I suggest you research font formats and the code required to render arbitrary unicode code points with case folding, reflow, ligatures, stacking, bidi, combinin

I suggest you research font formats and the code required to render arbitrary unicode code points with case folding, reflow, ligatures, stacking, bidi, combining marks, etc. TrueType and OpenType are turing-complete all by themselves. There are nearly 140,000 Unicode code points; exhaustively testing the combinations is computationally infeasible. It is the definition of a decidedly non-trivial problem. Furthermore layout passes often involve hundreds or even many thousands of string measurements. Every platform has had numerous bugs, including exploitable ones, in font and string handling.