duktape-1.5.2/src-separate/duk_util_bitdecoder.c

Go to the documentation of this file.

/*
 *  Bitstream decoder.
 */
 
#include "duk_internal.h"
 
/* Decode 'bits' bits from the input stream (bits must be 1...24).
 * When reading past bitstream end, zeroes are shifted in.  The result
 * is signed to match duk_bd_decode_flagged.
 */
DUK_INTERNAL duk_int32_t duk_bd_decode(duk_bitdecoder_ctx *ctx, duk_small_int_t bits) {
    duk_small_int_t shift;
    duk_uint32_t mask;
    duk_uint32_t tmp;
 
    /* Note: cannot read more than 24 bits without possibly shifting top bits out.
     * Fixable, but adds complexity.
     */
    DUK_ASSERT(bits >= 1 && bits <= 24);
 
    while (ctx->currbits < bits) {
#if 0
        DUK_DDD(DUK_DDDPRINT("decode_bits: shift more data (bits=%ld, currbits=%ld)",
                             (long) bits, (long) ctx->currbits));
#endif
        ctx->currval <<= 8;
        if (ctx->offset < ctx->length) {
            /* If ctx->offset >= ctx->length, we "shift zeroes in"
             * instead of croaking.
             */
            ctx->currval |= ctx->data[ctx->offset++];
        }
        ctx->currbits += 8;
    }
#if 0
    DUK_DDD(DUK_DDDPRINT("decode_bits: bits=%ld, currbits=%ld, currval=0x%08lx",
                         (long) bits, (long) ctx->currbits, (unsigned long) ctx->currval));
#endif
 
    /* Extract 'top' bits of currval; note that the extracted bits do not need
     * to be cleared, we just ignore them on next round.
     */
    shift = ctx->currbits - bits;
    mask = (1 << bits) - 1;
    tmp = (ctx->currval >> shift) & mask;
    ctx->currbits = shift;  /* remaining */
 
#if 0
    DUK_DDD(DUK_DDDPRINT("decode_bits: %ld bits -> 0x%08lx (%ld), currbits=%ld, currval=0x%08lx",
                         (long) bits, (unsigned long) tmp, (long) tmp, (long) ctx->currbits, (unsigned long) ctx->currval));
#endif
 
    return tmp;
}
 
DUK_INTERNAL duk_small_int_t duk_bd_decode_flag(duk_bitdecoder_ctx *ctx) {
    return (duk_small_int_t) duk_bd_decode(ctx, 1);
}
 
/* Decode a one-bit flag, and if set, decode a value of 'bits', otherwise return
 * default value.  Return value is signed so that negative marker value can be
 * used by caller as a "not present" value.
 */
DUK_INTERNAL duk_int32_t duk_bd_decode_flagged(duk_bitdecoder_ctx *ctx, duk_small_int_t bits, duk_int32_t def_value) {
    if (duk_bd_decode_flag(ctx)) {
        return (duk_int32_t) duk_bd_decode(ctx, bits);
    } else {
        return def_value;
    }
}