1 /*
2 * Bitstream decoder.
3 */
4
5 #include "duk_internal.h"
6
7 /* Decode 'bits' bits from the input stream (bits must be 1...24).
8 * When reading past bitstream end, zeroes are shifted in. The result
9 * is signed to match duk_bd_decode_flagged.
10 */
duk_bd_decode(duk_bitdecoder_ctx * ctx,duk_small_int_t bits)11 DUK_INTERNAL duk_int32_t duk_bd_decode(duk_bitdecoder_ctx *ctx, duk_small_int_t bits) {
12 duk_small_int_t shift;
13 duk_uint32_t mask;
14 duk_uint32_t tmp;
15
16 /* Note: cannot read more than 24 bits without possibly shifting top bits out.
17 * Fixable, but adds complexity.
18 */
19 DUK_ASSERT(bits >= 1 && bits <= 24);
20
21 while (ctx->currbits < bits) {
22 #if 0
23 DUK_DDD(DUK_DDDPRINT("decode_bits: shift more data (bits=%ld, currbits=%ld)",
24 (long) bits, (long) ctx->currbits));
25 #endif
26 ctx->currval <<= 8;
27 if (ctx->offset < ctx->length) {
28 /* If ctx->offset >= ctx->length, we "shift zeroes in"
29 * instead of croaking.
30 */
31 ctx->currval |= ctx->data[ctx->offset++];
32 }
33 ctx->currbits += 8;
34 }
35 #if 0
36 DUK_DDD(DUK_DDDPRINT("decode_bits: bits=%ld, currbits=%ld, currval=0x%08lx",
37 (long) bits, (long) ctx->currbits, (unsigned long) ctx->currval));
38 #endif
39
40 /* Extract 'top' bits of currval; note that the extracted bits do not need
41 * to be cleared, we just ignore them on next round.
42 */
43 shift = ctx->currbits - bits;
44 mask = (1 << bits) - 1;
45 tmp = (ctx->currval >> shift) & mask;
46 ctx->currbits = shift; /* remaining */
47
48 #if 0
49 DUK_DDD(DUK_DDDPRINT("decode_bits: %ld bits -> 0x%08lx (%ld), currbits=%ld, currval=0x%08lx",
50 (long) bits, (unsigned long) tmp, (long) tmp, (long) ctx->currbits, (unsigned long) ctx->currval));
51 #endif
52
53 return tmp;
54 }
55
duk_bd_decode_flag(duk_bitdecoder_ctx * ctx)56 DUK_INTERNAL duk_small_int_t duk_bd_decode_flag(duk_bitdecoder_ctx *ctx) {
57 return (duk_small_int_t) duk_bd_decode(ctx, 1);
58 }
59
60 /* Decode a one-bit flag, and if set, decode a value of 'bits', otherwise return
61 * default value. Return value is signed so that negative marker value can be
62 * used by caller as a "not present" value.
63 */
duk_bd_decode_flagged(duk_bitdecoder_ctx * ctx,duk_small_int_t bits,duk_int32_t def_value)64 DUK_INTERNAL duk_int32_t duk_bd_decode_flagged(duk_bitdecoder_ctx *ctx, duk_small_int_t bits, duk_int32_t def_value) {
65 if (duk_bd_decode_flag(ctx)) {
66 return (duk_int32_t) duk_bd_decode(ctx, bits);
67 } else {
68 return def_value;
69 }
70 }
71
