new argument: --batches <batches>: process splits in parallel instead of linearly

[helpimnotdrowning]

New argument --batches <batches: int || 4> will parallelize the splits using p-limit (3.1, compatible with node 10). This runs much faster on my machine: using the font "IBM Plex Sans JP" complete/hinted at -c 128, current takes about 1minute/split for 157 splits (took too long for me to keep measuring, est. 2h40m); using the default batch size of 4 brings it up to around 3minutes/split, taking only 44 minutes.

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[gemini-code-assist]

Summary of Changes

Hello @helpimnotdrowning, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a major performance enhancement by parallelizing the font splitting process. It adds a new --batches argument, allowing users to specify how many font subsets should be generated concurrently. By switching to asynchronous execution and utilizing a concurrency control library, the tool can now process multiple splits in parallel, leading to a substantial reduction in overall processing time, especially for fonts with many splits.

Highlights

• Parallel Processing: Introduced a new command-line argument --batches <batches> (default: 4) to enable parallel processing of font splits, significantly reducing execution time for large font files.
• Asynchronous Execution: Migrated from synchronous execFileSync to asynchronous execFile using util.promisify for external process calls, improving responsiveness and allowing for parallel operations.
• Concurrency Control: Integrated the p-limit library to manage the concurrency of font subset generation, ensuring that a specified number of tasks run in parallel without overwhelming system resources.

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[gemini-code-assist]

Code Review

This pull request introduces parallel processing for font subset generation, which is a great performance improvement. The implementation uses p-limit to control concurrency, which is appropriate for this task. I've made a few suggestions to improve code clarity and maintainability, such as removing unused parameters and commented-out code. I also have a suggestion to improve the progress logging to make it more accurate in a parallel execution context. Overall, this is a solid contribution.

[helpimnotdrowning]

the swap-around of the logging order mentioned above was an opinionated change based on personal belief that logs should state what the program is about to do rather than what it has just done (I find it helps in debugging and tracking down issue sources).

I made this swap before I thought to publish my changes at all; if you (maintainer) would like me to put it back to before I can do that, but I think it's for the best to stay like it is anyways.

(the AI explanation is also wrong, it says that "several tasks might be logged as 'started' while they are actually waiting in a queue": limit() will wait for the whole inner function to complete, so only the currently-working tasks are logged, any queued tasks are not logged)