only compile when it's actually necessary

[ritschwumm]

hi!

to minimize turnaround times in my typescript projects, i try to avoid as much work in my builds as possible - especially compiling anything when i have the output already and the sources did not change. this works well for most tools used in my builds, with one exception: lessc, which therefore eats about 95% of my build time.

i tried to abuse --depends to implement all this in a wrapper script, without much success though. --depends is just too specialized to the usecase of generating makefiles to be really useful, and more importantly cannot be used at the same time as writing output files. discussing #2925, i came to the conclusion this cannot be done cleanly without the help of lessc itself.

so, what do you need to avoid compilation if nothing changed? not much actually, and all of it is known after lessc is done compiling.

• the version of lessc
• which command line options were used
• the paths of output files like
  • the generated .css
  • the generated .css.map
• the path and modification time of
  • the entrypoint (e.g. index.less) and
  • all files transitively imported by the entrypoint

if now some previous compilation saved the above information in some file named state.json, lessc could easily determine there is nothing to do and exit early by checking that

• the state.json exists and can be loaded
• the lessc version is identical
• the command line options are identical
• all output files already exist
• all input files still exist
• all input files still have the same modification time

on a side note, a modification time is not a great indicator for whether a file changed, but often good enough.
one could investigate using a checksum instead - no idea how much that would impact performance.