From 98b540ea48d90631a3d6e442b1f73a8b3ddf2d27 Mon Sep 17 00:00:00 2001
From: Ming Ming <nkming2@gmail.com>
Date: Sun, 2 Jan 2022 01:25:50 +0800
Subject: [PATCH] Slice lists

---
 lib/list_extension.dart       | 20 +++++++++++
 test/list_extension_test.dart | 65 +++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)

diff --git a/lib/list_extension.dart b/lib/list_extension.dart
index edc10403..f9d4d764 100644
--- a/lib/list_extension.dart
+++ b/lib/list_extension.dart
@@ -1,3 +1,23 @@
+import 'dart:math' as math;
+
 extension ListExtension<T> on List<T> {
   Iterable<T> takeIndex(List<int> indexes) => indexes.map((e) => this[e]);
+
+  List<T> slice(int start, [int? stop]) {
+    if (start < 0) {
+      start = math.max(length + start, 0);
+    }
+    if (stop != null && stop < 0) {
+      stop = math.max(length + stop, 0);
+    }
+    if (start >= length) {
+      return [];
+    } else if (stop == null) {
+      return sublist(start);
+    } else if (start >= stop) {
+      return [];
+    } else {
+      return sublist(start, math.min(stop, length));
+    }
+  }
 }
diff --git a/test/list_extension_test.dart b/test/list_extension_test.dart
index b798a9b5..59161dac 100644
--- a/test/list_extension_test.dart
+++ b/test/list_extension_test.dart
@@ -6,5 +6,70 @@ void main() {
     test("takeIndex", () {
       expect([1, 2, 3, 4, 5, 6].takeIndex([5, 4, 3, 1, 0]), [6, 5, 4, 2, 1]);
     });
+
+    group("slice", () {
+      // [1, ..., 9]
+      final list = List.generate(9, (i) => i + 1);
+
+      /// Expected: [4, ..., 9]
+      test("+start", () {
+        expect(list.slice(3), List.generate(6, (i) => i + 4));
+      });
+
+      /// Expected: []
+      test("+start > length", () {
+        expect(list.slice(999), const []);
+      });
+
+      /// Expected: [4, 5]
+      test("+start +stop", () {
+        expect(list.slice(3, 5), const [4, 5]);
+      });
+
+      /// Expected: [4, ..., 9]
+      test("+start +stop > length", () {
+        expect(list.slice(3, 999), List.generate(6, (i) => i + 4));
+      });
+
+      /// Expected: []
+      test("+start > +stop", () {
+        expect(list.slice(5, 3), const []);
+      });
+
+      /// Expected: [5, ..., 9]
+      test("-start", () {
+        expect(list.slice(-5), List.generate(5, (i) => i + 5));
+      });
+
+      /// Expected: [1, ..., 9]
+      test("-start < -length", () {
+        expect(list.slice(-999), List.generate(9, (i) => i + 1));
+      });
+
+      /// Expected: [5, 6]
+      test("-start -stop", () {
+        expect(list.slice(-5, -3), const [5, 6]);
+      });
+
+      /// Expected: []
+      test("-start -stop < -length", () {
+        expect(list.slice(-5, -999), const []);
+      });
+
+      /// Expected: []
+      test("-start < -stop", () {
+        expect(list.slice(-3, -5), const []);
+      });
+
+      /// Expected: [4]
+      test("+start -stop", () {
+        expect(list.slice(3, -5), [4]);
+      });
+
+      /// Expected: [5, ..., 9]
+      test("-start +stop", () {
+        expect(list.slice(-5, 9), List.generate(5, (i) => i + 5));
+      });
+    });
   });
 }