Python lookup table generation script

lookup_table_generator.py

import numpy as np
import matplotlib.pyplot as plt
from math import sin, pi, asin, tan, atan

def f(x):
    return ((1.0 - pow(x / 101325.0, 0.190284)) * 145366.45) * 0.3048


def finv(x):
    return 101325.0 * (1 - 2.25577 * 10 ** -5 * x) ** 5.25588

max_alt = 3048  # 10,000 feet
min_alt = -430  # -1400 feet (dead sea)

max = finv(min_alt)
min = finv(max_alt)
scale = 32767/f(min)
num_entries = 200

function_name = "pressure"

x = np.arange(min, max, (max - min) / num_entries)
lookup_table = [round(scale*f(i)) for i in x]

# Make sure we aren't going to overflow
for i in lookup_table:
    if abs(i) > 32767:
        print "going to overflow"


def fast_f(x):
    dx = (max - min) / num_entries
    global lookup_table
    t = (x - min) / (max - min) * num_entries
    index = int(t)
    dp = t - index

    if index >= num_entries:
        out = min
    elif index < num_entries - 1:
        out = lookup_table[index]/scale + dp * (lookup_table[index + 1] - lookup_table[index])/scale
    else:
        out = lookup_table[index]/scale + dp * (lookup_table[index] - lookup_table[index - 1])/scale

    return out

def fast_f_no_interp(x):
    dx = (max - min) / num_entries
    global lookup_table
    t = (x - min) / (max - min) * num_entries
    index = int(t)
    return lookup_table[index]


y = np.arange(min, max, (max - min)/(num_entries*1000))

fast = [fast_f(i) for i in y]
truth = [f(i) for i in y]

error = [abs(i - j) for i, j in zip(fast, truth)]

file = open('table.txt', 'w')
count = 0
file.write("static const float %s_max_x = %f;\n" % (function_name , max))
file.write("static const float %s_min_x = %f;\n" % (function_name , min))
file.write("static const float %s_scale_factor = %f;\n" % (function_name , scale))
file.write("static const int16_t %s_num_entries = %d;\n" % (function_name , num_entries))
file.write("static const int16_t %s_lookup_table[%s] = {\n" % (function_name , int(len(lookup_table))))
for i in lookup_table:
    count += 1
    if count == 10:
        file.write("%s,\n" % int(i))
        count = 0
    else:
        file.write("%s,\t" % int(i))
file.write("};")



plt.figure(1)
# plt.plot(y, [fast_alt_no_interp(i) for i in y])
plt.plot(y, [fast_f(i) for i in y], "o")
plt.plot(y, [f(i) for i in y])

plt.figure(2)
plt.plot(y, error)
plt.show()

Only had to change the equation within f to make this work right. I used the values from the wiki. Also added a conversion back to meters.
https://gist.github.com/len0rd/194cb419349d7025afba5bce53ff376a

or just this line:

def f(pressure):
    return ((1.0 - pow(pressure/101325.0, 0.190284)) * 145366.45) * 0.3048

Also added a few lines so that a comma isnt appended onto the final value in the array. This isn't a huge deal since it wasnt causing build errors before, more of an aesthetic thing I guess?

https://gist.github.com/len0rd/194cb419349d7025afba5bce53ff376a