✨ feat: Add _pop and _shift. Refactor pop and shift.

Fixes #47.

Author: make-github-pseudonymous-again

src/_pop.js

@@ -0,0 +1,16 @@
+import assert from 'assert';
+import Node from './Node.js';
+import _remove from './_remove.js';
+
+/**
+ * Removes last {@link Node} from a non-empty list.
+ *
+ * @param {Node} x First node (not null).
+ * @return {Node} New list (possibly null).
+ */
+export default function _pop(x) {
+	assert(x instanceof Node);
+	if (x === x.prev) return null;
+	_remove(x.prev);
+	return x;
+}

src/_shift.js

@@ -0,0 +1,16 @@
+import assert from 'assert';
+import Node from './Node.js';
+import _remove from './_remove.js';
+
+/**
+ * Removes first {@link Node} from a non-empty list.
+ *
+ * @param {Node} x First node (not null).
+ * @return {Node} New list (possibly null).
+ */
+export default function _shift(x) {
+	assert(x instanceof Node);
+	if (x === x.next) return null;
+	_remove(x);
+	return x.next;
+}

src/index.js

@@ -4,7 +4,9 @@ export {default as _concat} from './_concat.js';
 export {default as _iter} from './_iter.js';
 export {default as _iter_fast} from './_iter_fast.js';
 export {default as _len} from './_len.js';
+export {default as _pop} from './_pop.js';
 export {default as _remove} from './_remove.js';
+export {default as _shift} from './_shift.js';
 export {default as concat} from './concat.js';
 export {default as empty} from './empty.js';
 export {default as from} from './from.js';

src/pop.js

@@ -1,6 +1,6 @@
 import assert from 'assert';
 import Node from './Node.js';
-import _remove from './_remove.js';
+import _pop from './_pop.js';
 
 /**
  * Removes last {@link Node} from a list. Throws if input list is empty.
@@ -12,6 +12,5 @@ export default function pop(x) {
 	if (x === null) throw new Error('input list is empty');
 	assert(x instanceof Node);
 	const node = x.prev;
-	_remove(node);
-	return [x === node ? null : x, node];
+	return [_pop(x), node];
 }

src/shift.js

@@ -1,6 +1,6 @@
 import assert from 'assert';
 import Node from './Node.js';
-import _remove from './_remove.js';
+import _shift from './_shift.js';
 
 /**
  * Removes first {@link Node} from a list. Throws if input list is empty.
@@ -11,6 +11,5 @@ import _remove from './_remove.js';
 export default function shift(x) {
 	if (x === null) throw new Error('input list is empty');
 	assert(x instanceof Node);
-	_remove(x);
-	return [x === x.next ? null : x.next, x];
+	return [_shift(x), x];
 }