this is a sketch, you may have to do some shifts or such, but the method should be crystal clear

you may need to change polarity (this is for active column high for row high)

// amalyze row 1 
row1 = 1;
if (Px & COL_MASK)
{ // column active in this row
  key = row1_table[(Px & COL_MASK)] // translate column to key code
  return; // we done (multiple keys pressed not supported)
}
row1 = 0; // done with row 1
// amalyze row 2 
row2 = 1;
....
....

Erik