Add targeted tests for byte-walking fast paths

36 tests covering the three optimization sites:

Expression.parse_number (13 tests):
  - Simple integers, negatives, floats, trailing dots
  - Multi-dot truncation (1.2.3 → 1.2)
  - Rejection of non-numeric input and trailing alpha (1.2.3a)

Expression.parse strip guard (7 tests):
  - Leading, trailing, both-sides whitespace
  - No-strip-needed case (no allocation)
  - Null byte stripping (matches String#strip behavior)

VariableLookup.simple_lookup? (8 tests):
  - Accepts: single names, dotted chains, question marks, hyphens
  - Rejects: brackets, empty, leading/trailing dots, double dots

VariableLookup fast path equivalence (7 tests):
  - name/lookups/command_flags match for simple and deep chains
  - Bracket inputs fall through to regex path correctly

BlockBody.try_parse_tag_token (10 tests):
  - Simple tags, whitespace control variants ({%-, -%}, both)
  - No-markup tags, hash comments, newline counting
  - Hyphenated names stop at hyphen (matching TagName = /\w+/)
  - Malformed tokens return nil (fallback to FullToken regex)

test/unit/fast_path_unit_test.rb

@@ -0,0 +1,256 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+# Tests for the byte-walking fast paths introduced in the ByteTables optimization.
+# Each fast path is tested for equivalence with the original regex-based code path
+# it replaces, covering accepted inputs, rejected inputs, and boundary cases.
+class FastPathUnitTest < Minitest::Test
+  # ── Expression.parse_number ────────────────────────────────────────
+  # Fast path: byte-walk with ByteTables::DIGIT
+  # Replaces: INTEGER_REGEX, FLOAT_REGEX, StringScanner loop
+
+  def test_parse_number_simple_integers
+    assert_equal(42, Liquid::Expression.parse_number("42"))
+    assert_equal(0, Liquid::Expression.parse_number("0"))
+    assert_equal(999, Liquid::Expression.parse_number("999"))
+  end
+
+  def test_parse_number_negative_integers
+    assert_equal(-7, Liquid::Expression.parse_number("-7"))
+    assert_equal(-0, Liquid::Expression.parse_number("-0"))
+    assert_equal(-123, Liquid::Expression.parse_number("-123"))
+  end
+
+  def test_parse_number_simple_floats
+    assert_equal(3.14, Liquid::Expression.parse_number("3.14"))
+    assert_equal(0.5, Liquid::Expression.parse_number("0.5"))
+    assert_equal(-0.5, Liquid::Expression.parse_number("-0.5"))
+    assert_equal(100.0, Liquid::Expression.parse_number("100.0"))
+  end
+
+  def test_parse_number_trailing_dot
+    # "123." → 123.0 (truncate before dot)
+    assert_equal(123.0, Liquid::Expression.parse_number("123."))
+    assert_equal(0.0, Liquid::Expression.parse_number("0."))
+  end
+
+  def test_parse_number_multi_dot_floats
+    # "1.2.3" → 1.2 (truncate at second dot)
+    assert_equal(1.2, Liquid::Expression.parse_number("1.2.3"))
+    assert_equal(1.2, Liquid::Expression.parse_number("1.2.3.4"))
+    assert_equal(0.0, Liquid::Expression.parse_number("0.0.0"))
+  end
+
+  def test_parse_number_rejects_non_numeric
+    assert_nil(Liquid::Expression.parse_number("hello"))
+    assert_nil(Liquid::Expression.parse_number(""))
+    assert_nil(Liquid::Expression.parse_number("abc123"))
+    assert_nil(Liquid::Expression.parse_number(".5"))
+    assert_nil(Liquid::Expression.parse_number("-.5"))
+  end
+
+  def test_parse_number_rejects_trailing_alpha_after_multi_dot
+    # "1.2.3a" must be nil, not 1.2 — these are not valid numbers
+    assert_nil(Liquid::Expression.parse_number("1.2.3a"))
+    assert_nil(Liquid::Expression.parse_number("1.2.3.4a"))
+    assert_nil(Liquid::Expression.parse_number("1.2.34a"))
+    assert_nil(Liquid::Expression.parse_number("-1.2.3a"))
+  end
+
+  def test_parse_number_rejects_bare_dash
+    assert_nil(Liquid::Expression.parse_number("-"))
+    assert_nil(Liquid::Expression.parse_number("-a"))
+  end
+
+  # ── Expression.parse strip guard ───────────────────────────────────
+  # Fast path: skip String#strip when no leading/trailing whitespace
+  # Must produce identical results to unconditional .strip
+
+  def test_parse_strips_leading_whitespace
+    assert_equal(42, Liquid::Expression.parse("  42"))
+    assert_equal(42, Liquid::Expression.parse("\t42"))
+    assert_equal(42, Liquid::Expression.parse("\n42"))
+  end
+
+  def test_parse_strips_trailing_whitespace
+    assert_equal(42, Liquid::Expression.parse("42  "))
+    assert_equal(42, Liquid::Expression.parse("42\t"))
+    assert_equal(42, Liquid::Expression.parse("42\n"))
+  end
+
+  def test_parse_strips_both_sides
+    assert_equal(42, Liquid::Expression.parse("  42  "))
+    assert_equal("hello", Liquid::Expression.parse("  'hello'  "))
+  end
+
+  def test_parse_no_strip_needed
+    assert_equal(42, Liquid::Expression.parse("42"))
+    assert_equal("hello", Liquid::Expression.parse("'hello'"))
+    assert_equal(true, Liquid::Expression.parse("true"))
+  end
+
+  def test_parse_strips_null_bytes
+    # String#strip removes \x00 — the WHITESPACE table must match
+    assert_equal(true, Liquid::Expression.parse("\x00true"))
+    assert_equal(true, Liquid::Expression.parse("true\x00"))
+  end
+
+  # ── VariableLookup.simple_lookup? ──────────────────────────────────
+  # Fast path: regex gate for simple a.b.c chains
+  # Must accept only inputs the byte-walk can handle correctly
+
+  def test_simple_lookup_accepts_single_names
+    assert(Liquid::VariableLookup.simple_lookup?("product"))
+    assert(Liquid::VariableLookup.simple_lookup?("x"))
+    assert(Liquid::VariableLookup.simple_lookup?("_private"))
+  end
+
+  def test_simple_lookup_accepts_dotted_chains
+    assert(Liquid::VariableLookup.simple_lookup?("product.title"))
+    assert(Liquid::VariableLookup.simple_lookup?("a.b.c.d"))
+  end
+
+  def test_simple_lookup_accepts_question_marks
+    assert(Liquid::VariableLookup.simple_lookup?("product.available?"))
+    assert(Liquid::VariableLookup.simple_lookup?("empty?"))
+  end
+
+  def test_simple_lookup_accepts_hyphens
+    assert(Liquid::VariableLookup.simple_lookup?("my-var"))
+    assert(Liquid::VariableLookup.simple_lookup?("my-var.some-field"))
+  end
+
+  def test_simple_lookup_rejects_brackets
+    refute(Liquid::VariableLookup.simple_lookup?("product[0]"))
+    refute(Liquid::VariableLookup.simple_lookup?("hash['key']"))
+    refute(Liquid::VariableLookup.simple_lookup?("[0]"))
+  end
+
+  def test_simple_lookup_rejects_empty_and_malformed
+    refute(Liquid::VariableLookup.simple_lookup?(""))
+    refute(Liquid::VariableLookup.simple_lookup?(".leading"))
+    refute(Liquid::VariableLookup.simple_lookup?("trailing."))
+    refute(Liquid::VariableLookup.simple_lookup?("a..b"))
+  end
+
+  # ── VariableLookup fast path equivalence ───────────────────────────
+  # The fast path must produce identical name, lookups, and command_flags
+  # to the original VariableParser regex path
+
+  def test_fast_path_simple_name
+    vl = Liquid::VariableLookup.new("product")
+    assert_equal("product", vl.name)
+    assert_equal([], vl.lookups)
+  end
+
+  def test_fast_path_dotted_chain
+    vl = Liquid::VariableLookup.new("product.title")
+    assert_equal("product", vl.name)
+    assert_equal(["title"], vl.lookups)
+  end
+
+  def test_fast_path_deep_chain
+    vl = Liquid::VariableLookup.new("a.b.c.d")
+    assert_equal("a", vl.name)
+    assert_equal(["b", "c", "d"], vl.lookups)
+  end
+
+  def test_fast_path_command_methods
+    vl = Liquid::VariableLookup.new("items.size")
+    assert_equal("items", vl.name)
+    assert_equal(["size"], vl.lookups)
+    assert(vl.lookup_command?(0))
+
+    vl2 = Liquid::VariableLookup.new("items.first")
+    assert(vl2.lookup_command?(0))
+
+    vl3 = Liquid::VariableLookup.new("items.last")
+    assert(vl3.lookup_command?(0))
+  end
+
+  def test_fast_path_non_command_lookups
+    vl = Liquid::VariableLookup.new("product.title")
+    refute(vl.lookup_command?(0))
+  end
+
+  def test_fast_path_question_mark
+    vl = Liquid::VariableLookup.new("product.available?")
+    assert_equal("product", vl.name)
+    assert_equal(["available?"], vl.lookups)
+  end
+
+  def test_bracket_lookup_falls_to_regex_path
+    vl = Liquid::VariableLookup.new("product[0]")
+    assert_equal("product", vl.name)
+    assert_equal([0], vl.lookups)
+  end
+
+  # ── BlockBody.try_parse_tag_token ──────────────────────────────────
+  # Fast path: byte-walk tag tokens instead of FullToken regex
+  # Must produce identical [tag_name, markup, newline_count] or nil
+
+  def test_tag_token_simple
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{% if x %}")
+    assert_equal(["if", "x ", 0], result)
+  end
+
+  def test_tag_token_whitespace_control_leading
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{%- if x %}")
+    assert_equal(["if", "x ", 0], result)
+  end
+
+  def test_tag_token_whitespace_control_trailing
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{% if x -%}")
+    assert_equal(["if", "x ", 0], result)
+  end
+
+  def test_tag_token_whitespace_control_both
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{%- if x -%}")
+    assert_equal(["if", "x ", 0], result)
+  end
+
+  def test_tag_token_no_markup
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{% endif %}")
+    assert_equal(["endif", "", 0], result)
+  end
+
+  def test_tag_token_hash_comment
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{% # this is a comment %}")
+    assert_equal(["#", "this is a comment ", 0], result)
+  end
+
+  def test_tag_token_with_newlines
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{% \n if \n x %}")
+    assert_equal(["if", "x ", 2], result)
+  end
+
+  def test_tag_token_hyphenated_name_stops_at_hyphen
+    # TagName = /\w+/ does not include hyphens
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{% my-tag markup %}")
+    assert_equal("my", result[0])
+  end
+
+  def test_tag_token_complex_markup
+    body = Liquid::BlockBody.new
+    result = body.send(:try_parse_tag_token, "{% for item in collection reversed %}")
+    assert_equal("for", result[0])
+    assert_equal("item in collection reversed ", result[1])
+  end
+
+  def test_tag_token_malformed_returns_nil
+    body = Liquid::BlockBody.new
+    # Token too short
+    assert_nil(body.send(:try_parse_tag_token, "{%"))
+    # No valid tag name start (digit)
+    assert_nil(body.send(:try_parse_tag_token, "{% 123 %}"))
+  end
+end