/*
 * jfdctfst.c
 *
 * This file was part of the Independent JPEG Group's software:
 * Copyright (C) 1994-1996, Thomas G. Lane.
 * libjpeg-turbo Modifications:
 * Copyright (C) 2015, D. R. Commander.
 * For conditions of distribution and use, see the accompanying README.ijg
 * file.
 *
 * This file contains a fast, not so accurate integer implementation of the
 * forward DCT (Discrete Cosine Transform).
 *
 * A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
 * on each column.  Direct algorithms are also available, but they are
 * much more complex and seem not to be any faster when reduced to code.
 *
 * This implementation is based on Arai, Agui, and Nakajima's algorithm for
 * scaled DCT.  Their original paper (Trans. IEICE E-71(11):1095) is in
 * Japanese, but the algorithm is described in the Pennebaker & Mitchell
 * JPEG textbook (see REFERENCES section in file README.ijg).  The following
 * code is based directly on figure 4-8 in P&M.
 * While an 8-point DCT cannot be done in less than 11 multiplies, it is
 * possible to arrange the computation so that many of the multiplies are
 * simple scalings of the final outputs.  These multiplies can then be
 * folded into the multiplications or divisions by the JPEG quantization
 * table entries.  The AA&N method leaves only 5 multiplies and 29 adds
 * to be done in the DCT itself.
 * The primary disadvantage of this method is that with fixed-point math,
 * accuracy is lost due to imprecise representation of the scaled
 * quantization values.  The smaller the quantization table entry, the less
 * precise the scaled value, so this implementation does worse with high-
 * quality-setting files than with low-quality ones.
 */

#define JPEG_INTERNALS

#include "jinclude.h"
#include "jpeglib.h"
#include "jdct.h"               /* Private declarations for DCT subsystem */

#ifdef DCT_IFAST_SUPPORTED


/*
 * This module is specialized to the case DCTSIZE = 8.
 */

#if DCTSIZE != 8
Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */
#endif


/* Scaling decisions are generally the same as in the LL&M algorithm;
 * see jfdctint.c for more details.  However, we choose to descale
 * (right shift) multiplication products as soon as they are formed,
 * rather than carrying additional fractional bits into subsequent additions.
 * This compromises accuracy slightly, but it lets us save a few shifts.
 * More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
 * everywhere except in the multiplications proper; this saves a good deal
 * of work on 16-bit-int machines.
 *
 * Again to save a few shifts, the intermediate results between pass 1 and
 * pass 2 are not upscaled, but are represented only to integral precision.
 *
 * A final compromise is to represent the multiplicative constants to only
 * 8 fractional bits, rather than 13.  This saves some shifting work on some
 * machines, and may also reduce the cost of multiplication (since there
 * are fewer one-bits in the constants).
 */

#define CONST_BITS  8


/* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
 * causing a lot of useless floating-point operations at run time.
 * To get around this we use the following pre-calculated constants.
 * If you change CONST_BITS you may want to add appropriate values.
 * (With a reasonable C compiler, you can just rely on the FIX() macro...)
 */

#if CONST_BITS == 8
#define FIX_0_382683433  ((JLONG)   98)         /* FIX(0.382683433) */
#define FIX_0_541196100  ((JLONG)  139)         /* FIX(0.541196100) */
#define FIX_0_707106781  ((JLONG)  181)         /* FIX(0.707106781) */
#define FIX_1_306562965  ((JLONG)  334)         /* FIX(1.306562965) */
#else
#define FIX_0_382683433  FIX(0.382683433)
#define FIX_0_541196100  FIX(0.541196100)
#define FIX_0_707106781  FIX(0.707106781)
#define FIX_1_306562965  FIX(1.306562965)
#endif


/* We can gain a little more speed, with a further compromise in accuracy,
 * by omitting the addition in a descaling shift.  This yields an incorrectly
 * rounded result half the time...
 */

#ifndef USE_ACCURATE_ROUNDING
#undef DESCALE
#define DESCALE(x, n)  RIGHT_SHIFT(x, n)
#endif


/* Multiply a DCTELEM variable by an JLONG constant, and immediately
 * descale to yield a DCTELEM result.
 */

#define MULTIPLY(var, const)  ((DCTELEM) DESCALE((var) * (const), CONST_BITS))


/*
 * Perform the forward DCT on one block of samples.
 */

GLOBAL(void)
jpeg_fdct_ifast(DCTELEM *data) {
	DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
	DCTELEM tmp10, tmp11, tmp12, tmp13;
	DCTELEM z1, z2, z3, z4, z5, z11, z13;
	DCTELEM *dataptr;
	int ctr;
	SHIFT_TEMPS

	/* Pass 1: process rows. */

	dataptr = data;
	for (ctr = DCTSIZE - 1; ctr >= 0; ctr--) {
		tmp0 = dataptr[0] + dataptr[7];
		tmp7 = dataptr[0] - dataptr[7];
		tmp1 = dataptr[1] + dataptr[6];
		tmp6 = dataptr[1] - dataptr[6];
		tmp2 = dataptr[2] + dataptr[5];
		tmp5 = dataptr[2] - dataptr[5];
		tmp3 = dataptr[3] + dataptr[4];
		tmp4 = dataptr[3] - dataptr[4];

		/* Even part */

		tmp10 = tmp0 + tmp3;        /* phase 2 */
		tmp13 = tmp0 - tmp3;
		tmp11 = tmp1 + tmp2;
		tmp12 = tmp1 - tmp2;

		dataptr[0] = tmp10 + tmp11; /* phase 3 */
		dataptr[4] = tmp10 - tmp11;

		z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
		dataptr[2] = tmp13 + z1;    /* phase 5 */
		dataptr[6] = tmp13 - z1;

		/* Odd part */

		tmp10 = tmp4 + tmp5;        /* phase 2 */
		tmp11 = tmp5 + tmp6;
		tmp12 = tmp6 + tmp7;

		/* The rotator is modified from fig 4-8 to avoid extra negations. */
		z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
		z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
		z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
		z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */

		z11 = tmp7 + z3;            /* phase 5 */
		z13 = tmp7 - z3;

		dataptr[5] = z13 + z2;      /* phase 6 */
		dataptr[3] = z13 - z2;
		dataptr[1] = z11 + z4;
		dataptr[7] = z11 - z4;

		dataptr += DCTSIZE;         /* advance pointer to next row */
	}

	/* Pass 2: process columns. */

	dataptr = data;
	for (ctr = DCTSIZE - 1; ctr >= 0; ctr--) {
		tmp0 = dataptr[DCTSIZE * 0] + dataptr[DCTSIZE * 7];
		tmp7 = dataptr[DCTSIZE * 0] - dataptr[DCTSIZE * 7];
		tmp1 = dataptr[DCTSIZE * 1] + dataptr[DCTSIZE * 6];
		tmp6 = dataptr[DCTSIZE * 1] - dataptr[DCTSIZE * 6];
		tmp2 = dataptr[DCTSIZE * 2] + dataptr[DCTSIZE * 5];
		tmp5 = dataptr[DCTSIZE * 2] - dataptr[DCTSIZE * 5];
		tmp3 = dataptr[DCTSIZE * 3] + dataptr[DCTSIZE * 4];
		tmp4 = dataptr[DCTSIZE * 3] - dataptr[DCTSIZE * 4];

		/* Even part */

		tmp10 = tmp0 + tmp3;        /* phase 2 */
		tmp13 = tmp0 - tmp3;
		tmp11 = tmp1 + tmp2;
		tmp12 = tmp1 - tmp2;

		dataptr[DCTSIZE * 0] = tmp10 + tmp11; /* phase 3 */
		dataptr[DCTSIZE * 4] = tmp10 - tmp11;

		z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
		dataptr[DCTSIZE * 2] = tmp13 + z1; /* phase 5 */
		dataptr[DCTSIZE * 6] = tmp13 - z1;

		/* Odd part */

		tmp10 = tmp4 + tmp5;        /* phase 2 */
		tmp11 = tmp5 + tmp6;
		tmp12 = tmp6 + tmp7;

		/* The rotator is modified from fig 4-8 to avoid extra negations. */
		z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
		z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
		z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
		z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */

		z11 = tmp7 + z3;            /* phase 5 */
		z13 = tmp7 - z3;

		dataptr[DCTSIZE * 5] = z13 + z2; /* phase 6 */
		dataptr[DCTSIZE * 3] = z13 - z2;
		dataptr[DCTSIZE * 1] = z11 + z4;
		dataptr[DCTSIZE * 7] = z11 - z4;

		dataptr++;                  /* advance pointer to next column */
	}
}

#endif /* DCT_IFAST_SUPPORTED */
