diff --git a/lib/protobuff/parser.rb b/lib/protobuff/parser.rb
new file mode 100644
index 0000000..428cae0
--- /dev/null
+++ b/lib/protobuff/parser.rb
@@ -0,0 +1,283 @@
+# Parser for the protobuf (proto3) language:
+# https://protobuf.dev/programming-guides/proto3/
+
+# Note: Google has a unittest.proto test file
+# We should aim to be able to parse it
+# https://github.com/protocolbuffers/protoscope/blob/main/testdata/unittest.proto
+
+# Notes about protobuf language (these will be removed later):
+# - There are // style single-line comments
+# - Messages have a list of fields
+#   - Fields have field numbers
+#   - Fields can be optional, repeated, map (key/value pairs)
+#   - Fields can have a default value
+# - There are enums, which are integer types
+#   - Enum variants have integer values (is this optional?)
+#   - There must be a zero value (the default value)
+#   - Multiple different enums can share the same value if you set "option allow_alias = true;"
+#   - Enumerator constants must be in the range of a 32-bit integer.
+#   enum Foo {
+#     reserved 2, 15, 9 to 11, 40 to max;
+#     reserved "FOO", "BAR";
+#   }
+# - There's also oneof, which is a kind of union/enum
+# - You can import definitions
+#   import "myproject/other_protos.proto";
+
+# Represents an input string/file
+# Works as a tokenizer
+class Input
+  def initialize(src)
+    @src = src
+    @cur_idx = 0
+  end
+
+  # Check if we're at the end of the input
+  def eof?
+    return @cur_idx >= @src.size
+  end
+
+  # Check if the input start with a given string
+  def start_with?(str)
+    return @src[@cur_idx...(@cur_idx + str.size)] == str
+  end
+
+  # Check if the input matches a given string/keyword
+  # If there is a match, consume the string
+  def match(str)
+    eat_ws
+    if start_with?(str)
+      @cur_idx += str.size
+      true
+    else
+      false
+    end
+  end
+
+  # Raise an exception if we can't match a specific string
+  def expect(str)
+    if !match(str)
+      raise "expected \"#{str}\""
+    end
+  end
+
+  # Peek at the next input character
+  def peek_ch()
+    @src[@cur_idx]
+  end
+
+  # Consume one character from the input
+  def eat_ch()
+    ch = @src[@cur_idx]
+    @cur_idx += 1
+    ch
+  end
+
+  # Consume whitespace
+  def eat_ws()
+    loop do
+      if eof?
+        break
+      end
+
+      ch = peek_ch
+
+      if ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r'
+        eat_ch
+      else
+        break
+      end
+    end
+  end
+
+  # Read an identifier/name, eg:
+  # foo
+  # bar_bif
+  # foo123
+  def read_ident
+    name = ''
+
+    loop do
+      if eof?
+        break
+      end
+
+      ch = peek_ch
+
+      if !ch.match(/[A-Za-z0-9_]/)
+        break
+      end
+
+      name << eat_ch
+    end
+
+    if name.size == 0
+      raise "expected variable name"
+    end
+
+    return name
+  end
+
+  # Read a string constant, eg:
+  # 'foobar'
+  def read_string
+    str = ''
+
+    # The string must start with an opening quote
+    expect '"'
+
+    loop do
+      if eof?
+        raise "unexpected end of input"
+      end
+
+      # End of string
+      if match("\"")
+        break
+      end
+
+      # TODO: more complete support for string escaping
+      if match("\\'")
+        str << "\'"
+        continue
+      end
+
+      str << eat_ch
+    end
+
+    return str
+  end
+
+  # Read an integer constant
+  def read_int
+    value = 0
+
+    loop do
+      if eof?
+        raise "unexpected end of input"
+      end
+
+      ch = peek_ch
+
+      if ch < '0' || ch > '9'
+        break
+      end
+
+      # Decimal digit value
+      digit = ch.getbyte(0) - 0x30
+      value = 10 * value + digit
+      eat_ch
+    end
+
+    value
+  end
+end
+
+Unit = Struct.new(:messages)
+
+Message = Struct.new(:name, :fields)
+
+# TODO: repeated fields
+Field = Struct.new(:optional, :name, :number)
+
+# Parse an entire source unit (e.g. input file)
+def parse_unit(input)
+  messages = []
+
+  loop do
+    input.eat_ws
+
+    if input.eof?
+      break
+    end
+
+    ident = input.read_ident
+
+    # Syntax mode
+    if ident == "syntax"
+      input.eat_ws
+      mode = input.read_string
+      input.expect ';'
+      if mode != "proto3"
+        raise "syntax mode must be proto3"
+      end
+    end
+
+    # Message definition
+    if ident == "message"
+      parse_message(input)
+    end
+  end
+
+  Unit.new(messages)
+end
+
+def parse_string(str)
+  parse_unit(Input.new(str))
+end
+
+def parse_file(name)
+  raise "TODO"
+end
+
+# Parse a message definition
+def parse_message(input)
+  fields = []
+
+  input.eat_ws
+  name = input.read_ident
+  input.expect '{'
+
+  loop do
+    if input.match '}'
+      break
+    end
+
+    optional = false
+    if input.match 'optional'
+      optional = true
+    end
+
+    # Field type and name
+    input.eat_ws
+    type = input.read_ident
+    input.eat_ws
+    name = input.read_ident
+    input.expect '='
+    input.eat_ws
+    number = input.read_int
+    input.expect ';'
+
+    if number < 0 || number > 0xFF_FF_FF_FF
+      raise "field number should be in uint32 range"
+    end
+
+    fields << Field.new(optional, name, number)
+  end
+
+  # TODO: determine if an empty message with no fields is valid protobuf
+
+  Message.new(name, fields)
+end
+
+parse_string('')
+parse_string('syntax "proto3";')
+parse_string('syntax "proto3"; message Foo {}')
+parse_string('message Test1 { optional int32 a = 1; }')
+parse_string('message TestMessage { string id = 1; uint64 shop_id = 2; bool boolean = 3; }')
+
+
+
+
+
+
+# TODO: write some tests, try to parse:
+# ./test/fixtures/test.proto
+
+
+
+
+
+
+
+