How does kati check if build.ninja is up-to-date?

Kati re-generates build.ninja when either of the followings has been changed:

• command line parameters passed to kati
• Makefiles (e.g., Android.mk) used.
• environment variables used during evaluation
• results of $(wildcard xxx)
• results of $(shell yyy)

The last one is tricky. $(shell find ...) is slow (~18 secs in total). Initializing the find emulator takes 1 or 2 secs.

Solution: When kati generates build.ninja, kati records the timestamps of directories traversed by find commands. When kati checks if re-generation is necessary, kati runs find commands only when the timestamps of these directories have been changed.