feat: add Prism and Assets

lib/prism/assets/compiler.rb

@@ -0,0 +1,68 @@
+require 'fileutils'
+
+module Prism
+  module Assets
+    # Sprockets/Propshaft-lite precompile step: walks every file under
+    # `source`, content-addresses it (sha256, first 8 hex chars) into
+    # `name-digest.ext` under `output`, and writes a manifest.json mapping
+    # the original relative path ("css/application.css") to the
+    # fingerprinted one ("css/application-3f2a9c1e.css"). Re-running with
+    # unchanged file content reproduces the exact same fingerprinted name
+    # (pure function of the bytes, no timestamps involved), and any
+    # fingerprinted file left over from a PREVIOUS compile that no logical
+    # path points to anymore is deleted - the same cleanup
+    # `rake assets:clean` does in Rails, done automatically on every compile
+    # instead of as a separate step.
+    class Compiler
+      DIGEST_LENGTH = 8
+
+      def initialize(source:, output:)
+        @source = File.expand_path(source)
+        @output = File.expand_path(output)
+      end
+
+      def compile!
+        FileUtils.mkdir_p(@output)
+        previous = Manifest.load(manifest_path).to_h.values
+        manifest = Manifest.new
+
+        each_source_file do |absolute, logical_path|
+          fingerprinted = fingerprint(logical_path, absolute)
+          destination = File.join(@output, fingerprinted)
+          FileUtils.mkdir_p(File.dirname(destination))
+          FileUtils.cp(absolute, destination)
+          manifest[logical_path] = fingerprinted
+        end
+
+        stale = previous - manifest.to_h.values
+        stale.each { |path| FileUtils.rm_f(File.join(@output, path)) }
+
+        manifest.save(manifest_path)
+        manifest
+      end
+
+      def manifest_path
+        File.join(@output, 'manifest.json')
+      end
+
+      private
+
+      def each_source_file
+        return unless File.directory?(@source)
+
+        Dir.glob(File.join(@source, '**', '*')).sort.each do |absolute|
+          next unless File.file?(absolute)
+
+          yield absolute, absolute.delete_prefix("#{@source}/")
+        end
+      end
+
+      def fingerprint(logical_path, absolute)
+        digest = Digest::SHA256.file(absolute).hexdigest[0, DIGEST_LENGTH]
+        ext = File.extname(logical_path)
+        base = logical_path.delete_suffix(ext)
+        "#{base}-#{digest}#{ext}"
+      end
+    end
+  end
+end

lib/prism/assets/manifest.rb

@@ -0,0 +1,50 @@
+module Prism
+  module Assets
+    # Maps logical asset paths ("application.css") to fingerprinted paths
+    # ("application-3f2a9c1e.css") the way Sprockets/Propshaft's
+    # manifest.json does. Purely a lookup table plus JSON (de)serialization -
+    # Compiler is what actually populates one.
+    class Manifest
+      AssetNotFound = Class.new(StandardError)
+
+      def initialize(assets = {})
+        @assets = assets.transform_keys(&:to_s)
+      end
+
+      attr_reader :assets
+
+      def []=(logical_path, fingerprinted_path)
+        @assets[logical_path.to_s] = fingerprinted_path
+      end
+
+      # Raises when the manifest is authoritative (production-style,
+      # precompiled ahead of time) so a typo in a view never silently links
+      # to a 404 - the same tradeoff Rails makes with
+      # config.assets.unknown_asset_fallback = false.
+      def lookup(logical_path)
+        @assets.fetch(logical_path.to_s) do
+          raise AssetNotFound, "asset #{logical_path.inspect} is not in the manifest " \
+                                "(known: #{@assets.keys.sort.join(', ')})"
+        end
+      end
+
+      def key?(logical_path)
+        @assets.key?(logical_path.to_s)
+      end
+
+      def to_h
+        @assets.dup
+      end
+
+      def save(path)
+        File.write(path, JSON.pretty_generate(@assets))
+      end
+
+      def self.load(path)
+        return new unless File.file?(path)
+
+        new(JSON.parse(File.read(path)))
+      end
+    end
+  end
+end