Data update

Task/Descending-primes/ALGOL-W/descending-primes.alg

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+begin % find all primes with strictly descending digits - translation of Lua %
+
+    % quicksorts v, the bounds of v must be specified in lb and ub           %
+    procedure quicksort ( integer array v( * )
+                        ; integer value lb, ub
+                        ) ;
+        if ub > lb then begin
+            % more than one element, so must sort %
+            integer left, right, pivot;
+            left   := lb;
+            right  := ub;
+            % choosing the middle element of the array as the pivot %
+            pivot  := v( left + ( ( right + 1 ) - left ) div 2 );
+            while begin
+                while left  <= ub and v( left  ) < pivot do left  := left  + 1;
+                while right >= lb and v( right ) > pivot do right := right - 1;
+                left <= right
+            end do begin
+                integer swap;
+                swap       := v( left  );
+                v( left  ) := v( right );
+                v( right ) := swap;
+                left       := left  + 1;
+                right      := right - 1
+            end while_left_le_right ;
+            quicksort( v, lb,   right );
+            quicksort( v, left, ub    )
+        end quicksort ;
+
+    % returns true if n is prime, false otherwise                            %
+    logical procedure is_prime( integer value n ) ;
+        if      n  <  2     then false
+        else if n rem 2 = 0 then n = 2
+        else if n rem 3 = 0 then n = 3
+        else begin
+            logical prime; prime := true;
+            for f := 5 step 6 until entier( sqrt( n ) ) do begin
+                if n rem f = 0 or n rem ( f + 2 ) = 0 then begin
+                    prime := false;
+                    goto done
+                end if_n_rem_f_eq_0_or_n_rem_f_plus_2_eq_0
+            end for_f;
+done:       prime
+        end is_prime ;
+
+    % increments n and also returns its new value                            %
+    integer procedure inc ( integer value result n ) ; begin n := n + 1; n end;
+
+    % sets primes to the list of descending primes and lenPrimes to the      %
+    % number of descending primes - primes must be big enough, e.g. have 511 %
+    % elements                                                               %
+    procedure descending_primes ( integer array primes ( * )
+                                ; integer result lenPrimes
+                                ) ;
+    begin
+        integer array digits     ( 1 ::    9 );
+        integer array candidates ( 1 :: 6000 );
+        integer lenCandidates;
+        candidates( 1 ) := 0;
+        lenCandidates   := 1;
+        lenPrimes       := 0;
+        for i := 1 until 9 do digits( i ) := 10 - i;
+        for i := 1 until 9 do begin
+            for j := 1 until lenCandidates do begin
+                integer cValue; cValue := candidates( j ) * 10 + digits( i );
+                if is_prime( cValue ) then primes( inc( lenPrimes ) ) := cValue;
+                candidates( inc( lenCandidates ) ) := cValue
+            end for_j
+        end for_i ;
+        quickSort( primes, 1, lenPrimes );
+    end descending_primes ;
+
+    begin % find the descending primes and print them                        %
+        integer array primes ( 1 :: 512 );
+        integer lenPrimes;
+        descending_primes( primes, lenPrimes );
+        for i := 1 until lenPrimes do begin
+            writeon( i_w := 8, s_w := 0, " ", primes( i ) );
+            if i rem 10 = 0 then write()
+        end for_i
+    end
+end.