Add nth-prime exercise

config.json

@@ -136,6 +136,14 @@
         "prerequisites": [],
         "difficulty": 4
       },
+      {
+        "slug": "nth-prime",
+        "name": "Nth Prime",
+        "uuid": "799a17cb-9f29-481b-94be-b532f46c8024",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "pangram",
         "name": "Pangram",

exercises/practice/nth-prime/.docs/instructions.md

@@ -0,0 +1,7 @@
+# Instructions
+
+Given a number n, determine what the nth prime is.
+
+By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
+
+If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

exercises/practice/nth-prime/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "nth_prime.zig"
+    ],
+    "test": [
+      "test_nth_prime.zig"
+    ],
+    "example": [
+      ".meta/example.zig"
+    ]
+  },
+  "blurb": "Given a number n, determine what the nth prime is.",
+  "source": "A variation on Problem 7 at Project Euler",
+  "source_url": "https://projecteuler.net/problem=7"
+}

exercises/practice/nth-prime/.meta/example.zig

@@ -0,0 +1,37 @@
+const std = @import("std");
+const math = std.math;
+const mem = std.mem;
+
+pub fn prime(allocator: mem.Allocator, number: usize) !usize {
+    // The first two primes are 2 and 3.
+    if (number <= 2) {
+        return number + 1;
+    }
+
+    const limit = number * math.log2_int_ceil(usize, number);
+    const table = try allocator.alloc(bool, limit);
+    defer allocator.free(table);
+    @memset(table, false);
+
+    var remaining = number - 2;
+    var p: usize = 1;
+    var step: usize = 4;
+    while (true) {
+        p += step; // skip multiples of 2 and multiples of 3
+        step = 6 - step;
+        if (table[p]) {
+            continue;
+        }
+
+        remaining -= 1;
+        if (remaining == 0) {
+            return p;
+        }
+
+        var q = p * p;
+        while (q < limit) {
+            table[q] = true;
+            q += 2 * p;
+        }
+    }
+}

exercises/practice/nth-prime/.meta/tests.toml

@@ -0,0 +1,26 @@
+# 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.
+
+[75c65189-8aef-471a-81de-0a90c728160c]
+description = "first prime"
+
+[2c38804c-295f-4701-b728-56dea34fd1a0]
+description = "second prime"
+
+[56692534-781e-4e8c-b1f9-3e82c1640259]
+description = "sixth prime"
+
+[fce1e979-0edb-412d-93aa-2c744e8f50ff]
+description = "big prime"
+
+[bd0a9eae-6df7-485b-a144-80e13c7d55b2]
+description = "there is no zeroth prime"
+include = false

exercises/practice/nth-prime/nth_prime.zig

@@ -0,0 +1,8 @@
+const std = @import("std");
+const mem = std.mem;
+
+pub fn prime(allocator: mem.Allocator, number: usize) !usize {
+    _ = allocator;
+    _ = number;
+    @compileError("please implement the prime function");
+}

exercises/practice/nth-prime/test_nth_prime.zig

@@ -0,0 +1,44 @@
+const std = @import("std");
+const testing = std.testing;
+
+const nth_prime = @import("nth_prime.zig");
+
+test "first prime" {
+    const p = try nth_prime.prime(testing.allocator, 1);
+    try testing.expectEqual(2, p);
+}
+
+test "second prime" {
+    const p = try nth_prime.prime(testing.allocator, 2);
+    try testing.expectEqual(3, p);
+}
+
+test "third prime" {
+    const p = try nth_prime.prime(testing.allocator, 3);
+    try testing.expectEqual(5, p);
+}
+
+test "fourth prime" {
+    const p = try nth_prime.prime(testing.allocator, 4);
+    try testing.expectEqual(7, p);
+}
+
+test "fifth prime" {
+    const p = try nth_prime.prime(testing.allocator, 5);
+    try testing.expectEqual(11, p);
+}
+
+test "sixth prime" {
+    const p = try nth_prime.prime(testing.allocator, 6);
+    try testing.expectEqual(13, p);
+}
+
+test "seventh prime" {
+    const p = try nth_prime.prime(testing.allocator, 7);
+    try testing.expectEqual(17, p);
+}
+
+test "big prime" {
+    const p = try nth_prime.prime(testing.allocator, 10001);
+    try testing.expectEqual(104743, p);
+}