598 lines
24 KiB
C
598 lines
24 KiB
C
/* enough.c -- determine the maximum size of inflate's Huffman code tables over
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* all possible valid and complete prefix codes, subject to a length limit.
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* Copyright (C) 2007, 2008, 2012, 2018, 2024 Mark Adler
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* Version 1.6 29 July 2024 Mark Adler
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*/
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/* Version history:
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1.0 3 Jan 2007 First version (derived from codecount.c version 1.4)
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1.1 4 Jan 2007 Use faster incremental table usage computation
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Prune examine() search on previously visited states
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1.2 5 Jan 2007 Comments clean up
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As inflate does, decrease root for short codes
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Refuse cases where inflate would increase root
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1.3 17 Feb 2008 Add argument for initial root table size
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Fix bug for initial root table size == max - 1
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Use a macro to compute the history index
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1.4 18 Aug 2012 Avoid shifts more than bits in type (caused endless loop!)
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Clean up comparisons of different types
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Clean up code indentation
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1.5 5 Aug 2018 Clean up code style, formatting, and comments
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Show all the codes for the maximum, and only the maximum
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1.6 29 Jul 2024 Avoid use of uintmax_t
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*/
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/*
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Examine all possible prefix codes for a given number of symbols and a
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maximum code length in bits to determine the maximum table size for zlib's
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inflate. Only complete prefix codes are counted.
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Two codes are considered distinct if the vectors of the number of codes per
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length are not identical. So permutations of the symbol assignments result
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in the same code for the counting, as do permutations of the assignments of
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the bit values to the codes (i.e. only canonical codes are counted).
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We build a code from shorter to longer lengths, determining how many symbols
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are coded at each length. At each step, we have how many symbols remain to
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be coded, what the last code length used was, and how many bit patterns of
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that length remain unused. Then we add one to the code length and double the
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number of unused patterns to graduate to the next code length. We then
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assign all portions of the remaining symbols to that code length that
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preserve the properties of a correct and eventually complete code. Those
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properties are: we cannot use more bit patterns than are available; and when
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all the symbols are used, there are exactly zero possible bit patterns left
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unused.
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The inflate Huffman decoding algorithm uses two-level lookup tables for
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speed. There is a single first-level table to decode codes up to root bits
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in length (root == 9 for literal/length codes and root == 6 for distance
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codes, in the current inflate implementation). The base table has 1 << root
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entries and is indexed by the next root bits of input. Codes shorter than
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root bits have replicated table entries, so that the correct entry is
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pointed to regardless of the bits that follow the short code. If the code is
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longer than root bits, then the table entry points to a second-level table.
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The size of that table is determined by the longest code with that root-bit
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prefix. If that longest code has length len, then the table has size 1 <<
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(len - root), to index the remaining bits in that set of codes. Each
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subsequent root-bit prefix then has its own sub-table. The total number of
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table entries required by the code is calculated incrementally as the number
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of codes at each bit length is populated. When all of the codes are shorter
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than root bits, then root is reduced to the longest code length, resulting
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in a single, smaller, one-level table.
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The inflate algorithm also provides for small values of root (relative to
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the log2 of the number of symbols), where the shortest code has more bits
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than root. In that case, root is increased to the length of the shortest
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code. This program, by design, does not handle that case, so it is verified
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that the number of symbols is less than 1 << (root + 1).
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In order to speed up the examination (by about ten orders of magnitude for
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the default arguments), the intermediate states in the build-up of a code
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are remembered and previously visited branches are pruned. The memory
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required for this will increase rapidly with the total number of symbols and
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the maximum code length in bits. However this is a very small price to pay
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for the vast speedup.
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First, all of the possible prefix codes are counted, and reachable
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intermediate states are noted by a non-zero count in a saved-results array.
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Second, the intermediate states that lead to (root + 1) bit or longer codes
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are used to look at all sub-codes from those junctures for their inflate
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memory usage. (The amount of memory used is not affected by the number of
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codes of root bits or less in length.) Third, the visited states in the
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construction of those sub-codes and the associated calculation of the table
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size is recalled in order to avoid recalculating from the same juncture.
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Beginning the code examination at (root + 1) bit codes, which is enabled by
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identifying the reachable nodes, accounts for about six of the orders of
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magnitude of improvement for the default arguments. About another four
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orders of magnitude come from not revisiting previous states. Out of
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approximately 2x10^16 possible prefix codes, only about 2x10^6 sub-codes
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need to be examined to cover all of the possible table memory usage cases
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for the default arguments of 286 symbols limited to 15-bit codes.
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Note that unsigned long long is used for counting. It is quite easy to
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exceed the capacity of an eight-byte integer with a large number of symbols
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and a large maximum code length, so multiple-precision arithmetic would need
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to replace the integer arithmetic in that case. This program will abort if
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an overflow occurs. The big_t type identifies where the counting takes
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place.
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The unsigned long long type is also used for calculating the number of
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possible codes remaining at the maximum length. This limits the maximum code
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length to the number of bits in a long long minus the number of bits needed
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to represent the symbols in a flat code. The code_t type identifies where
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the bit-pattern counting takes place.
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <stdarg.h>
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#include <assert.h>
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#define local static
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// Special data types.
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typedef unsigned long long big_t; // type for code counting
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#define PRIbig "llu" // printf format for big_t
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typedef big_t code_t; // type for bit pattern counting
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struct tab { // type for been-here check
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size_t len; // allocated length of bit vector in octets
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char *vec; // allocated bit vector
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};
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/* The array for saving results, num[], is indexed with this triplet:
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syms: number of symbols remaining to code
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left: number of available bit patterns at length len
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len: number of bits in the codes currently being assigned
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Those indices are constrained thusly when saving results:
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syms: 3..totsym (totsym == total symbols to code)
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left: 2..syms - 1, but only the evens (so syms == 8 -> 2, 4, 6)
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len: 1..max - 1 (max == maximum code length in bits)
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syms == 2 is not saved since that immediately leads to a single code. left
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must be even, since it represents the number of available bit patterns at
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the current length, which is double the number at the previous length. left
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ends at syms-1 since left == syms immediately results in a single code.
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(left > sym is not allowed since that would result in an incomplete code.)
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len is less than max, since the code completes immediately when len == max.
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The offset into the array is calculated for the three indices with the first
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one (syms) being outermost, and the last one (len) being innermost. We build
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the array with length max-1 lists for the len index, with syms-3 of those
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for each symbol. There are totsym-2 of those, with each one varying in
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length as a function of sym. See the calculation of index in map() for the
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index, and the calculation of size in main() for the size of the array.
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For the deflate example of 286 symbols limited to 15-bit codes, the array
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has 284,284 entries, taking up 2.17 MB for an 8-byte big_t. More than half
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of the space allocated for saved results is actually used -- not all
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possible triplets are reached in the generation of valid prefix codes.
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*/
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/* The array for tracking visited states, done[], is itself indexed identically
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to the num[] array as described above for the (syms, left, len) triplet.
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Each element in the array is further indexed by the (mem, rem) doublet,
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where mem is the amount of inflate table space used so far, and rem is the
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remaining unused entries in the current inflate sub-table. Each indexed
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element is simply one bit indicating whether the state has been visited or
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not. Since the ranges for mem and rem are not known a priori, each bit
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vector is of a variable size, and grows as needed to accommodate the visited
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states. mem and rem are used to calculate a single index in a triangular
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array. Since the range of mem is expected in the default case to be about
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ten times larger than the range of rem, the array is skewed to reduce the
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memory usage, with eight times the range for mem than for rem. See the
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calculations for offset and bit in been_here() for the details.
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For the deflate example of 286 symbols limited to 15-bit codes, the bit
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vectors grow to total 5.5 MB, in addition to the 4.3 MB done array itself.
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*/
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// Type for a variable-length, allocated string.
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typedef struct {
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char *str; // pointer to allocated string
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size_t size; // size of allocation
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size_t len; // length of string, not including terminating zero
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} string_t;
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// Clear a string_t.
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local void string_clear(string_t *s) {
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s->str[0] = 0;
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s->len = 0;
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}
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// Initialize a string_t.
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local void string_init(string_t *s) {
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s->size = 16;
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s->str = malloc(s->size);
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assert(s->str != NULL && "out of memory");
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string_clear(s);
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}
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// Release the allocation of a string_t.
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local void string_free(string_t *s) {
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free(s->str);
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s->str = NULL;
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s->size = 0;
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s->len = 0;
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}
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// Save the results of printf with fmt and the subsequent argument list to s.
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// Each call appends to s. The allocated space for s is increased as needed.
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local void string_printf(string_t *s, char *fmt, ...) {
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va_list ap;
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va_start(ap, fmt);
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size_t len = s->len;
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int ret = vsnprintf(s->str + len, s->size - len, fmt, ap);
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assert(ret >= 0 && "out of memory");
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s->len += ret;
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if (s->size < s->len + 1) {
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do {
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s->size <<= 1;
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assert(s->size != 0 && "overflow");
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} while (s->size < s->len + 1);
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s->str = realloc(s->str, s->size);
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assert(s->str != NULL && "out of memory");
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vsnprintf(s->str + len, s->size - len, fmt, ap);
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}
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va_end(ap);
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}
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// Globals to avoid propagating constants or constant pointers recursively.
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struct {
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int max; // maximum allowed bit length for the codes
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int root; // size of base code table in bits
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int large; // largest code table so far
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size_t size; // number of elements in num and done
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big_t tot; // total number of codes with maximum tables size
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string_t out; // display of subcodes for maximum tables size
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int *code; // number of symbols assigned to each bit length
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big_t *num; // saved results array for code counting
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struct tab *done; // states already evaluated array
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} g;
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// Index function for num[] and done[].
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local inline size_t map(int syms, int left, int len) {
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return ((size_t)((syms - 1) >> 1) * ((syms - 2) >> 1) +
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(left >> 1) - 1) * (g.max - 1) +
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len - 1;
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}
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// Free allocated space in globals.
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local void cleanup(void) {
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if (g.done != NULL) {
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for (size_t n = 0; n < g.size; n++)
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if (g.done[n].len)
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free(g.done[n].vec);
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g.size = 0;
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free(g.done); g.done = NULL;
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}
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free(g.num); g.num = NULL;
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free(g.code); g.code = NULL;
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string_free(&g.out);
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}
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// Return the number of possible prefix codes using bit patterns of lengths len
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// through max inclusive, coding syms symbols, with left bit patterns of length
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// len unused -- return -1 if there is an overflow in the counting. Keep a
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// record of previous results in num to prevent repeating the same calculation.
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local big_t count(int syms, int left, int len) {
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// see if only one possible code
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if (syms == left)
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return 1;
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// note and verify the expected state
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assert(syms > left && left > 0 && len < g.max);
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// see if we've done this one already
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size_t index = map(syms, left, len);
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big_t got = g.num[index];
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if (got)
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return got; // we have -- return the saved result
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// we need to use at least this many bit patterns so that the code won't be
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// incomplete at the next length (more bit patterns than symbols)
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int least = (left << 1) - syms;
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if (least < 0)
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least = 0;
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// we can use at most this many bit patterns, lest there not be enough
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// available for the remaining symbols at the maximum length (if there were
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// no limit to the code length, this would become: most = left - 1)
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int most = (((code_t)left << (g.max - len)) - syms) /
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(((code_t)1 << (g.max - len)) - 1);
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// count all possible codes from this juncture and add them up
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big_t sum = 0;
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for (int use = least; use <= most; use++) {
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got = count(syms - use, (left - use) << 1, len + 1);
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sum += got;
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if (got == (big_t)-1 || sum < got) // overflow
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return (big_t)-1;
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}
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// verify that all recursive calls are productive
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assert(sum != 0);
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// save the result and return it
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g.num[index] = sum;
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return sum;
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}
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// Return true if we've been here before, set to true if not. Set a bit in a
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// bit vector to indicate visiting this state. Each (syms,len,left) state has a
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// variable size bit vector indexed by (mem,rem). The bit vector is lengthened
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// as needed to allow setting the (mem,rem) bit.
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local int been_here(int syms, int left, int len, int mem, int rem) {
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// point to vector for (syms,left,len), bit in vector for (mem,rem)
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size_t index = map(syms, left, len);
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mem -= 1 << g.root; // mem always includes the root table
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mem >>= 1; // mem and rem are always even
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rem >>= 1;
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size_t offset = (mem >> 3) + rem;
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offset = ((offset * (offset + 1)) >> 1) + rem;
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int bit = 1 << (mem & 7);
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// see if we've been here
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size_t length = g.done[index].len;
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if (offset < length && (g.done[index].vec[offset] & bit) != 0)
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return 1; // done this!
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// we haven't been here before -- set the bit to show we have now
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// see if we need to lengthen the vector in order to set the bit
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if (length <= offset) {
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// if we have one already, enlarge it, zero out the appended space
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char *vector;
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if (length) {
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do {
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length <<= 1;
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} while (length <= offset);
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vector = realloc(g.done[index].vec, length);
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assert(vector != NULL && "out of memory");
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memset(vector + g.done[index].len, 0, length - g.done[index].len);
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}
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// otherwise we need to make a new vector and zero it out
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else {
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length = 16;
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while (length <= offset)
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length <<= 1;
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vector = calloc(length, 1);
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assert(vector != NULL && "out of memory");
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}
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// install the new vector
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g.done[index].len = length;
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g.done[index].vec = vector;
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}
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// set the bit
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g.done[index].vec[offset] |= bit;
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return 0;
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}
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// Examine all possible codes from the given node (syms, len, left). Compute
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// the amount of memory required to build inflate's decoding tables, where the
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// number of code structures used so far is mem, and the number remaining in
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// the current sub-table is rem.
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local void examine(int syms, int left, int len, int mem, int rem) {
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// see if we have a complete code
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if (syms == left) {
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// set the last code entry
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g.code[len] = left;
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// complete computation of memory used by this code
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while (rem < left) {
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left -= rem;
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rem = 1 << (len - g.root);
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mem += rem;
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}
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assert(rem == left);
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// if this is at the maximum, show the sub-code
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if (mem >= g.large) {
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// if this is a new maximum, update the maximum and clear out the
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// printed sub-codes from the previous maximum
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if (mem > g.large) {
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g.large = mem;
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string_clear(&g.out);
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}
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// compute the starting state for this sub-code
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syms = 0;
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left = 1 << g.max;
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for (int bits = g.max; bits > g.root; bits--) {
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syms += g.code[bits];
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left -= g.code[bits];
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assert((left & 1) == 0);
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left >>= 1;
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}
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// print the starting state and the resulting sub-code to g.out
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string_printf(&g.out, "<%u, %u, %u>:",
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syms, g.root + 1, ((1 << g.root) - left) << 1);
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for (int bits = g.root + 1; bits <= g.max; bits++)
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if (g.code[bits])
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string_printf(&g.out, " %d[%d]", g.code[bits], bits);
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string_printf(&g.out, "\n");
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}
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// remove entries as we drop back down in the recursion
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g.code[len] = 0;
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return;
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}
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// prune the tree if we can
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if (been_here(syms, left, len, mem, rem))
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return;
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// we need to use at least this many bit patterns so that the code won't be
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// incomplete at the next length (more bit patterns than symbols)
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int least = (left << 1) - syms;
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if (least < 0)
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least = 0;
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// we can use at most this many bit patterns, lest there not be enough
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// available for the remaining symbols at the maximum length (if there were
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// no limit to the code length, this would become: most = left - 1)
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int most = (((code_t)left << (g.max - len)) - syms) /
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(((code_t)1 << (g.max - len)) - 1);
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// occupy least table spaces, creating new sub-tables as needed
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int use = least;
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while (rem < use) {
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use -= rem;
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rem = 1 << (len - g.root);
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mem += rem;
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}
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rem -= use;
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// examine codes from here, updating table space as we go
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for (use = least; use <= most; use++) {
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g.code[len] = use;
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examine(syms - use, (left - use) << 1, len + 1,
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mem + (rem ? 1 << (len - g.root) : 0), rem << 1);
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if (rem == 0) {
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rem = 1 << (len - g.root);
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mem += rem;
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}
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rem--;
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}
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// remove entries as we drop back down in the recursion
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g.code[len] = 0;
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}
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// Look at all sub-codes starting with root + 1 bits. Look at only the valid
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// intermediate code states (syms, left, len). For each completed code,
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// calculate the amount of memory required by inflate to build the decoding
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// tables. Find the maximum amount of memory required and show the codes that
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// require that maximum.
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local void enough(int syms) {
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// clear code
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for (int n = 0; n <= g.max; n++)
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g.code[n] = 0;
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// look at all (root + 1) bit and longer codes
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string_clear(&g.out); // empty saved results
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g.large = 1 << g.root; // base table
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if (g.root < g.max) // otherwise, there's only a base table
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for (int n = 3; n <= syms; n++)
|
|
for (int left = 2; left < n; left += 2) {
|
|
// look at all reachable (root + 1) bit nodes, and the
|
|
// resulting codes (complete at root + 2 or more)
|
|
size_t index = map(n, left, g.root + 1);
|
|
if (g.root + 1 < g.max && g.num[index]) // reachable node
|
|
examine(n, left, g.root + 1, 1 << g.root, 0);
|
|
|
|
// also look at root bit codes with completions at root + 1
|
|
// bits (not saved in num, since complete), just in case
|
|
if (g.num[index - 1] && n <= left << 1)
|
|
examine((n - left) << 1, (n - left) << 1, g.root + 1,
|
|
1 << g.root, 0);
|
|
}
|
|
|
|
// done
|
|
printf("maximum of %d table entries for root = %d\n", g.large, g.root);
|
|
fputs(g.out.str, stdout);
|
|
}
|
|
|
|
// Examine and show the total number of possible prefix codes for a given
|
|
// maximum number of symbols, initial root table size, and maximum code length
|
|
// in bits -- those are the command arguments in that order. The default values
|
|
// are 286, 9, and 15 respectively, for the deflate literal/length code. The
|
|
// possible codes are counted for each number of coded symbols from two to the
|
|
// maximum. The counts for each of those and the total number of codes are
|
|
// shown. The maximum number of inflate table entries is then calculated across
|
|
// all possible codes. Each new maximum number of table entries and the
|
|
// associated sub-code (starting at root + 1 == 10 bits) is shown.
|
|
//
|
|
// To count and examine prefix codes that are not length-limited, provide a
|
|
// maximum length equal to the number of symbols minus one.
|
|
//
|
|
// For the deflate literal/length code, use "enough". For the deflate distance
|
|
// code, use "enough 30 6".
|
|
int main(int argc, char **argv) {
|
|
// set up globals for cleanup()
|
|
g.code = NULL;
|
|
g.num = NULL;
|
|
g.done = NULL;
|
|
string_init(&g.out);
|
|
|
|
// get arguments -- default to the deflate literal/length code
|
|
int syms = 286;
|
|
g.root = 9;
|
|
g.max = 15;
|
|
if (argc > 1) {
|
|
syms = atoi(argv[1]);
|
|
if (argc > 2) {
|
|
g.root = atoi(argv[2]);
|
|
if (argc > 3)
|
|
g.max = atoi(argv[3]);
|
|
}
|
|
}
|
|
if (argc > 4 || syms < 2 || g.root < 1 || g.max < 1) {
|
|
fputs("invalid arguments, need: [sym >= 2 [root >= 1 [max >= 1]]]\n",
|
|
stderr);
|
|
return 1;
|
|
}
|
|
|
|
// if not restricting the code length, the longest is syms - 1
|
|
if (g.max > syms - 1)
|
|
g.max = syms - 1;
|
|
|
|
// determine the number of bits in a code_t
|
|
int bits = 0;
|
|
for (code_t word = 1; word; word <<= 1)
|
|
bits++;
|
|
|
|
// make sure that the calculation of most will not overflow
|
|
if (g.max > bits || (code_t)(syms - 2) >= ((code_t)-1 >> (g.max - 1))) {
|
|
fputs("abort: code length too long for internal types\n", stderr);
|
|
return 1;
|
|
}
|
|
|
|
// reject impossible code requests
|
|
if ((code_t)(syms - 1) > ((code_t)1 << g.max) - 1) {
|
|
fprintf(stderr, "%d symbols cannot be coded in %d bits\n",
|
|
syms, g.max);
|
|
return 1;
|
|
}
|
|
|
|
// allocate code vector
|
|
g.code = calloc(g.max + 1, sizeof(int));
|
|
assert(g.code != NULL && "out of memory");
|
|
|
|
// determine size of saved results array, checking for overflows,
|
|
// allocate and clear the array (set all to zero with calloc())
|
|
if (syms == 2) // iff max == 1
|
|
g.num = NULL; // won't be saving any results
|
|
else {
|
|
g.size = syms >> 1;
|
|
int n = (syms - 1) >> 1;
|
|
assert(g.size <= (size_t)-1 / n && "overflow");
|
|
g.size *= n;
|
|
n = g.max - 1;
|
|
assert(g.size <= (size_t)-1 / n && "overflow");
|
|
g.size *= n;
|
|
g.num = calloc(g.size, sizeof(big_t));
|
|
assert(g.num != NULL && "out of memory");
|
|
}
|
|
|
|
// count possible codes for all numbers of symbols, add up counts
|
|
big_t sum = 0;
|
|
for (int n = 2; n <= syms; n++) {
|
|
big_t got = count(n, 2, 1);
|
|
sum += got;
|
|
assert(got != (big_t)-1 && sum >= got && "overflow");
|
|
}
|
|
printf("%"PRIbig" total codes for 2 to %d symbols", sum, syms);
|
|
if (g.max < syms - 1)
|
|
printf(" (%d-bit length limit)\n", g.max);
|
|
else
|
|
puts(" (no length limit)");
|
|
|
|
// allocate and clear done array for been_here()
|
|
if (syms == 2)
|
|
g.done = NULL;
|
|
else {
|
|
g.done = calloc(g.size, sizeof(struct tab));
|
|
assert(g.done != NULL && "out of memory");
|
|
}
|
|
|
|
// find and show maximum inflate table usage
|
|
if (g.root > g.max) // reduce root to max length
|
|
g.root = g.max;
|
|
if ((code_t)syms < ((code_t)1 << (g.root + 1)))
|
|
enough(syms);
|
|
else
|
|
fputs("cannot handle minimum code lengths > root", stderr);
|
|
|
|
// done
|
|
cleanup();
|
|
return 0;
|
|
}
|