add practice exercise palindrome-products

config.json

@@ -993,6 +993,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 2
+      },
+      {
+        "slug": "palindrome-products",
+        "name": "Palindrome Products",
+        "uuid": "7ab2d0e6-6a39-4edf-86e9-1e49dfb8908e",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
       }
     ]
   },

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 +1,17 @@
+{
+  "authors": [],
+  "files": {
+    "solution": [
+      "palindrome-products.jl"
+    ],
+    "test": [
+      "runtests.jl"
+    ],
+    "example": [
+      ".meta/example.jl"
+    ]
+  },
+  "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.jl

@@ -0,0 +1,27 @@
+getfactors(n, low, high) = [[i, n÷i] for i in low:isqrt(n) if iszero(n%i) && low ≤ n÷i ≤ high]
+
+function nextpal(n, smallest)
+    smallest && occursin(r"^9+$", "$n") && return n + 2
+    !smallest && occursin(r"^10*1$", "$n") && return n - 2
+    half = "$(parse(Int, "$n"[1:(ndigits(n)+1)÷2]) + (-1)^!smallest)"
+    parse(Int, half * reverse(half)[1+isodd(ndigits(n)):end])
+end
+
+function nearestvalidpal(bound, smallest)
+    half = "$bound"[1:(ndigits(bound)+1)÷2]
+    pal = parse(Int, half * reverse(half)[1+isodd(ndigits(bound)):end])
+    inrange = smallest ? bound ≤ pal : pal ≤ bound
+    inrange ? pal : nextpal(pal, smallest)
+end
+
+function pal_n_facts(pal, low, high, smallest)
+    ((smallest && pal > high^2) || (!smallest && pal < low^2)) && return nothing, []
+    facts = getfactors(pal, low, high)
+    isempty(facts) ? pal_n_facts(nextpal(pal, smallest), low, high, smallest) : (pal, facts)
+end
+
+function palindromeproducts(low, high, smallest)
+    high < low && throw(ArgumentError("low is greater than high: $low > $high"))
+    bound = smallest ? low^2 : high^2
+    pal_n_facts(nearestvalidpal(bound, smallest), low, high, smallest)
+end

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

@@ -0,0 +1,49 @@
+# 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"
+
+[eeeb5bff-3f47-4b1e-892f-05829277bd74]
+description = "error result for largest if min is more than max"
+
+[16481711-26c4-42e0-9180-e2e4e8b29c23]
+description = "smallest product does not use the smallest factor"

exercises/practice/palindrome-products/palindrome-products.jl

@@ -0,0 +1,4 @@
+function palindromeproducts(low::Int, high::Int, smallest::Bool)
+    # Your code here
+    palindrome, factors
+end

exercises/practice/palindrome-products/runtests.jl

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+using Test
+
+include("palindrome-products.jl")
+
+@testset verbose = true "tests" begin
+    @testset "find the smallest palindrome from single digit factors" begin
+        palindrome, factors = palindromeproducts(1, 9, true)
+        @test palindrome == 1
+        @test factors == [[1, 1]]
+    end
+
+    @testset "find the largest palindrome from single digit factors" begin
+        palindrome, factors = palindromeproducts(1, 9, false)
+        @test palindrome == 9
+        @test sort(factors) == [[1, 9], [3, 3]]
+    end
+
+    @testset "find the smallest palindrome from double digit factors" begin
+        palindrome, factors = palindromeproducts(10, 99, true)
+        @test palindrome == 121
+        @test factors == [[11, 11]]
+    end
+
+    @testset "find the largest palindrome from double digit factors" begin
+        palindrome, factors = palindromeproducts(10, 99, false)
+        @test palindrome == 9009
+        @test factors == [[91, 99]]
+    end
+
+    @testset "find the smallest palindrome from triple digit factors" begin
+        palindrome, factors = palindromeproducts(100, 999, true)
+        @test palindrome == 10201
+        @test factors == [[101, 101]]
+    end
+
+    @testset "find the largest palindrome from triple digit factors" begin
+        palindrome, factors = palindromeproducts(100, 999, false)
+        @test palindrome == 906609
+        @test factors == [[913, 993]]
+    end
+
+    @testset "find the smallest palindrome from four digit factors" begin
+        palindrome, factors = palindromeproducts(1000, 9999, true)
+        @test palindrome == 1002001
+        @test factors == [[1001, 1001]]
+    end
+
+    @testset "find the largest palindrome from four digit factors" begin
+        palindrome, factors = palindromeproducts(1000, 9999, false)
+        @test palindrome == 99000099
+        @test factors == [[9901, 9999]]
+    end
+
+    @testset "empty result for smallest if no palindrome in the range" begin
+        palindrome, factors = palindromeproducts(1002, 1003, true)
+        @test isnothing(palindrome)
+        @test isempty(factors)
+    end
+
+    @testset "empty result for largest if no palindrome in the range" begin
+        palindrome, factors = palindromeproducts(15, 15, false)
+        @test isnothing(palindrome)
+        @test isempty(factors)
+    end
+
+    @testset "error result for smallest if min is more than max" begin
+        @test_throws ArgumentError palindromeproducts(10000, 1, true)
+    end
+
+    @testset "error result for largest if min is more than max" begin
+        @test_throws ArgumentError palindromeproducts(10000, 1, false)
+    end
+
+    @testset "smallest product does not use the smallest factor" begin
+        palindrome, factors = palindromeproducts(3215, 4000, true)
+        @test palindrome == 10988901
+        @test factors == [[3297, 3333]]
+    end
+end