ReasonableRTF: Parsing gigabytes (sometimes) of RTF per second

So you're using C# and you want to convert some RTF to text. The solution is easy: You reach for the WinForms RichTextBox. Load your RTF in, access the Text property, and presto, it's all there. Mostly. Except smiley faces become the letter J. And sometimes non-ASCII text becomes gibberish even though old versions used to display it fine. And it's really, really slow. Also it leaks native memory.

You try the WPF version. Wait, did that one file take twenty-five seconds to load just because it had a 240x180 image in it?!

Forget it! You need something better. You need...

... the converter that's consistently over a hundred times faster than RichTextBox. 1.48 megs a second? That's unreasonable. 214 megs a second is slightly less unreasonable! That's like step 2½ out of 8 in Context is Everything!

Features

Wingdings 1, 2 and 3, Webdings, Symbol, and Zapf Dingbats all converted to equivalent Unicode characters.
Non-ASCII text correctly converted where RichTextBox can't.
Got huge files with tons of images? No problem. We blaze past image data so fast it may as well not exist.

Benchmarks


BenchmarkDotNet v0.13.12, Windows 10 (10.0.19045.3448/22H2/2022Update)
AMD Ryzen 9 3950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.204
  [Host]     : .NET 8.0.4 (8.0.424.16909), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.4 (8.0.424.16909), X64 RyuJIT AVX2

Method	Mean	Error	StdDev	Performance
RichTextBox_FullSet	5,556.20 ms	8.693 ms	8.132 ms	1x
ReasonableRTF_FullSet	42.31 ms	0.188 ms	0.176 ms	131x
RichTextBox_NoImageSet	2,389.84 ms	3.342 ms	3.126 ms	1x
ReasonableRTF_NoImageSet	16.52 ms	0.023 ms	0.019 ms	145x

Supported

All basic plain text, hex-encoded chars, Unicode-encoded chars
Symbol fonts (the abovementioned ones) converted to Unicode equivalents
Characters specified as "SYMBOL" field instructions
Undocumented use of the \langN keyword to specify character encoding - old versions of RichTextBox used to support this

Partially supported

Tables: Cells and rows have spaces between them, but not much functionality beyond that.
Lists: Numbers and bullets show up (that's better than RichTextBox most of the time), but indentation usually doesn't.

Not currently supported