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diff --git a/src/lzw.c b/src/lzw.c
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+/*
+ * This file is part of NetSurf's LibNSGIF, http://www.netsurf-browser.org/
+ * Licensed under the MIT License,
+ * http://www.opensource.org/licenses/mit-license.php
+ *
+ * Copyright 2017 Michael Drake <michael.drake@codethink.co.uk>
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+#include "lzw.h"
+
+/**
+ * \file
+ * \brief LZW decompression (implementation)
+ *
+ * Decoder for GIF LZW data.
+ */
+
+
+/**
+ * Context for reading LZW data.
+ *
+ * LZW data is split over multiple sub-blocks. Each sub-block has a
+ * byte at the start, which says the sub-block size, and then the data.
+ * Zero-size sub-blocks have no data, and the biggest sub-block size is
+ * 255, which means there are 255 bytes of data following the sub-block
+ * size entry.
+ *
+ * Note that an individual LZW code can be split over up to three sub-blocks.
+ */
+struct lzw_read_ctx {
+ const uint8_t *data; /**< Pointer to start of input data */
+ uint32_t data_len; /**< Input data length */
+ uint32_t data_sb_next; /**< Offset to sub-block size */
+
+ const uint8_t *sb_data; /**< Pointer to current sub-block in data */
+ uint32_t sb_bit; /**< Current bit offset in sub-block */
+ uint32_t sb_bit_count; /**< Bit count in sub-block */
+};
+
+/**
+ * LZW dictionary entry.
+ *
+ * Records in the dictionary are composed of 1 or more entries.
+ * Entries point to previous entries which can be followed to compose
+ * the complete record. To compose the record in reverse order, take
+ * the `last_value` from each entry, and move to the previous entry.
+ * If the previous_entry's index is < the current clear_code, then it
+ * is the last entry in the record.
+ */
+struct lzw_dictionary_entry {
+ uint8_t last_value; /**< Last value for record ending at entry. */
+ uint8_t first_value; /**< First value for entry's record. */
+ uint16_t previous_entry; /**< Offset in dictionary to previous entry. */
+};
+
+/**
+ * LZW decompression context.
+ */
+struct lzw_ctx {
+ /** Input reading context */
+ struct lzw_read_ctx input;
+
+ uint32_t previous_code; /**< Code read from input previously. */
+ uint32_t previous_code_first; /**< First value of previous code. */
+
+ uint32_t initial_code_size; /**< Starting LZW code size. */
+ uint32_t current_code_size; /**< Current LZW code size. */
+ uint32_t current_code_size_max; /**< Max code value for current size. */
+
+ uint32_t clear_code; /**< Special Clear code value */
+ uint32_t eoi_code; /**< Special End of Information code value */
+
+ uint32_t current_entry; /**< Next position in table to fill. */
+
+ /** Output value stack. */
+ uint8_t stack_base[1 << LZW_CODE_MAX];
+
+ /** LZW decode dictionary. Generated during decode. */
+ struct lzw_dictionary_entry table[1 << LZW_CODE_MAX];
+};
+
+
+/* Exported function, documented in lzw.h */
+lzw_result lzw_context_create(struct lzw_ctx **ctx)
+{
+ struct lzw_ctx *c = malloc(sizeof(*c));
+ if (c == NULL) {
+ return LZW_NO_MEM;
+ }
+
+ *ctx = c;
+ return LZW_OK;
+}
+
+
+/* Exported function, documented in lzw.h */
+void lzw_context_destroy(struct lzw_ctx *ctx)
+{
+ free(ctx);
+}
+
+
+/**
+ * Advance the context to the next sub-block in the input data.
+ *
+ * \param[in] ctx LZW reading context, updated on success.
+ * \return LZW_OK or LZW_OK_EOD on success, appropriate error otherwise.
+ */
+static lzw_result lzw__block_advance(struct lzw_read_ctx *ctx)
+{
+ uint32_t block_size;
+ uint32_t next_block_pos = ctx->data_sb_next;
+ const uint8_t *data_next = ctx->data + next_block_pos;
+
+ if (next_block_pos >= ctx->data_len) {
+ return LZW_NO_DATA;
+ }
+
+ block_size = *data_next;
+
+ if ((next_block_pos + block_size) >= ctx->data_len) {
+ return LZW_NO_DATA;
+ }
+
+ ctx->sb_bit = 0;
+ ctx->sb_bit_count = block_size * 8;
+
+ if (block_size == 0) {
+ ctx->data_sb_next += 1;
+ return LZW_OK_EOD;
+ }
+
+ ctx->sb_data = data_next + 1;
+ ctx->data_sb_next += block_size + 1;
+
+ return LZW_OK;
+}
+
+
+/**
+ * Get the next LZW code of given size from the raw input data.
+ *
+ * Reads codes from the input data stream coping with GIF data sub-blocks.
+ *
+ * \param[in] ctx LZW reading context, updated.
+ * \param[in] code_size Size of LZW code to get from data.
+ * \param[out] code_out Returns an LZW code on success.
+ * \return LZW_OK or LZW_OK_EOD on success, appropriate error otherwise.
+ */
+static inline lzw_result lzw__next_code(
+ struct lzw_read_ctx *ctx,
+ uint8_t code_size,
+ uint32_t *code_out)
+{
+ uint32_t code = 0;
+ uint8_t current_bit = ctx->sb_bit & 0x7;
+ uint8_t byte_advance = (current_bit + code_size) >> 3;
+
+ if (ctx->sb_bit + code_size < ctx->sb_bit_count) {
+ /* Fast path: code fully inside this sub-block */
+ const uint8_t *data = ctx->sb_data + (ctx->sb_bit >> 3);
+ switch (byte_advance) {
+ case 2: code |= data[2] << 16;
+ case 1: code |= data[1] << 8;
+ case 0: code |= data[0] << 0;
+ }
+ ctx->sb_bit += code_size;
+ } else {
+ /* Slow path: code spans sub-blocks */
+ uint8_t byte = 0;
+ uint8_t bits_remaining_0 = (code_size < (8 - current_bit)) ?
+ code_size : (8 - current_bit);
+ uint8_t bits_remaining_1 = code_size - bits_remaining_0;
+ uint8_t bits_used[3] = {
+ [0] = bits_remaining_0,
+ [1] = bits_remaining_1 < 8 ? bits_remaining_1 : 8,
+ [2] = bits_remaining_1 - 8,
+ };
+
+ while (true) {
+ const uint8_t *data = ctx->sb_data;
+ lzw_result res;
+
+ /* Get any data from end of this sub-block */
+ while (byte <= byte_advance &&
+ ctx->sb_bit < ctx->sb_bit_count) {
+ code |= data[ctx->sb_bit >> 3] << (byte << 3);
+ ctx->sb_bit += bits_used[byte];
+ byte++;
+ }
+
+ /* Check if we have all we need */
+ if (byte > byte_advance) {
+ break;
+ }
+
+ /* Move to next sub-block */
+ res = lzw__block_advance(ctx);
+ if (res != LZW_OK) {
+ return res;
+ }
+ }
+ }
+
+ *code_out = (code >> current_bit) & ((1 << code_size) - 1);
+ return LZW_OK;
+}
+
+
+/**
+ * Clear LZW code dictionary.
+ *
+ * \param[in] ctx LZW reading context, updated.
+ * \param[out] stack_pos_out Returns current stack position.
+ * \return LZW_OK or error code.
+ */
+static lzw_result lzw__clear_codes(
+ struct lzw_ctx *ctx,
+ const uint8_t ** const stack_pos_out)
+{
+ uint32_t code;
+ uint8_t *stack_pos;
+
+ /* Reset dictionary building context */
+ ctx->current_code_size = ctx->initial_code_size + 1;
+ ctx->current_code_size_max = (1 << ctx->current_code_size) - 1;;
+ ctx->current_entry = (1 << ctx->initial_code_size) + 2;
+
+ /* There might be a sequence of clear codes, so process them all */
+ do {
+ lzw_result res = lzw__next_code(&ctx->input,
+ ctx->current_code_size, &code);
+ if (res != LZW_OK) {
+ return res;
+ }
+ } while (code == ctx->clear_code);
+
+ /* The initial code must be from the initial dictionary. */
+ if (code > ctx->clear_code) {
+ return LZW_BAD_ICODE;
+ }
+
+ /* Record this initial code as "previous" code, needed during decode. */
+ ctx->previous_code = code;
+ ctx->previous_code_first = code;
+
+ /* Reset the stack, and add first non-clear code added as first item. */
+ stack_pos = ctx->stack_base;
+ *stack_pos++ = code;
+
+ *stack_pos_out = stack_pos;
+ return LZW_OK;
+}
+
+
+/* Exported function, documented in lzw.h */
+lzw_result lzw_decode_init(
+ struct lzw_ctx *ctx,
+ const uint8_t *compressed_data,
+ uint32_t compressed_data_len,
+ uint32_t compressed_data_pos,
+ uint8_t code_size,
+ const uint8_t ** const stack_base_out,
+ const uint8_t ** const stack_pos_out)
+{
+ struct lzw_dictionary_entry *table = ctx->table;
+
+ /* Initialise the input reading context */
+ ctx->input.data = compressed_data;
+ ctx->input.data_len = compressed_data_len;
+ ctx->input.data_sb_next = compressed_data_pos;
+
+ ctx->input.sb_bit = 0;
+ ctx->input.sb_bit_count = 0;
+
+ /* Initialise the dictionary building context */
+ ctx->initial_code_size = code_size;
+
+ ctx->clear_code = (1 << code_size) + 0;
+ ctx->eoi_code = (1 << code_size) + 1;
+
+ /* Initialise the standard dictionary entries */
+ for (uint32_t i = 0; i < ctx->clear_code; ++i) {
+ table[i].first_value = i;
+ table[i].last_value = i;
+ }
+
+ *stack_base_out = ctx->stack_base;
+ return lzw__clear_codes(ctx, stack_pos_out);
+}
+
+
+/* Exported function, documented in lzw.h */
+lzw_result lzw_decode(struct lzw_ctx *ctx,
+ const uint8_t ** const stack_pos_out)
+{
+ lzw_result res;
+ uint32_t code_new;
+ uint32_t code_out;
+ 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;
+ struct lzw_dictionary_entry * const table = ctx->table;
+
+ /* Get a new code from the input */
+ res = lzw__next_code(&ctx->input, ctx->current_code_size, &code_new);
+ if (res != LZW_OK) {
+ return res;
+ }
+
+ if (code_new == clear_code) {
+ /* Got Clear code */
+ return lzw__clear_codes(ctx, stack_pos_out);
+
+ } else if (code_new == ctx->eoi_code) {
+ /* Got End of Information code */
+ return LZW_EOI_CODE;
+ }
+
+ if (code_new > current_entry) {
+ /* Code is invalid */
+ return LZW_BAD_CODE;
+ } else if (code_new < current_entry) {
+ /* Code is in table */
+ code_out = code_new;
+ last_value = table[code_new].first_value;
+ } else {
+ /* Code not in table */
+ *stack_pos++ = ctx->previous_code_first;
+ code_out = ctx->previous_code;
+ last_value = ctx->previous_code_first;
+ }
+
+ /* Add to the dictionary, only if there's space */
+ if (current_entry < (1 << LZW_CODE_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++;
+ }
+
+ /* Ensure code size is increased, if needed. */
+ if (current_entry == ctx->current_code_size_max) {
+ if (ctx->current_code_size < LZW_CODE_MAX) {
+ ctx->current_code_size++;
+ ctx->current_code_size_max =
+ (1 << ctx->current_code_size) - 1;
+ }
+ }
+
+ ctx->previous_code_first = table[code_new].first_value;
+ ctx->previous_code = code_new;
+
+ /* Put rest of data for this code on output stack.
+ * Note, in the case of "code not in table", the last entry of the
+ * current code has already been placed on the stack above. */
+ while (code_out > clear_code) {
+ struct lzw_dictionary_entry *entry = table + code_out;
+ *stack_pos++ = entry->last_value;
+ code_out = entry->previous_entry;
+ }
+ *stack_pos++ = table[code_out].last_value;
+
+ *stack_pos_out = stack_pos;
+ return LZW_OK;
+}
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