- Title
- LWG 3809 changes behavior of some existing std::subtract_with_carry_engine code
- Status
- new
- Section
- [rand.eng.sub]
- Submitter
- Matt Stephanson

Created on **2023-11-15.00:00:00**
last changed **1 week ago**

Date: 2023-11-19.11:01:08

**Proposed resolution:**

This wording is relative to N4964 *after the wording changes applied* by LWG 3809,
which had been accepted into the working paper during the Kona 2023-11 meeting.

Modify [rand.eng.sub] as indicated:

explicit subtract_with_carry_engine(result_type value);

-7-

*Effects*: Sets the values of ${X}_{-r},\dots ,{X}_{-1}$, in that order, as specified below. If ${X}_{-1}$ is then $0$, sets $c$ to $1$; otherwise sets $c$ to $0$.To set the values ${X}_{k}$, first construct

`e`, a`linear_congruential_engine`object, as if by the following definition:linear_congruential_engine<uint_least32_t, 40014u,0u,2147483563u> e(value == 0u ? default_seed : static_cast<uint_least32_t>(value % 2147483563u));

Then, to set each ${X}_{k}$, obtain new values ${z}_{0},\dots ,{z}_{n-1}$ from $n=\lceil w/32\rceil $ successive invocations of

`e`. Set ${X}_{k}$ to $({\sum}_{j=0}^{n-1}{z}_{j}\bullet {2}^{32j})modm$.

Date: 2023-11-15.00:00:00

Issue 3809 pointed out that `subtract_with_carry_engine<T>` can be seeded with values
from a `linear_congruential_engine<T, 40014u, 0u, 2147483563u>` object, which results in narrowing
when `T` is less than 32 bits. Part of the resolution was to modify the LCG seed sequence as follows:

explicit subtract_with_carry_engine(result_type value);-7-

Effects: Sets the values of ${X}_{-r},\dots ,{X}_{-1}$, in that order, as specified below. If ${X}_{-1}$ is then $0$, sets $c$ to $1$; otherwise sets $c$ to $0$.To set the values ${X}_{k}$, first construct

e, alinear_congruential_engineobject, as if by the following definition:linear_congruential_engine<~~result_type~~uint_least32_t, 40014u,0u,2147483563u> e(value == 0u ? default_seed : value);Then, to set each ${X}_{k}$, obtain new values ${z}_{0},\dots ,{z}_{n-1}$ from $n=\lceil w/32\rceil $ successive invocations of

e. Set ${X}_{k}$ to $({\sum}_{j=0}^{n-1}{z}_{j}\bullet {2}^{32j})modm$.

Inside `linear_congruential_engine`, the seed is reduced modulo 2147483563, so `uint_least32_t`
is fine from that point on. This resolution, however, forces `value`, the user-provided seed, to be
truncated from `result_type` to `uint_least32_t` before the reduction, which generally will
change the result. It also breaks the existing behavior that two seeds are equivalent if they're in the same
congruence class modulo the divisor.

History | |||
---|---|---|---|

Date | User | Action | Args |

2023-11-19 11:01:08 | admin | set | messages: + msg13869 |

2023-11-15 00:00:00 | admin | create |