Re: DConf '22: No-Allocated 0-terminated path strings
On Friday, 21 October 2022 at 14:34:47 UTC, ag0aep6g wrote:
Nitpick: You cannot iterate a true input range twice. You need
a forward range for that.
Nitpick²: you don't actually need to iterate the range twice
```d
//version=AllowMalloc;
auto toCStringThen(alias dg, Range)(Range src) /*nothrow*/ if
(isInputRange!Range && !isInfinite!Range) {
char[10] small = void;
size_t i = 0;
while(!src.empty && i < small.length) {
small[i++] = src.front;
src.popFront;
}
if(i == small.length) {
version(AllowMalloc) {
import std.container.array;
Array!char large = small[];
large ~= src;
large ~= '\0';
return dg(large.data);
} else {
throw new Exception(
"C string buffer overflow (%s >= %s)"
.format(i+src.walkLength, small.length-1)
);
}
} else {
small[i] = '\0';
return dg(small[0..i]);
}
}
```
Unfortunately this version does multiple allocations if contents
of the range do not fit into the small buffer, but you could
avoid that by detecting if `src` is a forward range/defines
`.length` and doing `large.reserve` or something similar.
Re: DConf '22: No-Allocated 0-terminated path strings
On Friday, 21 October 2022 at 13:49:01 UTC, cc wrote:
```d
//version=AllowMalloc;
auto toCStringThen(alias dg, Range)(Range src) /*nothrow*/ if
(isInputRange!Range && !isInfinite!Range) {
```
[...]
May need to be cleaned up for character types and needs to
iterate twice if allocations are going to be allowed
Nitpick: You cannot iterate a true input range twice. You need a
forward range for that.
DConf '22: No-Allocated 0-terminated path strings
Catching up on the DConf '22 videos, really enjoyed the tricks
Walter presents here for no-allocation strings:
[DConf '22: Strawberries and Cream aka Delightful Emergent
Properties of D -- Walter
Bright](https://www.youtube.com/watch?v=iuP-AWUyjp8)
In the Q segment, the first question asked whether the filename
in the path example needs memory allocation to be passed to C,
and is told it does, however the two methods presented can be
easily combined to provide no-allocation 0-terminated strings
from `chain()`ed paths by passing an `InputRange` rather than a
`const(char)` to `toCStringThen`:
```d
//version=AllowMalloc;
auto toCStringThen(alias dg, Range)(Range src) /*nothrow*/ if
(isInputRange!Range && !isInfinite!Range) {
const len = src.walkLength;
char[512] small = void;
version(AllowMalloc) {
import dmd.common.string : SmallBuffer;
auto sb = SmallBuffer!char(len + 1, small[]);
scope ptr = sb[];
} else {
enforce(len < small.length, format!"C string buffer overflow
(%s >= %s)"(len, small.length));
scope ptr = small[];
}
size_t i = 0;
foreach (char c; src)
ptr[i++] = c;
ptr[len] = '\0';
return dg(ptr);
}
void main() {
string path = "include/";
string name = "file";
string ext = ".ext";
auto filename = chain(path, name, ext);
filename.writeln;
filename.byChar.toCStringThen!(
(str) => printf("printf: {%s}\n", str.ptr)
);
}
```
May need to be cleaned up for character types and needs to
iterate twice if allocations are going to be allowed, and
walkLength/chain()'s range functions aren't nothrow as far as I
can tell. But otherwise thought this was kind of neat, just
posting it here in case anyone else finds it handy.
