test(WO80/A4): rulings for contexts, maybe, and capture (#820)

Six more normative rulings pinned as vectors: before/after are
zero-width lookbehind/lookahead; maybe is zero-or-one within
longest-match; capture/ref round-trips in sequential rules; capture
inside parallel raises (the WO80 capture-support gap -- now with two
motivating cases: the RTGS compound vowels and the parallel capture
crash).

Author: ronaldtse

spec/isc/semantics_spec.rb

@@ -56,3 +56,86 @@ def with_map(name, body)
     end
   end
 end
+
+RSpec.describe "ISC semantics — contexts, maybe, capture" do
+  def with_map(name, body)
+    dir = Dir.mktmpdir
+    File.write(File.join(dir, name), body)
+    Interscript.load_path.unshift(dir)
+    yield
+  ensure
+    Interscript.load_path.delete(dir) if dir
+  end
+
+  def context_map(id, rule)
+    <<~ISC
+      system "TST:xx-Latn:Latn:#{id}" {
+        metadata {
+          authority_id tst
+          id #{id}
+          language iso-639-2:xx
+          source_script Latn
+          destination_script Latn
+        }
+        stage main { parallel { sub { #{rule} } } }
+      }
+    ISC
+  end
+
+  it "rules: before is a zero-width lookbehind (not consumed)" do
+    with_map("tst-before-2.isc", context_map(11, 'from "a" to "X" before "b"')) do
+      expect(Interscript.transliterate("tst-before-2", "ba")).to eq("bX")
+      expect(Interscript.transliterate("tst-before-2", "ab")).to eq("ab")
+    end
+  end
+
+  it "rules: after is a zero-width lookahead (not consumed)" do
+    with_map("tst-after-2.isc", context_map(12, 'from "b" to "Y" after "a"')) do
+      expect(Interscript.transliterate("tst-after-2", "ba")).to eq("Ya")
+      expect(Interscript.transliterate("tst-after-2", "ab")).to eq("ab")
+    end
+  end
+
+  it "rules: maybe is zero-or-one and participates in longest-match" do
+    with_map("tst-maybe-2.isc", <<~ISC) do
+      system "TST:xx-Latn:Latn:13" {
+        metadata {
+          authority_id tst
+          id 13
+          language iso-639-2:xx
+          source_script Latn
+          destination_script Latn
+        }
+        stage main { parallel { sub { from "k" + maybe("h") to "X" } } }
+      }
+    ISC
+      expect(Interscript.transliterate("tst-maybe-2", "k")).to eq("X")
+      expect(Interscript.transliterate("tst-maybe-2", "kh")).to eq("X")
+      expect(Interscript.transliterate("tst-maybe-2", "kk")).to eq("XX")
+    end
+  end
+
+  it "rules: capture/ref round-trips in sequential rules" do
+    with_map("tst-capseq-2.isc", <<~ISC) do
+      system "TST:xx-Latn:Latn:14" {
+        metadata {
+          authority_id tst
+          id 14
+          language iso-639-2:xx
+          source_script Latn
+          destination_script Latn
+        }
+        stage main { sub { from capture(any("0".."9")) to ref(1) } }
+      }
+    ISC
+      expect(Interscript.transliterate("tst-capseq-2", "77a9")).to eq("77a9")
+    end
+  end
+
+  it "rules: capture inside parallel is pending capture support (WO80)" do
+    with_map("tst-cappar-1.isc", context_map(15, 'from capture(any("0".."9")) to ref(1)')) do
+      expect { Interscript.transliterate("tst-cappar-1", "5") }
+        .to raise_error(NoMethodError, /max_length/)
+    end
+  end
+end