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Fix Range#size for unsigned edge cases #14978

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Merged
straight-shoota merged 2 commits into from
Feb 28, 2025
Merged

Fix Range#size for unsigned edge cases #14978

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3d77ab9
Fix `Range#size` for unsigned edge cases
straight-shoota May 13, 2024
09de8a0
Fix convert small types to Int32
straight-shoota Dec 2, 2024
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47 changes: 43 additions & 4 deletions spec/std/range_spec.cr
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Original file line number Diff line number Diff line change
Expand Up @@ -669,10 +669,49 @@ describe "Range" do
end

describe "#size" do
it "optimizes for int range" do
(5..12).size.should eq(8)
(5...12).size.should eq(7)
(5..4).size.should eq(0)
describe "Int" do
it { (5..12).size.should eq(8) }
it { (5...12).size.should eq(7) }
it { (5..4).size.should eq(0) }
it { (0..0).size.should eq(1) }
it { (0...0).size.should eq(0) }
it { (1..1).size.should eq(1) }
it { (1...1).size.should eq(0) }

it { (-12..-5).size.should eq(8) }
it { (-12...-5).size.should eq(7) }
it { (-4..-5).size.should eq(0) }
it { (-1..-1).size.should eq(1) }
it { (-1...-1).size.should eq(0) }

it { (-3..3).size.should eq(7) }
it { (-3...3).size.should eq(6) }
it { (-3..0).size.should eq(4) }
it { (-3...0).size.should eq(3) }
it { (3..-3).size.should eq(0) }
it { (3...-3).size.should eq(0) }
it { (-128_i8..0_i8).size.should eq(129) }
it { (-128_i8...0_i8).size.should eq(128) }

it { (Int32::MAX..Int32::MAX).size.should eq(1) }
it { (Int32::MAX...Int32::MAX).size.should eq(0) }
it { (-Int32::MAX..-Int32::MAX).size.should eq(1) }
it { (-Int32::MAX...-Int32::MAX).size.should eq(0) }

it { (5_u8..12_u8).size.should eq(8) }
it { (5_u8...12_u8).size.should eq(7) }
it { (5_u8..4_u8).size.should eq(0) }
it { (0_u8..0_u8).size.should eq(1) }
it { (0_u8...0_u8).size.should eq(0) }
it { (1_u8..1_u8).size.should eq(1) }
it { (1_u8...1_u8).size.should eq(0) }
it { (UInt8::MAX..UInt8::MAX).size.should eq(1) }
it { (UInt8::MAX...UInt8::MAX).size.should eq(0) }

it { (-32768_i16..254_u8).size.should eq(33023) }
it { (-128_i8..127_i8).size.should eq(256) }
it { (Int32::MIN..-127_i8).size.should eq(2_147_483_522) }
it { ((Int16::MIN.to_i32 - 1)..127_i16).size.should eq(32897) }
end

it "works for other types" do
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14 changes: 11 additions & 3 deletions src/range.cr
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Expand Up @@ -489,9 +489,17 @@ struct Range(B, E)

# Optimized implementation for int range
if b.is_a?(Int) && e.is_a?(Int)
e -= 1 if @exclusive
n = e - b + 1
n < 0 ? 0 : n.to_i32
return 0 if e < b

# Convert `e` to `Int32` in order to ensure that `e &- b` doesn't get
# truncated due to the smaller type of `e`.
if e.is_a?(UInt8 | Int8 | UInt16 | Int16)
e = e.to_i32!
end

diff = (e &- b).to_i32.abs

@oprypin oprypin Sep 6, 2024

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This can't be right.. how can it be safe to just not check for overflows 🤔
We should double-check it, I'll probably do so today

@straight-shoota straight-shoota Sep 6, 2024 •
edited
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Good catch. e < b only excludes the obvious case. 🤦
We can still end up with a difference that doesn't fit into E (e.g. (Int8::MIN..1i8).size). But it would fit into Int32.

I suppose we could use non-wrapping arithmetics and cast e to Int32 if it's a smaller type. That should probably cover every constellation where the result fits into Int32.

@straight-shoota straight-shoota Sep 6, 2024 •
edited
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There are so many edge cases to take into account 🤯
I'm wondering if we should collect a set of
for testing Range methods with optimized implementations for Int (i.e. the ones mentioned in #13648: size, sum, step.sum, map; maybe also sample).

diff &+= 1 unless @exclusive
diff
else
if b.nil? || e.nil?
raise ArgumentError.new("Can't calculate size of an open range")
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