bug#28753: 25.3; Functions to get alist from hash table and vice versa

[Drew Adams <drew.adams@oracle.com>]

Dunno whether functions like these might be useful.  I use something
similar.  If you think they're useful, consider adding them.

(cl-defun alist-to-hash-table (alist &optional use-last-p
                                     &key (test 'eql) weakness (size 65)
                                     (rehash-size 1.5) (rehash-threshold 0.8))
  "Create and return a hash table created from ALIST.
By default, if the same alist key is used in more than one alist entry
then only the first entry is used for the hash table.  Non-nil
USE-LAST-P means override this to use only the last entry for a given
key.

See `make-hash-table' for the keyword arguments you can use and their
default values."
  (let ((ht  (make-hash-table :test test :weakness weakness :size size
                              :rehash-size rehash-size :rehash-threshold rehash-threshold))
        key val)
    (dolist (key.val  alist)
      (setq key  (car key.val)
            val  (cdr key.val))
      (when (or use-last-p  (not (gethash key ht)))
        (puthash key val ht)))
    ht))

(defun hash-table-to-alist (hash-table)
  "Create and return an alist created from HASH-TABLE.
The order of alist entries is the same as the order of hash-table
entries (which normally is the order in which the entries were added
to the table)."
  (let ((al  ()))
    (maphash (lambda (key val) (push (cons key val) al)) hash-table)
    (nreverse al)))


In GNU Emacs 25.3.1 (x86_64-w64-mingw32)
 of 2017-09-26
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --without-compress-install 'CFLAGS=-O2
 -static -g3' PKG_CONFIG_PATH=/mingw64/lib/pkgconfig'