Create the Sieve of Eratosthenes exercise. (#101)

Also, a script to automate adding new exercises.

bin/add-exercise

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+#!/usr/bin/env bash
+
+set -euo pipefail
+
+if (( $# != 1 )); then
+    echo >&2 "Populates a new directory for a practice exercise."
+    echo >&2 "Usage: $0 <exercise-slug>"
+    exit 1
+fi
+
+die() { echo >&2 "$*"; exit 1; }
+
+required_tool() {
+    command -v "$1" >/dev/null 2>&1 ||
+        die "$1 is required but not installed. Please install it and make sure it's in your PATH."
+}
+required_tool jq
+required_tool curl
+
+[[ -f ./bin/fetch-configlet ]] || die "run this script from the repo's root directory."
+
+
+slug="${1}"
+name=$(echo "${slug}" | sed 's/\b\w/\u&/g; s/-/ /g')    # assumes GNU sed
+
+it_exists=$(
+    jq --arg slug "${slug}" '
+        .exercises.practice
+        | map(select(.slug == $slug))
+        | length > 0
+    ' config.json
+)
+[[ ${it_exists} == false ]] || die "${slug} already exists in config.json"
+
+# Add entry for exercise in config.json
+./bin/fetch-configlet
+jq  --arg slug "${slug}"                 \
+    --arg uuid "$(./bin/configlet uuid)" \
+    --arg name "${name}"                 \
+    '
+        .exercises.practice += [
+            {
+                slug: $slug,
+                name: $name,
+                uuid: $uuid,
+                practices: [],
+                prerequisites: [],
+                difficulty: 1
+            }
+        ]
+    ' config.json > config.json.tmp \
+&& mv config.json.tmp config.json
+
+# Sync the exercise
+./bin/configlet sync --update --yes  \
+                     --tests include \
+                     --metadata      \
+                     --docs          \
+                     --exercise "${slug}"
+
+cp lib/test-words.8th "exercises/practice/${slug}"
+
+cat << END_TEST > "exercises/practice/${slug}/test.8th"
+"test-words.8th" dup . cr f:include
+"${slug}.8th" dup . cr f:include
+"${slug}-tests.8th" dup . cr f:include
+bye
+END_TEST
+
+touch "exercises/practice/${slug}/${slug}.8th"
+touch "exercises/practice/${slug}/.meta/example.8th"
+curl --silent "https://raw.githubusercontent.com/exercism/problem-specifications/main/exercises/${slug}/canonical-data.json" > "exercises/practice/${slug}/${slug}-tests.8th"
+
+echo 
+read -rp 'Your github username: ' author
+conf="exercises/practice/${slug}/.meta/config.json"
+jq  --arg slug "${slug}"     \
+    --arg author "${author}" \
+    '
+        .authors = [$author] |
+        .files = {
+            solution: [$slug + ".8th"],
+            test: [$slug + "-tests.8th"],
+            example: [".meta/example.8th"]
+        }
+    ' "${conf}" > "${conf}.tmp" \
+&& mv "${conf}.tmp" "${conf}"
+
+echo
+ls -laR "exercises/practice/${slug}"
+
+cat << NEXT_STEPS
+
+Your next steps are:
+- Create the test suite in 'exercises/practice/${slug}/${slug}-tests.8th'
+  based on the canonical data at 'https://github.com/exercism/problem-specifications/blob/main/exercises/${slug}/canonical-data.json'
+  - Any test cases you don't implement, mark them in 'exercises/practice/${slug}/.meta/tests.toml' with "include = false"
+- Create the example solution in 'exercises/practice/${slug}/.meta/example.8th'
+- Verify the example solution by running 'bin/test-no-docker ${slug}'
+- Create the stub solution in 'exercises/practice/${slug}/${slug}.8th'
+- Update the 'difficulty' value for the exercise's entry in the 'config.json' file
+- Validate CI using 'bin/configlet lint'
+NEXT_STEPS

config.json

@@ -234,6 +234,14 @@
         "prerequisites": [],
         "difficulty": 9,
         "practices": []
+      },
+      {
+        "slug": "sieve",
+        "name": "Sieve",
+        "uuid": "e31eac0d-cece-440e-a2be-1431e36284bf",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 8
       }
     ]
   },

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

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+# Instructions
+
+Your task is to create a program that implements the Sieve of Eratosthenes algorithm to find prime numbers.
+
+A prime number is a number that is only divisible by 1 and itself.
+For example, 2, 3, 5, 7, 11, and 13 are prime numbers.
+
+The Sieve of Eratosthenes is an ancient algorithm that works by taking a list of numbers and crossing out all the numbers that aren't prime.
+
+A number that is **not** prime is called a "composite number".
+
+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. This is a prime number.
+2. Mark all the multiples of that prime number as composite (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
+[Wikipedia's Sieve of Eratosthenes article][eratosthenes] has a useful graphic that explains the algorithm.
+
+The tests don't check that you've implemented the algorithm, only that you've come up with the correct list of primes.
+A good first test is to check that you do not use division or remainder operations.
+
+[eratosthenes]: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
+~~~~

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

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+# 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

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+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "sieve.8th"
+    ],
+    "test": [
+      "sieve-tests.8th"
+    ],
+    "example": [
+      ".meta/example.8th"
+    ]
+  },
+  "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.8th

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+(* from https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes#Algorithm_and_variants
+ *
+ *  algorithm Sieve of Eratosthenes is
+ *      input: an integer n > 1.
+ *      output: all prime numbers from 2 through n.
+ *
+ *      let A be an array of Boolean values, indexed by integers 2 to n,
+ *      initially all set to true.
+ *
+ *      for i = 2, 3, 4, ..., not exceeding √n do
+ *          if A[i] is true
+ *              for j = i2, i2+i, i2+2i, i2+3i, ..., not exceeding n do
+ *                  set A[j] := false
+ *
+ *      return all i such that A[i] is true.
+ *)
+
+: make-flag-array \ -- a
+    ( drop true ) 0 r@ a:generate
+    0 false a:!
+    1 false a:!
+;
+
+: mark-multiples-of   \ a i -- a
+    2dup a:@ !if 2drop ;; then
+    drop
+    dup dup n:*       \ a i idx (initial idx is i^2)
+    repeat
+        third over    \ a i idx a idx
+        false a:!     \ a i idx a
+        drop over n:+
+        dup r@ n:>
+    until!
+    2drop
+;
+
+: mark-multiples \ a -- a
+    ' mark-multiples-of 2 r@ n:sqrt loop
+;
+
+: extract-primes \ a1 -- a2
+    a:new >r
+    ( if r> a:push >r else drop then ) a:each
+    drop r>
+;
+
+: primes \ n -- a
+    >r
+    make-flag-array
+    mark-multiples
+    extract-primes
+    rdrop
+;

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

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+# 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-tests.8th

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+5 tests
+
+"no primes under two"
+    []
+    ( 1 primes )
+    test_array_eq_num
+
+SKIP-REST-OF-TESTS
+
+"find first prime"
+    [2]
+    ( 2 primes )
+    test_array_eq_num
+
+"find primes up to 10"
+    [2, 3, 5, 7]
+    ( 10 primes )
+    test_array_eq_num
+
+"limit is prime"
+    [2, 3, 5, 7, 11, 13]
+    ( 13 primes )
+    test_array_eq_num
+
+"find primes up to 1000"
+    [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]
+    ( 1000 primes )
+    test_array_eq_num
+
+end-of-tests

exercises/practice/sieve/sieve.8th

@@ -0,0 +1,2 @@
+: primes \ n -- a
+;