/// Whether `c` is between `0-9`
pub fn is_digit(c: char) -> bool {
    '0' <= c && c <= '9'
}

/// Whether `c` is between `0-9a-fA-F`
pub fn is_hex_digit(c: char) -> bool {
    is_digit(c) || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F'
}

/// Joins a String and a char
pub fn str_append(current: String, c: char) -> String {
    format!("{}{}", current, c)
}

/// Whether `c` is an operator char.
pub fn is_operator(c: char) -> bool {
    c == '+'
        || c == '-'
        || c == '='
        || c == '*'
        || c == '!'
        // TODO: ??? where is this used??
        || c == '\\'
        || c == '/'
        || c == '|'
        || c == '@'
        || c == '#'
        || c == '$'
        || c == '~'
        || c == '%'
        || c == '&'
        || c == '?'
        || c == '<'
        || c == '>'
        || c == '^'
        || c == '.'
        || c == ':'
}

/// Whether `c` is between `a-z`
pub fn is_lowercase(c: char) -> bool {
    c >= 'a' && c <= 'z'
}

/// Whether `c` is between `A-Z`
pub fn is_uppercase(c: char) -> bool {
    c >= 'A' && c <= 'Z'
}

/// Whether `c` is between `a-zA-Z_0-9`
pub fn is_identifier_char(c: char) -> bool {
    is_lowercase(c) || is_uppercase(c) || c == '_' || is_digit(c)
}