love2d-tank/libs/windfield/mlib/mlib.lua

--[[ License
	A math library made in Lua
	copyright (C) 2014 Davis Claiborne
	This program is free software; you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation; either version 2 of the License, or
	(at your option) any later version.
	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU General Public License for more details.
	You should have received a copy of the GNU General Public License along
	with this program; if not, write to the Free Software Foundation, Inc.,
	51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
	Contact me at davisclaib@gmail.com
]]

-- Local Utility Functions ---------------------- {{{
local unpack = table.unpack or unpack

-- Used to handle variable-argument functions and whether they are passed as func{ table } or func( unpack( table ) )
local function checkInput(...)
	local input = {}
	if type(...) ~= "table" then
		input = { ... }
	else
		input = ...
	end
	return input
end

-- Deals with floats / verify false false values. This can happen because of significant figures.
local function checkFuzzy(number1, number2)
	return (number1 - 0.00001 <= number2 and number2 <= number1 + 0.00001)
end

-- Remove multiple occurrences from a table.
local function removeDuplicatePairs(tab)
	for index1 = #tab, 1, -1 do
		local first = tab[index1]
		for index2 = #tab, 1, -1 do
			local second = tab[index2]
			if index1 ~= index2 then
				if
					type(first[1]) == "number"
					and type(second[1]) == "number"
					and type(first[2]) == "number"
					and type(second[2]) == "number"
				then
					if checkFuzzy(first[1], second[1]) and checkFuzzy(first[2], second[2]) then
						table.remove(tab, index1)
					end
				elseif first[1] == second[1] and first[2] == second[2] then
					table.remove(tab, index1)
				end
			end
		end
	end
	return tab
end

local function removeDuplicates4Points(tab)
	for index1 = #tab, 1, -1 do
		local first = tab[index1]
		for index2 = #tab, 1, -1 do
			local second = tab[index2]
			if index1 ~= index2 then
				if type(first[1]) ~= type(second[1]) then
					return false
				end
				if
					type(first[2]) == "number"
					and type(second[2]) == "number"
					and type(first[3]) == "number"
					and type(second[3]) == "number"
				then
					if checkFuzzy(first[2], second[2]) and checkFuzzy(first[3], second[3]) then
						table.remove(tab, index1)
					end
				elseif
					checkFuzzy(first[1], second[1])
					and checkFuzzy(first[2], second[2])
					and checkFuzzy(first[3], second[3])
				then
					table.remove(tab, index1)
				end
			end
		end
	end
	return tab
end

-- Add points to the table.
local function addPoints(tab, x, y)
	tab[#tab + 1] = x
	tab[#tab + 1] = y
end

-- Like removeDuplicatePairs but specifically for numbers in a flat table
local function removeDuplicatePointsFlat(tab)
	for i = #tab, 1 - 2 do
		for ii = #tab - 2, 3, -2 do
			if i ~= ii then
				local x1, y1 = tab[i], tab[i + 1]
				local x2, y2 = tab[ii], tab[ii + 1]
				if checkFuzzy(x1, x2) and checkFuzzy(y1, y2) then
					table.remove(tab, ii)
					table.remove(tab, ii + 1)
				end
			end
		end
	end
	return tab
end

-- Check if input is actually a number
local function validateNumber(n)
	if type(n) ~= "number" then
		return false
	elseif n ~= n then
		return false -- nan
	elseif math.abs(n) == math.huge then
		return false
	else
		return true
	end
end

local function cycle(tab, index)
	return tab[(index - 1) % #tab + 1]
end

local function getGreatestPoint(points, offset)
	offset = offset or 1
	local start = 2 - offset
	local greatest = points[start]
	local least = points[start]
	for i = 2, #points / 2 do
		i = i * 2 - offset
		if points[i] > greatest then
			greatest = points[i]
		end
		if points[i] < least then
			least = points[i]
		end
	end
	return greatest, least
end

local function isWithinBounds(min, num, max)
	return num >= min and num <= max
end

local function distance2(x1, y1, x2, y2) -- Faster since it does not use math.sqrt
	local dx, dy = x1 - x2, y1 - y2
	return dx * dx + dy * dy
end -- }}}

-- Points -------------------------------------- {{{
local function rotatePoint(x, y, rotation, ox, oy)
	ox, oy = ox or 0, oy or 0
	return (x - ox) * math.cos(rotation) + ox - (y - oy) * math.sin(rotation),
		(x - ox) * math.sin(rotation) + (y - oy) * math.cos(rotation) + oy
end

local function scalePoint(x, y, scale, ox, oy)
	ox, oy = ox or 0, oy or 0
	return (x - ox) * scale + ox, (y - oy) * scale + oy
end
-- }}}

-- Lines --------------------------------------- {{{
-- Returns the length of a line.
local function getLength(x1, y1, x2, y2)
	local dx, dy = x1 - x2, y1 - y2
	return math.sqrt(dx * dx + dy * dy)
end

-- Gives the midpoint of a line.
local function getMidpoint(x1, y1, x2, y2)
	return (x1 + x2) / 2, (y1 + y2) / 2
end

-- Gives the slope of a line.
local function getSlope(x1, y1, x2, y2)
	if checkFuzzy(x1, x2) then
		return false
	end -- Technically it's undefined, but this is easier to program.
	return (y1 - y2) / (x1 - x2)
end

-- Gives the perpendicular slope of a line.
-- x1, y1, x2, y2
-- slope
local function getPerpendicularSlope(...)
	local input = checkInput(...)
	local slope

	if #input ~= 1 then
		slope = getSlope(unpack(input))
	else
		slope = unpack(input)
	end

	if not slope then
		return 0 -- Vertical lines become horizontal.
	elseif checkFuzzy(slope, 0) then
		return false -- Horizontal lines become vertical.
	else
		return -1 / slope
	end
end

-- Gives the y-intercept of a line.
-- x1, y1, x2, y2
-- x1, y1, slope
local function getYIntercept(x, y, ...)
	local input = checkInput(...)
	local slope

	if #input == 1 then
		slope = input[1]
	else
		slope = getSlope(x, y, unpack(input))
	end

	if not slope then
		return x, true
	end -- This way we have some information on the line.
	return y - slope * x, false
end

-- Gives the intersection of two lines.
-- slope1, slope2, x1, y1, x2, y2
-- slope1, intercept1, slope2, intercept2
-- x1, y1, x2, y2, x3, y3, x4, y4
local function getLineLineIntersection(...)
	local input = checkInput(...)
	local x1, y1, x2, y2, x3, y3, x4, y4
	local slope1, intercept1
	local slope2, intercept2
	local x, y

	if #input == 4 then -- Given slope1, intercept1, slope2, intercept2.
		slope1, intercept1, slope2, intercept2 = unpack(input)

		-- Since these are lines, not segments, we can use arbitrary points, such as ( 1, y ), ( 2, y )
		y1 = slope1 and slope1 * 1 + intercept1 or 1
		y2 = slope1 and slope1 * 2 + intercept1 or 2
		y3 = slope2 and slope2 * 1 + intercept2 or 1
		y4 = slope2 and slope2 * 2 + intercept2 or 2
		x1 = slope1 and (y1 - intercept1) / slope1 or intercept1
		x2 = slope1 and (y2 - intercept1) / slope1 or intercept1
		x3 = slope2 and (y3 - intercept2) / slope2 or intercept2
		x4 = slope2 and (y4 - intercept2) / slope2 or intercept2
	elseif #input == 6 then -- Given slope1, intercept1, and 2 points on the other line.
		slope1, intercept1 = input[1], input[2]
		slope2 = getSlope(input[3], input[4], input[5], input[6])
		intercept2 = getYIntercept(input[3], input[4], input[5], input[6])

		y1 = slope1 and slope1 * 1 + intercept1 or 1
		y2 = slope1 and slope1 * 2 + intercept1 or 2
		y3 = input[4]
		y4 = input[6]
		x1 = slope1 and (y1 - intercept1) / slope1 or intercept1
		x2 = slope1 and (y2 - intercept1) / slope1 or intercept1
		x3 = input[3]
		x4 = input[5]
	elseif #input == 8 then -- Given 2 points on line 1 and 2 points on line 2.
		slope1 = getSlope(input[1], input[2], input[3], input[4])
		intercept1 = getYIntercept(input[1], input[2], input[3], input[4])
		slope2 = getSlope(input[5], input[6], input[7], input[8])
		intercept2 = getYIntercept(input[5], input[6], input[7], input[8])

		x1, y1, x2, y2, x3, y3, x4, y4 = unpack(input)
	end

	if not slope1 and not slope2 then -- Both are vertical lines
		if x1 == x3 then -- Have to have the same x positions to intersect
			return true
		else
			return false
		end
	elseif not slope1 then -- First is vertical
		x = x1 -- They have to meet at this x, since it is this line's only x
		y = slope2 and slope2 * x + intercept2 or 1
	elseif not slope2 then -- Second is vertical
		x = x3 -- Vice-Versa
		y = slope1 * x + intercept1
	elseif checkFuzzy(slope1, slope2) then -- Parallel (not vertical)
		if checkFuzzy(intercept1, intercept2) then -- Same intercept
			return true
		else
			return false
		end
	else -- Regular lines
		x = (-intercept1 + intercept2) / (slope1 - slope2)
		y = slope1 * x + intercept1
	end

	return x, y
end

-- Gives the closest point on a line to a point.
-- perpendicularX, perpendicularY, x1, y1, x2, y2
-- perpendicularX, perpendicularY, slope, intercept
local function getClosestPoint(perpendicularX, perpendicularY, ...)
	local input = checkInput(...)
	local x, y, x1, y1, x2, y2, slope, intercept

	if #input == 4 then -- Given perpendicularX, perpendicularY, x1, y1, x2, y2
		x1, y1, x2, y2 = unpack(input)
		slope = getSlope(x1, y1, x2, y2)
		intercept = getYIntercept(x1, y1, x2, y2)
	elseif #input == 2 then -- Given perpendicularX, perpendicularY, slope, intercept
		slope, intercept = unpack(input)
		x1, y1 = 1, slope and slope * 1 + intercept or 1 -- Need x1 and y1 in case of vertical/horizontal lines.
	end

	if not slope then -- Vertical line
		x, y = x1, perpendicularY -- Closest point is always perpendicular.
	elseif checkFuzzy(slope, 0) then -- Horizontal line
		x, y = perpendicularX, y1
	else
		local perpendicularSlope = getPerpendicularSlope(slope)
		local perpendicularIntercept = getYIntercept(perpendicularX, perpendicularY, perpendicularSlope)
		x, y = getLineLineIntersection(slope, intercept, perpendicularSlope, perpendicularIntercept)
	end

	return x, y
end

-- Gives the intersection of a line and a line segment.
-- x1, y1, x2, y2, x3, y3, x4, y4
-- x1, y1, x2, y2, slope, intercept
local function getLineSegmentIntersection(x1, y1, x2, y2, ...)
	local input = checkInput(...)

	local slope1, intercept1, x, y, lineX1, lineY1, lineX2, lineY2
	local slope2, intercept2 = getSlope(x1, y1, x2, y2), getYIntercept(x1, y1, x2, y2)

	if #input == 2 then -- Given slope, intercept
		slope1, intercept1 = input[1], input[2]
		lineX1, lineY1 = 1, slope1 and slope1 + intercept1
		lineX2, lineY2 = 2, slope1 and slope1 * 2 + intercept1
	else -- Given x3, y3, x4, y4
		lineX1, lineY1, lineX2, lineY2 = unpack(input)
		slope1 = getSlope(unpack(input))
		intercept1 = getYIntercept(unpack(input))
	end

	if not slope1 and not slope2 then -- Vertical lines
		if checkFuzzy(x1, lineX1) then
			return x1, y1, x2, y2
		else
			return false
		end
	elseif not slope1 then -- slope1 is vertical
		x, y = input[1], slope2 * input[1] + intercept2
	elseif not slope2 then -- slope2 is vertical
		x, y = x1, slope1 * x1 + intercept1
	else
		x, y = getLineLineIntersection(slope1, intercept1, slope2, intercept2)
	end

	local length1, length2, distance
	if x == true then -- Lines are collinear.
		return x1, y1, x2, y2
	elseif x then -- There is an intersection
		length1, length2 = getLength(x1, y1, x, y), getLength(x2, y2, x, y)
		distance = getLength(x1, y1, x2, y2)
	else -- Lines are parallel but not collinear.
		if checkFuzzy(intercept1, intercept2) then
			return x1, y1, x2, y2
		else
			return false
		end
	end

	if length1 <= distance and length2 <= distance then
		return x, y
	else
		return false
	end
end

-- Checks if a point is on a line.
-- Does not support the format using slope because vertical lines would be impossible to check.
local function checkLinePoint(x, y, x1, y1, x2, y2)
	local m = getSlope(x1, y1, x2, y2)
	local b = getYIntercept(x1, y1, m)

	if not m then -- Vertical
		return checkFuzzy(x, x1)
	end
	return checkFuzzy(y, m * x + b)
end -- }}}

-- Segment -------------------------------------- {{{
-- Gives the perpendicular bisector of a line.
local function getPerpendicularBisector(x1, y1, x2, y2)
	local slope = getSlope(x1, y1, x2, y2)
	local midpointX, midpointY = getMidpoint(x1, y1, x2, y2)
	return midpointX, midpointY, getPerpendicularSlope(slope)
end

-- Gives whether or not a point lies on a line segment.
local function checkSegmentPoint(px, py, x1, y1, x2, y2)
	-- Explanation around 5:20: https://www.youtube.com/watch?v=A86COO8KC58
	local x = checkLinePoint(px, py, x1, y1, x2, y2)
	if not x then
		return false
	end

	local lengthX = x2 - x1
	local lengthY = y2 - y1

	if checkFuzzy(lengthX, 0) then -- Vertical line
		if checkFuzzy(px, x1) then
			local low, high
			if y1 > y2 then
				low = y2
				high = y1
			else
				low = y1
				high = y2
			end

			if py >= low and py <= high then
				return true
			else
				return false
			end
		else
			return false
		end
	elseif checkFuzzy(lengthY, 0) then -- Horizontal line
		if checkFuzzy(py, y1) then
			local low, high
			if x1 > x2 then
				low = x2
				high = x1
			else
				low = x1
				high = x2
			end

			if px >= low and px <= high then
				return true
			else
				return false
			end
		else
			return false
		end
	end

	local distanceToPointX = (px - x1)
	local distanceToPointY = (py - y1)
	local scaleX = distanceToPointX / lengthX
	local scaleY = distanceToPointY / lengthY

	if (scaleX >= 0 and scaleX <= 1) and (scaleY >= 0 and scaleY <= 1) then -- Intersection
		return true
	end
	return false
end

-- Gives the point of intersection between two line segments.
local function getSegmentSegmentIntersection(x1, y1, x2, y2, x3, y3, x4, y4)
	local slope1, intercept1 = getSlope(x1, y1, x2, y2), getYIntercept(x1, y1, x2, y2)
	local slope2, intercept2 = getSlope(x3, y3, x4, y4), getYIntercept(x3, y3, x4, y4)

	if ((slope1 and slope2) and checkFuzzy(slope1, slope2)) or (not slope1 and not slope2) then -- Parallel lines
		if checkFuzzy(intercept1, intercept2) then -- The same lines, possibly in different points.
			local points = {}
			if checkSegmentPoint(x1, y1, x3, y3, x4, y4) then
				addPoints(points, x1, y1)
			end
			if checkSegmentPoint(x2, y2, x3, y3, x4, y4) then
				addPoints(points, x2, y2)
			end
			if checkSegmentPoint(x3, y3, x1, y1, x2, y2) then
				addPoints(points, x3, y3)
			end
			if checkSegmentPoint(x4, y4, x1, y1, x2, y2) then
				addPoints(points, x4, y4)
			end

			points = removeDuplicatePointsFlat(points)
			if #points == 0 then
				return false
			end
			return unpack(points)
		else
			return false
		end
	end

	local x, y = getLineLineIntersection(x1, y1, x2, y2, x3, y3, x4, y4)
	if x and checkSegmentPoint(x, y, x1, y1, x2, y2) and checkSegmentPoint(x, y, x3, y3, x4, y4) then
		return x, y
	end
	return false
end -- }}}

-- Math ----------------------------------------- {{{
-- Get the root of a number (i.e. the 2nd (square) root of 4 is 2)
local function getRoot(number, root)
	return number ^ (1 / root)
end

-- Checks if a number is prime.
local function isPrime(number)
	if number < 2 then
		return false
	end

	for i = 2, math.sqrt(number) do
		if number % i == 0 then
			return false
		end
	end
	return true
end

-- Rounds a number to the xth decimal place (round( 3.14159265359, 4 ) --> 3.1416)
local function round(number, place)
	local pow = 10 ^ (place or 0)
	return math.floor(number * pow + 0.5) / pow
end

-- Gives the summation given a local function
local function getSummation(start, stop, func)
	local returnValues = {}
	local sum = 0
	for i = start, stop do
		local value = func(i, returnValues)
		returnValues[i] = value
		sum = sum + value
	end
	return sum
end

-- Gives the percent of change.
local function getPercentOfChange(old, new)
	if old == 0 and new == 0 then
		return 0
	else
		return (new - old) / math.abs(old)
	end
end

-- Gives the percentage of a number.
local function getPercentage(percent, number)
	return percent * number
end

-- Returns the quadratic roots of an equation.
local function getQuadraticRoots(a, b, c)
	local discriminant = b ^ 2 - (4 * a * c)
	if discriminant < 0 then
		return false
	end
	discriminant = math.sqrt(discriminant)
	local denominator = (2 * a)
	return (-b - discriminant) / denominator, (-b + discriminant) / denominator
end

-- Gives the angle between three points.
local function getAngle(x1, y1, x2, y2, x3, y3)
	local a = getLength(x3, y3, x2, y2)
	local b = getLength(x1, y1, x2, y2)
	local c = getLength(x1, y1, x3, y3)

	return math.acos((a * a + b * b - c * c) / (2 * a * b))
end -- }}}

-- Circle --------------------------------------- {{{
-- Gives the area of the circle.
local function getCircleArea(radius)
	return math.pi * (radius * radius)
end

-- Checks if a point is within the radius of a circle.
local function checkCirclePoint(x, y, circleX, circleY, radius)
	return getLength(circleX, circleY, x, y) <= radius
end

-- Checks if a point is on a circle.
local function isPointOnCircle(x, y, circleX, circleY, radius)
	return checkFuzzy(getLength(circleX, circleY, x, y), radius)
end

-- Gives the circumference of a circle.
local function getCircumference(radius)
	return 2 * math.pi * radius
end

-- Gives the intersection of a line and a circle.
local function getCircleLineIntersection(circleX, circleY, radius, x1, y1, x2, y2)
	slope = getSlope(x1, y1, x2, y2)
	intercept = getYIntercept(x1, y1, slope)

	if slope then
		local a = (1 + slope ^ 2)
		local b = (-2 * circleX + (2 * slope * intercept) - (2 * circleY * slope))
		local c = (circleX ^ 2 + intercept ^ 2 - 2 * circleY * intercept + circleY ^ 2 - radius ^ 2)

		x1, x2 = getQuadraticRoots(a, b, c)

		if not x1 then
			return false
		end

		y1 = slope * x1 + intercept
		y2 = slope * x2 + intercept

		if checkFuzzy(x1, x2) and checkFuzzy(y1, y2) then
			return "tangent", x1, y1
		else
			return "secant", x1, y1, x2, y2
		end
	else -- Vertical Lines
		local lengthToPoint1 = circleX - x1
		local remainingDistance = lengthToPoint1 - radius
		local intercept = math.sqrt(-(lengthToPoint1 ^ 2 - radius ^ 2))

		if -(lengthToPoint1 ^ 2 - radius ^ 2) < 0 then
			return false
		end

		local bottomX, bottomY = x1, circleY - intercept
		local topX, topY = x1, circleY + intercept

		if topY ~= bottomY then
			return "secant", topX, topY, bottomX, bottomY
		else
			return "tangent", topX, topY
		end
	end
end

-- Gives the type of intersection of a line segment.
local function getCircleSegmentIntersection(circleX, circleY, radius, x1, y1, x2, y2)
	local Type, x3, y3, x4, y4 = getCircleLineIntersection(circleX, circleY, radius, x1, y1, x2, y2)
	if not Type then
		return false
	end

	local slope, intercept = getSlope(x1, y1, x2, y2), getYIntercept(x1, y1, x2, y2)

	if isPointOnCircle(x1, y1, circleX, circleY, radius) and isPointOnCircle(x2, y2, circleX, circleY, radius) then -- Both points are on line-segment.
		return "chord", x1, y1, x2, y2
	end

	if slope then
		if
			checkCirclePoint(x1, y1, circleX, circleY, radius) and checkCirclePoint(x2, y2, circleX, circleY, radius)
		then -- Line-segment is fully in circle.
			return "enclosed", x1, y1, x2, y2
		elseif x3 and x4 then
			if checkSegmentPoint(x3, y3, x1, y1, x2, y2) and not checkSegmentPoint(x4, y4, x1, y1, x2, y2) then -- Only the first of the points is on the line-segment.
				return "tangent", x3, y3
			elseif checkSegmentPoint(x4, y4, x1, y1, x2, y2) and not checkSegmentPoint(x3, y3, x1, y1, x2, y2) then -- Only the second of the points is on the line-segment.
				return "tangent", x4, y4
			else -- Neither of the points are on the circle (means that the segment is not on the circle, but "encasing" the circle)
				if checkSegmentPoint(x3, y3, x1, y1, x2, y2) and checkSegmentPoint(x4, y4, x1, y1, x2, y2) then
					return "secant", x3, y3, x4, y4
				else
					return false
				end
			end
		elseif not x4 then -- Is a tangent.
			if checkSegmentPoint(x3, y3, x1, y1, x2, y2) then
				return "tangent", x3, y3
			else -- Neither of the points are on the line-segment (means that the segment is not on the circle or "encasing" the circle).
				local length = getLength(x1, y1, x2, y2)
				local distance1 = getLength(x1, y1, x3, y3)
				local distance2 = getLength(x2, y2, x3, y3)

				if length > distance1 or length > distance2 then
					return false
				elseif length < distance1 and length < distance2 then
					return false
				else
					return "tangent", x3, y3
				end
			end
		end
	else
		local lengthToPoint1 = circleX - x1
		local remainingDistance = lengthToPoint1 - radius
		local intercept = math.sqrt(-(lengthToPoint1 ^ 2 - radius ^ 2))

		if -(lengthToPoint1 ^ 2 - radius ^ 2) < 0 then
			return false
		end

		local topX, topY = x1, circleY - intercept
		local bottomX, bottomY = x1, circleY + intercept

		local length = getLength(x1, y1, x2, y2)
		local distance1 = getLength(x1, y1, topX, topY)
		local distance2 = getLength(x2, y2, topX, topY)

		if bottomY ~= topY then -- Not a tangent
			if
				checkSegmentPoint(topX, topY, x1, y1, x2, y2) and checkSegmentPoint(bottomX, bottomY, x1, y1, x2, y2)
			then
				return "chord", topX, topY, bottomX, bottomY
			elseif checkSegmentPoint(topX, topY, x1, y1, x2, y2) then
				return "tangent", topX, topY
			elseif checkSegmentPoint(bottomX, bottomY, x1, y1, x2, y2) then
				return "tangent", bottomX, bottomY
			else
				return false
			end
		else -- Tangent
			if checkSegmentPoint(topX, topY, x1, y1, x2, y2) then
				return "tangent", topX, topY
			else
				return false
			end
		end
	end
end

-- Checks if one circle intersects another circle.
local function getCircleCircleIntersection(circle1x, circle1y, radius1, circle2x, circle2y, radius2)
	local length = getLength(circle1x, circle1y, circle2x, circle2y)
	if length > radius1 + radius2 then
		return false
	end -- If the distance is greater than the two radii, they can't intersect.
	if checkFuzzy(length, 0) and checkFuzzy(radius1, radius2) then
		return "equal"
	end
	if checkFuzzy(circle1x, circle2x) and checkFuzzy(circle1y, circle2y) then
		return "collinear"
	end

	local a = (radius1 * radius1 - radius2 * radius2 + length * length) / (2 * length)
	local h = math.sqrt(radius1 * radius1 - a * a)

	local p2x = circle1x + a * (circle2x - circle1x) / length
	local p2y = circle1y + a * (circle2y - circle1y) / length
	local p3x = p2x + h * (circle2y - circle1y) / length
	local p3y = p2y - h * (circle2x - circle1x) / length
	local p4x = p2x - h * (circle2y - circle1y) / length
	local p4y = p2y + h * (circle2x - circle1x) / length

	if not validateNumber(p3x) or not validateNumber(p3y) or not validateNumber(p4x) or not validateNumber(p4y) then
		return "inside"
	end

	if checkFuzzy(length, radius1 + radius2) or checkFuzzy(length, math.abs(radius1 - radius2)) then
		return "tangent", p3x, p3y
	end
	return "intersection", p3x, p3y, p4x, p4y
end

-- Checks if circle1 is entirely inside of circle2.
local function isCircleCompletelyInsideCircle(circle1x, circle1y, circle1radius, circle2x, circle2y, circle2radius)
	if not checkCirclePoint(circle1x, circle1y, circle2x, circle2y, circle2radius) then
		return false
	end
	local Type = getCircleCircleIntersection(circle2x, circle2y, circle2radius, circle1x, circle1y, circle1radius)
	if Type ~= "tangent" and Type ~= "collinear" and Type ~= "inside" then
		return false
	end
	return true
end

-- Checks if a line-segment is entirely within a circle.
local function isSegmentCompletelyInsideCircle(circleX, circleY, circleRadius, x1, y1, x2, y2)
	local Type = getCircleSegmentIntersection(circleX, circleY, circleRadius, x1, y1, x2, y2)
	return Type == "enclosed"
end -- }}}

-- Polygon -------------------------------------- {{{
-- Gives the signed area.
-- If the points are clockwise the number is negative, otherwise, it's positive.
local function getSignedPolygonArea(...)
	local points = checkInput(...)

	-- Shoelace formula (https://en.wikipedia.org/wiki/Shoelace_formula).
	points[#points + 1] = points[1]
	points[#points + 1] = points[2]

	return (
		0.5
		* getSummation(1, #points / 2, function(index)
			index = index * 2 - 1 -- Convert it to work properly.
			return ((points[index] * cycle(points, index + 3)) - (cycle(points, index + 2) * points[index + 1]))
		end)
	)
end

-- Simply returns the area of the polygon.
local function getPolygonArea(...)
	return math.abs(getSignedPolygonArea(...))
end

-- Gives the height of a triangle, given the base.
-- base, x1, y1, x2, y2, x3, y3, x4, y4
-- base, area
local function getTriangleHeight(base, ...)
	local input = checkInput(...)
	local area

	if #input == 1 then
		area = input[1] -- Given area.
	else
		area = getPolygonArea(input)
	end -- Given coordinates.

	return (2 * area) / base, area
end

-- Gives the centroid of the polygon.
local function getCentroid(...)
	local points = checkInput(...)

	points[#points + 1] = points[1]
	points[#points + 1] = points[2]

	local area = getSignedPolygonArea(points) -- Needs to be signed here in case points are counter-clockwise.

	-- This formula: https://en.wikipedia.org/wiki/Centroid#Centroid_of_polygon
	local centroidX = (1 / (6 * area))
		* (
			getSummation(1, #points / 2, function(index)
				index = index * 2 - 1 -- Convert it to work properly.
				return (
					(points[index] + cycle(points, index + 2))
					* ((points[index] * cycle(points, index + 3)) - (cycle(points, index + 2) * points[index + 1]))
				)
			end)
		)

	local centroidY = (1 / (6 * area))
		* (
			getSummation(1, #points / 2, function(index)
				index = index * 2 - 1 -- Convert it to work properly.
				return (
					(points[index + 1] + cycle(points, index + 3))
					* ((points[index] * cycle(points, index + 3)) - (cycle(points, index + 2) * points[index + 1]))
				)
			end)
		)

	return centroidX, centroidY
end

-- Returns whether or not a line intersects a polygon.
-- x1, y1, x2, y2, polygonPoints
local function getPolygonLineIntersection(x1, y1, x2, y2, ...)
	local input = checkInput(...)
	local choices = {}

	local slope = getSlope(x1, y1, x2, y2)
	local intercept = getYIntercept(x1, y1, slope)

	local x3, y3, x4, y4
	if slope then
		x3, x4 = 1, 2
		y3, y4 = slope * x3 + intercept, slope * x4 + intercept
	else
		x3, x4 = x1, x1
		y3, y4 = y1, y2
	end

	for i = 1, #input, 2 do
		local x1, y1, x2, y2 =
			getLineSegmentIntersection(input[i], input[i + 1], cycle(input, i + 2), cycle(input, i + 3), x3, y3, x4, y4)
		if x1 and not x2 then
			choices[#choices + 1] = { x1, y1 }
		elseif x1 and x2 then
			choices[#choices + 1] = { x1, y1, x2, y2 }
		end
		-- No need to check 2-point sets since they only intersect each poly line once.
	end

	local final = removeDuplicatePairs(choices)
	return #final > 0 and final or false
end

-- Returns if the line segment intersects the polygon.
-- x1, y1, x2, y2, polygonPoints
local function getPolygonSegmentIntersection(x1, y1, x2, y2, ...)
	local input = checkInput(...)
	local choices = {}

	for i = 1, #input, 2 do
		local x1, y1, x2, y2 = getSegmentSegmentIntersection(
			input[i],
			input[i + 1],
			cycle(input, i + 2),
			cycle(input, i + 3),
			x1,
			y1,
			x2,
			y2
		)
		if x1 and not x2 then
			choices[#choices + 1] = { x1, y1 }
		elseif x2 then
			choices[#choices + 1] = { x1, y1, x2, y2 }
		end
	end

	local final = removeDuplicatePairs(choices)
	return #final > 0 and final or false
end

-- Checks if the point lies INSIDE the polygon not on the polygon.
local function checkPolygonPoint(px, py, ...)
	local points = { unpack(checkInput(...)) } -- Make a new table, as to not edit values of previous.

	local greatest, least = getGreatestPoint(points, 0)
	if not isWithinBounds(least, py, greatest) then
		return false
	end
	greatest, least = getGreatestPoint(points)
	if not isWithinBounds(least, px, greatest) then
		return false
	end

	local count = 0
	for i = 1, #points, 2 do
		if checkFuzzy(points[i + 1], py) then
			points[i + 1] = py + 0.001 -- Handles vertices that lie on the point.
			-- Not exactly mathematically correct, but a lot easier.
		end
		if points[i + 3] and checkFuzzy(points[i + 3], py) then
			points[i + 3] = py + 0.001 -- Do not need to worry about alternate case, since points[2] has already been done.
		end
		local x1, y1 = points[i], points[i + 1]
		local x2, y2 = points[i + 2] or points[1], points[i + 3] or points[2]

		if getSegmentSegmentIntersection(px, py, greatest, py, x1, y1, x2, y2) then
			count = count + 1
		end
	end

	return count and count % 2 ~= 0
end

-- Returns if the line segment is fully or partially inside.
-- x1, y1, x2, y2, polygonPoints
local function isSegmentInsidePolygon(x1, y1, x2, y2, ...)
	local input = checkInput(...)

	local choices = getPolygonSegmentIntersection(x1, y1, x2, y2, input) -- If it's partially enclosed that's all we need.
	if choices then
		return true
	end

	if checkPolygonPoint(x1, y1, input) or checkPolygonPoint(x2, y2, input) then
		return true
	end
	return false
end

-- Returns whether two polygons intersect.
local function getPolygonPolygonIntersection(polygon1, polygon2)
	local choices = {}

	for index1 = 1, #polygon1, 2 do
		local intersections = getPolygonSegmentIntersection(
			polygon1[index1],
			polygon1[index1 + 1],
			cycle(polygon1, index1 + 2),
			cycle(polygon1, index1 + 3),
			polygon2
		)
		if intersections then
			for index2 = 1, #intersections do
				choices[#choices + 1] = intersections[index2]
			end
		end
	end

	for index1 = 1, #polygon2, 2 do
		local intersections = getPolygonSegmentIntersection(
			polygon2[index1],
			polygon2[index1 + 1],
			cycle(polygon2, index1 + 2),
			cycle(polygon2, index1 + 3),
			polygon1
		)
		if intersections then
			for index2 = 1, #intersections do
				choices[#choices + 1] = intersections[index2]
			end
		end
	end

	choices = removeDuplicatePairs(choices)
	for i = #choices, 1, -1 do
		if type(choices[i][1]) == "table" then -- Remove co-linear pairs.
			table.remove(choices, i)
		end
	end

	return #choices > 0 and choices
end

-- Returns whether the circle intersects the polygon.
-- x, y, radius, polygonPoints
local function getPolygonCircleIntersection(x, y, radius, ...)
	local input = checkInput(...)
	local choices = {}

	for i = 1, #input, 2 do
		local Type, x1, y1, x2, y2 =
			getCircleSegmentIntersection(x, y, radius, input[i], input[i + 1], cycle(input, i + 2), cycle(input, i + 3))
		if x2 then
			choices[#choices + 1] = { Type, x1, y1, x2, y2 }
		elseif x1 then
			choices[#choices + 1] = { Type, x1, y1 }
		end
	end

	local final = removeDuplicates4Points(choices)

	return #final > 0 and final
end

-- Returns whether the circle is inside the polygon.
-- x, y, radius, polygonPoints
local function isCircleInsidePolygon(x, y, radius, ...)
	local input = checkInput(...)
	return checkPolygonPoint(x, y, input)
end

-- Returns whether the polygon is inside the polygon.
local function isPolygonInsidePolygon(polygon1, polygon2)
	local bool = false
	for i = 1, #polygon2, 2 do
		local result = false
		result = isSegmentInsidePolygon(
			polygon2[i],
			polygon2[i + 1],
			cycle(polygon2, i + 2),
			cycle(polygon2, i + 3),
			polygon1
		)
		if result then
			bool = true
			break
		end
	end
	return bool
end

-- Checks if a segment is completely inside a polygon
local function isSegmentCompletelyInsidePolygon(x1, y1, x2, y2, ...)
	local polygon = checkInput(...)
	if
		not checkPolygonPoint(x1, y1, polygon)
		or not checkPolygonPoint(x2, y2, polygon)
		or getPolygonSegmentIntersection(x1, y1, x2, y2, polygon)
	then
		return false
	end
	return true
end

-- Checks if a polygon is completely inside another polygon
local function isPolygonCompletelyInsidePolygon(polygon1, polygon2)
	for i = 1, #polygon1, 2 do
		local x1, y1 = polygon1[i], polygon1[i + 1]
		local x2, y2 = polygon1[i + 2] or polygon1[1], polygon1[i + 3] or polygon1[2]
		if not isSegmentCompletelyInsidePolygon(x1, y1, x2, y2, polygon2) then
			return false
		end
	end
	return true
end

-------------- Circle w/ Polygons --------------
-- Gets if a polygon is completely within a circle
-- circleX, circleY, circleRadius, polygonPoints
local function isPolygonCompletelyInsideCircle(circleX, circleY, circleRadius, ...)
	local input = checkInput(...)
	local function isDistanceLess(px, py, x, y, circleRadius) -- Faster, does not use math.sqrt
		local distanceX, distanceY = px - x, py - y
		return distanceX * distanceX + distanceY * distanceY < circleRadius * circleRadius -- Faster. For comparing distances only.
	end

	for i = 1, #input, 2 do
		if not checkCirclePoint(input[i], input[i + 1], circleX, circleY, circleRadius) then
			return false
		end
	end
	return true
end

-- Checks if a circle is completely within a polygon
-- circleX, circleY, circleRadius, polygonPoints
local function isCircleCompletelyInsidePolygon(circleX, circleY, circleRadius, ...)
	local input = checkInput(...)
	if not checkPolygonPoint(circleX, circleY, ...) then
		return false
	end

	local rad2 = circleRadius * circleRadius

	for i = 1, #input, 2 do
		local x1, y1 = input[i], input[i + 1]
		local x2, y2 = input[i + 2] or input[1], input[i + 3] or input[2]
		if distance2(x1, y1, circleX, circleY) <= rad2 then
			return false
		end
		if getCircleSegmentIntersection(circleX, circleY, circleRadius, x1, y1, x2, y2) then
			return false
		end
	end
	return true
end -- }}}

-- Statistics ----------------------------------- {{{
-- Gets the average of a list of points
-- points
local function getMean(...)
	local input = checkInput(...)

	mean = getSummation(1, #input, function(i, t)
		return input[i]
	end) / #input

	return mean
end

local function getMedian(...)
	local input = checkInput(...)

	table.sort(input)

	local median
	if #input % 2 == 0 then -- If you have an even number of terms, you need to get the average of the middle 2.
		median = getMean(input[#input / 2], input[#input / 2 + 1])
	else
		median = input[#input / 2 + 0.5]
	end

	return median
end

-- Gets the mode of a number.
local function getMode(...)
	local input = checkInput(...)

	table.sort(input)
	local sorted = {}
	for i = 1, #input do
		local value = input[i]
		sorted[value] = sorted[value] and sorted[value] + 1 or 1
	end

	local occurrences, least = 0, {}
	for i, value in pairs(sorted) do
		if value > occurrences then
			least = { i }
			occurrences = value
		elseif value == occurrences then
			least[#least + 1] = i
		end
	end

	if #least >= 1 then
		return least, occurrences
	else
		return false
	end
end

-- Gets the range of the numbers.
local function getRange(...)
	local input = checkInput(...)
	local high, low = math.max(unpack(input)), math.min(unpack(input))
	return high - low
end

-- Gets the variance of a set of numbers.
local function getVariance(...)
	local input = checkInput(...)
	local mean = getMean(...)
	local sum = 0
	for i = 1, #input do
		sum = sum + (mean - input[i]) * (mean - input[i])
	end
	return sum / #input
end

-- Gets the standard deviation of a set of numbers.
local function getStandardDeviation(...)
	return math.sqrt(getVariance(...))
end

-- Gets the central tendency of a set of numbers.
local function getCentralTendency(...)
	local mode, occurrences = getMode(...)
	return mode, occurrences, getMedian(...), getMean(...)
end

-- Gets the variation ratio of a data set.
local function getVariationRatio(...)
	local input = checkInput(...)
	local numbers, times = getMode(...)
	times = times * #numbers -- Account for bimodal data
	return 1 - (times / #input)
end

-- Gets the measures of dispersion of a data set.
local function getDispersion(...)
	return getVariationRatio(...), getRange(...), getStandardDeviation(...)
end -- }}}

return {
	_VERSION = "MLib 0.10.0",
	_DESCRIPTION = "A math and shape-intersection detection library for Lua",
	_URL = "https://github.com/davisdude/mlib",
	point = {
		rotate = rotatePoint,
		scale = scalePoint,
	},
	line = {
		getLength = getLength,
		getMidpoint = getMidpoint,
		getSlope = getSlope,
		getPerpendicularSlope = getPerpendicularSlope,
		getYIntercept = getYIntercept,
		getIntersection = getLineLineIntersection,
		getClosestPoint = getClosestPoint,
		getSegmentIntersection = getLineSegmentIntersection,
		checkPoint = checkLinePoint,

		-- Aliases
		getDistance = getLength,
		getCircleIntersection = getCircleLineIntersection,
		getPolygonIntersection = getPolygonLineIntersection,
		getLineIntersection = getLineLineIntersection,
	},
	segment = {
		checkPoint = checkSegmentPoint,
		getPerpendicularBisector = getPerpendicularBisector,
		getIntersection = getSegmentSegmentIntersection,

		-- Aliases
		getCircleIntersection = getCircleSegmentIntersection,
		getPolygonIntersection = getPolygonSegmentIntersection,
		getLineIntersection = getLineSegmentIntersection,
		getSegmentIntersection = getSegmentSegmentIntersection,
		isSegmentCompletelyInsideCircle = isSegmentCompletelyInsideCircle,
		isSegmentCompletelyInsidePolygon = isSegmentCompletelyInsidePolygon,
	},
	math = {
		getRoot = getRoot,
		isPrime = isPrime,
		round = round,
		getSummation = getSummation,
		getPercentOfChange = getPercentOfChange,
		getPercentage = getPercentage,
		getQuadraticRoots = getQuadraticRoots,
		getAngle = getAngle,
	},
	circle = {
		getArea = getCircleArea,
		checkPoint = checkCirclePoint,
		isPointOnCircle = isPointOnCircle,
		getCircumference = getCircumference,
		getLineIntersection = getCircleLineIntersection,
		getSegmentIntersection = getCircleSegmentIntersection,
		getCircleIntersection = getCircleCircleIntersection,
		isCircleCompletelyInside = isCircleCompletelyInsideCircle,
		isPolygonCompletelyInside = isPolygonCompletelyInsideCircle,
		isSegmentCompletelyInside = isSegmentCompletelyInsideCircle,

		-- Aliases
		getPolygonIntersection = getPolygonCircleIntersection,
		isCircleInsidePolygon = isCircleInsidePolygon,
		isCircleCompletelyInsidePolygon = isCircleCompletelyInsidePolygon,
	},
	polygon = {
		getSignedArea = getSignedPolygonArea,
		getArea = getPolygonArea,
		getTriangleHeight = getTriangleHeight,
		getCentroid = getCentroid,
		getLineIntersection = getPolygonLineIntersection,
		getSegmentIntersection = getPolygonSegmentIntersection,
		checkPoint = checkPolygonPoint,
		isSegmentInside = isSegmentInsidePolygon,
		getPolygonIntersection = getPolygonPolygonIntersection,
		getCircleIntersection = getPolygonCircleIntersection,
		isCircleInside = isCircleInsidePolygon,
		isPolygonInside = isPolygonInsidePolygon,
		isCircleCompletelyInside = isCircleCompletelyInsidePolygon,
		isSegmentCompletelyInside = isSegmentCompletelyInsidePolygon,
		isPolygonCompletelyInside = isPolygonCompletelyInsidePolygon,

		-- Aliases
		isCircleCompletelyOver = isPolygonCompletelyInsideCircle,
	},
	statistics = {
		getMean = getMean,
		getMedian = getMedian,
		getMode = getMode,
		getRange = getRange,
		getVariance = getVariance,
		getStandardDeviation = getStandardDeviation,
		getCentralTendency = getCentralTendency,
		getVariationRatio = getVariationRatio,
		getDispersion = getDispersion,
	},
}