Consider the following code. It counts the number of
subdirectories in directories provided via commandline args.
import std.stdio;
import std.algorithm;
import std.file;
import std.range;
import std.exception;
@trusted bool isDirNothrow(string dir) nothrow
{
bool ok;
collectException(dir.isDir(), ok);
return ok;
}
auto subDirs(Range)(Range searchDirs)
if(is(ElementType!Range : string))
{
return searchDirs
.filter!(isDirNothrow).map!(function(dir) {
return dir.dirEntries(SpanMode.shallow)
.filter!(isDirNothrow);
}).cache().joiner;
}
int main(string[] args)
{
auto r = args[1..$].subDirs;
writefln("Found %s subdirs", count(r));
return 0;
}
Using strace I inspect how many stat calls it uses. Like this:
strace -C -e stat path/to/binary
strace reports some duplicated stat calls, specifically on the
first provided directory and first subdirectory of each
directories.
How can I avoid redundant stat calls? Adding array() after each
filter() removes duplicates but it leads to allocations
obviously. Changing the cache() to array() before joiner also
solve the issue.
I suspect there's something wrong in combination of filters and
joiner.
