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diff --git a/lib/bundler/compact_index_client.rb b/lib/bundler/compact_index_client.rb
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+++ b/lib/bundler/compact_index_client.rb
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+# frozen_string_literal: true
+
+require "set"
+
+module Bundler
+  # The CompactIndexClient is responsible for fetching and parsing the compact index.
+  #
+  # The compact index is a set of caching optimized files that are used to fetch gem information.
+  # The files are:
+  # - names: a list of all gem names
+  # - versions: a list of all gem versions
+  # - info/[gem]: a list of all versions of a gem
+  #
+  # The client is instantiated with:
+  # - `directory`: the root directory where the cache files are stored.
+  # - `fetcher`: (optional) an object that responds to #call(uri_path, headers) and returns an http response.
+  # If the `fetcher` is not provided, the client will only read cached files from disk.
+  #
+  # The client is organized into:
+  # - `Updater`: updates the cached files on disk using the fetcher.
+  # - `Cache`: calls the updater, caches files, read and return them from disk
+  # - `Parser`: parses the compact index file data
+  # - `CacheFile`: a concurrency safe file reader/writer that verifies checksums
+  #
+  # The client is intended to optimize memory usage and performance.
+  # It is called 100s or 1000s of times, parsing files with hundreds of thousands of lines.
+  # It may be called concurrently without global interpreter lock in some Rubies.
+  # As a result, some methods may look more complex than necessary to save memory or time.
+  class CompactIndexClient
+    SUPPORTED_DIGESTS = { "sha-256" => :SHA256 }.freeze
+    DEBUG_MUTEX = Thread::Mutex.new
+
+    # info returns an Array of INFO Arrays. Each INFO Array has the following indices:
+    INFO_NAME = 0
+    INFO_VERSION = 1
+    INFO_PLATFORM = 2
+    INFO_DEPS = 3
+    INFO_REQS = 4
+
+    def self.debug
+      return unless ENV["DEBUG_COMPACT_INDEX"]
+      DEBUG_MUTEX.synchronize { warn("[#{self}] #{yield}") }
+    end
+
+    class Error < StandardError; end
+
+    require_relative "compact_index_client/cache"
+    require_relative "compact_index_client/cache_file"
+    require_relative "compact_index_client/parser"
+    require_relative "compact_index_client/updater"
+
+    def initialize(directory, fetcher = nil)
+      @cache = Cache.new(directory, fetcher)
+      @parser = Parser.new(@cache)
+    end
+
+    def names
+      Bundler::CompactIndexClient.debug { "names" }
+      @parser.names
+    end
+
+    def versions
+      Bundler::CompactIndexClient.debug { "versions" }
+      @parser.versions
+    end
+
+    def dependencies(names)
+      Bundler::CompactIndexClient.debug { "dependencies(#{names})" }
+      names.map {|name| info(name) }
+    end
+
+    def info(name)
+      Bundler::CompactIndexClient.debug { "info(#{name})" }
+      @parser.info(name)
+    end
+
+    def latest_version(name)
+      Bundler::CompactIndexClient.debug { "latest_version(#{name})" }
+      @parser.info(name).map {|d| Gem::Version.new(d[INFO_VERSION]) }.max
+    end
+
+    def available?
+      Bundler::CompactIndexClient.debug { "available?" }
+      @parser.available?
+    end
+
+    def reset!
+      Bundler::CompactIndexClient.debug { "reset!" }
+      @cache.reset!
+    end
+  end
+end