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lzw: Split out dictionary augmentation.

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This commit is contained in:
Michael Drake 2021-03-31 19:40:35 +01:00 committed by Michael Drake
parent 47a441f86b
commit 1714cf6c3a
1 changed files with 21 additions and 14 deletions
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35
libvips/foreign/libnsgif/lzw.c
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@ -303,6 +303,24 @@ lzw_result lzw_decode_init(
return lzw__clear_codes(ctx, stack_pos_out);
}
/**
* Create new dictionary entry.
*
* \param[in] ctx LZW reading context, updated.
* \param[in] code Last value code for new dictionary entry.
*/
static inline void lzw__dictionary_add_entry(
struct lzw_ctx *ctx,
uint32_t code)
{
struct lzw_dictionary_entry *entry = &ctx->table[ctx->current_entry];
entry->last_value = code;
entry->first_value = ctx->previous_code_first;
entry->previous_entry = ctx->previous_code;
ctx->current_entry++;
}
/* Exported function, documented in lzw.h */
lzw_result lzw_decode(struct lzw_ctx *ctx,
@ -310,7 +328,6 @@ lzw_result lzw_decode(struct lzw_ctx *ctx,
{
lzw_result res;
uint32_t code_new;
uint8_t last_value;
uint8_t *stack_pos = ctx->stack_base;
uint32_t clear_code = ctx->clear_code;
uint32_t current_entry = ctx->current_entry;
@ -334,22 +351,12 @@ lzw_result lzw_decode(struct lzw_ctx *ctx,
} else if (code_new > current_entry) {
/* Code is invalid */
return LZW_BAD_CODE;
} else if (code_new < current_entry) {
/* Code is in table */
last_value = table[code_new].first_value;
} else {
/* Code not in table */
last_value = ctx->previous_code_first;
}
/* Add to the dictionary, only if there's space */
if (current_entry < LZW_TABLE_ENTRY_MAX) {
struct lzw_dictionary_entry *entry = table + current_entry;
entry->last_value = last_value;
entry->first_value = ctx->previous_code_first;
entry->previous_entry = ctx->previous_code;
ctx->current_entry++;
lzw__dictionary_add_entry(ctx, (code_new < current_entry) ?
table[code_new].first_value :
ctx->previous_code_first);
}
/* Ensure code size is increased, if needed. */