[ note followups ] Mark Carter <[EMAIL PROTECTED]> writes:
> I'd like to propose a coding challenge of my own. The challenge is to > reproduce the TEA (Tiny Encryption Algorith): > http://www.simonshepherd.supanet.com/tea.htm > in your language of choice. Here's mine, in Common Lisp. (defmacro define-tea-pair ((encrypt decrypt) (delta n) (i1 j1 i2 j2)) `(macrolet ((32bitize (form) `(logand ,form #xffffffff)) (tea-kernel (x i j) `(logxor (+ (ash ,x 4) (aref key ,i)) (+ ,x sum) (+ (ash ,x -5) (aref key ,j)))) (tea-loop ((text n sum) &body body) `(let ((result (make-array 2 :element-type '(unsigned-byte 32))) (y (aref ,text 0)) (z (aref ,text 1))) (do ((n ,n (- n 1)) (sum ,sum)) ((<= n 0) (prog1 result (setf (aref result 0) y (aref result 1) z))) ,@body)))) (defun ,encrypt (plaintext key &aux (delta ,delta)) (declare (type (simple-array (unsigned-byte 32) (2)) plaintext) (type (simple-array (unsigned-byte 32) (4)) key)) (tea-loop (plaintext ,n 0) (setq sum (32bitize (+ sum delta)) y (32bitize (+ y (tea-kernel z ,i1 ,j1))) z (32bitize (+ z (tea-kernel y ,i2 ,j2)))))) (defun ,decrypt (ciphertext key &aux (delta ,delta)) (declare (type (simple-array (unsigned-byte 32) (2)) ciphertext) (type (simple-array (unsigned-byte 32) (4)) key)) (tea-loop (ciphertext ,n (32bitize (* ,n delta))) (setq z (32bitize (- z (tea-kernel y ,i2 ,j2))) y (32bitize (- y (tea-kernel z ,i1 ,j1))) sum (32bitize (- sum delta))))))) (define-tea-pair (encipher decipher) (#x9e3779b9 32) (0 1 2 3)) So far, so ordinary; only marginally shorter than the C version; I'm certainly nowhere near Pascal's 14 lines, although my version has the advantage over his that each constant is mentioned only once, and all quantities derived from it are computed at compile-time; I can define a different pair with (define-tea-pair (eprime dprime) (#xabcdef01) (3 1 2 0)) and the new functions are inverses of each other as before. The other thing that might be of interest is the inner loop. There are no declarations other than the argument declarations, and all the code that I have written is portable Common Lisp, and should work in any conforming implemenation. In SBCL (on the PowerPC; other platforms are similar), the inner loop for ENCIPHER is addis $nl0,$nl3,-25033 addi $nl3,$nl0,31161 rlwinm $nl5,$nl2,4,0,27 lwz $nl6,1($fdefn) add $nl6,$nl5,$nl6 add $nl0,$nl2,$nl3 xor $cfunc,$nl6,$nl0 rlwinm $nl0,$nl2,27,5,31 mr $nl5,$nl0 lwz $nl6,5($fdefn) add $nl6,$nl5,$nl6 xor $nl6,$cfunc,$nl6 add $nl1,$nl1,$nl6 rlwinm $nl5,$nl1,4,0,27 lwz $nl6,9($fdefn) add $nl6,$nl5,$nl6 add $nl0,$nl1,$nl3 xor $cfunc,$nl6,$nl0 rlwinm $nl0,$nl1,27,5,31 mr $nl5,$nl0 lwz $nl6,13($fdefn) add $nl6,$nl5,$nl6 xor $nl6,$cfunc,$nl6 add $nl2,$nl2,$nl6 addi $nl4,$nl4,-4 cmpwi cr0,$nl4,0 bt gt,l0 and while this may be opaque to some readers, the point is that it is pretty much comparable to the C code in performance (the differences between this disassembly and gcc -O2 lie in the fact that SBCL's instruction scheduler is pretty much nonexistent on the PowerPC architecture). Christophe -- http://mail.python.org/mailman/listinfo/python-list
