemble!(
        cache::NodalCache,
        assembler::NodalAssembler,
        kernel::AbstractKernel,
        mesh::AbstractMesh
    ) -> Nothing

Assemble global system using nodal traversal **in-place, zero allocations**.

**Status**: Not yet implemented. Placeholder for future GPU-based assembly.

# Algorithm (Planned)

1. Reset cache
2. Loop over nodes (parallelizable on GPU):
   a. Get all elements touching this node
   b. For each touching element:
      - Compute full element stiffness (or use cached value)
      - Extract only rows/columns for this node
      - Accumulate to global system (atomic add on GPU)

# GPU Parallelization

Each node processed by one GPU thread:
```cuda
__global__ void assemble_nodal(nodes, elements, K, f) {
    int node_id = blockIdx.x * blockDim.x + threadIdx.x;
    if (node_id >= nnodes) return;

    // Get touching elements
    for (elem in touching_elements[node_id]) {
        // Compute element contribution for this node
        // Atomic add to K, f
    }
}
```

# Arguments
- `cache`: Pre-allocated nodal cache
- `assembler`: Nodal assembler
- `kernel`: Domain kernel
- `mesh`: Finite element mesh

# Zero-Allocation Guarantee

All arrays pre-allocated. Atomic operations on GPU ensure thread-safety.
"""
function assemble!(
    cache::NodalCache,
    assembler::NodalAssembler,
    kernel::AbstractKernel,
    mesh::AbstractMesh
)
    error("NodalAssembler not yet implemented. Use COOAssembler or CSCAssembler.")

    # Planned implementation:
    # 1. Reset cache
    # 2. Loop over nodes
    # 3. For each node, get touching elements from cache.node_to_elements
    # 4. Accumulate contributions from all touching elements
    # 5. Write to global K, f (atomic on GPU)

    return nothing
end

# ============================================================================
# HELPER FUNCTIONS (for future implementation)
# ============================================================================

"""
    create_cache(assembler::NodalAssembler, mesh::AbstractMesh, kernel::AbstractKernel) -> NodalCache

Create pre-allocated cache for nodal assembly.

Builds node-to-elements map (inverse connectivity) for efficient nodal traversal.

# Arguments
- `assembler`: Nodal assembler
- `mesh`: Finite element mesh
- `kernel`: Domain kernel

# Returns
- Pre-allocated nodal cache with inverse connectivity
"""
function create_cache(assembler::NodalAssembler, mesh::AbstractMesh, kernel::AbstractKernel)
    return NodalCache(mesh, kernel)
end

"""
    compute_node_contributions!(
        node_cache::NodeCache,
        element_cache::ElementCache,
        node_id::Int,
        kernel::AbstractKernel,
        mesh::AbstractMesh,
        node_to_elements::NodeToElementsMap
    ) -> Nothing

Compute contributions to this node from all touching elements **in-place**.

# Algorithm

1. Get all elements touching this node
2. For each element:
   a. Compute full element stiffness (using `compute_element_stiffness!`)
   b. Find local node index within element
   c. Extract rows/columns corresponding to this node's DOFs
   d. Accumulate to node contributions

# Arguments
- `node_cache`: Node workspace (output)
- `element_cache`: Element workspace (for kernel calls)
- `node_id`: Node index
- `kernel`: Domain kernel
- `mesh`: Finite element mesh
- `node_to_elements`: Inverse connectivity

# Returns

Nothing - writes to `node_cache` in-place.
"""
function compute_node_contributions!(
    node_cache::NodeCache,
    element_cache::ElementCache,
    node_id::Int,
    kernel::AbstractKernel,
    mesh::AbstractMesh,
    node_to_elements::NodeToElementsMap
)
    error("compute_node_contributions! not yet implemented")

    # Planned implementation:
    # 1. Get touching elements: get_node_spider(node_to_elements, node_id)
    # 2. For each element:
    #    - Compute element stiffness: compute_element_stiffness!(element_cache, ...)
    #    - Find local node index in element
    #    - Extract node DOF rows/columns from Ke, fe
    #    - Accumulate to node_cache
    # 3. Return node_cache (contains all contributions for this node)

    return nothing
end

# ============================================================================
# GPU KERNEL STUBS (for future CUDA implementation)
# ============================================================================

# """
#     assemble_nodal_gpu!(K, f, mesh, kernel, node_to_elements)
#
# GPU kernel for nodal assembly.
#
# Launches one thread per node. Each thread:
# 1. Gets touching elements for its node
# 2. Computes contributions from all elements
# 3. Atomically adds to global K, f
#
# Requires CUDA.jl or similar GPU framework.
# """
# function assemble_nodal_gpu! end