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

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" 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.

Casaurius is an excellent and performant Rust implementation of Kiwi.

This rock can produce a LuaJIT FFI binding for both the C++ and Rust implementations. The Rust implementation is not currently built by default. It can be built by setting the FRUST environment variable to any non-empty value. If the Rust implementation is built it will take precedence over the C++ implementation. The C++ implementation can be forced at runtime by setting the global KIWI_CKIWI prior to loading the kiwi package.

The C++ and Rust implementations provide a nearly semantically equivalent API. One difference is that the Rust implementation tracks constraints by value, not identity, so adding an equivalent constraint will raise an error while the C++ implementation does not notice this mistake.

There is also a Lua C API binding with support for 5.1 through 5.4. Currently only the C++ implementation is wrapped by the Lua C API binding.

This package has no mandatory dependencies other than a supported C++14 compiler to compile the included Kiwi library and a small C wrapper.

Example

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.