[src/core/Cool-str.pm] 500% speedup of .trans

Keep a hash with the next index of each substring to be substituted,
and then pick the smallest one through each iteration. (Or, more
informally, "skip the boring parts".) This makes the number of
iterations through the main loop proportional to the number of
substitutions actually made.

src/core/Cool-str.pm

@@ -176,59 +176,35 @@ augment class Cool {
         }
 
         my %c;
-        my %prefixes;
         for (@changes) -> $p {
             die "$p.perl is not a Pair" unless $p ~~ Pair;
             my @from = expand $p.key;
             my @to   = expand $p.value;
-#            warn "Substitution is longer than pattern\n" if @to > @from;
             if @to {
                 @to = @to xx ceiling(@from / @to);
             } else {
                 @to = '' xx @from;
             }
             for @from Z @to -> $f, $t {
                 if %c.exists($f) && %c{$f} ne $t {
-#                    warn "Ambiguous transliteration rule for '$f'; "
-#                         ~ "using the first one (transliteration to '$t')";
                 } else {
-                    if $f.chars > 1 {
-                        %prefixes{$f.substr(0, 1)} //= [];
-                        %prefixes{$f.substr(0, 1)}.push($f);
-                    }
                     %c{$f} = $t;
                 }
             }
         }
 
-        # should be replaced by a proper trie implementation
-        # at some point
-        for %prefixes.keys {
-            %prefixes{$_}.=sort({-.chars});
+        my $i = 0;
+        my $r = "";
+        my %h = %c.keys Z=> map { self.index($_) // Inf }, %c.keys;
+        while ($_ = %h.pairs.sort({-.chars}).min: *.value).value < Inf {
+            %h{.key} = self.index(.key, .value + 1) // Inf;
+            next if .value < $i;
+            $r ~= self.substr($i, .value - $i) ~ %c{.key};
+            $i = .value + .key.chars;
         }
+        $r ~= self.substr($i);
 
-        my @res;
-        my $l = $.chars;
-        loop (my $i = 0; $i < $l; ++$i) {
-            my $c = $.substr($i, 1);
-            my $success = 0;
-            if %prefixes.exists($c) {
-                for %prefixes{$c}.list {
-                    if self.substr($i, .chars) eq $_ {
-                        @res.push: %c{$_};
-                        $success = 1;
-                        $i += .chars - 1;
-                        last;
-                    }
-                }
-            }
-            unless $success {
-                @res.push: %c.exists($c)
-                            ?? %c{$c}
-                            !! $c;
-            }
-        }
-        @res.join: '';
+        return $r;
     }