hinv.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

#include	<u.h>
#include	<libc.h>
#include	<bio.h>
#include	"sky.h"

static	void	unshuffle(Pix*, int, int, Pix*);
static	void	unshuffle1(Pix*, int, Pix*);

void
hinv(Pix *a, int nx, int ny)
{
	int nmax, log2n, h0, hx, hy, hc, i, j, k;
	int nxtop, nytop, nxf, nyf, c;
	int oddx, oddy;
	int shift;
	int s10, s00;
	Pix *tmp;

	/*
	 * log2n is log2 of max(nx, ny) rounded up to next power of 2
	 */
	nmax = ny;
	if(nx > nmax)
		nmax = nx;
	log2n = log(nmax)/LN2 + 0.5;
	if(nmax > (1<<log2n))
		log2n++;

	/*
	 * get temporary storage for shuffling elements
	 */
	tmp = (Pix*)malloc(((nmax+1)/2) * sizeof(*tmp));
	if(tmp == nil) {
		fprint(2, "hinv: insufficient memory\n");
		exits("memory");
	}

	/*
	 * do log2n expansions
	 *
	 * We're indexing a as a 2-D array with dimensions (nx,ny).
	 */
	shift = 1;
	nxtop = 1;
	nytop = 1;
	nxf = nx;
	nyf = ny;
	c = 1<<log2n;
	for(k = log2n-1; k>=0; k--) {
		/*
		 * this somewhat cryptic code generates the sequence
		 * ntop[k-1] = (ntop[k]+1)/2, where ntop[log2n] = n
		 */
		c = c>>1;
		nxtop = nxtop<<1;
		nytop = nytop<<1;
		if(nxf <= c)
			nxtop--;
		else
			nxf -= c;
		if(nyf <= c)
			nytop--;
		else
			nyf -= c;

		/*
		 * halve divisors on last pass
		 */
		if(k == 0)
			shift = 0;

		/*
		 * unshuffle in each dimension to interleave coefficients
		 */
		for(i = 0; i<nxtop; i++)
			unshuffle1(&a[ny*i], nytop, tmp);
		for(j = 0; j<nytop; j++)
			unshuffle(&a[j], nxtop, ny, tmp);
		oddx = nxtop & 1;
		oddy = nytop & 1;
		for(i = 0; i<nxtop-oddx; i += 2) {
			s00 = ny*i;			/* s00 is index of a[i,j]	*/
			s10 = s00+ny;			/* s10 is index of a[i+1,j]	*/
			for(j = 0; j<nytop-oddy; j += 2) {
				/*
				 * Multiply h0,hx,hy,hc by 2 (1 the last time through).
				 */
				h0 = a[s00  ] << shift;
				hx = a[s10  ] << shift;
				hy = a[s00+1] << shift;
				hc = a[s10+1] << shift;

				/*
				 * Divide sums by 4 (shift right 2 bits).
				 * Add 1 to round -- note that these values are always
				 * positive so we don't need to do anything special
				 * for rounding negative numbers.
				 */
				a[s10+1] = (h0 + hx + hy + hc + 2) >> 2;
				a[s10  ] = (h0 + hx - hy - hc + 2) >> 2;
				a[s00+1] = (h0 - hx + hy - hc + 2) >> 2;
				a[s00  ] = (h0 - hx - hy + hc + 2) >> 2;
				s00 += 2;
				s10 += 2;
			}
			if(oddy) {
				/*
				 * do last element in row if row length is odd
				 * s00+1, s10+1 are off edge
				 */
				h0 = a[s00  ] << shift;
				hx = a[s10  ] << shift;
				a[s10  ] = (h0 + hx + 2) >> 2;
				a[s00  ] = (h0 - hx + 2) >> 2;
			}
		}
		if(oddx) {
			/*
			 * do last row if column length is odd
			 * s10, s10+1 are off edge
			 */
			s00 = ny*i;
			for(j = 0; j<nytop-oddy; j += 2) {
				h0 = a[s00  ] << shift;
				hy = a[s00+1] << shift;
				a[s00+1] = (h0 + hy + 2) >> 2;
				a[s00  ] = (h0 - hy + 2) >> 2;
				s00 += 2;
			}
			if(oddy) {
				/*
				 * do corner element if both row and column lengths are odd
				 * s00+1, s10, s10+1 are off edge
				 */
				h0 = a[s00  ] << shift;
				a[s00  ] = (h0 + 2) >> 2;
			}
		}
	}
	free(tmp);
}

static
void
unshuffle(Pix *a, int n, int n2, Pix *tmp)
{
	int i;
	int nhalf, twon2, n2xnhalf;
	Pix *p1, *p2, *pt;

	twon2 = n2<<1;
	nhalf = (n+1)>>1;
	n2xnhalf = n2*nhalf;		/* pointer to a[i] */

	/*
	 * copy 2nd half of array to tmp
	 */
	pt = tmp;
	p1 = &a[n2xnhalf];		/* pointer to a[i] */
	for(i=nhalf; i<n; i++) {
		*pt = *p1;
		pt++;
		p1 += n2;
	}

	/*
	 * distribute 1st half of array to even elements
	 */
	p2 = &a[n2xnhalf];		/* pointer to a[i] */
	p1 = &a[n2xnhalf<<1];		/* pointer to a[2*i] */
	for(i=nhalf-1; i>=0; i--) {
		p1 -= twon2;
		p2 -= n2;
		*p1 = *p2;
	}

	/*
	 * now distribute 2nd half of array (in tmp) to odd elements
	 */
	pt = tmp;
	p1 = &a[n2];			/* pointer to a[i] */
	for(i=1; i<n; i+=2) {
		*p1 = *pt;
		p1 += twon2;
		pt++;
	}
}

static
void
unshuffle1(Pix *a, int n, Pix *tmp)
{
	int i;
	int nhalf;
	Pix *p1, *p2, *pt;

	nhalf = (n+1) >> 1;

	/*
	 * copy 2nd half of array to tmp
	 */
	pt = tmp;
	p1 = &a[nhalf];				/* pointer to a[i]			*/
	for(i=nhalf; i<n; i++) {
		*pt = *p1;
		pt++;
		p1++;
	}

	/*
	 * distribute 1st half of array to even elements
	 */
	p2 = &a[nhalf];		/* pointer to a[i]			*/
	p1 = &a[nhalf<<1];		/* pointer to a[2*i]		*/
	for(i=nhalf-1; i>=0; i--) {
		p1 -= 2;
		p2--;
		*p1 = *p2;
	}

	/*
	 * now distribute 2nd half of array (in tmp) to odd elements
	 */
	pt = tmp;
	p1 = &a[1];					/* pointer to a[i]			*/
	for(i=1; i<n; i+=2) {
		*p1 = *pt;
		p1 += 2;
		pt++;
	}
}