JuliaFEM.jl/docs/contributor/coding_standards.md

title, description, date, author, categories, keywords, audience, level, type, status

title	description	date	author	categories	keywords	audience	level	type	status
JuliaFEM Coding Standards	Required coding conventions and style guide for all contributors	2025-11-09	Jukka Aho	development standards style guide	juliafem coding standards style conventions best practices	developers	required	standards	active

This document defines the coding standards and conventions for JuliaFEM development.

Last Updated: November 9, 2025
Status: Active - all new code must follow these standards

---

Core Principles

1. Readability over cleverness - Code should be understandable by FEM practitioners
2. Type stability first - Performance depends on it (100x difference)
3. Zero allocations in hot paths - Profiling required
4. Explicit over implicit - No magic, show what happens
5. Composition over inheritance - Structs and free functions, not OOP

---

Variable Naming Conventions

No Greek Letters in Code (Critical!)

Rule: Never use Greek letters (ξ, η, ζ, α, β, γ, etc.) in code.

Rationale:

• Keyboard accessibility - Not all keyboards support Greek input
• Editor compatibility - Some editors struggle with Unicode math symbols
• Copy-paste issues - Greek letters cause encoding problems
• Search/replace problems - Text tools may not handle Unicode correctly
• Terminal rendering - SSH sessions may not display correctly
• Internationalization - Non-Western keyboards make editing difficult
• Git diffs - Unicode can cause merge conflicts or display issues
• Accessibility - Screen readers struggle with Greek letters

Correct:

# Reference element coordinates
function eval_basis(u::Float64, v::Float64, w::Float64)
    # Natural coordinates u, v, w ∈ [-1, 1]
    N1 = (1 - u) * (1 - v) * (1 - w) / 8
    return N1
end

# Physical coordinates
function map_to_physical(x::Vec, y::Vec, z::Vec, u::Float64, v::Float64, w::Float64)
    # Map from natural (u,v,w) to physical (x,y,z)
end

Incorrect:

# ❌ DON'T DO THIS
function eval_basis(ξ::Float64, η::Float64, ζ::Float64)
    N1 = (1 - ξ) * (1 - η) * (1 - ζ) / 8
    return N1
end

Exception: Greek letters are acceptable in:

• Comments - Mathematical notation for clarity: # Shape function: Nᵢ(ξ)
• Documentation - Latex math blocks: $\\xi \\in [-1, 1]$
• String literals - Plot labels: xlabel="ξ coordinate"
• Error messages - User-facing text: "Invalid ξ coordinate"

Standard variable names:

• Reference coordinates: u, v, w (not ξ, η, ζ)
• Physical coordinates: x, y, z
• Derivatives: du, dv, dw or dudx, dudy, etc.
• Jacobian: J or jac (not ∂)
• Determinant: detJ (not |J|)
• Inverse: invJ or Jinv (not J⁻¹)

---

Type Naming

Structs and Types

• PascalCase - All type names: Element, Problem, Material
• No abbreviations - Quadrilateral not Quad (unless established convention)
• Descriptive suffixes - Purpose clear from name

Basis types - Append "Basis" suffix to distinguish from topology:

# ✅ Correct - No name collision
struct Tri3Basis <: AbstractBasis{2} end  # Interpolation scheme
struct Tri3 <: AbstractTopology end        # Element geometry

# ❌ Wrong - Name collision!
struct Tri3 <: AbstractBasis{2} end
struct Tri3 <: AbstractTopology end  # ERROR: type Tri3 already defined

Functions

• snake_case - All function names: assemble_element, solve_static
• Verb-noun pattern - Action clear: compute_stiffness, evaluate_basis
• Boolean predicates - is_* or has_*: is_converged, has_contact

Constants

• UPPER_CASE - Module-level constants: MAX_ITERATIONS, TOLERANCE
• Type-stable - Always specify type: const MAX_ITER::Int = 100

Internal/Private

• Underscore prefix - Not exported: _compute_internal_forces
• Not API - Can change between versions

---

Code Organization

File Structure

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

"""
Brief one-line description of file purpose.

Extended description if needed.
"""

# Imports (grouped)
using LinearAlgebra
using SparseArrays

# Local imports
using ..JuliaFEM: AbstractElement, AbstractBasis

# Type definitions
struct MyType
    # ...
end

# Function implementations
function my_function(args)
    # ...
end

Import Style

# ✅ Correct - Explicit imports
using LinearAlgebra: norm, dot, cross
using SparseArrays: sparse, spzeros

# ❌ Avoid - Blanket imports (pollutes namespace)
using LinearAlgebra
using SparseArrays

---

Performance Guidelines

Type Stability

# ✅ Type-stable - Return type inferrable
function compute_mass(element::Quad4, density::Float64)
    m::Float64 = 0.0
    # ...
    return m
end

# ❌ Type-unstable - Return type changes!
function compute_mass(element, density)
    if density > 0
        return 1.0      # Float64
    else
        return nothing  # Nothing - type-unstable!
    end
end

Zero Allocations

# ✅ Pre-allocated cache
struct AssemblyCache
    K_local::Matrix{Float64}
    f_local::Vector{Float64}
end

function assemble!(cache::AssemblyCache, element)
    fill!(cache.K_local, 0.0)
    # Reuse cache.K_local - no allocations
end

# ❌ Allocates every call
function assemble(element)
    K_local = zeros(8, 8)  # Allocates!
    return K_local
end

Tuple Returns (Zero Allocation)

# ✅ Tuple return - no allocation
function eval_basis(element::Tri3, u::Float64, v::Float64)
    N1 = 1 - u - v
    N2 = u
    N3 = v
    return (N1, N2, N3)  # NTuple{3,Float64} - stack allocated
end

# ❌ Vector return - allocates!
function eval_basis(element::Tri3, u::Float64, v::Float64)
    return [1 - u - v, u, v]  # Vector{Float64} - heap allocation
end

---

Documentation Style

Docstrings

Use Julia's docstring format with standard sections:

"""
    assemble_element(element::Quad4, u::Vector{Float64}) -> Matrix{Float64}

Assemble element stiffness matrix for 4-node quadrilateral element.

# Arguments
- `element::Quad4`: Quadrilateral element with nodal connectivity
- `u::Vector{Float64}`: Nodal displacement vector (8 DOFs: u1,v1,u2,v2,...)

# Returns
- `K::Matrix{Float64}`: 8×8 element stiffness matrix in global DOFs

# Theory
Uses 2×2 Gauss quadrature with bilinear shape functions:
```math
K_{ij} = \\int_{\\Omega_e} B_i^T D B_j \\, d\\Omega
```

# Example
```julia
nodes = [Node(0,0), Node(1,0), Node(1,1), Node(0,1)]
element = Quad4(nodes)
u = zeros(8)
K = assemble_element(element, u)
```

# Performance
This function allocates. For zero-allocation assembly, use `assemble_element!` 
with pre-allocated cache.
"""
function assemble_element(element::Quad4, u::Vector{Float64})
    # Implementation
end

Comments

# ✅ Good comments - Why, not what
# Use RCM ordering to minimize bandwidth (10x faster solve)
perm = rcm_permutation(mesh)

# Check convergence: ||Δu|| < ε||u||
# Relative norm prevents scale-dependent tolerance
if norm(Δu) < tol * norm(u)
    break
end

# ❌ Bad comments - Obvious from code
# Loop over elements
for element in elements
    # Add to global matrix
    K_global += K_local
end

---

Testing Standards

Test Organization

@testset "Quad4 element" begin
    @testset "Stiffness matrix" begin
        # Unit square, E=1, ν=0.3
        element = Quad4([Node(0,0), Node(1,0), Node(1,1), Node(0,1)])
        K = stiffness_matrix(element, E=1.0, ν=0.3)
        
        # Symmetry
        @test issymmetric(K)
        
        # Positive definite (after BC)
        @test all(eigvals(K[3:end, 3:end]) .> 0)
    end
    
    @testset "Patch test" begin
        # Linear displacement field must be exact
        # ... validation test ...
    end
end

Floating Point Comparisons

# ✅ Use tolerances
@test result ≈ expected atol=1e-10
@test isapprox(result, expected, rtol=1e-6)

# ❌ Never exact equality for floats
@test result == expected  # Fragile!

---

Anti-Patterns (Don't Do This!)

1. Dict Without Type Parameters

# ❌ Type-unstable Dict (100x slower!)
fields = Dict("displacement" => u, "velocity" => v)

# ✅ Type-stable alternative
fields = (displacement=u, velocity=v)  # NamedTuple
# or
struct Fields
    displacement::Vector{Float64}
    velocity::Vector{Float64}
end

2. Abstract Types in Structs

# ❌ Type-unstable struct
struct Element
    nodes::AbstractVector  # Type-unstable!
end

# ✅ Parametric struct
struct Element{N}
    nodes::NTuple{N, Node}  # Type-stable, zero-allocation
end

3. Global Variables

# ❌ Global mutable state
global_stiffness = zeros(1000, 1000)

function assemble!(element)
    global_stiffness .+= K_local  # Spooky action at a distance!
end

# ✅ Explicit parameters
function assemble!(K_global::Matrix, element)
    K_global .+= K_local  # Clear data flow
end

4. Type Piracy

# ❌ Extending methods on types you don't own
Base.+(a::Vector, b::Matrix) = ...  # DON'T!

# ✅ Wrapper type or different function
struct MyVector
    data::Vector
end
Base.+(a::MyVector, b::Matrix) = ...  # OK - our type

---

Git Commit Style

Commit Messages

feat(topology): Add Pyr5 pyramid element topology

- Implement 5-node pyramid reference element
- Add connectivity information (faces, edges)
- Zero-allocation tuple interface
- Tests: reference coordinates, edge/face queries

Closes #123

Format: <type>(<scope>): <subject>

Types:

• feat: New feature
• fix: Bug fix
• docs: Documentation only
• refactor: Code restructuring (no behavior change)
• perf: Performance improvement
• test: Adding tests
• chore: Tooling, dependencies

Scope: Module or component (topology, assembly, solver, etc.)

Subject:

• Imperative mood: "Add feature" not "Added feature"
• No period at end
• Max 72 characters

---

Editor Configuration

Recommended Settings

# .editorconfig
[*.jl]
charset = utf-8
end_of_line = lf
indent_style = space
indent_size = 4
trim_trailing_whitespace = true
insert_final_newline = true

JuliaFormatter.jl

# .JuliaFormatter.toml
indent = 4
margin = 92
always_for_in = true
whitespace_typedefs = true
whitespace_ops_in_indices = true
remove_extra_newlines = true

---

Summary Checklist

Before submitting code, verify:

• No Greek letters in variable names (use u, v, w)
• All types are PascalCase
• All functions are snake_case
• Type-stable (check with @code_warntype)
• Zero allocations in hot paths (check with @allocations or @btime)
• Docstrings for exported functions
• Tests pass locally
• Comments explain "why" not "what"
• No type piracy
• No global mutable state

---

References

• Performance tips: https://docs.julialang.org/en/v1/manual/performance-tips/
• Style guide: https://docs.julialang.org/en/v1/manual/style-guide/
• JuliaFEM vision: llm/VISION_2.0.md
• Architecture: llm/ARCHITECTURE.md
• Technical lessons: llm/TECHNICAL_VISION.md

---

Enforcement: These standards are enforced through code review. All PRs must follow these conventions.

Evolution: This document evolves with the project. Propose changes via pull request.