## Feature #18809

closed### Add Numeric#ceildiv

**Description**

pull request: https://github.com/ruby/ruby/pull/5965

I have needed to implement "rounding up division" several times.

("rounding up division" means getting a quotient of division which is rounded up to the nearest integer.)

Typically, this is implemented as follows:

```
# notice that b > 0 is assumed
def rounding_up_division(a, b)
(a + b - 1) / b
end
```

But for me, this is difficult to write without careful consideration.

Every time I implement this, I need to think for a few minutes on paper.

So I propose to add a new method `Numeric#ceildiv`

.

Typical examples where this is necessary are counting groups and pagination.

e.g. There are 123 items. If you display 10 items on each page, how many pages are there?

```
123.ceildiv(10) # => 13
```

We can find several examples of this division also in the Ruby's source code. (Try `grep -r -E -e '([^ ]+) *- *1\) */ *\1' .`

)

```
./internal.h:#define roomof(x, y) (((x) + (y) - 1) / (y))
./array.c: len = (len + ustep - 1) / ustep;
./include/ruby/internal/memory.h: const size_t cnt = (total_size + sizeof(VALUE) - 1) / sizeof(VALUE);
./ext/bigdecimal/missing/dtoa.c:#define PRIVATE_mem ((PRIVATE_MEM+sizeof(double)-1)/sizeof(double))
./ext/bigdecimal/bigdecimal.c: nc += (nc + mc - 1) / mc + 1;
./ext/bigdecimal/bigdecimal.c: mx = (mx + BASE_FIG - 1) / BASE_FIG; /* Determine allocation unit. */
./ext/bigdecimal/bigdecimal.c: mf = (mf + BASE_FIG - 1) / BASE_FIG + 2; /* Needs 1 more for div */
./ext/bigdecimal/bigdecimal.c: nalloc = (ni + nf + BASE_FIG - 1) / BASE_FIG + 1; /* set effective allocation */
./ext/bigdecimal/bigdecimal.c: size_t const round_limit = (VpGetPrecLimit() + BASE_FIG - 1) / BASE_FIG;
./ext/bigdecimal/bigdecimal.c: if ((ix + BASE_FIG - 1) / BASE_FIG > ixDigit + 1) return 0;
./ext/bigdecimal/bits.h:#define roomof(x, y) (((x) + (y) - 1) / (y))
./internal/numeric.h: VALUE values[(SIZEOF_DOUBLE + SIZEOF_VALUE - 1) / SIZEOF_VALUE];
./regcomp.c: OnigDistance str_len = (byte_len + mb_len - 1) / mb_len;
./bignum.c: size_t num_bdigits = (num_bits + BITSPERDIG - 1) / BITSPERDIG;
./missing/dtoa.c:#define PRIVATE_mem ((PRIVATE_MEM+sizeof(double)-1)/sizeof(double))
./numeric.c: char buf[float_dig + (decimal_mant + CHAR_BIT - 1) / CHAR_BIT + 10];
./gc.c:#define CEILDIV(i, mod) (((i) + (mod) - 1)/(mod))
```

Naming:

I was not sure whether to name it `ceildiv`

or `divceil`

because there are both `divmod`

and `fdiv`

.

Since `divmod`

is a method that returns two elements, the quotient and the remainder,

while `fdiv`

is a method that performs Float division, I decided to follow `fdiv`

.

#### Updated by nobu (Nobuyoshi Nakada) 8 months ago

I'm positive.

It may be nice to alias `div`

as `floordiv`

too?

#### Updated by mrkn (Kenta Murata) 8 months ago

#### Updated by Dan0042 (Daniel DeLorme) 7 months ago

Why not simply use `a.fdiv(b).ceil`

?

It expresses the intent of the code clearly, and I doubt there would be a measurable difference in performance except in the tightest of tight loops.

#### Updated by sawa (Tsuyoshi Sawada) 7 months ago

Dan0042 (Daniel DeLorme) wrote in #note-3:

Why not simply use

`a.fdiv(b).ceil`

?

It expresses the intent of the code clearly, and I doubt there would be a measurable difference in performance except in the tightest of tight loops.

```
a = 99999999999999999
b = 1
(a + b - 1) / b # => 99999999999999999
a.fdiv(b).ceil # => 100000000000000000
```

#### Updated by matz (Yukihiro Matsumoto) 6 months ago

Let's add `Integer#ceildiv`

.

Matz.

#### Updated by mame (Yusuke Endoh) 6 months ago

Additional information.

- We do introduce only
`Integer#ceildiv`

. - We do not introduce
`Numeric#ceildiv`

until we see the need. There is already`Numeric#div`

, but a consistency with it is not a sufficient reason to introduce it. - We do not introduce
`Numeric#floordiv`

. -
`3.ceildiv(-2)`

should return -1, which is ceil(-(3/2)). Note that the naive implementation of`(a + b - 1) / b`

, which returns (3 + (-2) - 1) / (-2) = 0. (As far as we glanced at the PR, it is implemented correctly.)

#### Updated by kyanagi (Kouhei Yanagita) 6 months ago

Thank you for accepting.

I updated the PR. The PR contains only `Integer#ceildiv`

.

#### Updated by kyanagi (Kouhei Yanagita) 6 months ago

Are there any blocking issues?

If exist, I will work to resolve them.

#### Updated by nobu (Nobuyoshi Nakada) 6 months ago

**Status**changed from*Open*to*Closed*

Applied in changeset git|0617cba197cdff626ee9c74cece480df31d384ef.

[DOC] Add the link to [Feature #18809]