[rubygems/rubygems] Reduce allocations when parsing compact index

This still allocates a ton (a string for each line, plus a bunch of
splits into arrays), but it helps a bit when Bundler has to go through
dependency resolution.

```
==> memprof.after.txt <==
Total allocated: 194.14 MB (2317172 objects)
Total retained:  60.81 MB (593164 objects)

==> memprof.before.txt <==
Total allocated: 211.97 MB (2404890 objects)
Total retained:  62.85 MB (640342 objects)
```

https://github.com/rubygems/rubygems/commit/c68b41b0e5

1 files changed, 7 insertions, 3 deletions

diff --git a/lib/rubygems/resolver/api_set/gem_parser.rb b/lib/rubygems/resolver/api_set/gem_parser.rb
index 685c39558d..643b857107 100644
--- a/lib/rubygems/resolver/api_set/gem_parser.rb
+++ b/lib/rubygems/resolver/api_set/gem_parser.rb
@@ -1,12 +1,15 @@
 # frozen_string_literal: true
 
 class Gem::Resolver::APISet::GemParser
+  EMPTY_ARRAY = [].freeze
+  private_constant :EMPTY_ARRAY
+
   def parse(line)
     version_and_platform, rest = line.split(" ", 2)
     version, platform = version_and_platform.split("-", 2)
-    dependencies, requirements = rest.split("|", 2).map {|s| s.split(",") } if rest
-    dependencies = dependencies ? dependencies.map {|d| parse_dependency(d) } : []
-    requirements = requirements ? requirements.map {|d| parse_dependency(d) } : []
+    dependencies, requirements = rest.split("|", 2).map! {|s| s.split(",") } if rest
+    dependencies = dependencies ? dependencies.map! {|d| parse_dependency(d) } : EMPTY_ARRAY
+    requirements = requirements ? requirements.map! {|d| parse_dependency(d) } : EMPTY_ARRAY
     [version, platform, dependencies, requirements]
   end
 
@@ -15,6 +18,7 @@ class Gem::Resolver::APISet::GemParser
   def parse_dependency(string)
     dependency = string.split(":")
     dependency[-1] = dependency[-1].split("&") if dependency.size > 1
+    dependency[0] = -dependency[0]
     dependency
   end
 end