Encode an NEC IR command into code for Tuya ZS06/ZS08/TS1201

tuya_ir_encode.js

/*
Encode an NEC IR command into code for Tuya ZS06/ZS08/TS1201

Usage: encode_nec("<four bytes in hex, two bytes of address followed by two bytes of command>")
If your address and/or command is just one byte (8 bits), append the complement of the byte after it to make it two bytes.

Example:
encode_nec("04fb08f7") // encodes nec address 0x04 and command 0x08

See:
https://www.sbprojects.net/knowledge/ir/nec.php
https://www.zigbee2mqtt.io/devices/ZS06.html
https://github.com/Koenkk/zigbee2mqtt/issues/11633

Tuya ZS06 uses a custom encoding of ir codes. I figured enough of the encoding to generate arbitary ir codes, but there are additional encoding features to save bytes that I haven't figured out.

The encoding is base64 encoding of any number of concatenated command blocks, where each block is:
- 1 byte of [length]-1 (length <= 32 bytes is supported)
- [length] number of bytes of little endian 16 bit integers
  - Each integer describes the length of time in microseconds to keep the tramsitter's current on/off state, before flipping

The initial state is on, so the first 16 bit integer describes on time, the second off time, the third on time again, etc.
*/

function encode_nec(hex) {
    function le(x) {
        x = x & 0xffff;
        return [ x & 0xff, x >> 8 ];
    }

    let output = [ 4-1, ...le(9000), ...le(4500) ];

    for (const x of Buffer.from(hex, 'hex')) {
        output.push(32-1);
        for (let i = 0; i < 8; i++) {
            output.push(...le(560));
            if (x & (1 << i)) {
                output.push(...le(2250-560));
            } else {
                output.push(...le(1125-560));
            }
        }
    }

    output.push(2-1, ...le(560));

    return Buffer.from(new Uint8Array(output)).toString('base64');
}