Add "[" to the possible characters of TRANSITION_FNS.

Introducing "[" now we will start to write the code to parse the
character class expressions, i.e. [a-z0-9].  The `context` key will
contain a `set` with all the literal characters that were found, and all
the ranges too.  For parsing the ranges, a `range` key equivalent to the
one for the {m,n} range is used.  Despite the superficial syntax being
simmilar, its logic, semantic and implementation will be different.

* src/paca.mjs (TRANSITION_FNS) <"[">: Add new transition function for
  handling the start of a character class expression.
* tests/paca.mjs (TRANSITION_FNS): Add a singular test entry, that
  exercises the conditionless body of the function.

2 files changed, 30 insertions, 0 deletions

diff --git a/src/paca.mjs b/src/paca.mjs
index c1bead1..6843c20 100644
--- a/src/paca.mjs
+++ b/src/paca.mjs
@@ -121,6 +121,18 @@ const TRANSITION_FNS = {
 			where: "from",
 		},
 	}),
+	"[": ({ out, _state, _context }, _char, _index, _next) => ({
+		out,
+		state: ConcatStep.CLASS,
+		context: {
+			set:   [],
+			range: {
+				from:  [],
+				to:    [],
+				where: "from",
+			},
+		},
+	}),
 };
 
 const stateTransitionOperators = new Set(Object.keys(TRANSITION_FNS));
diff --git a/tests/paca.mjs b/tests/paca.mjs
index b259968..a7cd929 100644
--- a/tests/paca.mjs
+++ b/tests/paca.mjs
@@ -73,6 +73,24 @@ const test_TRANSITION_FNS = t => {
 			},
 		);
 	});
+
+	t.testing("\"[\" only depends on `out`", () => {
+		t.assertEq(
+			TRANSITION_FNS["["]({ out: 123 }, null, null, null),
+			{
+				out:     123,
+				state:   "class",
+				context: {
+					set:   [],
+					range: {
+						from:  [],
+						to:    [],
+						where: "from",
+					},
+				},
+			},
+		);
+	});
 };
 
 const test_shouldConcat = t => {