Add README

Author: mic-e

README.md

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+# Age of Empires II scrollbug fix
+
+## What is the scrollbug?
+
+Sometimes when you tab out of the game, after tabbing back in the game will be stuck in a state where it is constantly scrolling in some direction.
+
+This makes the game unplayable and requires a restart of the game. This is especially annoying in multiplayer games.
+
+The scrollbug happens mostly when running the game in Wine, but has supposedly also been observed on Windows.
+
+## How do I reliably reproduce the scrollbug?
+
+- Use Wine. I don't know if these steps work on Windows.
+- Start a game, e.g. a standard single-player random map game
+- Hold the left-arrow key for 3 seconds
+- Release the left-arrow key
+- Wait for one second
+- Tab out of the game
+- Wait for one second
+- Tab back into the game
+- Congratulations; you now have a scroll bug
+
+## Why is there a scrollbug?
+
+Age of Empires II uses the `GetKeyboardState()` and `GetKeyState()` Win32 API functions to determine whether one of the scrolling keys is pressed.
+
+In the result of these functions, the key stats are reported as a single byte.
+
+According to the MSDN documentation,
+
+- Bit 7 of the byte says whether the key is currently pressed
+- Bit 0 of the byte toggles each time the key is pressed
+- All other bits are not documented
+
+Age of Empires II wants to check whether the key is currently pressed. To do this it checks whether the byte value is `>= 1` instead of checking if Bit 7 is set (`& 0x80`).
+
+This works as long as bits 1..6 are zero.
+
+Unfortunately, wine seems to use bit 2 to provide some other metadata, obviously related to tabbing out of the window, so it doesn't guarantee that bits 1..6 are always 0.
+
+Windows also seems to sometimes use some of the other bits.
+
+## Which versions of Age of Empires II have the scrollbug?
+
+Every single one, i.e.:
+
+- AoK
+- AoC
+- Forgotten Empires
+- UserPatch
+- AoE2:HD
+- AoE2:DE
+
+## How does this fix work?
+
+This uses the Microsoft Research [Detours](https://github.com/microsoft/Detours) library to inject replacements of `GetKeyState()` and `GetKeyboardState()` into the running Age of Empires II binary.
+
+The replacement functions mask out bits 1..6 in the results.
+
+The fix consists of two binaries:
+
+- `age2_x1_fixed.exe` reads the configuration from `age2_x1_fixed.ini` and calls the `exe` that is specified in the `ini` file with the injector `dll` file that is specified in the `ini` file.
+- `fix_scrollbug32.dll` injects fixed versions of `GetKeyState()` and `GetKeyboardState()` into any 32-bit Windows executable it is used with.
+
+## How do I build this fix?
+
+- Use Windows (please, somebody port this to winegcc or mingw or _something_ that works on Linux)
+- Install Visual C++ Build Tools 2015
+- Get this source code (duh.)
+- Get `detours.lib` and `detours.h`, e.g. by building the Detours library from source by invoking `nmake`, or by downloading the binaries from this Github repo. Place these two files in a subfolder `detours/` of this source code
+- Call `build.bat`
+- You will now have a whole bunch of garbage files, plus
+  - `fix_scrollbug32.dll`
+  - `dll_inject.exe`
+  - `dll_inject.ini`
+
+## How do I use this fix?
+
+`dll_inject.exe` can be renamed to whatever name. It is recommended to rename it to `{name_of_the_exe_you_want_to_fix}_fixed.exe`.
+
+When launched, `dll_inject.exe` will
+
+- open the `.ini` file with the same name
+- in that `.ini` file in the section `[dll_inject]`, read the keys `exe_file` and `dll_file`
+- launch the specified `exe` file and inject the specified `dll` file
+- wait until the exe file has finished
+
+If you use the pre-compiled and pre-configured `age2_x1_fixed.exe`, `age2_x1_fixed.ini`, `fix_scrollbug32.dll` you can just place them in the `age2_x1` folder and launch `age2_x1_fixed.exe` instead of `age2_x1.exe`.