#include "stdafx.h"

//-----------------------------------------------------------------------------
//
//	Generate a Taylor expansion for expr.
//
//	Input:	tos-4	expr
//
//		tos-3	function var
//
//		tos-2	number of terms
//
//		tos-1	expansion point
//
//	Output:	Result on stack
//
//	Example:
//
//		taylor(cos(x),x,8,a)
//
//		tos-4	cos(x)
//
//		tos-3	x
//
//		tos-2	8
//
//		tos-1	a
//
//-----------------------------------------------------------------------------

#include "defs.h"

static void __taylor(void);

void
taylor(void)
{
	save();
	__taylor();
	restore();
}

#define F p1
#define X p2
#define N p3
#define A p4
#define C p5

static void
__taylor(void)
{
	int i, k;

	A = pop();
	N = pop();
	X = pop();
	F = pop();

	if (X->k != SYM)
		stop("taylor: symbol expected, 2nd arg is not a symbol");

	push(N);
	k = pop_integer();
	if (k == (int) 0x80000000) {
		push_symbol(TAYLOR);
		push(F);
		push(X);
		push(N);
		push(A);
		list(5);
		return;
	}

	push(F);	// f(a)
	push(X);
	push(A);
	evalat();

	C = one;

	for (i = 1; i <= k; i++) {

		push(F);	// f = f'
		push(X);
		derivative();
		F = pop();

		if (iszero(F))
			break;

		push(C);	// c = c * (x - a)
		push(X);
		push(A);
		subtract();
		multiply();
		C = pop();

		push(F);	// f(a)
		push(X);
		push(A);
		evalat();

		push(C);
		multiply();
		push_integer(i);
		factorial();
		divide();

		add();
	}
}

static char *s[] = {

	"taylor(1/(5+4*cos(x)),x,6,0)",
	"1/9+2/81*x^2+5/1458*x^4+49/131220*x^6",

	"taylor(1/(5+4*cos(x)),x,6)",
	"1/9+2/81*x^2+5/1458*x^4+49/131220*x^6",

	"taylor(ln(x),x,8)",
	"Stop: undefined function",

	// make sure x is restored after taylor breaks

	"x",
	"x",
};

void
test_taylor(void)
{
	test(__FILE__, s, sizeof s / sizeof (char *));
}