#include <u.h>
#include <base.h>

static const uint16 tab[128] = {
0xb451,0xb2f0,0xb196,0xb044,0xaef9,0xadb6,0xac79,0xab43,
0xaa14,0xa8eb,0xa7c8,0xa6aa,0xa592,0xa480,0xa373,0xa26b,
0xa168,0xa06a,0x9f70,0x9e7b,0x9d8a,0x9c9d,0x9bb5,0x9ad1,
0x99f0,0x9913,0x983a,0x9765,0x9693,0x95c4,0x94f8,0x9430,
0x936b,0x92a9,0x91ea,0x912e,0x9075,0x8fbe,0x8f0a,0x8e59,
0x8daa,0x8cfe,0x8c54,0x8bac,0x8b07,0x8a64,0x89c4,0x8925,
0x8889,0x87ee,0x8756,0x86c0,0x862b,0x8599,0x8508,0x8479,
0x83ec,0x8361,0x82d8,0x8250,0x81c9,0x8145,0x80c2,0x8040,
0xff02,0xfd0e,0xfb25,0xf947,0xf773,0xf5aa,0xf3ea,0xf234,
0xf087,0xeee3,0xed47,0xebb3,0xea27,0xe8a3,0xe727,0xe5b2,
0xe443,0xe2dc,0xe17a,0xe020,0xdecb,0xdd7d,0xdc34,0xdaf1,
0xd9b3,0xd87b,0xd748,0xd61a,0xd4f1,0xd3cd,0xd2ad,0xd192,
0xd07b,0xcf69,0xce5b,0xcd51,0xcc4a,0xcb48,0xca4a,0xc94f,
0xc858,0xc764,0xc674,0xc587,0xc49d,0xc3b7,0xc2d4,0xc1f4,
0xc116,0xc03c,0xbf65,0xbe90,0xbdbe,0xbcef,0xbc23,0xbb59,
0xba91,0xb9cc,0xb90a,0xb84a,0xb78c,0xb6d0,0xb617,0xb560,
};

static inline uint32
mul32(uint32 a, uint32 b)
{
	return (uint64)a*b >> 32;
}

float
math·sqrtf(float x)
{
	uint32 ix, m, m1, m0, even, ey;
    union{float f; uint32 i;} phi;

	phi.f=x, ix=phi.i;
	if(ix - 0x00800000 >= 0x7f800000 - 0x00800000){
		/* x < 0x1p-126 or inf or nan.  */
		if(ix * 2 == 0)
			return x;
		if(ix == 0x7f800000)
			return x;
		if(ix > 0x7f800000)
			return (x-x)/(x-x);
		/* x is subnormal, normalize it.  */
		phi.f=x * 0x1p23f, ix=phi.i;
		ix -= 23 << 23;
	}

	/* x = 4^e m; with int e and m in [1, 4).  */
	even = ix & 0x00800000;
	m1 = (ix << 8) | 0x80000000;
	m0 = (ix << 7) & 0x7fffffff;
	m = even ? m0 : m1;

	/* 2^e is the exponent part of the return value.  */
	ey = ix >> 1;
	ey += 0x3f800000 >> 1;
	ey &= 0x7f800000;

	/* compute r ~ 1/sqrt(m), s ~ sqrt(m) with 2 goldschmidt iterations.  */
	static const uint32 three = 0xc0000000;
	uint32 r, s, d, u, i;
	i = (ix >> 17) % 128;
	r = (uint32)tab[i] << 16;
	/* |r*sqrt(m) - 1| < 0x1p-8 */
	s = mul32(m, r);
	/* |s/sqrt(m) - 1| < 0x1p-8 */
	d = mul32(s, r);
	u = three - d;
	r = mul32(r, u) << 1;
	/* |r*sqrt(m) - 1| < 0x1.7bp-16 */
	s = mul32(s, u) << 1;
	/* |s/sqrt(m) - 1| < 0x1.7bp-16 */
	d = mul32(s, r);
	u = three - d;
	s = mul32(s, u);
	/* -0x1.03p-28 < s/sqrt(m) - 1 < 0x1.fp-31 */
	s = (s - 1)>>6;
	/* s < sqrt(m) < s + 0x1.08p-23 */

	/* compute nearest rounded result.  */
	uint32 d0, d1, d2;
	float y, t;
	d0 = (m << 16) - s*s;
	d1 = s - d0;
	d2 = d1 + s + 1;
	s += d1 >> 31;
	s &= 0x007fffff;
	s |= ey;
	phi.i = s, y = phi.f;

    /* handle rounding and inexact exception. */
    uint32 tiny = (d2==0) ? 0 : 0x01000000;
    tiny |= (d1^d2) & 0x80000000;
    phi.i=tiny, t=phi.f;
    y = y + t;

	return y;
}