bug#63671: Add function to test equality of hash tables

[Joseph Turner]

Hello!

Would y'all be open to adding something like this?

(defun hash-equal (hash1 hash2)
  "Return non-nil when the contents of HASH1 and HASH2 are equal.
Table values are compared using `equal' unless they are both hash
tables themselves, in which case `hash-equal' is used.
Does not compare equality predicates."
  (and (= (hash-table-count hash1)
          (hash-table-count hash2))
       (catch 'flag (maphash (lambda (key hash1-value)
                               (let ((hash2-value (gethash key hash2)))
                                 (or (if (and (hash-table-p hash1-value)
                                              (hash-table-p hash2-value))
                                         (hash-equal hash1-value hash2-value)
                                       (equal hash1-value hash2-value))
                                     (throw 'flag nil))))
                             hash1)
              t)))

Rudimentary test:

(let ((hash1 (make-hash-table))
      (hash2 (make-hash-table))
      (hash3 (make-hash-table))
      (hash4 (make-hash-table)))
  (puthash 'foo "foo" hash1)
  (puthash 'foo "foo" hash2)
  (puthash 'bar "foo" hash3)
  (puthash 'bar "foo" hash4)
  (puthash 'baz hash3 hash1)
  (puthash 'baz hash4 hash2)
  (hash-equal hash1 hash2))

We could use hash-table-test to compare predicates, perhaps
dependent on the presence of a 'compare-tests flag?

Best,

Joseph