katex.js

const katex = require('katex');
const assert = require('assert').ok;

let inline = {
    throwOnError: false,
    displayMode: false,
    output: 'html'
};

let display = {
    throwOnError: false,
    displayMode: true,
    output: 'html'
};

function detex(block) {
    if (block.t === 'Math') {
        assert(block.c.length === 2);
        assert((block.c[0].t === 'InlineMath') || (block.c[0].t === 'DisplayMath'));
        assert(typeof(block.c[1]) === 'string');
        block.t = 'RawInline';
        block.c = ['html', katex.renderToString(block.c[1], block.c[0].t === 'InlineMath' ? inline : display)];
    } else if (Array.isArray(block.c)) {
        block.c.forEach(detex);
    }
}

let json = '';

process.stdin.setEncoding('utf-8');

process.stdin.on('readable', () => {
    for (let chunk = process.stdin.read(); chunk; chunk = process.stdin.read()) {
        json += chunk;
    }
});

process.stdin.on('end', () => {
    let data = JSON.parse(json);
    data.blocks.forEach(detex);
    process.stdout.write(JSON.stringify(data));
});

panpipe.c

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <io.h>
#include <fcntl.h>

#define EAGAIN 11

/*
 * NOTE: While this looks POSIX-y, it is _not_ Linux compatible!
 *
 * An apparently undocumented feature of Windows pipes opened
 * with _popen is that they are bidirectional--you can read
 * from and write to the same pipe. Linux does not allow this.
 *
 * The weird way of checking for eof is due to
 * http://cbloomrants.blogspot.com/2018/04/race-in-eof-on-windows-pipes.html
 * which I never encountered, but, hell.
 */

int main(int argc, char **argv)
{
    (void)(argc, argv);

#if 1
    FILE *pandoc = _popen("/path/to/pandoc --katex --filter=katex", "wb");
#else
    FILE *pandoc = _popen("path/to/node pandoc\\katex.js", "wb");
#endif

    if (pandoc == 0) {
        exit(1);
    }

    int in = _fileno(stdin);
    int out = _fileno(stdout);
    int pan = _fileno(pandoc);

    _setmode(in, O_BINARY);
    _setmode(out, O_BINARY);

    char buf[BUFSIZ];

    for (;;) {
        int n = _read(in, buf, sizeof(buf));
        if (n > 0) {
            _write(pan, buf, n);
        } else if (n == 0) {
            if (_eof(in)) {
                break;
            }
        } else {
            if (errno == EAGAIN) {
                continue;
            }

            if (_eof(in)) {
                break;
            }
        }
    }

    for (;;) {
        int n = _read(pan, buf, sizeof(buf));
        if (n > 0) {
            _write(out, buf, n);
        } else if (n == 0) {
            if (_eof(in)) {
                break;
            }
        } else {
            if (errno == EAGAIN) {
                continue;
            }

            if (_eof(in)) {
                break;
            }
        }
    }

    return 0;
}