Add palindrome-products (#507)

Author: keiravillekode

config.json

@@ -1083,6 +1083,14 @@
         "prerequisites": [],
         "difficulty": 7
       },
+      {
+        "slug": "palindrome-products",
+        "name": "Palindrome Products",
+        "uuid": "4f4d859e-18c2-4a80-ad95-5f2427dafc9d",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 8
+      },
       {
         "slug": "word-count",
         "name": "Word Count",

exercises/practice/palindrome-products/.docs/instructions.append.md

@@ -0,0 +1,5 @@
+# Instructions append
+
+## Zig-specific Notes
+
+You may assume the range `[min, max]` is valid: `min <= max`.

exercises/practice/palindrome-products/.docs/instructions.md

@@ -0,0 +1,36 @@
+# Instructions
+
+Detect palindrome products in a given range.
+
+A palindromic number is a number that remains the same when its digits are reversed.
+For example, `121` is a palindromic number but `112` is not.
+
+Given a range of numbers, find the largest and smallest palindromes which
+are products of two numbers within that range.
+
+Your solution should return the largest and smallest palindromes, along with the factors of each within the range.
+If the largest or smallest palindrome has more than one pair of factors within the range, then return all the pairs.
+
+## Example 1
+
+Given the range `[1, 9]` (both inclusive)...
+
+And given the list of all possible products within this range:
+`[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 15, 21, 24, 27, 20, 28, 32, 36, 25, 30, 35, 40, 45, 42, 48, 54, 49, 56, 63, 64, 72, 81]`
+
+The palindrome products are all single digit numbers (in this case):
+`[1, 2, 3, 4, 5, 6, 7, 8, 9]`
+
+The smallest palindrome product is `1`.
+Its factors are `(1, 1)`.
+The largest palindrome product is `9`.
+Its factors are `(1, 9)` and `(3, 3)`.
+
+## Example 2
+
+Given the range `[10, 99]` (both inclusive)...
+
+The smallest palindrome product is `121`.
+Its factors are `(11, 11)`.
+The largest palindrome product is `9009`.
+Its factors are `(91, 99)`.

exercises/practice/palindrome-products/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "palindrome_products.zig"
+    ],
+    "test": [
+      "test_palindrome_products.zig"
+    ],
+    "example": [
+      ".meta/example.zig"
+    ]
+  },
+  "blurb": "Detect palindrome products in a given range.",
+  "source": "Problem 4 at Project Euler",
+  "source_url": "https://projecteuler.net/problem=4"
+}

exercises/practice/palindrome-products/.meta/example.zig

@@ -0,0 +1,110 @@
+const std = @import("std");
+const math = std.math;
+const mem = std.mem;
+
+pub const Pair = struct {
+    first: u64,
+    second: u64,
+};
+
+pub const Palindrome = struct {
+    value: u64,
+    factors: []Pair,
+};
+
+fn palindrome(number: u64) bool {
+    var buffer: [20]u64 = undefined;
+    buffer[0] = number % 10;
+    var end: usize = 1;
+    var n: u64 = number / 10;
+    while (n != 0) {
+        buffer[end] = n % 10;
+        end += 1;
+        n = n / 10;
+    }
+
+    var begin: usize = 0;
+    while (begin + 1 < end) {
+        end -= 1;
+        if (buffer[begin] != buffer[end]) {
+            return false;
+        }
+        begin += 1;
+    }
+    return true;
+}
+
+pub fn smallest(allocator: mem.Allocator, min: u64, max: u64) mem.Allocator.Error!?Palindrome {
+    std.debug.assert(min <= max);
+    var value: u64 = math.maxInt(u64);
+    var factors = std.array_list.Managed(Pair).init(allocator);
+    errdefer factors.deinit();
+
+    for (min..max) |i| {
+        const first = @as(u64, @intCast(i));
+        for (first..max) |j| {
+            const second = @as(u64, @intCast(j));
+            const product = first * second;
+            if (product > value) {
+                break;
+            }
+
+            if (!palindrome(product)) {
+                continue;
+            }
+
+            if (product < value) {
+                factors.clearRetainingCapacity();
+                value = product;
+            }
+
+            try factors.append(.{ .first = first, .second = second });
+        }
+    }
+    if (factors.items.len == 0) {
+        return null;
+    }
+
+    return .{
+        .value = value,
+        .factors = try factors.toOwnedSlice(),
+    };
+}
+
+pub fn largest(allocator: mem.Allocator, min: u64, max: u64) mem.Allocator.Error!?Palindrome {
+    std.debug.assert(min <= max);
+    var value: u64 = 0;
+
+    var factors = std.array_list.Managed(Pair).init(allocator);
+    errdefer factors.deinit();
+
+    for (0..(max - min + 1)) |i| {
+        const second = @as(u64, @intCast(max - i));
+        for (0..(second - min + 1)) |j| {
+            const first = @as(u64, @intCast(second - j));
+            const product = first * second;
+            if (product < value) {
+                break;
+            }
+
+            if (!palindrome(product)) {
+                continue;
+            }
+
+            if (product > value) {
+                factors.clearRetainingCapacity();
+                value = product;
+            }
+
+            try factors.append(.{ .first = first, .second = second });
+        }
+    }
+    if (factors.items.len == 0) {
+        return null;
+    }
+
+    return .{
+        .value = value,
+        .factors = try factors.toOwnedSlice(),
+    };
+}

exercises/practice/palindrome-products/.meta/tests.toml

@@ -0,0 +1,51 @@
+# 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.
+
+[5cff78fe-cf02-459d-85c2-ce584679f887]
+description = "find the smallest palindrome from single digit factors"
+
+[0853f82c-5fc4-44ae-be38-fadb2cced92d]
+description = "find the largest palindrome from single digit factors"
+
+[66c3b496-bdec-4103-9129-3fcb5a9063e1]
+description = "find the smallest palindrome from double digit factors"
+
+[a10682ae-530a-4e56-b89d-69664feafe53]
+description = "find the largest palindrome from double digit factors"
+
+[cecb5a35-46d1-4666-9719-fa2c3af7499d]
+description = "find the smallest palindrome from triple digit factors"
+
+[edab43e1-c35f-4ea3-8c55-2f31dddd92e5]
+description = "find the largest palindrome from triple digit factors"
+
+[4f802b5a-9d74-4026-a70f-b53ff9234e4e]
+description = "find the smallest palindrome from four digit factors"
+
+[787525e0-a5f9-40f3-8cb2-23b52cf5d0be]
+description = "find the largest palindrome from four digit factors"
+
+[58fb1d63-fddb-4409-ab84-a7a8e58d9ea0]
+description = "empty result for smallest if no palindrome in the range"
+
+[9de9e9da-f1d9-49a5-8bfc-3d322efbdd02]
+description = "empty result for largest if no palindrome in the range"
+
+[12e73aac-d7ee-4877-b8aa-2aa3dcdb9f8a]
+description = "error result for smallest if min is more than max"
+include = false
+
+[eeeb5bff-3f47-4b1e-892f-05829277bd74]
+description = "error result for largest if min is more than max"
+include = false
+
+[16481711-26c4-42e0-9180-e2e4e8b29c23]
+description = "smallest product does not use the smallest factor"

exercises/practice/palindrome-products/palindrome_products.zig

@@ -0,0 +1,26 @@
+const std = @import("std");
+const mem = std.mem;
+
+pub const Pair = struct {
+    first: u64,
+    second: u64,
+};
+
+pub const Palindrome = struct {
+    value: u64,
+    factors: []Pair,
+};
+
+pub fn smallest(allocator: mem.Allocator, min: u64, max: u64) mem.Allocator.Error!?Palindrome {
+    _ = allocator;
+    _ = min;
+    _ = max;
+    @compileError("please implement the smallest function");
+}
+
+pub fn largest(allocator: mem.Allocator, min: u64, max: u64) mem.Allocator.Error!?Palindrome {
+    _ = allocator;
+    _ = min;
+    _ = max;
+    @compileError("please implement the largest function");
+}