ord and chr for nix 🤠

char.nix

{ lib ? (import <nixpkgs> {}).lib }:

let

  # ord-data.nix is an auto generated nix file
  # which contains single byte strings from
  # 0x01 to 0xff in a list where there index
  # is equal to their byte-value.
  #
  # The string at index 0 is empty because nix
  # apparently thinks a NUL byte is EOF.
  ordData = import ./gen/ord-data.nix;

  inherit (lib)
    stringToCharacters
    concatMapStrings
    ;

  inherit (builtins)
    map
    elemAt
    ;

  elemIndex = e: l:
    let
      tailCall = e: l: i:
        if l == []
        then -1
        else if builtins.head l == e
        then i
        else tailCall e (builtins.tail l) (i + 1);
    in tailCall e l 0;

  ord = char:
    let
      i = elemIndex char ordData;
    in if i == -1 then 0 else i;   # can't have a NUL byte in a string apparently

  chr = i:
    if i < 1 || i > 255
    then throw "chr: only 0x01-0xff is supported"
    else elemAt ordData i;

  stringToIntegers = str:
    map ord (stringToCharacters str);

  integersToString = is: concatMapStrings chr is;

in

{
  inherit
    ord
    chr
    stringToIntegers
    integersToString
    ;
}

ord-data.nix

[
  ""
  "
"
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  "\t"
  "\n"
  ""
  ""
  "\r"
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  ""
  " "
  "!"
  "\""
  "#"
  "$"
  "%"
  "&"
  "'"
  "("
  ")"
  "*"
  "+"
  ","
  "-"
  "."
  "/"
  "0"
  "1"
  "2"
  "3"
  "4"
  "5"
  "6"
  "7"
  "8"
  "9"
  ":"
  ";"
  "<"
  "="
  ">"
  "?"
  "@"
  "A"
  "B"
  "C"
  "D"
  "E"
  "F"
  "G"
  "H"
  "I"
  "J"
  "K"
  "L"
  "M"
  "N"
  "O"
  "P"
  "Q"
  "R"
  "S"
  "T"
  "U"
  "V"
  "W"
  "X"
  "Y"
  "Z"
  "["
  "\\"
  "]"
  "^"
  "_"
  "`"
  "a"
  "b"
  "c"
  "d"
  "e"
  "f"
  "g"
  "h"
  "i"
  "j"
  "k"
  "l"
  "m"
  "n"
  "o"
  "p"
  "q"
  "r"
  "s"
  "t"
  "u"
  "v"
  "w"
  "x"
  "y"
  "z"
  "{"
  "|"
  "}"
  "~"
  ""
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]

z-generate-ord-data.c

#include <stdio.h>

int main(void) {
  fputs("[\n  \"\"\n", stdout);
  for(int i = 1; i <= 0xff; i++) {
    for(int j = 0; j < 2; j++)
      putchar(' ');

    putchar('\"');
    switch(i) {
      case '\n':
        fputs("\\n", stdout);
        break;
      case '\t':
        fputs("\\t", stdout);
        break;
      case '\\':
        fputs("\\\\", stdout);
        break;
      case '\"':
        fputs("\\\"", stdout);
        break;
      case '\r':
        fputs("\\r", stdout);
        break;
      default:
        putchar(i);
    }
    putchar('\"');
    putchar('\n');
  }
  fputs("]\n", stdout);

  return 0;
}