Implement giant component and percolation

[Zepeacedust]

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

Closes #2736

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.

Project coverage is 75.37%. Comparing base (fc6860b) to head (6db2bab).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
src/misc/percolation.c	0.00%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2746      +/-   ##
==========================================
- Coverage   75.38%   75.37%   -0.02%     
==========================================
  Files         379      380       +1     
  Lines       55128    55179      +51     
  Branches    12320    12330      +10     
==========================================
+ Hits        41559    41589      +30     
- Misses      10017    10040      +23     
+ Partials     3552     3550       -2     

Files with missing lines	Coverage Δ
src/misc/percolation.c	0.00% <0.00%> (ø)

... and 9 files with indirect coverage changes

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[szhorvat]

Let's move these to igraph_components.h

[szhorvat]

First step is to discuss the prototype. It's best if we do this in the issue. I'll post there.

[szhorvat]

Suggested change

	IGRAPH_CHECK(igraph_vector_int_init(&sizes, vert_count));
	igraph_vector_int_t links;
	IGRAPH_CHECK(igraph_vector_int_init(&links, vert_count));
	int edge_count = igraph_vector_int_size(edges) / 2;
	IGRAPH_CHECK(igraph_vector_int_resize(output, edge_count));
	for (int i = 0; i < edge_count; i++) {
	igraph_i_percolate_edge(&links, &sizes, &biggest, VECTOR(*edges)[2*i], VECTOR(*edges)[2*i+1]);
	VECTOR(*output)[i] = biggest;
	}
	igraph_vector_int_destroy(&sizes);
	igraph_vector_int_destroy(&links);
	IGRAPH_CHECK(igraph_vector_int_init(&sizes, vert_count));
	IGRAPH_FINALLY(igraph_vector_int_destroy, &sizes);
	igraph_vector_int_t links;
	IGRAPH_CHECK(igraph_vector_int_init(&links, vert_count));
	IGRAPH_FINALLY(igraph_vector_int_destroy, &links);
	int edge_count = igraph_vector_int_size(edges) / 2;
	IGRAPH_CHECK(igraph_vector_int_resize(output, edge_count));
	for (int i = 0; i < edge_count; i++) {
	igraph_i_percolate_edge(&links, &sizes, &biggest, VECTOR(*edges)[2*i], VECTOR(*edges)[2*i+1]);
	VECTOR(*output)[i] = biggest;
	}
	igraph_vector_int_destroy(&sizes);
	igraph_vector_int_destroy(&links);
	IGRAPH_FINALLY_CLEAN(2);

IGRAPH_FINALLY places an object and a destructor on a stack which we call the "finally" stack.

If an error occurs, all destructors currently on the stack are called. This way allocated resources are freed even if the function returns early because of an error.

IGRAPH_FINALLY_CLEAN(2) removes the elements from this stack. This always comes after you manually called destructors, in this case two destructors.

There's a shorthand for the following pattern:

    IGRAPH_CHECK(igraph_vector_int_init(&sizes, vert_count));
    IGRAPH_FINALLY(igraph_vector_int_destroy, &sizes);

We usually just write

   IGRAPH_VECTOR_INT_INIT_FINALLY(&sizes, vert_count);

This is what you should use here, as well as with vector types, but such a macro is not available for all data structures, so I wanted to show you the separated init + FINALLY pattern as well.

[szhorvat]

Continued in #2778