[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

4 files changed, 23 insertions, 10 deletions

diff --git a/lib/bundler/compact_index_client/cache.rb b/lib/bundler/compact_index_client/cache.rb
index 0b43581c11..b5607c8751 100644
--- a/lib/bundler/compact_index_client/cache.rb
+++ b/lib/bundler/compact_index_client/cache.rb
@@ -32,11 +32,11 @@ module Bundler
           name, versions_string, info_checksum = line.split(" ", 3)
           info_checksums_by_name[name] = info_checksum || ""
           versions_string.split(",").each do |version|
-            if version.start_with?("-")
-              version = version[1..-1].split("-", 2).unshift(name)
+            delete = version.delete_prefix!("-")
+            version = version.split("-", 2).unshift(name)
+            if delete
               versions_by_name[name].delete(version)
             else
-              version = version.split("-", 2).unshift(name)
               versions_by_name[name] << version
             end
           end
diff --git a/lib/bundler/compact_index_client/gem_parser.rb b/lib/bundler/compact_index_client/gem_parser.rb
index e7bf4c6001..60a1817607 100644
--- a/lib/bundler/compact_index_client/gem_parser.rb
+++ b/lib/bundler/compact_index_client/gem_parser.rb
@@ -6,12 +6,15 @@ module Bundler
       GemParser = Gem::Resolver::APISet::GemParser
     else
       class 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
 
@@ -20,6 +23,7 @@ module Bundler
         def parse_dependency(string)
           dependency = string.split(":")
           dependency[-1] = dependency[-1].split("&") if dependency.size > 1
+          dependency[0] = -dependency[0]
           dependency
         end
       end
diff --git a/lib/bundler/compact_index_client/updater.rb b/lib/bundler/compact_index_client/updater.rb
index 0f7bf9bb50..3b75d5c129 100644
--- a/lib/bundler/compact_index_client/updater.rb
+++ b/lib/bundler/compact_index_client/updater.rb
@@ -95,7 +95,12 @@ module Bundler
         # because we need to preserve \n line endings on windows when calculating
         # the checksum
         SharedHelpers.filesystem_access(path, :read) do
-          SharedHelpers.digest(:MD5).hexdigest(File.read(path))
+          File.open(path, "rb") do |f|
+            digest = SharedHelpers.digest(:MD5).new
+            buf = String.new(:capacity => 16_384, :encoding => Encoding::BINARY)
+            digest << buf while f.read(16_384, buf)
+            digest.hexdigest
+          end
         end
       end
 
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