add bytes/2 combinator

This commit adds a new combinator bytes/2. The combinator will
pull the next n bytes from the input. It combiles to a byte-size
pattern match on the input.

lib/nimble_parsec.ex

@@ -237,6 +237,7 @@ defmodule NimbleParsec do
   @typep bound_combinator ::
            {:bin_segment, [inclusive_range], [exclusive_range], bin_modifier}
            | {:string, binary}
+           | {:bytes, pos_integer}
            | :eos
 
   @typep maybe_bound_combinator ::
@@ -380,6 +381,11 @@ defmodule NimbleParsec do
     generate(parsecs, mod, gen_times(t, Enum.random(0..max), mod, acc))
   end
 
+  defp generate([{:bytes, count} | parsecs], mod, acc) do
+    bytes = bytes_random(count)
+    generate(parsecs, mod, [bytes | acc])
+  end
+
   defp generate([], _mod, acc), do: Enum.reverse(acc)
 
   defp gen_export(mod, fun) do
@@ -437,6 +443,10 @@ defmodule NimbleParsec do
   defp weighted_random([_ | list], [weight | weights], chosen),
     do: weighted_random(list, weights, chosen - weight)
 
+  defp bytes_random(count) when is_integer(count) do
+    :crypto.strong_rand_bytes(count)
+  end
+
   @doc ~S"""
   Returns an empty combinator.
 
@@ -1774,6 +1784,30 @@ defmodule NimbleParsec do
     choice(combinator, [optional, empty()])
   end
 
+  @doc """
+  Defines a combinator to consume the next `n` bytes from the input.
+
+  ## Examples
+
+        defmodule MyParser do
+          import NimbleParsec
+
+          defparsec :three_bytes = bytes(3)
+        end
+
+        MyParser.three_bytes("abc")
+        #=> {:ok, ["abc"], "", %{}, {1, 0}, 3}
+
+        MyParser.three_bytes("ab")
+        #=> {:error, "expected 3 bytes", "ab", %{}, {1, 0}, 0}
+  """
+  @spec bytes(pos_integer) :: t
+  @spec bytes(t, pos_integer) :: t
+  def bytes(combinator \\ empty(), count)
+    when is_combinator(combinator) and is_integer(count) and count > 0 do
+    [{:bytes, count} | combinator]
+  end
+
   ## Helpers
 
   defp validate_min_and_max!(count_or_opts, required_min \\ 0)

lib/nimble_parsec/compiler.ex

@@ -900,6 +900,15 @@ defmodule NimbleParsec.Compiler do
     end
   end
 
+  defp bound_combinator({:bytes, count}, metadata) do
+    %{counter: counter, offset: offset} = metadata
+    {var, counter} = build_var(counter)
+    input = quote do unquote(var)::binary-size(unquote(count)) end
+    offset = add_offset(offset, count)
+    metadata = %{metadata | counter: counter, offset: offset}
+    {:ok, [input], [], [var], metadata}
+  end
+
   defp bound_combinator(_, _) do
     :error
   end
@@ -1025,6 +1034,10 @@ defmodule NimbleParsec.Compiler do
     Atom.to_string(name)
   end
 
+  defp label({:bytes, count}) do
+    "#{inspect(count)} bytes"
+  end
+
   ## Bin segments
 
   defp compile_bin_ranges(var, ors, ands) do

test/nimble_generator_test.exs

@@ -107,6 +107,11 @@ defmodule NimbleGeneratorTest do
     assert times(string("foo"), min: 2, gen_times: 3) |> generate() == "foofoofoofoofoo"
   end
 
+  test "bytes" do
+    parsec = bytes(3)
+    assert byte_size(generate(parsec)) === 3
+  end
+
   defparsec :string_foo, string("foo"), export_metadata: true
   defparsec :string_choice, choice([parsec(:string_foo), string("bar")]), export_metadata: true
 

test/nimble_parsec_test.exs

@@ -1467,6 +1467,18 @@ defmodule NimbleParsecTest do
     end
   end
 
+  describe "bytes/2 combinator" do
+    defparsec :parse_bytes, bytes(3)
+
+    test "succeeds if input has sufficient bytes" do
+      assert parse_bytes("abc") == {:ok, ["abc"], "", %{}, {1, 0}, 3}
+    end
+
+    test "fails if input has insufficent bytes" do
+      assert parse_bytes("ab") == {:error, "expected 3 bytes", "ab", %{}, {1, 0}, 0}
+    end
+  end
+
   describe "continuing parser" do
     defparsecp :digits, [?0..?9] |> ascii_char() |> times(min: 1) |> label("digits")
     defparsecp :chars, [?a..?z] |> ascii_char() |> times(min: 1) |> label("chars")