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add practice exercise palindrome-products (#758)
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depial authored Jul 20, 2024
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8 changes: 8 additions & 0 deletions config.json
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"practices": [],
"prerequisites": [],
"difficulty": 2
},
{
"slug": "palindrome-products",
"name": "Palindrome Products",
"uuid": "7ab2d0e6-6a39-4edf-86e9-1e49dfb8908e",
"practices": [],
"prerequisites": [],
"difficulty": 4
}
]
},
36 changes: 36 additions & 0 deletions exercises/practice/palindrome-products/.docs/instructions.md
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# 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)`.
17 changes: 17 additions & 0 deletions exercises/practice/palindrome-products/.meta/config.json
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{
"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"
}
27 changes: 27 additions & 0 deletions exercises/practice/palindrome-products/.meta/example.jl
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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
49 changes: 49 additions & 0 deletions exercises/practice/palindrome-products/.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.

[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"
4 changes: 4 additions & 0 deletions exercises/practice/palindrome-products/palindrome-products.jl
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function palindromeproducts(low::Int, high::Int, smallest::Bool)
# Your code here
palindrome, factors
end
79 changes: 79 additions & 0 deletions 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

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