Improve IndexOfStartOf, add more tests for it

aannenko · Jan 3, 2025 · 7108065 · 7108065
1 parent fc2ab3f
commit 7108065
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Showing 2 changed files with 24 additions and 11 deletions.
diff --git a/src/Tests/TransmissionManager.TorrentWebPages.Tests/ReadOnlySpanExtensionsTests.cs b/src/Tests/TransmissionManager.TorrentWebPages.Tests/ReadOnlySpanExtensionsTests.cs
@@ -6,34 +6,42 @@ namespace TransmissionManager.TorrentWebPages.Tests;
 internal sealed class ReadOnlySpanExtensionsTests
 {
     [Test]
-    public void IndexOfStartOf_WhenGivenNormalSpanAndFullyExistingValue_ReturnsIndexOfValue() =>
+    public void IndexOfStartOf_GivenNormalSpanAndFullyExistingValue_ReturnsIndexOfValue() =>
         Assert.That("testing"u8.IndexOfStartOf("ing"u8), Is.EqualTo(4));
 
     [Test]
-    public void IndexOfStartOf_WhenGivenNormalSpanAndValueEqualToSpan_ReturnsIndexOfValue() =>
+    public void IndexOfStartOf_GivenNormalSpanAndValueEqualToSpan_ReturnsIndexOfValue() =>
         Assert.That("testing"u8.IndexOfStartOf("testing"u8), Is.EqualTo(0));
 
     [Test]
-    public void IndexOfStartOf_WhenGivenNormalSpanAndPartiallyExistingValue_ReturnsIndexOfValue() =>
+    public void IndexOfStartOf_GivenNormalSpanAndPartiallyExistingValue_ReturnsIndexOfValue() =>
         Assert.That("testin"u8.IndexOfStartOf("ing"u8), Is.EqualTo(4));
 
     [Test]
-    public void IndexOfStartOf_WhenGivenShorterSpanAndPartiallyExistingValue_ReturnsIndexOfValue() =>
+    public void IndexOfStartOf_GivenNormalSpanWithDuplicatesAndPartiallyExistingValue_ReturnsIndexOfValue() =>
+        Assert.That("aaaab"u8.IndexOfStartOf("aabc"u8), Is.EqualTo(2));
+
+    [Test]
+    public void IndexOfStartOf_GivenNormalSpanWithRepeatingPatternAndPartiallyExistingValue_ReturnsIndexOfValue() =>
+        Assert.That("ababa"u8.IndexOfStartOf("abac"u8), Is.EqualTo(2));
+
+    [Test]
+    public void IndexOfStartOf_GivenShorterSpanAndPartiallyExistingValue_ReturnsIndexOfValue() =>
         Assert.That("in"u8.IndexOfStartOf("ing"u8), Is.EqualTo(0));
 
     [Test]
-    public void IndexOfStartOf_WhenGivenNormalSpanAndEmptyValue_ReturnsIndexOfValue() =>
+    public void IndexOfStartOf_GivenNormalSpanAndEmptyValue_ReturnsIndexOfValue() =>
         Assert.That("testing"u8.IndexOfStartOf(""u8), Is.EqualTo(0));
 
     [Test]
-    public void IndexOfStartOf_WhenGivenEmptySpanAndEmptyValue_ReturnsIndexOfValue() =>
+    public void IndexOfStartOf_GivenEmptySpanAndEmptyValue_ReturnsIndexOfValue() =>
         Assert.That(""u8.IndexOfStartOf(""u8), Is.EqualTo(0));
 
     [Test]
-    public void IndexOfStartOf_WhenGivenNormalSpanAndNonExistingValue_ReturnsMinusOne() =>
+    public void IndexOfStartOf_GivenNormalSpanAndNonExistingValue_ReturnsMinusOne() =>
         Assert.That("testing"u8.IndexOfStartOf("asdfg"u8), Is.EqualTo(-1));
 
     [Test]
-    public void IndexOfStartOf_WhenGivenEmptySpanAndNonEmptyValue_ReturnsMinusOne() =>
+    public void IndexOfStartOf_GivenEmptySpanAndNonEmptyValue_ReturnsMinusOne() =>
         Assert.That(""u8.IndexOfStartOf("asdf"u8), Is.EqualTo(-1));
 }
diff --git a/src/TransmissionManager.TorrentWebPages/Extensions/ReadOnlySpanExtensions.cs b/src/TransmissionManager.TorrentWebPages/Extensions/ReadOnlySpanExtensions.cs
@@ -11,10 +11,15 @@ public static int IndexOfStartOf(this ReadOnlySpan<byte> span, ReadOnlySpan<byte
         if (index is not -1)
             return index;
 
-        for (var valueLength = Math.Min(span.Length, value.Length - 1); valueLength > 0; valueLength--)
+        for (var end = span[^Math.Min(span.Length, value.Length - 1)..]; end.Length > 0; end = end[1..])
         {
-            if (span.EndsWith(value[..valueLength]))
-                return span.Length - valueLength;
+            index = end.IndexOf(value[0]);
+            if (index is -1)
+                return -1;
+
+            end = end[index..];
+            if (end.SequenceEqual(value[..end.Length]))
+                return span.Length - end.Length;
         }
 
         return -1;