Feature #18809
closedAdd 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) over 2 years ago
I'm positive.
It may be nice to alias div
as floordiv
too?
Updated by mrkn (Kenta Murata) over 2 years ago
Updated by Dan0042 (Daniel DeLorme) over 2 years 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) over 2 years 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) over 2 years ago
Let's add Integer#ceildiv
.
Matz.
Updated by mame (Yusuke Endoh) over 2 years ago
Additional information.
- We do introduce only
Integer#ceildiv
. - We do not introduce
Numeric#ceildiv
until we see the need. There is alreadyNumeric#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) over 2 years ago
Thank you for accepting.
I updated the PR. The PR contains only Integer#ceildiv
.
Updated by kyanagi (Kouhei Yanagita) over 2 years ago
Are there any blocking issues?
If exist, I will work to resolve them.
Updated by nobu (Nobuyoshi Nakada) over 2 years ago
- Status changed from Open to Closed
Applied in changeset git|0617cba197cdff626ee9c74cece480df31d384ef.
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