From 7ff3416737a5a05242114945c6ddc34393ddfd54 Mon Sep 17 00:00:00 2001
From: tenplus1 <kinsellaja@yahoo.com>
Date: Wed, 20 May 2015 10:22:04 +0100
Subject: NEW growing routine added

---
 statistics.lua | 150 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 150 insertions(+)
 create mode 100644 statistics.lua

(limited to 'statistics.lua')

diff --git a/statistics.lua b/statistics.lua
new file mode 100644
index 0000000..aaa4f51
--- /dev/null
+++ b/statistics.lua
@@ -0,0 +1,150 @@
+local statistics = {}
+
+local ROOT_2 = math.sqrt(2.0)
+
+-- Approximations for erf(x) and erfInv(x) from
+--    https://en.wikipedia.org/wiki/Error_function
+local erf
+local erf_inv
+
+local A = 8 * (math.pi - 3.0)/(3.0 * math.pi * (4.0 - math.pi))
+local B = 4.0 / math.pi
+local C = 2.0/(math.pi * A)
+local D = 1.0 / A
+
+erf = function(x)
+	if x == 0 then return 0; end
+	local xSq  = x * x
+	local aXSq = A * xSq
+	local v = math.sqrt(1.0 - math.exp(-xSq * (B + aXSq)/(1.0 + aXSq)))
+	return (x > 0 and v) or -v
+end
+
+erf_inv = function(x)
+	if x == 0 then return 0; end
+	if x <= -1 or x >= 1 then return nil; end
+	local y = math.log(1 - x * x)
+	local u = C + 0.5 * y
+	local v = math.sqrt(math.sqrt(u * u - D * y) - u)
+	return (x > 0 and v) or -v
+end
+
+local function std_normal(u)
+	return ROOT_2 * erf_inv(2.0 * u - 1.0)
+end
+
+local poisson
+local cdf_table = {}
+
+local function generate_cdf(lambda_index, lambda)
+	local max = math.ceil(4 * lambda)
+	local pdf = math.exp(-lambda)
+	local cdf = pdf
+	local t = { [0] = pdf }
+
+	for i = 1, max - 1 do
+		pdf = pdf * lambda / i
+		cdf = cdf + pdf
+		t[i] = cdf
+	end
+
+	return t
+end
+
+for li = 1, 100 do
+	cdf_table[li] = generate_cdf(li, 0.25 * li)
+end
+
+poisson = function(lambda, max)
+	if max < 2 then
+		return (math.random() < math.exp(-lambda) and 0) or 1
+	elseif lambda >= 2 * max then
+		return max
+	end
+
+	local u = math.random()
+	local lambda_index = math.floor(4 * lambda + 0.5)
+	local cdfs = cdf_table[lambda_index]
+
+	if cdfs then
+		lambda = 0.25 * lambda_index
+
+		if u < cdfs[0] then return 0; end
+		if max > #cdfs then max = #cdfs + 1 else max = math.floor(max); end
+		if u >= cdfs[max - 1] then return max; end
+
+		if max > 4 then  -- Binary search
+			local s = 0
+			while s + 1 < max do
+				local m = math.floor(0.5 * (s + max))
+				if u < cdfs[m] then max = m; else s = m; end
+			end
+		else
+			for i = 1, max - 1 do
+				if u < cdfs[i] then return i; end
+			end
+		end
+
+		return max
+	else
+		local x = lambda + math.sqrt(lambda) * std_normal(u)
+		return (x < 0.5 and 0) or (x >= max - 0.5 and max) or math.floor(x + 0.5)
+	end
+end
+
+-- Error function.
+statistics.erf = erf
+
+-- Inverse error function.
+statistics.erf_inv = erf_inv
+
+--- Standard normal distribution function (mean 0, standard deviation 1).
+ --
+ -- @return
+ --    Any real number (actually between -3.0 and 3.0).
+ --
+statistics.std_normal = function()
+	local u = math.random()
+	if u < 0.001 then
+		return -3.0
+	elseif u > 0.999 then
+		return 3.0
+	end
+	return std_normal(u)
+end
+
+--- Standard normal distribution function (mean 0, standard deviation 1).
+ --
+ -- @param mu
+ --    The distribution mean.
+ -- @param sigma
+ --    The distribution standard deviation.
+ -- @return
+ --    Any real number (actually between -3*sigma and 3*sigma).
+ --
+statistics.normal = function(mu, sigma)
+	local u = math.random()
+	if u < 0.001 then
+		return mu - 3.0 * sigma
+	elseif u > 0.999 then
+		return mu + 3.0 * sigma
+	end
+	return mu + sigma * std_normal(u)
+end
+
+--- Poisson distribution function.
+ --
+ -- @param lambda
+ --    The distribution mean and variance.
+ -- @param max
+ --    The distribution maximum.
+ -- @return
+ --    An integer between 0 and max (both inclusive).
+ --
+statistics.poisson = function(lambda, max)
+	lambda, max = tonumber(lambda), tonumber(max)
+	if not lambda or not max or lambda <= 0 or max < 1 then return 0; end
+	return poisson(lambda, max)
+end
+
+return statistics
-- 
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