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+# Some useful building blocks
+
+Apps can be composed of a wide variety of building blocks that you
+can use in your functions, including a small number of functions that get
+automatically called for you as appropriate.
+
+## Variables you can read
+
+* `App.screen`
+  * `width` and `height` -- integer dimensions for the app window in pixels.
+  * `flags` -- some properties of the app window. See [`flags` in `love.graphics.getMode`](https://love2d.org/wiki/love.window.getMode)
+    for details.
+
+## Functions that get automatically called
+
+* `App.initialize_globals()` -- called before running each test and also
+  before the app starts up. As the name suggests, use this to initialize all
+  your global variables to something consistent. I also find it useful to be
+  able to see all my global variables in one place, and avoid defining
+  top-level variables anywhere else (unless they're constants and never going
+  to be modified).
+
+* `App.initialize(arg)` -- called when app starts up after
+  `App.initialize_globals`. Provides in `arg` an array of words typed in if
+  you ran it from a terminal window.
+  (Based on [LÖVE](https://love2d.org/wiki/love.load).)
+
+* `App.quit()` -- called before the app shuts down.
+  (Based on [LÖVE](https://love2d.org/wiki/love.quit).)
+
+* `App.focus(start?)` -- called when the app starts or stops receiving
+  keypresses. `start?` will be `true` when app starts receiving keypresses and
+  `false` when keypresses move to another window.
+  (Based on [LÖVE](https://love2d.org/wiki/love.focus).)
+
+* `App.resize(w,h)` -- called when you resize the app window. Provides new
+  window dimensions in `w` and `h`. Don't bother updating `App.screen.width`
+  and `App.screen.height`, that will happen automatically before calling
+  `on.resize`.
+  (Based on [LÖVE](https://love2d.org/wiki/love.resize))
+
+* `App.filedropped(file)` -- called when a file icon is dragged and dropped on
+  the app window. Provides in `file` an object representing the file that was
+  dropped, that will respond to the following messages:
+
+    * `file:getFilename()` returning a string name
+    * `file:read()` returning the entire file contents in a single string
+
+  (Based on [LÖVE](https://love2d.org/wiki/love.filedropped).)
+
+* `App.draw()` -- called to draw on the window, around 30 times a second.
+  (Based on [LÖVE](https://love2d.org/wiki/love.draw).)
+
+* `App.update(dt)` -- called after every call to `on.draw`. Make changes to
+  your app's variables here rather than in `on.draw`. Provides in `dt` the
+  time since the previous call to `on.update`, which can be useful for things
+  like smooth animations.
+  (Based on [LÖVE](https://love2d.org/wiki/love.update).)
+
+* `App.mousepressed(x,y, mouse_button)` -- called when you press down on a
+  mouse button. Provides in `x` and `y` the point on the screen at which the
+  click occurred, and in `mouse_button` an integer id of the mouse button
+  pressed.
+  `1` is the primary mouse button (the left button on a right-handed mouse),
+  `2` is the secondary button (the right button on a right-handed mouse),
+  and `3` is the middle button. Further buttons are mouse-dependent.
+  (Based on [LÖVE](https://love2d.org/wiki/love.mousepressed).)
+
+* `App.mousereleased(x,y, mouse_button)` -- called when you release a mouse
+  button. Provides the same arguments as `on.mouse_press()` above.
+  (Based on [LÖVE](https://love2d.org/wiki/love.mousereleased).)
+
+* `App.wheelmoved(dx,dy)` -- called when you use the scroll wheel on a mouse
+  that has it. Provides in `dx` and `dy` an indication of how fast the wheel
+  is being scrolled. Positive values for `dx` indicate movement to the right.
+  Positive values for `dy` indicate upward movement.
+  (Based on [LÖVE](https://love2d.org/wiki/love.wheelmoved).)
+
+* `App.keychord_press(chord, key)` -- called when you press a key-combination.
+  Provides in `key` a string name for the key most recently pressed ([valid
+  values](https://love2d.org/wiki/KeyConstant)). Provides in `chord` a
+  string representation of the current key combination, consisting of the key
+  with the following prefixes:
+    * `C-` if one of the `ctrl` keys is pressed,
+    * `M-` if one of the `alt` keys is pressed,
+    * `S-` if one of the `shift` keys is pressed, and
+    * `s-` if the `windows`/`cmd`/`super` key is pressed.
+
+* `App.textinput(t)` -- called when you press a key combination that yields
+  (roughly) a printable character. For example, `shift` and `a` pressed
+  together will call `on.textinput` with `A`.
+  (Based on [LÖVE](https://love2d.org/wiki/love.textinput).)
+
+* `App.keyrelease(key)` -- called when you press a key on the keyboard.
+  Provides in `key` a string name for the key ([valid values](https://love2d.org/wiki/KeyConstant)).
+  (Based on [LÖVE](https://love2d.org/wiki/love.keyreleased), including other
+  variants.)
+
+## Functions you can call
+
+Everything in the [LÖVE](https://love2d.org/wiki/Main_Page) and
+[Lua](https://www.lua.org/manual/5.1/manual.html) guides is available to you,
+but here's a brief summary of the most useful primitives. Some primitives have
+new, preferred names under the `App` namespace, often because these variants
+are more testable. If you run them within a test you'll be able to make
+assertions on their side-effects.
+
+### regarding the app window
+
+* `width, height, flags = App.screen.size()` -- returns the dimensions and
+  some properties of the app window.
+  (Based on [LÖVE](https://love2d.org/wiki/love.window.getMode).)
+
+* `App.screen.resize(width, height, flags)` -- modify the size and properties
+  of the app window. The OS may or may not act on the request.
+  (Based on [LÖVE](https://love2d.org/wiki/love.window.setMode).)
+
+* `x, y, displayindex = App.screen.position()` -- returns the coordinates and
+  monitor index (if you have more than one monitor) for the top-left corner of
+  the app window.
+  (Based on [LÖVE](https://love2d.org/wiki/love.window.getPosition).)
+
+* `App.screen.move(x, y, displayindex)` -- moves the app window so its
+  top-left corner is at the specified coordinates of the specified monitor.
+  The OS may or may not act on the request.
+  (Based on [LÖVE](https://love2d.org/wiki/love.window.setPosition).)
+
+### drawing to the app window
+
+* `App.screen.print(text, x,y)` -- print the given `text` in the current font
+  using the current color so its top-left corner is at the specified
+  coordinates of the app window.
+  (Based on [LÖVE](https://love2d.org/wiki/love.graphics.print).)
+
+* `love.graphics.getFont()` -- returns a representation of the current font.
+  (From [LÖVE](https://love2d.org/wiki/love.graphics.getFont).)
+
+* `love.graphics.setFont(font)` -- switches the current font to `font`.
+  (From [LÖVE](https://love2d.org/wiki/love.graphics.setFont).)
+
+* `love.graphics.newFont(filename)` -- creates a font from the given font
+  file.
+  (From [LÖVE](https://love2d.org/wiki/love.graphics.newFont), including other
+  variants.)
+
+* `App.width(text)` returns the width of `text` in pixels when rendered using
+  the current font.
+  (Based on [LÖVE](https://love2d.org/wiki/Font:getWidth).)
+
+* `App.color(color)` -- sets the current color based on the fields `r`, `g`,
+  `b` and `a` (for opacity) of the table `color`.
+  (Based on [LÖVE](https://love2d.org/wiki/love.graphics.setColor).)
+
+* `love.graphics.line(x1,y1, x2,y2)` -- draws a line from (`x1`,`y1`) to
+  (`x2`, `y2`) in the app window using the current color, clipping data for
+  negative coordinates and coordinates outside (`App.screen.width`,
+  `App.screen.height`)
+  (From [LÖVE](https://love2d.org/wiki/love.graphics.line), including other
+  variants.)
+
+* `love.graphics.rectangle(mode, x, y, w, h)` -- draws a rectangle using the
+  current color, with a top-left corner at (`x`, `y`), with dimensions `width`
+  along the x axis and `height` along the y axis
+  (though check out https://love2d.org/wiki/love.graphics for ways to scale
+  and rotate shapes).
+  `mode` is a string, either `'line'` (to draw just the outline) and `'fill'`.
+  (From [LÖVE](https://love2d.org/wiki/love.graphics.circle), including other
+  variants.)
+
+* `love.graphics.circle(mode, x, y, r)` -- draws a circle using the current
+  color, centered at (`x`, `y`) and with radius `r`.
+  `mode` is a string, either `'line'` and `'fill'`.
+  (From [LÖVE](https://love2d.org/wiki/love.graphics.circle), including other
+  variants.)
+
+* `love.graphics.arc(mode, x, y, r, angle1, angle2)` -- draws an arc of a
+  circle using the current color, centered at (`x`, `y`) and with radius `r`.
+  `mode` is a string, either `'line'` and `'fill'`.
+  `angle1` and `angle2` are in [radians](https://en.wikipedia.org/wiki/Radian).
+  (From [LÖVE](https://love2d.org/wiki/love.graphics.circle), including other
+  variants.)
+
+There's much more I could include here; check out [the LÖVE manual](https://love2d.org/wiki/love.graphics).
+
+### mouse primitives
+
+* `App.mouse_move(x, y)` -- sets the current position of the mouse to (`x`,
+  `y`).
+  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.setPosition).)
+
+* `App.mouse_down(mouse_button)` -- returns `true` if the button
+  `mouse_button` is pressed. See `on.mouse_press` for `mouse_button` codes.
+  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.isDown).)
+
+* `App.mouse_x()` -- returns the x coordinate of the current position of the
+  mouse.
+  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.getX).)
+
+* `App.mouse_y()` -- returns the x coordinate of the current position of the
+  mouse.
+  (Based on [LÖVE](https://love2d.org/wiki/love.mouse.getY).)
+
+### keyboard primitives
+
+* `App.modifier_down(key)` -- returns `true` if the given key (doesn't
+  actually have to be just a modifier) is currently pressed.
+  (Based on [LÖVE](https://love2d.org/wiki/love.keyboard.isDown).)
+
+### interacting with files
+
+* `App.open_for_reading(filename)` -- returns a file handle that you can
+  [`read()`](https://www.lua.org/manual/5.1/manual.html#pdf-file:read) from.
+  Make sure `filename` is an absolute path so that your app can work reliably
+  by double-clicking on it.
+  (Based on [Lua](https://www.lua.org/manual/5.1/manual.html#pdf-io.open).)
+
+* `App.open_for_writing(filename)` -- returns a file handle that you can
+  [`write()`](https://www.lua.org/manual/5.1/manual.html#pdf-file:write) to.
+  Make sure `filename` is an absolute path so that your app can work reliably
+  by double-clicking on it.
+  (Based on [Lua](https://www.lua.org/manual/5.1/manual.html#pdf-io.open).)
+
+* `json.encode(obj)` -- returns a JSON string for an object `obj` that will
+  recreate `obj` when passed to `json.decode`. `obj` can be of most types but
+  has some exceptions.
+  (From [json.lua](https://github.com/rxi/json.lua).)
+
+* `json.decode(obj)` -- turns a JSON string into a Lua object.
+  (From [json.lua](https://github.com/rxi/json.lua).)
+
+* `love.filesystem.getDirectoryItems(dir)` -- returns an unsorted array of the
+  files and directories available under `dir`. `dir` must be relative to
+  [LÖVE's save directory](https://love2d.org/wiki/love.filesystem.getSaveDirectory).
+  There is no easy, portable way in Lua/LÖVE to list directories outside the
+  save dir.
+  (From [LÖVE](https://love2d.org/wiki/love.filesystem.getDirectoryItems).]
+
+* `love.filesystem.getInfo(filename)` -- returns some information about
+  `filename`, particularly whether it exists (non-`nil` return value) or not.
+  `filename` must be relative to [LÖVE's save directory](https://love2d.org/wiki/love.filesystem.getSaveDirectory).
+  (From [LÖVE](https://love2d.org/wiki/love.filesystem.getInfo).]
+
+* `os.remove(filename)` -- removes a file or empty directory. Definitely make
+  sure `filename` is an absolute path.
+  (From [Lua](https://www.lua.org/manual/5.1/manual.html#pdf-os.remove).)
+
+There's much more I could include here; check out [the LÖVE manual](https://love2d.org/wiki/love.filesystem)
+and [the Lua manual](https://www.lua.org/manual/5.1/manual.html#5.7).
+
+### desiderata
+
+* `App.getTime()` -- returns the number of seconds elapsed since some
+  unspecified start time.
+  (Based on [LÖVE](https://love2d.org/wiki/love.timer.getTime).)
+
+* `App.getClipboardText()` -- returns a string with the current clipboard
+  contents.
+  (Based on [LÖVE](https://love2d.org/wiki/love.system.getClipboardText).)
+
+* `App.setClipboardText(text)` -- stores the string `text` in the clipboard.
+  (Based on [LÖVE](https://love2d.org/wiki/love.system.setClipboardText).)
+
+There's much more I could include here; check out [the LÖVE manual](https://love2d.org/wiki)
+and [the Lua manual](https://www.lua.org/manual/5.1/manual.html).