Data commit

Task/Prime-decomposition/Eiffel/prime-decomposition.e

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+class
+    PRIME_DECOMPOSITION
+
+feature
+
+    factor (p: INTEGER): ARRAY [INTEGER]
+            -- Prime decomposition of 'p'.
+        require
+            p_positive: p > 0
+        local
+            div, i, next, rest: INTEGER
+        do
+            create Result.make_empty
+            if p = 1 then
+                Result.force (1, 1)
+            end
+            div := 2
+            next := 3
+            rest := p
+            from
+                i := 1
+            until
+                rest = 1
+            loop
+                from
+                until
+                    rest \\ div /= 0
+                loop
+                    Result.force (div, i)
+                    rest := (rest / div).floor
+                    i := i + 1
+                end
+                div := next
+                next := next + 2
+            end
+        ensure
+            is_divisor: across Result as r all p \\ r.item = 0 end
+            is_prime: across Result as r all prime (r.item) end
+        end