[ondrik]

Fixes #22.

The bug

With --loopsum, every amplitude is replaced by a variable v encoded as vR + i·vI. After the fix point, eval_symbol puts the real amplitudes back in. It bound vI to FiveTuple::imag(), which is i·Im(x) and not the real number Im(x). The term's coefficient already carries the i, so it contributed i·i·Im(x) = −Im(x), and every summarized iteration ran on Re(x) − Im(x) instead of x. Any loop whose iterations see a complex amplitude gave wrong states. These come from a complex precondition or from gates like s, t, y or sdg in the body. The error changed ver verdicts in both directions.

The fix

src/loop_summarization.cc: bind vI to imag() rotated a quarter turn clockwise, which multiplies it by −i and gives Im(x). clockwise() is part of the is_complex concept, so this doesn't depend on the FiveTuple representation.

I left FiveTuple::imag() alone. The HSL imag(...) function also uses it and has the same i·Im(x) quirk, but changing that would change how existing .hsl files are read, so it should get its own PR.

Test

A new unit test, for_loop_summarization_complex_amplitudes, runs each loop both unrolled and summarized and requires exactly the same printed set of states:

Precondition	Loop body	Iterations
(|0⟩ + i|1⟩)/√2 (the issue's example)	h	1
|0⟩	h; t	3
|1⟩	s	2
(|00⟩ + e^{iπ/4}|11⟩)/√2	y; h; cz; tdg	3

All four fail without the fix and pass with it. The existing GroverFor test still passes, and ctest passes (2/2).

I also checked against the CLI:

• The issue's ver example now gives [OK] / [failed] with --loopsum, the same as without it.
• (H·T)^n on |0⟩ for n = 1, 2, 3, 4 and 8 matches unrolling exactly. Before the fix, every n ≥ 2 was wrong and not even normalised.

Not fixed here: --loopsum hangs on multi-state preconditions

While testing, I found that --loopsum never finishes when the precondition contains more than one state, for example {|0>, |1>} with a single x. The same happens on main without this change. It is filed as #66.

🤖 Generated with Claude Code

https://claude.ai/code/session_013gtFQRPe1eSSpmA7EJ2KNY