Make CURL::File faster

This gets us back the time we lost in the spectest recently.  Part of the
speedup is attained by only going through the directory of the CompUnitRepo
only once: this behaviour may need tweaking in certain situations.

src/core/CompUnitRepo/Local/File.pm

@@ -1,57 +1,61 @@
 class CompUnitRepo::Local::File does CompUnitRepo::Locally {
+    has Hash $!potentials;
+
+    my $precomp := $*VM.precomp-ext;
+    my %extensions =
+      perl6 => [$precomp,'pm6','pm'],
+      perl5 => [$precomp,'pm5','pm'],
+      nqp   => [$precomp,'nqp'];
+    my $anyextensions = any($precomp,<pm6 pm5 pm nqp>);
 
     method install($source, $from?) { ... }
     method files($file, :$name, :$auth, :$ver) { ... }
 
-    method candidates($name, :$from = 'perl6', :$file, :$auth, :$ver) {
-        my @extensions = $from eq 'perl6'
-          ?? ($*VM.precomp-ext,'pm6','pm')
-          !! $from eq 'perl5'
-            ?? ($*VM.precomp-ext,'pm5','pm')
-            !! $from eq 'nqp'
-               ?? ($*VM.precomp-ext,'nqp')
-               !! die "Don't know how to handle :from<$from>";
-        my $anyextensions = any(@extensions);
+    method candidates($name = /./, :$from = 'perl6', :$file, :$auth, :$ver) {
+        my @extensions := %extensions{$from};
+
         my @candidates;
-        my %seen;
+        for ( $!potentials //= self.potentials ).keys -> $root {
+            next unless $root ~~ $name;             # not right name
+
+            my $candidate := $!potentials{$root};
+            for @extensions -> $extension {
+                if $candidate{$extension} -> $sig { # use this extension
+                    @candidates.push: CompUnit.new(|$sig, :$extension);
+                    last;
+                }
+            }
+        }
+        @candidates;
+    }
+
+    method short-id() { 'file' }
+
+    submethod potentials {
+        my Hash $potentials = {};
 
         for $!path.contents -> $path {
             my $file = ~$path;
 
             for $file.rindex(".") -> $i {
-                last unless $i.defined; # could not find any extension
+                last unless $i.defined;               # could not find any ext
 
                 my $ext = $file.substr($i + 1);
-                last unless $ext ~~ $anyextensions; # not right ext
+                last unless $ext ~~ $anyextensions;   # not right ext
 
                 my $root = $file.substr(0,$i);
                 my $j := $root.rindex(IO::Spec.rootdir);
                 $root = $root.substr($j + 1) if $j.defined;
-                last unless $root ~~ $name;         # not right name
-
-                if %seen{$root} -> $seenroot {            # seen name before
-                    $seenroot{$ext} := \($file, :name($root) );
-                }
 
-                else {                                    # first time
-                    %seen{$root}{$ext} := \($file, :name($root) );
-                    @candidates.push: %seen{$root};
+                if $potentials{$root} -> $seenroot { # seen name before
+                    $potentials{$ext} := \($file, :name($root) );
                 }
-            }
-        }
 
-        CANDIDATE:
-        for @candidates <-> $can {
-            for @extensions -> $extension {
-                if $can{$extension} -> $sig { # use this extension
-                    $can = CompUnit.new(|$sig, :$extension);
-                    next CANDIDATE;
+                else {                               # first time
+                    $potentials{$root}{$ext} := \($file, :name($root) );
                 }
             }
         }
-
-        @candidates;
+        $potentials;
     }
-
-    method short-id() { 'file' }
 }