Add sieve exercise (#376)

* Add sieve exercise

* reformat across multiple lines, set difficulty 3

config.json

@@ -476,6 +476,18 @@
           "strings"
         ]
       },
+      {
+        "slug": "sieve",
+        "name": "Sieve",
+        "uuid": "21172aa9-a155-493c-ad9f-ab4884596488",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3,
+        "topics": [
+          "control_flow_loops",
+          "math"
+        ]
+      },
       {
         "slug": "largest-series-product",
         "name": "Largest Series Product",

exercises/practice/sieve/.docs/instructions.md

@@ -0,0 +1,42 @@
+# Instructions
+
+Your task is to create a program that implements the Sieve of Eratosthenes algorithm to find all prime numbers less than or equal to a given number.
+
+A prime number is a number larger than 1 that is only divisible by 1 and itself.
+For example, 2, 3, 5, 7, 11, and 13 are prime numbers.
+By contrast, 6 is _not_ a prime number as it not only divisible by 1 and itself, but also by 2 and 3.
+
+To use the Sieve of Eratosthenes, you first create a list of all the numbers between 2 and your given number.
+Then you repeat the following steps:
+
+1. Find the next unmarked number in your list (skipping over marked numbers).
+   This is a prime number.
+2. Mark all the multiples of that prime number as **not** prime.
+
+You keep repeating these steps until you've gone through every number in your list.
+At the end, all the unmarked numbers are prime.
+
+~~~~exercism/note
+The tests don't check that you've implemented the algorithm, only that you've come up with the correct list of primes.
+To check you are implementing the Sieve correctly, a good first test is to check that you do not use division or remainder operations.
+~~~~
+
+## Example
+
+Let's say you're finding the primes less than or equal to 10.
+
+- List out 2, 3, 4, 5, 6, 7, 8, 9, 10, leaving them all unmarked.
+- 2 is unmarked and is therefore a prime.
+  Mark 4, 6, 8 and 10 as "not prime".
+- 3 is unmarked and is therefore a prime.
+  Mark 6 and 9 as not prime _(marking 6 is optional - as it's already been marked)_.
+- 4 is marked as "not prime", so we skip over it.
+- 5 is unmarked and is therefore a prime.
+  Mark 10 as not prime _(optional - as it's already been marked)_.
+- 6 is marked as "not prime", so we skip over it.
+- 7 is unmarked and is therefore a prime.
+- 8 is marked as "not prime", so we skip over it.
+- 9 is marked as "not prime", so we skip over it.
+- 10 is marked as "not prime", so we stop as there are no more numbers to check.
+
+You've examined all numbers and found 2, 3, 5, and 7 are still unmarked, which means they're the primes less than or equal to 10.

exercises/practice/sieve/.docs/introduction.md

@@ -0,0 +1,7 @@
+# Introduction
+
+You bought a big box of random computer parts at a garage sale.
+You've started putting the parts together to build custom computers.
+
+You want to test the performance of different combinations of parts, and decide to create your own benchmarking program to see how your computers compare.
+You choose the famous "Sieve of Eratosthenes" algorithm, an ancient algorithm, but one that should push your computers to the limits.

exercises/practice/sieve/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "sieve.el"
+    ],
+    "test": [
+      "sieve-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "blurb": "Use the Sieve of Eratosthenes to find all the primes from 2 up to a given number.",
+  "source": "Sieve of Eratosthenes at Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes"
+}

exercises/practice/sieve/.meta/example.el

@@ -0,0 +1,25 @@
+;;; sieve.el --- Sieve (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl-lib)
+
+(defun primes (limit)
+  (let ((table (make-bool-vector (1+ limit) t))
+        (result nil))
+    (cl-loop for p from 2
+      for psq = (* p p)
+      until (< limit psq)
+      do (when (aref table p)
+           (cl-loop for m from psq to limit by p
+             do (aset table m nil))))
+    (cl-loop for p from limit downto 2
+      do (when (aref table p)
+           (setq result (cons p result))))
+    result))
+
+(provide 'sieve)
+;;; sieve.el ends here
+

exercises/practice/sieve/.meta/tests.toml

@@ -0,0 +1,25 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[88529125-c4ce-43cc-bb36-1eb4ddd7b44f]
+description = "no primes under two"
+
+[4afe9474-c705-4477-9923-840e1024cc2b]
+description = "find first prime"
+
+[974945d8-8cd9-4f00-9463-7d813c7f17b7]
+description = "find primes up to 10"
+
+[2e2417b7-3f3a-452a-8594-b9af08af6d82]
+description = "limit is prime"
+
+[92102a05-4c7c-47de-9ed0-b7d5fcd00f21]
+description = "find primes up to 1000"

exercises/practice/sieve/sieve-test.el

@@ -0,0 +1,43 @@
+;;; sieve-test.el --- Tests for Sieve (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(load-file "sieve.el")
+(declare-function primes "sieve.el" (limit))
+
+
+(ert-deftest no-primes-under-two ()
+  (should (equal '() (primes 1))))
+
+
+(ert-deftest find-first-prime ()
+  (should (equal '(2) (primes 2))))
+
+
+(ert-deftest find-primes-up-to-10 ()
+  (should (equal '(2 3 5 7) (primes 10))))
+
+
+(ert-deftest limit-is-prime ()
+  (should (equal '(2 3 5 7 11 13) (primes 13))))
+
+
+(ert-deftest find-primes-up-to-1000 ()
+  (should (equal '(2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73
+                   79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157
+                   163 167 173 179 181 191 193 197 199 211 223 227 229 233 239
+                   241 251 257 263 269 271 277 281 283 293 307 311 313 317 331
+                   337 347 349 353 359 367 373 379 383 389 397 401 409 419 421
+                   431 433 439 443 449 457 461 463 467 479 487 491 499 503 509
+                   521 523 541 547 557 563 569 571 577 587 593 599 601 607 613
+                   617 619 631 641 643 647 653 659 661 673 677 683 691 701 709
+                   719 727 733 739 743 751 757 761 769 773 787 797 809 811 821
+                   823 827 829 839 853 857 859 863 877 881 883 887 907 911 919
+                   929 937 941 947 953 967 971 977 983 991 997)
+                 (primes 1000))))
+
+
+(provide 'sieve-test)
+;;; sieve-test.el ends here

exercises/practice/sieve/sieve.el

@@ -0,0 +1,14 @@
+;;; sieve.el --- Sieve (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(defun primes (limit)
+  (error "Delete this S-Expression and write your own implementation"))
+
+
+(provide 'sieve)
+;;; sieve.el ends here
+