From fada1a850cb1e88b9c19e45029092311ab7f869c Mon Sep 17 00:00:00 2001 From: Michael Drake Date: Sun, 26 Sep 2021 20:16:32 +0100 Subject: lzw: Optimisation: Pre-multiply relative offsets by struct size. --- src/lzw.c | 25 +++++++++++++++++++------ 1 file changed, 19 insertions(+), 6 deletions(-) diff --git a/src/lzw.c b/src/lzw.c index 59c74a6..fe79556 100644 --- a/src/lzw.c +++ b/src/lzw.c @@ -364,7 +364,7 @@ static inline void lzw__table_add_entry( entry->value = code; entry->first = ctx->prev_code_first; entry->count = ctx->prev_code_count + 1; - entry->extends = ctx->table_size - ctx->prev_code; + entry->extends = (ctx->table_size - ctx->prev_code) * sizeof(*entry); ctx->table_size++; } @@ -443,6 +443,19 @@ static inline lzw_result lzw__decode( return LZW_OK; } +/** + * Get the LZW table entry that the given entry extends. + * + * \param[in] entry The starting entry/ + * \return new table entry. + */ +static inline const struct lzw_table_entry *lzw__entry_get_extends( + const struct lzw_table_entry *entry) +{ + return (const struct lzw_table_entry *) + (((const uint8_t *)(entry)) - entry->extends); +} + /** * Write values for this code to the output stack. * @@ -483,13 +496,13 @@ static inline uint32_t lzw__write_fn(struct lzw_ctx *ctx, /* Skip over any values we don't have space for. */ for (unsigned i = left; i != 0; i--) { - entry -= entry->extends; + entry = lzw__entry_get_extends(entry); } output_pos += count; for (unsigned i = count; i != 0; i--) { *--output_pos = entry->value; - entry -= entry->extends; + entry = lzw__entry_get_extends(entry); } return count; @@ -561,7 +574,7 @@ static inline uint32_t lzw__map_write_fn(struct lzw_ctx *ctx, ctx->output_left = left; for (unsigned i = left; i != 0; i--) { - entry -= entry->extends; + entry = lzw__entry_get_extends(entry); } output_pos += count; @@ -571,12 +584,12 @@ static inline uint32_t lzw__map_write_fn(struct lzw_ctx *ctx, if (entry->value != ctx->transparency_idx) { *output_pos = ctx->colour_map[entry->value]; } - entry -= entry->extends; + entry = lzw__entry_get_extends(entry); } } else { for (unsigned i = count; i != 0; i--) { *--output_pos = ctx->colour_map[entry->value]; - entry -= entry->extends; + entry = lzw__entry_get_extends(entry); } } -- cgit v1.2.3