lib/zlib_inflate/inftrees.c

Source file repositories/reference/linux-study-clean/lib/zlib_inflate/inftrees.c

File Facts

System
Linux kernel
Corpus path
lib/zlib_inflate/inftrees.c
Extension
.c
Size
13335 bytes
Lines
316
Domain
Kernel Services
Bucket
lib
Inferred role
Kernel Services: implementation source
Status
source implementation candidate

Why This File Exists

Shared kernel service surface used by multiple subsystems, including helpers, cryptography, virtualization support, and async I/O infrastructure.

Dependency Surface

Detected Declarations

Annotated Snippet

if (max == 0) {                     /* no symbols to code at all */
        this.op = (unsigned char)64;    /* invalid code marker */
        this.bits = (unsigned char)1;
        this.val = (unsigned short)0;
        *(*table)++ = this;             /* make a table to force an error */
        *(*table)++ = this;
        *bits = 1;
        return 0;     /* no symbols, but wait for decoding to report error */
    }
    for (min = 1; min < MAXBITS; min++)
        if (count[min] != 0) break;
    if (root < min) root = min;

    /* check for an over-subscribed or incomplete set of lengths */
    left = 1;
    for (len = 1; len <= MAXBITS; len++) {
        left <<= 1;
        left -= count[len];
        if (left < 0) return -1;        /* over-subscribed */
    }
    if (left > 0 && (type == CODES || max != 1))
        return -1;                      /* incomplete set */

    /* generate offsets into symbol table for each length for sorting */
    offs[1] = 0;
    for (len = 1; len < MAXBITS; len++)
        offs[len + 1] = offs[len] + count[len];

    /* sort symbols by length, by symbol order within each length */
    for (sym = 0; sym < codes; sym++)
        if (lens[sym] != 0) work[offs[lens[sym]]++] = (unsigned short)sym;

    /*
       Create and fill in decoding tables.  In this loop, the table being
       filled is at next and has curr index bits.  The code being used is huff
       with length len.  That code is converted to an index by dropping drop
       bits off of the bottom.  For codes where len is less than drop + curr,
       those top drop + curr - len bits are incremented through all values to
       fill the table with replicated entries.

       root is the number of index bits for the root table.  When len exceeds
       root, sub-tables are created pointed to by the root entry with an index
       of the low root bits of huff.  This is saved in low to check for when a
       new sub-table should be started.  drop is zero when the root table is
       being filled, and drop is root when sub-tables are being filled.

       When a new sub-table is needed, it is necessary to look ahead in the
       code lengths to determine what size sub-table is needed.  The length
       counts are used for this, and so count[] is decremented as codes are
       entered in the tables.

       used keeps track of how many table entries have been allocated from the
       provided *table space.  It is checked when a LENS table is being made
       against the space in *table, ENOUGH, minus the maximum space needed by
       the worst case distance code, MAXD.  This should never happen, but the
       sufficiency of ENOUGH has not been proven exhaustively, hence the check.
       This assumes that when type == LENS, bits == 9.

       sym increments through all symbols, and the loop terminates when
       all codes of length max, i.e. all codes, have been processed.  This
       routine permits incomplete codes, so another loop after this one fills
       in the rest of the decoding tables with invalid code markers.
     */

    /* set up for code type */
    switch (type) {
    case CODES:
        base = extra = work;    /* dummy value--not used */
        end = 19;
        break;
    case LENS:
        base = lbase;
        base -= 257;
        extra = lext;
        extra -= 257;
        end = 256;
        break;
    default:            /* DISTS */
        base = dbase;
        extra = dext;
        end = -1;
    }

    /* initialize state for loop */
    huff = 0;                   /* starting code */
    sym = 0;                    /* starting code symbol */
    len = min;                  /* starting code length */
    next = *table;              /* current table to fill in */
    curr = root;                /* current table index bits */
    drop = 0;                   /* current bits to drop from code for index */

Annotation

Implementation Notes