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README.md

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+ljkiwi - Free LuaJIT FFI 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 with either segfaults or incorrect results.
+Since the C++ Kiwi library is well tested and widely used it was simpler to provide a LuaJIT FFI wrapper and use that.
+This package has no dependencies other than a C++11 toolchain to compile the included Kiwi library and a small C wrapper.
+
+The Lua API has a pure Lua expression builder. There is of course some overhead to this, however in most cases expression building is infrequent and the underlying structures can be reused.
+
+The wrapper is quite close to the Kiwi C++/Python port with a few naming changes.
+
+## 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 are convenience constructors: `new_pair_ratio_constraint`, `new_pair_constraint`, and `new_single_constraint` to allow efficient construction of the most common simple expression types for GUI layout.
+
+## Documentation
+WIP - However the API is fully annotated and will work with lua-language-server. Documentation can also be generated with lua-language-server.