# A C++ shim for ergm

This vignette documents the C++ convenience wrappers for developing ERGM terms and proposals using modern C++ while interfacing with ergm’s core C data structures.

The API partially wraps the Terms and Proposals APIs and focuses on lightweight wrappers (no ownership) around existing C structs to provide: range-based iteration, safer array handling, and access to `->R` list elements/attributes.

WARNING: This API is experimental and is subject to change in response to evolving needs and user feedback, but some effort will be made to maintain backwards compatibility. In particular, see the item about namespace versioning below.

## Overview

- Headers live in `inst/include/cpp/` and complement the C headers in `inst/include/`.
- Wrappers are header-only and intended to be used in compiled code within `src/`, typically via `#include "cpp/ergm_network.h"`, `#include "cpp/ergm_changestat.h"`, etc.
- All C++ wrappers, aliases, and helper macros live in the `ergm` namespace (e.g., `ergm::ErgmCppNetwork`); either qualify their names or add `using` declarations in your translation units.
- The API is versioned via an inline namespace `ergm::v1` (currently defaulted); pin explicitly with `ergm::v1::ErgmCppNetwork` if you need to avoid future breaking changes.
- All classes are thin wrappers around existing C pointers; they do not allocate or free memory.

## Core Concepts

- Arrays you write/read: `mt.stat`, `mt.dinput`, `mt.iinput`, `mt.dattrib`, `mt.iattrib` and, for proposals, `p.dinput`, `p.iinput`. Treat these as array-like: use `[]` for access and `.size()` for length.
- Network iteration: use `nw.nodes()`, `nw.out_neighbors(i)`, `nw.in_neighbors(i)`, `nw.neighbors(i)`, and `nw.edges()` with range-based `for` loops; for weighted networks, neighbor/edge values include weights.
- Term/proposal storage: `mt.storage` / `p.storage` let you keep a user-defined pointer across calls; `mt.aux_storage[i]` / `p.aux_storage[i]` access auxiliaries at position `i`. (Cast to your type before use.)
- The elements of the R list returned by the `Init*Ergm*()` function can be accessed as `mt.R["name"]`, its attributes as `mt.R.attr["name"]`, and analogously for `mt.ext_state`. They return `SEXP` values; use the standard R API to handle them as needed.

## Network Wrappers: `ErgmCppNetwork` and `ErgmCppWtNetwork`

Headers: `#include "cpp/ergm_network.h"`, `#include "cpp/ergm_wtnetwork.h"`

These wrap `Network` (unweighted) and `WtNetwork` (weighted) to provide edge queries, degree access, and simple iteration.

- Construction: `ErgmCppNetwork nw(nwp);` and `ErgmCppWtNetwork nw(nwp);`
- Edge query: `nw(tail, head)` returns presence (`Rboolean`) or `double` weight (0 for no edge).
- Node ranges: `nw.nodes()`, bipartite halves: `nw.b1()`, `nw.b2()`; node ranges support `.size()` for O(1) range size.
- Neighbor ranges: `nw.out_neighbors(i)`, `nw.in_neighbors(i)`, `nw.neighbors(i)`.
- Degree access: `nw.out_degree(i)`, `nw.in_degree(i)`, `nw.degree(i)`.
- Edge range over all edges: `for (auto e : nw.edges())` yields `(tail, head)`.
- Directed accessors starting with `in_` and `out_` automatically fall back to the undirected network if `nw` is undirected.
- Valued neighbor iteration yields pairs `(neighbor, weight)`; `edges()` yields `(tail, head, weight)`.

Example:

    // You can also use nw.edges(), though for an undirected network, the following
    // code will visit each edge twice, once from each end.
    ErgmCppNetwork nw(nwp);
    for(Vertex i : nw.nodes()) {
      for(Vertex j : nw.out_neighbors(i)) {
        // process edge i->j
      }
    }

    ErgmCppWtNetwork nw(nwp);
    for(auto [j, w] : nw.neighbors(i)) {
      // weighted edge i->j of weight w
    }

    for(auto [i, j, w] : nw.edges()) {
      // weighted edge i->j of weight w
    }

## Model Terms: `ErgmCppModelTerm` and `ErgmCppWtModelTerm`

Headers: `#include "cpp/ergm_changestat.h"`, `#include "cpp/ergm_wtchangestat.h"`

- Constructed from `ModelTerm*` (or `WtModelTerm*`).
- Arrays (use `.size()` to get lengths):
  - `stat`: writable stats (`double`); length via `mt.stat.size()`.
  - `dinput`, `iinput`: numeric and integer inputs; lengths via `mt.dinput.size()` / `mt.iinput.size()`.
  - `dattrib`, `iattrib`: attribute slices of inputs if present; lengths via `.size()`.
- `storage`: user-defined pointer you manage across calls (`mt.storage`).
- `aux_storage`: access auxiliary storage by index, e.g., `auto* my_aux = static_cast<MyType*>(mt.aux_storage[0])`.
- `R` and `ext_state`: access term and extended state via `mt.R["name"]` / `mt.ext_state["name"]`, their attributes via `mt.R.attr["name"]` and `mt.ext_state.attr["name"]`

## Proposals: `ErgmCppProposal` and `ErgmCppWtProposal`

Headers: `#include "cpp/ergm_proposal.h"`, `#include "cpp/ergm_wtproposal.h"`

- Constructed from `MHProposal*`.
- Direct members:
  - `size` (`Edge&`): number of toggles (`ntoggles`).
  - `tail`, `head` (`Vertex*`), `weight` (`double*`, for valued networks): toggle arrays.
  - `logratio` (`double&`).
- Inputs: `dinput`, `iinput` are array-like; use `.size()` for lengths.
- `storage`: user-managed pointer reference (`p.storage`).
- `aux_storage`: access by index if present, e.g., `p.aux_storage[0]`.
- `R`: access proposal list elements if needed.

## Helper macros

The following macros define the C entry point and construct network and model term handles named `nw` and `mt` for use in `impl`:

- `C_CHANGESTAT_CPP(name, StorageType, impl)`
- `S_CHANGESTAT_CPP(name, StorageType, impl)`
- `D_CHANGESTAT_CPP(name, StorageType, impl)`
- `I_CHANGESTAT_CPP(name, StorageType, impl)`
- `U_CHANGESTAT_CPP(name, StorageType, impl)`
- `F_CHANGESTAT_CPP(name, StorageType, impl)`
- `W_CHANGESTAT_CPP(name, StorageType, impl)` (returns `SEXP`)
- `X_CHANGESTAT_CPP(name, StorageType, impl)`
- `Z_CHANGESTAT_CPP(name, StorageType, impl)`

`StorageType` can be omitted (i.e., passing only 2 arguments) if no private storage is used.

Weighted counterparts in `ergm_wtchangestat.h` are prefixed with `Wt` (e.g., `WtC_CHANGESTAT_CPP`).

## Examples

Binary terms: triangle and cycle counts in `src/cpp_changestats.cpp`.

Valued terms: transitive weights in `src/cpp_wtchangestats.cpp`.

Proposal: `src/MHproposals_triadic.cpp`.

## Notes and Best Practices

- All wrappers are non-owning; manage lifetimes via the usual ergm mechanisms.
- Prefer range-based loops for clarity and correctness.
- Use `FixedArray::size()` to avoid out-of-bounds when iterating inputs and stats.
- Cast `aux_storage` elements to the correct type before use.