1 files changed, 185 insertions, 0 deletions
diff --git a/lib/prism/pack.rb b/lib/prism/pack.rb
new file mode 100644
index 0000000000..83f5569923
--- /dev/null
+++ b/lib/prism/pack.rb
@@ -0,0 +1,185 @@
+# frozen_string_literal: true
+
+module YARP
+  module Pack
+    %i[
+      SPACE
+      COMMENT
+      INTEGER
+      UTF8
+      BER
+      FLOAT
+      STRING_SPACE_PADDED
+      STRING_NULL_PADDED
+      STRING_NULL_TERMINATED
+      STRING_MSB
+      STRING_LSB
+      STRING_HEX_HIGH
+      STRING_HEX_LOW
+      STRING_UU
+      STRING_MIME
+      STRING_BASE64
+      STRING_FIXED
+      STRING_POINTER
+      MOVE
+      BACK
+      NULL
+
+      UNSIGNED
+      SIGNED
+      SIGNED_NA
+
+      AGNOSTIC_ENDIAN
+      LITTLE_ENDIAN
+      BIG_ENDIAN
+      NATIVE_ENDIAN
+      ENDIAN_NA
+
+      SIZE_SHORT
+      SIZE_INT
+      SIZE_LONG
+      SIZE_LONG_LONG
+      SIZE_8
+      SIZE_16
+      SIZE_32
+      SIZE_64
+      SIZE_P
+      SIZE_NA
+
+      LENGTH_FIXED
+      LENGTH_MAX
+      LENGTH_RELATIVE
+      LENGTH_NA
+    ].each do |const|
+      const_set(const, const)
+    end
+
+    class Directive
+      attr_reader :version, :variant, :source, :type, :signed, :endian, :size, :length_type, :length
+
+      def initialize(version, variant, source, type, signed, endian, size, length_type, length)
+        @version = version
+        @variant = variant
+        @source = source
+        @type = type
+        @signed = signed
+        @endian = endian
+        @size = size
+        @length_type = length_type
+        @length = length
+      end
+
+      ENDIAN_DESCRIPTIONS = {
+        AGNOSTIC_ENDIAN: 'agnostic',
+        LITTLE_ENDIAN: 'little-endian (VAX)',
+        BIG_ENDIAN: 'big-endian (network)',
+        NATIVE_ENDIAN: 'native-endian',
+        ENDIAN_NA: 'n/a'
+      }
+
+      SIGNED_DESCRIPTIONS = {
+        UNSIGNED: 'unsigned',
+        SIGNED: 'signed',
+        SIGNED_NA: 'n/a'
+      }
+
+      SIZE_DESCRIPTIONS = {
+        SIZE_SHORT: 'short',
+        SIZE_INT: 'int-width',
+        SIZE_LONG: 'long',
+        SIZE_LONG_LONG: 'long long',
+        SIZE_8: '8-bit',
+        SIZE_16: '16-bit',
+        SIZE_32: '32-bit',
+        SIZE_64: '64-bit',
+        SIZE_P: 'pointer-width'
+      }
+
+      def describe
+        case type
+        when SPACE
+          'whitespace'
+        when COMMENT
+          'comment'
+        when INTEGER
+          if size == SIZE_8
+            base = "#{SIGNED_DESCRIPTIONS[signed]} #{SIZE_DESCRIPTIONS[size]} integer"
+          else
+            base = "#{SIGNED_DESCRIPTIONS[signed]} #{SIZE_DESCRIPTIONS[size]} #{ENDIAN_DESCRIPTIONS[endian]} integer"
+          end
+          case length_type
+          when LENGTH_FIXED
+            if length > 1
+              base + ", x#{length}"
+            else
+              base
+            end
+          when LENGTH_MAX
+            base + ', as many as possible'
+          end
+        when UTF8
+          'UTF-8 character'
+        when BER
+          'BER-compressed integer'
+        when FLOAT
+          "#{SIZE_DESCRIPTIONS[size]} #{ENDIAN_DESCRIPTIONS[endian]} float"
+        when STRING_SPACE_PADDED
+          'arbitrary binary string (space padded)'
+        when STRING_NULL_PADDED
+          'arbitrary binary string (null padded, count is width)'
+        when STRING_NULL_TERMINATED
+          'arbitrary binary string (null padded, count is width), except that null is added with *'
+        when STRING_MSB
+          'bit string (MSB first)'
+        when STRING_LSB
+          'bit string (LSB first)'
+        when STRING_HEX_HIGH
+          'hex string (high nibble first)'
+        when STRING_HEX_LOW
+          'hex string (low nibble first)'
+        when STRING_UU
+          'UU-encoded string'
+        when STRING_MIME
+          'quoted printable, MIME encoding'
+        when STRING_BASE64
+          'base64 encoded string'
+        when STRING_FIXED
+          'pointer to a structure (fixed-length string)'
+        when STRING_POINTER
+          'pointer to a null-terminated string'
+        when MOVE
+          'move to absolute position'
+        when BACK
+          'back up a byte'
+        when NULL
+          'null byte'
+        else
+          raise
+        end
+      end
+    end
+
+    class Format
+      attr_reader :directives, :encoding
+
+      def initialize(directives, encoding)
+        @directives = directives
+        @encoding = encoding
+      end
+
+      def describe
+        source_width = directives.map { |d| d.source.inspect.length }.max
+        directive_lines = directives.map do |directive|
+          if directive.type == SPACE
+            source = directive.source.inspect
+          else
+            source = directive.source
+          end
+          "  #{source.ljust(source_width)}  #{directive.describe}"
+        end
+
+        (['Directives:'] + directive_lines + ['Encoding:', "  #{encoding}"]).join("\n")
+      end
+    end
+  end
+end