import std.traits;
import std.stdio;

static
ubyte[] packVarint(T)( T value )
if( isIntegral!T && isUnsigned!T )
out( arr )
{
    T d;
    size_t size = d.unpackVarint( &arr[0] );

    writeln( "out contract, type=", typeid(T), " isUnsigned=", isUnsigned!T, " arg=", value, " result=", arr );
    stdout.flush;

    assert( size == arr.length );
    assert( d == value );
}
body
{
    writeln( "value inside of body: ", value, " type=", typeid( typeof(value) ) );
    stdout.flush;

    ubyte[] res;

    immutable ubyte maximal = 0b_1000_0000;

    while( value >= maximal )
    {
        res ~= cast( ubyte )( value | maximal );
        value >>= 7;
    }

    import std.conv: to;
    res ~= to!( ubyte )( value );

    return res;
}
unittest
{
    auto v = packVarint!ulong( 300 );
    assert( v.length == 2 );
    assert( v == [ 0b_10101100, 0b_00000010 ] );
}

pure size_t unpackVarint( T )( out T result, inout ubyte* data )
if( isIntegral!T && isUnsigned!T )
{
    size_t i;
    size_t res; // big sized type used also for overflow checking

    do {
        res |= ( data[i] & 0b_0111_1111 ) << 7 * i;
        enforce( res <= T.max, "Varint is too big for type " ~ T.stringof );
    } while( msbIsSet( &data[i++] ) );

    result = cast(T) res;

    return i;
}
unittest
{
    ubyte d[2] = [ 0b_10101100, 0b_00000010 ];
    size_t result;

    assert( result.unpackVarint( &d[0] ) == d.length );
    assert( result == 300 );
}

void main()
{
    for( long i = -100; i < 100; i++ )
        packVarint( i );
}