Replace O(n) window list scan with O(1) open-addressing hash table

[jEsuSdA]

Summary

Replaces the O(n) linear window list scan with an O(1) open-addressing hash table, providing the single biggest performance improvement for long-running sessions with many windows.

Problem

find_win() scans the entire window list linearly to locate a window by ID. For each frame, paint_all() calls find_win() for every event window lookup. On a typical desktop with 10–50 windows, this adds up to hundreds of O(n) scans per second. As the window count grows (e.g. after hours of browsing with many tabs), this becomes a significant bottleneck.

Solution

Open-addressing hash table

• Power-of-2 size — enables fast modulo via bitwise AND
• Linear probing — simple and cache-friendly
• Tombstones — for deletion without breaking probe chains. Auto-rehash when tombstones exceed live entries to prevent O(n) degradation over long sessions
• 75% load factor trigger — automatic resize
• OOM resilience — resize failure does not abort; insertion guards against infinite loops with a probe limit

Changes

• find_win() → win_hash_lookup() (O(1) amortized, filters out destroyed entries)
• find_win_any_state() — new, for lookups that include destroyed windows (needed for finish_destroy_win and anti-duplicate guard)
• add_win() — inserts into the hash table + anti-duplicate guard via find_win_any_state() (prevents duplicate win* for the same Window ID, which caused use-after-free crashes)
• finish_destroy_win() — removes from the hash table and now accepts win* directly instead of Window ID. This fixes the zombie window leak where find_win() filtered out destroyed entries and finish_destroy_win() could never find them again

Files changed

• cm-window.c — hash table implementation
• cm-window.h — declarations for find_win_any_state(), win_hash_insert(), win_hash_remove()
• fastcompmgr.c — updated add_win(), finish_destroy_win(), destroy_win(), destroy_callback()

Scope

• make clean && make produces zero warnings, zero errors

Impact

Single biggest performance improvement for long-running sessions. On a desktop with 50 windows, find_win() goes from ~25 pointer comparisons per call to ~1–2 probes. The effect compounds across hundreds of calls per frame.