ljkiwi - Free LuaJIT FFI and Lua C API kiwi (Cassowary derived) constraint solver.

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# Introduction

Kiwi is a reasonably efficient C++ implementation of the Cassowary constraint solving algorithm. It is an implementation of the algorithm as described in the paper ["The Cassowary Linear Arithmetic Constraint Solving Algorithm"](http://www.cs.washington.edu/research/constraints/cassowary/techreports/cassowaryTR.pdf) by Greg J. Badros and Alan Borning. The Kiwi implementation is not based on the original C++ implementation, but is a ground-up reimplementation with performance 10x to 500x faster in typical use.
Cassowary constraint solving is a technique that is particularly well suited to user interface layout. It is the algorithm Apple uses for iOS and OS X Auto Layout.

There are a few Lua implementations or attempts. The SILE typesetting system has a pure Lua implementation of the original Cassowary code, which appears to be correct but is quite slow. There are two extant Lua ports of Kiwi, one that is based on a C rewrite of Kiwi. However testing of these was not encouraging and they appear mostly unmaintained.
Since the C++ Kiwi library is well tested, it was simpler to provide a LuaJIT FFI wrapper. Now, there is also a Lua C API binding with support for 5.1 through 5.4, and since
in most common use cases all the heavy lifting is done in the library, there is usually
no practical perfomance difference between the two.
This package has no dependencies other than a supported C++14 compiler to compile the included Kiwi library and a small C wrapper.


## Example

```lua
local kiwi = require("kiwi")
local Var = kiwi.Var

local Button = setmetatable({}, {
   __call = function(_, identifier)
      return setmetatable({
         left = Var(identifier .. " left"),
         width = Var(identifier .. " width"),
      }, {
         __tostring = function(self)
            return "Button(" .. self.left:value() .. ", " .. self.width:value() .. ")"
         end,
      })
   end,
})

local b1 = Button("b1")
local b2 = Button("b2")

local left_edge = Var("left")
local right_edge = Var("width")

local STRONG = kiwi.Strength.STRONG

-- stylua: ignore start
local constraints = {
   left_edge    :eq(0.0),
   -- two buttons are the same width
   b1.width     :eq(b2.width),
   -- button1 starts 50 from the left margin
   b1.left      :eq(left_edge + 50),
   -- button2 ends 50 from the right margin
   right_edge   :eq(b2.left + b2.width + 50),
   -- button2 starts at least 100 from the end of button1. This is the "elastic" constraint
   b2.left      :ge(b1.left + b1.width + 100),
   -- button1 has a minimum width of 87
   b1.width     :ge(87),
   -- button1 has a preferred width of 87
   b1.width     :eq(87, STRONG),
   -- button2 has minimum width of 113
   b2.width     :ge(113),
   -- button2 has a preferred width of 113
   b2.width     :eq(113, STRONG),
}
-- stylua: ignore end

local solver = kiwi.Solver()

for _, c in ipairs(constraints) do
   solver:add_constraint(c)
end

solver:update_vars()

print(b1) -- Button(50, 113)
print(b2) -- Button(263, 113)
print(left_edge:value()) -- 0
print(right_edge:value()) -- 426

solver:add_edit_var(right_edge, STRONG)
solver:suggest_value(right_edge, 500)
solver:update_vars()
print(b1) -- Button(50, 113)
print(b2) -- Button(337, 113)
print(right_edge:value()) -- 500

```

In addition to the expression builder there is a convenience constraints submodule with: `pair_ratio`, `pair`, and `single` to allow efficient construction of the most common simple expression types for GUI layout.

## Documentation
The API is fully annotated and will work with lua-language-server. Documentation can also be generated with lua-language-server.