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Add Int.from_digits as inverse of Int#digits #16237

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Merged
merged 7 commits into from
Oct 23, 2025
Merged

Add Int.from_digits as inverse of Int#digits #16237

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e30d2c4
Add Int.undigits that is inverse of Int#digits
andrykonchin Oct 20, 2025
7b75945
Rename Int.undigits to from_digits
andrykonchin Oct 20, 2025
6b42516
Define from_digits on concrete integer types
andrykonchin Oct 20, 2025
0ff5326
Don't calculate multiplier upfront
andrykonchin Oct 20, 2025
5a0f0af
Use #each instead of #each_with_index as far as index is not used
andrykonchin Oct 20, 2025
051bd92
Fix formating issues
andrykonchin Oct 20, 2025
84ade81
Polish the comment and examples with raising exceptions
andrykonchin Oct 21, 2025
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53 changes: 53 additions & 0 deletions spec/std/int_spec.cr
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,19 @@ private macro it_converts_to_s(num, str, **opts)
end
end

private class IntEnumerable
include Enumerable(Int32)

def initialize(@elements : Array(Int32))
end

def each(&)
@elements.each do |e|
yield e
end
end
end

describe "Int" do
describe "#integer?" do
{% for int in BUILTIN_INTEGER_TYPES %}
Expand Down Expand Up @@ -1139,4 +1152,44 @@ describe "Int" do
end
end
end

describe "undigits" do
it "returns Int composed from given digits" do
Int.undigits([9, 8, 7, 6, 5, 4, 3, 2, 1]).should eq(123456789)
end

it "works with a base" do
Int.undigits([11, 7], 16).should eq(123)
Int.undigits([11, 7], base: 16).should eq(123)
end

it "accepts digits as Enumerable" do
enumerable = IntEnumerable.new([11, 7])
Int.undigits(enumerable, 16).should eq(123)
end

it "raises for base less than 2" do
[-1, 0, 1].each do |base|
expect_raises(ArgumentError, "Invalid base #{base}") do
Int.undigits([1, 2, 3], base)
end
end
end

it "raises for digits greater than base" do
expect_raises(ArgumentError, "Invalid digit 2 for base 2") do
Int.undigits([1, 0, 2], 2)
end

expect_raises(ArgumentError, "Invalid digit 10 for base 2") do
Int.undigits([1, 0, 10], 2)
end
end

it "raises for negative digits" do
expect_raises(ArgumentError, "Invalid digit -1") do
Int.undigits([1, 2, -1])
end
end
end
end
24 changes: 24 additions & 0 deletions src/int.cr
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Original file line number Diff line number Diff line change
Expand Up @@ -670,6 +670,30 @@ struct Int
ary
end

def self.undigits(digits : Enumerable(Int), base : Int = 10) : self
if base < 2
raise ArgumentError.new("Invalid base #{base}")
end

num = 0
multiplier = 1
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@HertzDevil HertzDevil Oct 20, 2025

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I think these should not be hardcoded to Int32

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@straight-shoota straight-shoota Oct 20, 2025

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Yes, I think this method should be defined on the concrete types (Int32, UInt8 etc.). Then self defines the return type.

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@andrykonchin andrykonchin Oct 20, 2025

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Is it OK to use macros in this case?

{% for type in %w(Int8 Int16 Int32 Int64 Int128 UInt8 UInt16 UInt32 UInt64 UInt128) %}
  def {{type.id}}.from_digits(digits : Enumerable(Int), base : Int = 10) : self
    if base < 2
      raise ArgumentError.new("Invalid base #{base}")
    end

    num = 0
    multiplier = 1

    # ...

    num
  end
{% end %}

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@straight-shoota straight-shoota Oct 20, 2025

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It might be okay to define it on Int. It will only work on its concrete subclasses though.
Int.from_io works that way as well. Calling it directly doesn't compile, but Int32.from_io etc. work fine.

Obviously, it's confusing to have the method defined on Int when it doesn't work there. So maybe the macro solution is better.

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@straight-shoota straight-shoota Oct 20, 2025

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We could use a macro inherited hook instead of declaring on each child type directly. But for some reason, that doesn't seem to work on Int. Maybe because the inheritance is defined in the compiler? 🤔

struct Int
  macro inherited
    def self.from_digits(digits : Enumerable(Int), base : Int = 10) : self
      # ...
      ZERO    
    end
  end
end

Int32.from_digits([1, 2, 3, 4]) # Error: undefined method 'from_digits' for Int32.class

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@andrykonchin andrykonchin Oct 20, 2025

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Done

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@HertzDevil HertzDevil Oct 20, 2025

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The inheritance is done in the compiler, so no hooks are called. See #13836


digits.each do |digit|
if digit < 0
raise ArgumentError.new("Invalid digit #{digit}")
end

if digit >= base
raise ArgumentError.new("Invalid digit #{digit} for base #{base}")
end

num += digit * multiplier
multiplier *= base
end

num
end

private DIGITS_DOWNCASE = "0123456789abcdefghijklmnopqrstuvwxyz"
private DIGITS_UPCASE = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
private DIGITS_BASE62 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
Expand Down
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