`#ceil` with negative argument produces incorrect results on certain values

[trinistr]

In my project I'm testing various numeric rounding methods, and I chanced upon this issue.

The failing case: 999.15.ceil(-1) should be 1000, but it returns 1010.

I tested several cases:

RSpec.describe "Float#ceil" do
  it "returns the ceiling of 999.15 correctly" do
    expect(999.15.ceil).to eq 1000
    expect(999.15.ceil(1)).to eq 999.2
    expect(999.15.ceil(-1)).to eq 1000
  end

  it "returns the ceiling of 1.25 correctly" do
    expect(1.25.ceil).to eq 2
    expect(1.25.ceil(1)).to eq 1.3
    expect(1.25.ceil(-1)).to eq 10
  end

  it "returns the ceiling of -999.15 correctly" do
    expect(-999.15.ceil).to eq(-999)
    expect(-999.15.ceil(1)).to eq(-999.1)
    expect(-999.15.ceil(-1)).to eq(-990)
  end

  it "returns the ceiling of -1.25 correctly" do
    expect(-1.25.ceil).to eq(-1)
    expect(-1.25.ceil(1)).to eq(-1.2)
    expect(-1.25.ceil(-1)).to eq(0)
  end
end

However, only 999.15 here exhibits the issue:

1) Float#ceil returns the ceiling of 999.15 correctly
     Failure/Error: expect(999.15.ceil(-1)).to eq 1000

       expected: 1000
            got: 1010

       (compared using ==)

I haven't encountered problems with any other rounding method or arguments, but there may be more issues lurking.

[trinistr]

Ah, found the problem. Float.ceil calls Integer.ceil, which has the following implementation:

  def ceil(precision = 0)
    return self unless precision < 0
    return 0 if self == 0

    x = 10 ** precision.abs
    ((self / x) + 1) * x
  end

The problem is the unconditional + 1, which is not supposed to happen if self % x == 0.

Will try to make a PR addressing this. Had to close the PR due to being unable to sign OCA.