idiomatic case-insensitive string comparison?

[Sam Steingold <sds@gnu.org>]

Hi,
Suppose I need to compare strings in a case-insensitive manner.

Common Lisp provides `string-equal', `string-lessp' and `string-greaterp'
(http://clhs.lisp.se/Body/f_stgeq_.htm).

However, in Emacs Lisp `string=' and `string-equal' are the same
(and are not affected by `case-fold-search').

I suppose I can define

--8<---------------cut here---------------start------------->8---
(defun string=-case-fold (a b)
  "case-insensitive string equality test"
  (string= (downcase a) (downcase b)))
--8<---------------cut here---------------end--------------->8---

but this has at least two problems:

1. It can break down for some non-ASCII characters (e.g., Eszett)

2. It allocates fresh memory unnecessarily

So, what is the Emacs Lisp idiom for case-insensitive string equality?

PS. Some languages offer strcasecmp()
(https://en.wikipedia.org/wiki/C_string_handling) others do not
(https://stackoverflow.com/q/319426/850781).

-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.2113
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