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Diffstat (limited to 'fs/ntfs3/lib/decompress_common.c')
| -rw-r--r-- | fs/ntfs3/lib/decompress_common.c | 319 |
1 files changed, 319 insertions, 0 deletions
diff --git a/fs/ntfs3/lib/decompress_common.c b/fs/ntfs3/lib/decompress_common.c new file mode 100644 index 000000000..e96652240 --- /dev/null +++ b/fs/ntfs3/lib/decompress_common.c @@ -0,0 +1,319 @@ +// SPDX-License-Identifier: GPL-2.0-or-later +/* + * decompress_common.c - Code shared by the XPRESS and LZX decompressors + * + * Copyright (C) 2015 Eric Biggers + */ + +#include "decompress_common.h" + +/* + * make_huffman_decode_table() - + * + * Build a decoding table for a canonical prefix code, or "Huffman code". + * + * This is an internal function, not part of the library API! + * + * This takes as input the length of the codeword for each symbol in the + * alphabet and produces as output a table that can be used for fast + * decoding of prefix-encoded symbols using read_huffsym(). + * + * Strictly speaking, a canonical prefix code might not be a Huffman + * code. But this algorithm will work either way; and in fact, since + * Huffman codes are defined in terms of symbol frequencies, there is no + * way for the decompressor to know whether the code is a true Huffman + * code or not until all symbols have been decoded. + * + * Because the prefix code is assumed to be "canonical", it can be + * reconstructed directly from the codeword lengths. A prefix code is + * canonical if and only if a longer codeword never lexicographically + * precedes a shorter codeword, and the lexicographic ordering of + * codewords of the same length is the same as the lexicographic ordering + * of the corresponding symbols. Consequently, we can sort the symbols + * primarily by codeword length and secondarily by symbol value, then + * reconstruct the prefix code by generating codewords lexicographically + * in that order. + * + * This function does not, however, generate the prefix code explicitly. + * Instead, it directly builds a table for decoding symbols using the + * code. The basic idea is this: given the next 'max_codeword_len' bits + * in the input, we can look up the decoded symbol by indexing a table + * containing 2**max_codeword_len entries. A codeword with length + * 'max_codeword_len' will have exactly one entry in this table, whereas + * a codeword shorter than 'max_codeword_len' will have multiple entries + * in this table. Precisely, a codeword of length n will be represented + * by 2**(max_codeword_len - n) entries in this table. The 0-based index + * of each such entry will contain the corresponding codeword as a prefix + * when zero-padded on the left to 'max_codeword_len' binary digits. + * + * That's the basic idea, but we implement two optimizations regarding + * the format of the decode table itself: + * + * - For many compression formats, the maximum codeword length is too + * long for it to be efficient to build the full decoding table + * whenever a new prefix code is used. Instead, we can build the table + * using only 2**table_bits entries, where 'table_bits' is some number + * less than or equal to 'max_codeword_len'. Then, only codewords of + * length 'table_bits' and shorter can be directly looked up. For + * longer codewords, the direct lookup instead produces the root of a + * binary tree. Using this tree, the decoder can do traditional + * bit-by-bit decoding of the remainder of the codeword. Child nodes + * are allocated in extra entries at the end of the table; leaf nodes + * contain symbols. Note that the long-codeword case is, in general, + * not performance critical, since in Huffman codes the most frequently + * used symbols are assigned the shortest codeword lengths. + * + * - When we decode a symbol using a direct lookup of the table, we still + * need to know its length so that the bitstream can be advanced by the + * appropriate number of bits. The simple solution is to simply retain + * the 'lens' array and use the decoded symbol as an index into it. + * However, this requires two separate array accesses in the fast path. + * The optimization is to store the length directly in the decode + * table. We use the bottom 11 bits for the symbol and the top 5 bits + * for the length. In addition, to combine this optimization with the + * previous one, we introduce a special case where the top 2 bits of + * the length are both set if the entry is actually the root of a + * binary tree. + * + * @decode_table: + * The array in which to create the decoding table. This must have + * a length of at least ((2**table_bits) + 2 * num_syms) entries. + * + * @num_syms: + * The number of symbols in the alphabet; also, the length of the + * 'lens' array. Must be less than or equal to 2048. + * + * @table_bits: + * The order of the decode table size, as explained above. Must be + * less than or equal to 13. + * + * @lens: + * An array of length @num_syms, indexable by symbol, that gives the + * length of the codeword, in bits, for that symbol. The length can + * be 0, which means that the symbol does not have a codeword + * assigned. + * + * @max_codeword_len: + * The longest codeword length allowed in the compression format. + * All entries in 'lens' must be less than or equal to this value. + * This must be less than or equal to 23. + * + * @working_space + * A temporary array of length '2 * (max_codeword_len + 1) + + * num_syms'. + * + * Returns 0 on success, or -1 if the lengths do not form a valid prefix + * code. + */ +int make_huffman_decode_table(u16 decode_table[], const u32 num_syms, + const u32 table_bits, const u8 lens[], + const u32 max_codeword_len, + u16 working_space[]) +{ + const u32 table_num_entries = 1 << table_bits; + u16 * const len_counts = &working_space[0]; + u16 * const offsets = &working_space[1 * (max_codeword_len + 1)]; + u16 * const sorted_syms = &working_space[2 * (max_codeword_len + 1)]; + int left; + void *decode_table_ptr; + u32 sym_idx; + u32 codeword_len; + u32 stores_per_loop; + u32 decode_table_pos; + u32 len; + u32 sym; + + /* Count how many symbols have each possible codeword length. + * Note that a length of 0 indicates the corresponding symbol is not + * used in the code and therefore does not have a codeword. + */ + for (len = 0; len <= max_codeword_len; len++) + len_counts[len] = 0; + for (sym = 0; sym < num_syms; sym++) + len_counts[lens[sym]]++; + + /* We can assume all lengths are <= max_codeword_len, but we + * cannot assume they form a valid prefix code. A codeword of + * length n should require a proportion of the codespace equaling + * (1/2)^n. The code is valid if and only if the codespace is + * exactly filled by the lengths, by this measure. + */ + left = 1; + for (len = 1; len <= max_codeword_len; len++) { + left <<= 1; + left -= len_counts[len]; + if (left < 0) { + /* The lengths overflow the codespace; that is, the code + * is over-subscribed. + */ + return -1; + } + } + + if (left) { + /* The lengths do not fill the codespace; that is, they form an + * incomplete set. + */ + if (left == (1 << max_codeword_len)) { + /* The code is completely empty. This is arguably + * invalid, but in fact it is valid in LZX and XPRESS, + * so we must allow it. By definition, no symbols can + * be decoded with an empty code. Consequently, we + * technically don't even need to fill in the decode + * table. However, to avoid accessing uninitialized + * memory if the algorithm nevertheless attempts to + * decode symbols using such a code, we zero out the + * decode table. + */ + memset(decode_table, 0, + table_num_entries * sizeof(decode_table[0])); + return 0; + } + return -1; + } + + /* Sort the symbols primarily by length and secondarily by symbol order. + */ + + /* Initialize 'offsets' so that offsets[len] for 1 <= len <= + * max_codeword_len is the number of codewords shorter than 'len' bits. + */ + offsets[1] = 0; + for (len = 1; len < max_codeword_len; len++) + offsets[len + 1] = offsets[len] + len_counts[len]; + + /* Use the 'offsets' array to sort the symbols. Note that we do not + * include symbols that are not used in the code. Consequently, fewer + * than 'num_syms' entries in 'sorted_syms' may be filled. + */ + for (sym = 0; sym < num_syms; sym++) + if (lens[sym]) + sorted_syms[offsets[lens[sym]]++] = sym; + + /* Fill entries for codewords with length <= table_bits + * --- that is, those short enough for a direct mapping. + * + * The table will start with entries for the shortest codeword(s), which + * have the most entries. From there, the number of entries per + * codeword will decrease. + */ + decode_table_ptr = decode_table; + sym_idx = 0; + codeword_len = 1; + stores_per_loop = (1 << (table_bits - codeword_len)); + for (; stores_per_loop != 0; codeword_len++, stores_per_loop >>= 1) { + u32 end_sym_idx = sym_idx + len_counts[codeword_len]; + + for (; sym_idx < end_sym_idx; sym_idx++) { + u16 entry; + u16 *p; + u32 n; + + entry = ((u32)codeword_len << 11) | sorted_syms[sym_idx]; + p = (u16 *)decode_table_ptr; + n = stores_per_loop; + + do { + *p++ = entry; + } while (--n); + + decode_table_ptr = p; + } + } + + /* If we've filled in the entire table, we are done. Otherwise, + * there are codewords longer than table_bits for which we must + * generate binary trees. + */ + decode_table_pos = (u16 *)decode_table_ptr - decode_table; + if (decode_table_pos != table_num_entries) { + u32 j; + u32 next_free_tree_slot; + u32 cur_codeword; + + /* First, zero out the remaining entries. This is + * necessary so that these entries appear as + * "unallocated" in the next part. Each of these entries + * will eventually be filled with the representation of + * the root node of a binary tree. + */ + j = decode_table_pos; + do { + decode_table[j] = 0; + } while (++j != table_num_entries); + + /* We allocate child nodes starting at the end of the + * direct lookup table. Note that there should be + * 2*num_syms extra entries for this purpose, although + * fewer than this may actually be needed. + */ + next_free_tree_slot = table_num_entries; + + /* Iterate through each codeword with length greater than + * 'table_bits', primarily in order of codeword length + * and secondarily in order of symbol. + */ + for (cur_codeword = decode_table_pos << 1; + codeword_len <= max_codeword_len; + codeword_len++, cur_codeword <<= 1) { + u32 end_sym_idx = sym_idx + len_counts[codeword_len]; + + for (; sym_idx < end_sym_idx; sym_idx++, cur_codeword++) { + /* 'sorted_sym' is the symbol represented by the + * codeword. + */ + u32 sorted_sym = sorted_syms[sym_idx]; + u32 extra_bits = codeword_len - table_bits; + u32 node_idx = cur_codeword >> extra_bits; + + /* Go through each bit of the current codeword + * beyond the prefix of length @table_bits and + * walk the appropriate binary tree, allocating + * any slots that have not yet been allocated. + * + * Note that the 'pointer' entry to the binary + * tree, which is stored in the direct lookup + * portion of the table, is represented + * identically to other internal (non-leaf) + * nodes of the binary tree; it can be thought + * of as simply the root of the tree. The + * representation of these internal nodes is + * simply the index of the left child combined + * with the special bits 0xC000 to distinguish + * the entry from direct mapping and leaf node + * entries. + */ + do { + /* At least one bit remains in the + * codeword, but the current node is an + * unallocated leaf. Change it to an + * internal node. + */ + if (decode_table[node_idx] == 0) { + decode_table[node_idx] = + next_free_tree_slot | 0xC000; + decode_table[next_free_tree_slot++] = 0; + decode_table[next_free_tree_slot++] = 0; + } + + /* Go to the left child if the next bit + * in the codeword is 0; otherwise go to + * the right child. + */ + node_idx = decode_table[node_idx] & 0x3FFF; + --extra_bits; + node_idx += (cur_codeword >> extra_bits) & 1; + } while (extra_bits != 0); + + /* We've traversed the tree using the entire + * codeword, and we're now at the entry where + * the actual symbol will be stored. This is + * distinguished from internal nodes by not + * having its high two bits set. + */ + decode_table[node_idx] = sorted_sym; + } + } + } + return 0; +} |