Encode bytes as hashtags

Problem: you want to encode arbitrary binary data as a hashtag on Mastodon.

A solution: transform to a hex string and shift the 0..f up to a..p so all the bytes are lowercase Latin characters.

First, load up {tidyverse}:

library(tidyverse)

I want to transform a string like “Hello” to a hex string:

chars2hexstr <- function(t) {
  t |>
    charToRaw() |>
    as.character() |>
    paste(collapse = "")
}

Testing:

chars2hexstr("Hello")

[1] "48656c6c6f"

Consulting an ASCII table, looks good. “H” = 0x48, “e” = 0x65, “l” = 0x6c, and “o” = 0x6f.

Let’s undo it:

hexstr2chars <- function(hexstr) {
  substring(hexstr,
            seq(1, nchar(hexstr) - 1, 2),
            seq(2, nchar(hexstr), 2)) |>
  strtoi(base = 16) |>
  as.raw() |>
  rawToChar()
}

And a test:

hexstr2chars("48656c6c6f")

[1] "Hello"

Next, I want to shift 0..f up to a..p. I’m sure there’s an easier way. Here’s how I’m doing it. First, setup a mapping:

charmap <- data.frame(
  hex = c(0:9, letters[1:6]),
  let = letters[1:16])
charmap

These two functions translate back and forth between 0..f (hex) and a..p (I’m going to call it a lethex), using a strsplit and join:

hex2lethex <- function(hstr) {
  data.frame(hex = (hstr |> strsplit(""))[[1]]) |>
    left_join(charmap, by = "hex") |>
    pull(let) |>
    paste(collapse = "")
}

lethex2hex <- function(lethex) {
  data.frame(let = (lethex |> strsplit(""))[[1]]) |>
    left_join(charmap, by = "let") |>
    pull(hex) |>
    paste(collapse = "")
}

Try it with “Hello”. Here’s the hex string:

hello_hex <- chars2hexstr("Hello")
hello_hex

[1] "48656c6c6f"

Now as a lethex:

hello_lethex <- hello_hex |> hex2lethex()
hello_lethex

[1] "eigfgmgmgp"

And back again:

hello_lethex |> lethex2hex()

[1] "48656c6c6f"

Finally, a couple of wrappers, which take a string to a lethex string and back again:

str2hashtag <- function(s) {
  s |> chars2hexstr() |> hex2lethex()
}

hashtag2str <- function(h) {
  h |> lethex2hex() |> hexstr2chars()
}

str2hashtag("Hello")

[1] "eigfgmgmgp"

hashtag2str("eigfgmgmgp")

[1] "Hello"

Finally:

hashtag2str("gihehehahddkcpcphhhhhhcohjgphfhehfgcgfcogdgpgncphhgbhegdgidphgdngefbhhdehhdjfhghfigdfb")