Split each page of a PDF document in two using Ghostscript and pdftk

a.mjs

#!/usr/bin/env zx

await $`gs -q -dNOPAUSE -dBATCH -o ${'out_' + argv.input} -sDEVICE=pdfwrite ${
    (await $`pdftk ${argv.input} dump_data`).stdout
        .split('\n')
        .filter(line => line.startsWith('PageMediaDimensions: '))
        .map(line => line.split(' ').slice(1).map(x => +x.replace(/,/g, '')))
        .reduce((acc, page, i) => {
            const w = Math.floor(page[0] / 2);
            const h = Math.floor(page[1]);
        
            return acc.concat([
                `-g${w * 10}x${h * 10}`, '-c', `<</PageOffset [-${w} 0]>> setpagedevice`,
                    `-dFirstPage=${i + 1}`, `-dLastPage=${i + 1}`, '-f', argv.input,
                `-g${w * 10}x${h * 10}`, '-c', `<</PageOffset [0 0]>> setpagedevice`,
                    `-dFirstPage=${i + 1}`, `-dLastPage=${i + 1}`, '-f', argv.input
            ]);
        }, [])
}`