How to read a certain number of characters (as opposed to bytes) in Crystal? - crystal-lang

Question

In Crystal, if I have a string (or a file), how do I read a certain number of characters at a time? Using functions like IO#read, IO#gets, IO#read_string, and IO#read_utf8, one can specify a certain number of bytes to read, but not a certain number of UTF-8 characters (or ones of another encoding).
In Python, for example, one might do this:
from io import StringIO
s = StringIO("abcdefgh")
while True:
chunk = s.read(4)
if not chunk: break
Or, in the case of a file, this:
with open("example.txt", 'r') as f:
while True:
chunk = f.read(4)
if not chunk: break
Generally, I'd expect IO::Memory to be the class to use for the string case, but as far as I can tell, its methods don't allow for this. How would one do this in an efficient and idiomatic fashion (for both strings and files – perhaps the answer is different for each) in Crystal?

Answer 1

There currently is no short cut implementation for this available in Crystal.
You can read individual chars with IO#read_char or consecutive ones with IO#each_char.
So a basic implementation would be:
io = IO::Memory.new("€abcdefgh")
string = String.build(4) do |builder|
4.times do
builder << io.read_char
end
end
puts string
Whether you use a memory IO or a file or any other IO is irrelevant, the behaviour is all the same.

Answer 2

io = IO::Memory.new("€€€abc€€€") #UTF-8 string from memory
or
io = File.open("test.txt","r") #UTF-8 string from file
iter = io.each_char.each_slice(4) #read max 4 chars at once
iter.each { |slice| #into a slice
puts slice
puts slice.join #join to a string
}
output:
['€', '€', '€', 'a']
€€€a
['b', 'c', '€', '€']
bc€€
['€']
€

Answer 3

In addition to the answers already given, for strings in Crystal, you can read X amount of characters with a range like this:
my_string = "A foo, a bar."
my_string[0..5] => "A foo"

Answer 4

This workaround seems to work for me:
io = IO::Memory.new("abcdefghz")
chars_to_read = 2 # Number of chars to read
while true
chunk = io.gets(chars_to_read) # Grab the chunk of type String?
break if chunk.nil? # Break if nothing else to read aka `nil`
end