[FFmpeg-devel] [PATCH] hardcoding for pcm alaw/ulaw tables

[Michael Niedermayer]

On Sun, Mar 21, 2010 at 06:43:38PM +0100, Reimar D?ffinger wrote:
> Hello,
> this allows hard-coding the linear-to-alaw and linear-to-ulaw tables,
> which have a size of 32 kB overall.

not specific to this patch but id like to point out that the table generation
is messy and it lacks documentation.


>  Makefile       |    1 
>  pcm.c          |   79 +-------------------------------------
>  pcm_tablegen.c |   51 ++++++++++++++++++++++++
>  pcm_tablegen.h |  118 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 173 insertions(+), 76 deletions(-)
> e47b001605eda9746a6afec01520084951af5157  pcmtables.diff
> Index: libavcodec/pcm_tablegen.c
> ===================================================================
> --- libavcodec/pcm_tablegen.c	(revision 0)
> +++ libavcodec/pcm_tablegen.c	(revision 0)
> @@ -0,0 +1,51 @@
> +/*
> + * Generate a header file for hardcoded PCM tables
> + *
> + * Copyright (c) 2010 Reimar D?ffinger <Reimar.Doeffinger at gmx.de>
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#include <stdlib.h>

> +#define av_cold
> +#define CONFIG_HARDCODED_TABLES 0

ugly


> +#include "pcm_tablegen.h"
> +#include "tableprint.h"
> +
> +void tableinit(void)
> +{
> +    pcm_alaw_tableinit();
> +    pcm_ulaw_tableinit();
> +}
> +
> +const struct tabledef tables[] = {
> +    {
> +        "static const uint8_t linear_to_alaw[1 << 14]",
> +        write_uint8_array,
> +        linear_to_alaw,
> +        1 << 14,
> +        0
> +    },


> +    {
> +        "static const uint8_t linear_to_ulaw[1 << 14]",
> +        write_uint8_array,
> +        linear_to_ulaw,
> +        1 << 14,
> +        0
> +    },

cleaner and with half the code:

printf("static const uint8_t linear_to_ulaw[1 << 14]= {\n");
for(i=0; i < 1<<14; i++)
    printf("%d,", linear_to_ulaw[i]);
printf("};\n");


[...]
> +static av_cold int ulaw2linear(unsigned char u_val)
> +{
> +        int t;
> +
> +        /* Complement to obtain normal u-law value. */
> +        u_val = ~u_val;
> +
> +        /*
> +         * Extract and bias the quantization bits. Then
> +         * shift up by the segment number and subtract out the bias.
> +         */
> +        t = ((u_val & QUANT_MASK) << 3) + BIAS;
> +        t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;
> +
> +        return (u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS);
> +}
> +
> +#if CONFIG_HARDCODED_TABLES

> +#define pcm_alaw_tableinit()
> +#define pcm_ulaw_tableinit()

ugly


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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