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solve.cal
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1995-07-17
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/*
* Copyright (c) 1993 David I. Bell
* Permission is granted to use, distribute, or modify this source,
* provided that this copyright notice remains intact.
*
* Solve the equation f(x) = 0 to within the desired error value for x.
* The function 'f' must be defined outside of this routine, and the low
* and high values are guesses which must produce values with opposite signs.
*/
define solve(low, high, epsilon)
{
local flow, fhigh, fmid, mid, places;
if (isnull(epsilon))
epsilon = epsilon();
if (epsilon <= 0)
quit "Non-positive epsilon value";
places = highbit(1 + int(1/epsilon)) + 1;
flow = f(low);
if (abs(flow) < epsilon)
return low;
fhigh = f(high);
if (abs(flow) < epsilon)
return high;
if (sgn(flow) == sgn(fhigh))
quit "Non-opposite signs";
while (1) {
mid = bround(high - fhigh * (high - low) / (fhigh - flow), places);
if ((mid == low) || (mid == high))
places++;
fmid = f(mid);
if (abs(fmid) < epsilon)
return mid;
if (sgn(fmid) == sgn(flow)) {
low = mid;
flow = fmid;
} else {
high = mid;
fhigh = fmid;
}
}
}
global lib_debug;
if (lib_debug >= 0) {
print "solve(low, high, epsilon) defined";
}