skip single letter final blocks

src/UglyToad.PdfPig.Tests/Filters/Ascii85FilterTests.cs

@@ -34,10 +34,39 @@ public void DecodesWikipediaExample()
                 text);
         }
 
-        [Fact]
-        public void ReplacesZWithEmptyBytes()
+        [Theory]
+        [InlineData("BE", "h")]
+        [InlineData("BOq", "he")]
+        [InlineData("BOtu", "hel")]
+        [InlineData("BOu!r", "hell")]
+        [InlineData("BOu!rDZ", "hello")]
+        [InlineData("BOu!rD]f", "hello ")]
+        [InlineData("BOu!rD]j6", "hello w")]
+        [InlineData("BOu!rD]j7B", "hello wo")]
+        [InlineData("BOu!rD]j7BEW", "hello wor")]
+        [InlineData("BOu!rD]j7BEbk", "hello worl")]
+        [InlineData("BOu!rD]j7BEbo7", "hello world")]
+        [InlineData("BOu!rD]j7BEbo80", "hello world!")]
+        public void DecodesHelloWorld(string encoded, string decoded)
         {
-            var bytes = Encoding.ASCII.GetBytes("9jqo^zBlbD-");
+            var result = filter.Decode(
+                Encoding.ASCII.GetBytes(encoded),
+                dictionary,
+                TestFilterProvider.Instance,
+                0);
+
+            Assert.Equal(decoded, Encoding.ASCII.GetString(result.ToArray()));
+        }
+
+        [Theory]
+        [InlineData("9jqo^zBlbD-", "Man \0\0\0\0is d")]
+        [InlineData("", "")]
+        [InlineData("z", "\0\0\0\0")]
+        [InlineData("zz", "\0\0\0\0\0\0\0\0")]
+        [InlineData("zzz", "\0\0\0\0\0\0\0\0\0\0\0\0")]
+        public void ReplacesZWithEmptyBytes(string encoded, string decoded)
+        {
+            var bytes = Encoding.ASCII.GetBytes(encoded);
 
             var result = filter.Decode(bytes, dictionary, TestFilterProvider.Instance, 1);
 
@@ -47,7 +76,7 @@ public void ReplacesZWithEmptyBytes()
             string text = Encoding.ASCII.GetString(result.Span);
 #endif
 
-            Assert.Equal("Man \0\0\0\0is d", text);
+            Assert.Equal(decoded, text);
         }
 
         [Fact]
@@ -60,14 +89,17 @@ public void ZInMiddleOf5CharacterSequenceThrows()
             Assert.Throws<InvalidOperationException>(action);
         }
 
-        [Fact]
-        public void SingleCharacterLastThrows()
+        [Theory]
+        [InlineData("@rH:%B", "cool")]
+        [InlineData("A~>", "")]
+        [InlineData("@rH:%A~>", "cool")]
+        public void SingleCharacterLastIgnores(string encoded, string decoded)
         {
-            var bytes = Encoding.ASCII.GetBytes("9jqo^B");
+            var bytes = Encoding.ASCII.GetBytes(encoded);
 
-            Action action = () => filter.Decode(bytes, dictionary, TestFilterProvider.Instance, 1);
+            var result = filter.Decode(bytes, dictionary, TestFilterProvider.Instance, 1);
 
-            Assert.Throws<ArgumentOutOfRangeException>(action);
+            Assert.Equal(decoded, Encoding.ASCII.GetString(result.ToArray()));
         }
 
         private const string PdfContent = @"1 0 obj

src/UglyToad.PdfPig/Filters/Ascii85Filter.cs

@@ -2,6 +2,7 @@
 {
     using System;
     using Core;
+    using System.Text;
     using Tokens;
 
     /// <summary>
@@ -13,7 +14,7 @@ public sealed class Ascii85Filter : IFilter
         private const byte Offset = (byte)'!';
         private const byte EmptyCharacterPadding = (byte)'u';
 
-        private static ReadOnlySpan<byte> EndOfDataBytes => [(byte)'~', (byte)'>'];
+        private static ReadOnlySpan<byte> EndOfDataBytes => "~>"u8;
 
         private static readonly int[] PowerByIndex =
         [
@@ -52,7 +53,7 @@ public Memory<byte> Decode(Memory<byte> input, DictionaryToken streamDictionary,
                     {
                         if (index > 0)
                         {
-                            WriteData(asciiBuffer, index, writer);
+                            WriteData(asciiBuffer, index, writer, true);
                         }
 
                         index = 0;
@@ -88,24 +89,36 @@ public Memory<byte> Decode(Memory<byte> input, DictionaryToken streamDictionary,
 
                 if (index == 5)
                 {
-                    WriteData(asciiBuffer, index, writer);
+                    WriteData(asciiBuffer, index, writer, false);
                     index = 0;
                 }
             }
 
             if (index > 0)
             {
-                WriteData(asciiBuffer, index, writer);
+                WriteData(asciiBuffer, index, writer, true);
             }
 
             return writer.WrittenMemory.ToArray();
         }
 
-        private static void WriteData(Span<byte> ascii, int index, ArrayPoolBufferWriter<byte> writer)
+        private static void WriteData(
+            Span<byte> ascii,
+            int index,
+            ArrayPoolBufferWriter<byte> writer,
+            bool isAtEnd)
         {
             if (index < 2)
             {
-                throw new ArgumentOutOfRangeException(nameof(index), "Cannot convert a block padded by 4 'u' characters.");
+                if (isAtEnd)
+                {
+                    return;
+                }
+
+                var bufferTxt = Encoding.ASCII.GetString(ascii);
+                var soFar = Encoding.ASCII.GetString(writer.GetSpan());
+                throw new ArgumentOutOfRangeException(nameof(index),
+                    $"Cannot convert a this block because we're not at the end of the stream. Chunk: '{bufferTxt}'. Content: '{soFar}'");
             }
 
             // Write any empty padding if the block ended early.