/* * CDDL HEADER START * * The contents of this file are subject to the terms of the * Common Development and Distribution License, Version 1.0 only * (the "License"). You may not use this file except in compliance * with the License. * * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE * or http://www.opensolaris.org/os/licensing. * See the License for the specific language governing permissions * and limitations under the License. * * When distributing Covered Code, include this CDDL HEADER in each * file and include the License file at usr/src/OPENSOLARIS.LICENSE. * If applicable, add the following below this CDDL HEADER, with the * fields enclosed by brackets "[]" replaced with your own identifying * information: Portions Copyright [yyyy] [name of copyright owner] * * CDDL HEADER END */ /* * Copyright (c) 1998 by Sun Microsystems, Inc. * All rights reserved. */ /* * NOTE: this file is compiled into the kernel, cprboot, and savecore. * Therefore it must compile in kernel, boot, and userland source context; * so if you ever change this code, avoid references to external symbols. * * This compression algorithm is a derivative of LZRW1, which I'll call * LZJB in the classic LZ* spirit. All LZ* (Lempel-Ziv) algorithms are * based on the same basic principle: when a "phrase" (sequences of bytes) * is repeated in a data stream, we can save space by storing a reference to * the previous instance of that phrase (a "copy item") rather than storing * the phrase itself (a "literal item"). The compressor remembers phrases * in a simple hash table (the "Lempel history") that maps three-character * sequences (the minimum match) to the addresses where they were last seen. * * A copy item must encode both the length and the location of the matching * phrase so that decompress() can reconstruct the original data stream. * For example, here's how we'd encode "yadda yadda yadda, blah blah blah" * (with "_" replacing spaces for readability): * * Original: * * y a d d a _ y a d d a _ y a d d a , _ b l a h _ b l a h _ b l a h * * Compressed: * * y a d d a _ 6 11 , _ b l a h 5 10 * * In the compressed output, the "6 11" simply means "to get the original * data, execute memmove(ptr, ptr - 6, 11)". Note that in this example, * the match at "6 11" actually extends beyond the current location and * overlaps it. That's OK; like memmove(), decompress() handles overlap. * * There's still one more thing decompress() needs to know, which is how to * distinguish literal items from copy items. We encode this information * in an 8-bit bitmap that precedes each 8 items of output; if the Nth bit * is set, then the Nth item is a copy item. Thus the full encoding for * the example above would be: * * 0x40 y a d d a _ 6 11 , 0x20 _ b l a h 5 10 * * Finally, the "6 11" isn't really encoded as the two byte values 6 and 11 * in the output stream because, empirically, we get better compression by * dedicating more bits to offset, fewer to match length. LZJB uses 6 bits * to encode the match length, 10 bits to encode the offset. Since copy-item * encoding consumes 2 bytes, we don't generate copy items unless the match * length is at least 3; therefore, we can store (length - 3) in the 6-bit * match length field, which extends the maximum match from 63 to 66 bytes. * Thus the 2-byte encoding for a copy item is as follows: * * byte[0] = ((length - 3) << 2) | (offset >> 8); * byte[1] = (uint8_t)offset; * * In our example above, an offset of 6 with length 11 would be encoded as: * * byte[0] = ((11 - 3) << 2) | (6 >> 8) = 0x20 * byte[1] = (uint8_t)6 = 0x6 * * Similarly, an offset of 5 with length 10 would be encoded as: * * byte[0] = ((10 - 3) << 2) | (5 >> 8) = 0x1c * byte[1] = (uint8_t)5 = 0x5 * * Putting it all together, the actual LZJB output for our example is: * * 0x40 y a d d a _ 0x2006 , 0x20 _ b l a h 0x1c05 * * The main differences between LZRW1 and LZJB are as follows: * * (1) LZRW1 is sloppy about buffer overruns. LZJB never reads past the * end of its input, and never writes past the end of its output. * * (2) LZJB allows a maximum match length of 66 (vs. 18 for LZRW1), with * the trade-off being a shorter look-behind (1K vs. 4K for LZRW1). * * (3) LZJB records only the low-order 16 bits of pointers in the Lempel * history (which is all we need since the maximum look-behind is 1K), * and uses only 256 hash entries (vs. 4096 for LZRW1). This makes * the compression hash small enough to allocate on the stack, which * solves two problems: (1) it saves 64K of kernel/cprboot memory, * and (2) it makes the code MT-safe without any locking, since we * don't have multiple threads sharing a common hash table. * * (4) LZJB is faster at both compression and decompression, has a * better compression ratio, and is somewhat simpler than LZRW1. * * Finally, note that LZJB is non-deterministic: given the same input, * two calls to compress() may produce different output. This is a * general characteristic of most Lempel-Ziv derivatives because there's * no need to initialize the Lempel history; not doing so saves time. */ #include <sys/types.h> #include <sys/param.h> #define MATCH_BITS 6 #define MATCH_MIN 3 #define MATCH_MAX ((1 << MATCH_BITS) + (MATCH_MIN - 1)) #define OFFSET_MASK ((1 << (16 - MATCH_BITS)) - 1) #define LEMPEL_SIZE 256 size_t compress(void *s_start, void *d_start, size_t s_len) { uchar_t *src = s_start; uchar_t *dst = d_start; uchar_t *cpy, *copymap = NULL; int copymask = 1 << (NBBY - 1); int mlen, offset; uint16_t *hp; uint16_t lempel[LEMPEL_SIZE]; /* uninitialized; see above */ while (src < (uchar_t *)s_start + s_len) { if ((copymask <<= 1) == (1 << NBBY)) { if (dst >= (uchar_t *)d_start + s_len - 1 - 2 * NBBY) { mlen = s_len; for (src = s_start, dst = d_start; mlen; mlen--) *dst++ = *src++; return (s_len); } copymask = 1; copymap = dst; *dst++ = 0; } if (src > (uchar_t *)s_start + s_len - MATCH_MAX) { *dst++ = *src++; continue; } hp = &lempel[((src[0] + 13) ^ (src[1] - 13) ^ src[2]) & (LEMPEL_SIZE - 1)]; offset = (intptr_t)(src - *hp) & OFFSET_MASK; *hp = (uint16_t)(uintptr_t)src; cpy = src - offset; if (cpy >= (uchar_t *)s_start && cpy != src && src[0] == cpy[0] && src[1] == cpy[1] && src[2] == cpy[2]) { *copymap |= copymask; for (mlen = MATCH_MIN; mlen < MATCH_MAX; mlen++) if (src[mlen] != cpy[mlen]) break; *dst++ = ((mlen - MATCH_MIN) << (NBBY - MATCH_BITS)) | (offset >> NBBY); *dst++ = (uchar_t)offset; src += mlen; } else { *dst++ = *src++; } } return (dst - (uchar_t *)d_start); } size_t decompress(void *s_start, void *d_start, size_t s_len, size_t d_len) { uchar_t *src = s_start; uchar_t *dst = d_start; uchar_t *s_end = (uchar_t *)s_start + s_len; uchar_t *d_end = (uchar_t *)d_start + d_len; uchar_t *cpy, copymap = '\0'; int copymask = 1 << (NBBY - 1); if (s_len >= d_len) { size_t d_rem = d_len; while (d_rem-- != 0) *dst++ = *src++; return (d_len); } while (src < s_end && dst < d_end) { if ((copymask <<= 1) == (1 << NBBY)) { copymask = 1; copymap = *src++; } if (copymap & copymask) { int mlen = (src[0] >> (NBBY - MATCH_BITS)) + MATCH_MIN; int offset = ((src[0] << NBBY) | src[1]) & OFFSET_MASK; src += 2; if ((cpy = dst - offset) >= (uchar_t *)d_start) while (--mlen >= 0 && dst < d_end) *dst++ = *cpy++; else /* * offset before start of destination buffer * indicates corrupt source data */ return (dst - (uchar_t *)d_start); } else { *dst++ = *src++; } } return (dst - (uchar_t *)d_start); } uint32_t checksum32(void *cp_arg, size_t length) { uchar_t *cp, *ep; uint32_t sum = 0; for (cp = cp_arg, ep = cp + length; cp < ep; cp++) sum = ((sum >> 1) | (sum << 31)) + *cp; return (sum); }
