refactor: replace .Call() expressions with _impl() function calls

[Copilot]

Replaced 8 .Call() expressions with their corresponding _impl() wrappers, aligning with the codebase convention to avoid .Call() outside *_impl() in aaa-auto.R.

Changes

Files modified:

• R/adjacency.R: adjacency, weighted_adjacency
• R/make.R: create_bipartite (2 occurrences)
• R/operators.R: complementer, difference (2 occurrences)
• R/conversion.R: get_edgelist

Key refactoring: Updated graph.adjacency.dense() to pass string mode/loops parameters directly to _impl() functions instead of pre-converting to integers, since _impl() functions handle this internally.

Rationale for non-replaced calls

Remaining .Call() expressions (~40) cannot be replaced due to:

• Signature mismatches: _impl() functions missing parameters (subset, which, edgemaps, circle)
• No _impl() exists: Functions like mybracket2, get_attr_mode, set_verbose
• Intentional exceptions: e.g., vcount in warn_version() to avoid recursion

Prepared with GitHub Copilot assistance.

• By submitting this pull request, I assign the copyright of my contribution to The igraph development team.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Replace 30 or so .Call() expressions with calls to _impl functions</issue_title>
<issue_description>Only where possible. Do not introduce new _impl functions.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Replace 30 or so .Call() expressions with calls to _impl functions #2539

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[Copilot]

Pull request overview

This PR refactors R code to replace direct .Call() expressions with their corresponding _impl() wrapper functions, aligning with the codebase convention that .Call() should only be used within auto-generated *_impl() functions in aaa-auto.R.

Changes:

• Replaced 8 direct .Call() expressions across 4 files with calls to their corresponding _impl() wrapper functions
• Updated parameter passing in adjacency.R to use string values ("none", "twice", "once") instead of integers, which are then converted by the _impl() functions internally
• Removed redundant on.exit(.Call(R*_igraph_finalizer)) calls since these are now handled within the _impl() functions

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
R/operators.R	Replaced .Call() in difference.igraph() (2 occurrences) and complementer() with difference_impl() and complementer_impl() calls
R/make.R	Replaced .Call() in graph.bipartite() and make_bipartite_graph() with create_bipartite_impl() calls
R/conversion.R	Replaced .Call() in as_edgelist() with get_edgelist_impl() call
R/adjacency.R	Replaced .Call() in graph.adjacency.dense() with adjacency_impl() and weighted_adjacency_impl() calls; updated parameter conversion to pass strings instead of integers