Use `shouldConcat()` in decision of `escapingStateSte()`

* src/paca.mjs
  (escapingStateStep): Use `shouldConcat()` instead of only checking if
  we're on the last char.  We abuse it a bit by passing `null` as the
  first argument, since it is being escaped.
  (nonConcatOperators, shouldConcat): Hoist the definition of both above
  `escapingStateStep()`, so that they're defined before being used.
* tests/paca.mjs (test_shouldConcat): Add test case where `null` is
  explicitly passed as the first argument.

2 files changed, 15 insertions, 11 deletions

diff --git a/src/paca.mjs b/src/paca.mjs
index b92bcdc..598c751 100644
--- a/src/paca.mjs
+++ b/src/paca.mjs
@@ -15,6 +15,15 @@ const ConcatStep = {
 	CLASS:     "class",
 };
 
+const nonConcatOperators = new Set(["*", "+", "?", "|", ")"]);
+
+const shouldConcat = (char, next) =>
+	next !== undefined &&
+	char !== "(" &&
+	char !== "|" &&
+	char !== "{" &&
+	!nonConcatOperators.has(next);
+
 const numFromDigits = digits =>
 	digits.length === 0
 		? -1
@@ -23,7 +32,7 @@ const numFromDigits = digits =>
 const escapingStateStep = ({ out, _state, context }, char, _index, next) => ({
 	out: out.concat(
 		char,
-		next !== undefined ? {operator: "concat"} : [],
+		shouldConcat(null, next) ? [{ operator: "concat" }] : [],
 	),
 	state: ConcatStep.ACCEPTING,
 	context,
@@ -265,15 +274,6 @@ const TRANSITION_FNS = {
 
 const stateTransitionOperators = new Set(Object.keys(TRANSITION_FNS));
 
-const nonConcatOperators = new Set(["*", "+", "?", "|", ")"]);
-
-const shouldConcat = (char, next) =>
-	next !== undefined &&
-	char !== "(" &&
-	char !== "|" &&
-	char !== "{" &&
-	!nonConcatOperators.has(next);
-
 const isOperator = char =>
 	nonConcatOperators.has(char) || char == "(";
 
@@ -302,7 +302,7 @@ const tokenizeRegexStep = chars => ({ out, state, context }, char, index) => {
 	return {
 		out: out.concat(
 			op,
-			shouldConcat(char, next) ? [{operator: "concat"}] : [],
+			shouldConcat(char, next) ? [{ operator: "concat" }] : [],
 		),
 		state,
 		context,
diff --git a/tests/paca.mjs b/tests/paca.mjs
index e9c3a6b..2c44f9e 100644
--- a/tests/paca.mjs
+++ b/tests/paca.mjs
@@ -511,6 +511,10 @@ const test_shouldConcat = t => {
 		t.assertEq(shouldConcat("a", "|"), false);
 		t.assertEq(shouldConcat("a", ")"), false);
 	});
+
+	t.testing("only consider the `next` char", () => {
+		t.assertEq(shouldConcat(null, "\\"), true);
+	});
 };
 
 const test_isOperator = t => {