Prove that git_mergebase + 2x git_revwalk can _not_ be used to find the "is 5 ahead 3 behind foo"

divergence.c

#include <assert.h>
#include <stdio.h>
#include <memory.h>

#include "git2.h"

char* gerr = 0;

int find_divergence_single(
		git_repository*	repo,
		const git_oid*	base,
		const git_oid*	dest,
		int*		steps )
{
	int		rc;
	git_revwalk*	walk = NULL;
	git_oid		current;

	assert( base && repo && dest && steps );

	if( git_oid_cmp( base, dest ) == 0 )
	{
		*steps = 0;
		return 0;
	}

	if( git_revwalk_new( &walk, repo ) < 0 )
	{
		gerr = "Cannot create a walk";
		return -1;
	}

	if( !git_revwalk_push( walk, dest ) < 0 )
	{
		gerr = "Cannot push base";
		return -1;
	}

	git_revwalk_sorting( walk, GIT_SORT_TOPOLOGICAL );

	while( ( rc = git_revwalk_next( &current, walk ) ) >= 0 )
	{
		if( rc == GIT_ITEROVER )
		{
			gerr = "dest does not follow to base";
			return -1;
		}
		else if( rc < 0 )
		{
			gerr = "Cannot walk";
			return -1;
		}

		if( git_oid_cmp( &current, base ) == 0 )
		{
			break;
		}

		++(*steps);
	}

	git_revwalk_free( walk );
	return 0;
}

int find_divergence(
		git_repository*	repo,
		const git_oid*	me,
		const git_oid*	they,
		int*		ahead,
		int*		behind )
{
	char oid_a[41], oid_b[41], oid_c[41];
	assert( repo && me && they && ahead && behind );

	*ahead = *behind = 0;

	git_oid merge_base;
	if( git_merge_base( &merge_base, repo, (git_oid*)me, (git_oid*)they ) < 0 )
	{
		const git_error* e = giterr_last();
		printf( "ERR: %s\n", e ? e->message : "???" );
		gerr = "Cannot find merge base";
		return -2;
	}

	git_oid_fmt( oid_a, me ); oid_a[ 40 ] = 0;
	git_oid_fmt( oid_b, they ); oid_b[ 40 ] = 0;
	git_oid_fmt( oid_c, &merge_base ); oid_c[ 40 ] = 0;

//	printf( "Base of '%s' and '%s' is '%s'\n", &oid_a[0], &oid_b[0], &oid_c[0] );

	if( find_divergence_single( repo, &merge_base, me, ahead ) < 0 )
	{
		return -1;
	}

	if( find_divergence_single( repo, &merge_base, they, behind ) < 0 )
	{
		return -1;
	}

	return 0;
}

git_oid tags[5];

int read_tags( git_repository* repo )
{
	int i;
	char oidstr[41];
	char tagname[] = "refs/tags/t0";

	for( i = 0; i < 5; i++ )
	{
		tagname[ 11 ] = '1' + i;
		if( git_reference_name_to_oid( &tags[ i ], repo, tagname ) < 0 )
		{
			return -1;
		}

		git_oid_fmt( oidstr, &tags[ i ] );
		printf( "Tag '%s' is at %s\n", &tagname[0], oidstr );
	}

	return 0;
}

int main(void)
{
	int a, b, i, j, rc;
	git_repository* repo = NULL;

	git_threads_init();

	if( git_repository_open( &repo, "../merge_base_test" ) < 0 )
	{
		puts( "Cannot find repo" );
		return -1;
	}

	read_tags( repo );

	for( i = 0; i < 5; i++ )
	{
		for( j = 0; j < 5; j++ )
		{
			if( ( rc = find_divergence( repo, &tags[ i ], &tags[ j ], &a, &b ) ) < 0 )
			{
				printf( "Error: %s\n", gerr );
				if( rc != -2 )
				{
					return -1;
				}
			}

			if( rc == -2 )
			{
				printf( "t%i is not related to t%i\n",
					i+1, j+1 );
			}
			else
			{
				printf( "t%i is %i ahead and %i behind of t%i\n",
					i+1, a, b, j+1 );
			}
			fflush(stdout);
		}
	}

	git_repository_free( repo );
	git_threads_shutdown();
	return 0;
}