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Minor formatting changes
* In the description about "three categories", explicitly number them instead of using a giant paragraph that is harder to follow. * Switch lists of items to consistently use American style commas. The American style lists is better for clarity purposes. Consider the following: -Each category of value (insert and copy lengths, literals and distances) +Each category of value (insert and copy lengths, literals, and distances) * Make sure not to break a hyphenated phrase with a newline. When the nroff file is processed, "insert-\nand-copy" becomes "insert- and-copy", making it inconsistent with other uses of the hyphenated phrase. * Consistently use the same hyphenated phrase if referred to as a single unit. "insert and copy" -> "insert-and-copy" "least significant" -> "least-significant" "most significant" -> "most-significant" "fixed length" -> "fixed-length" "block switch" -> "block-switch". * Consistently use "indexes" instead of "indices"
This commit is contained in:
Joe Tsai
parent
166edb0287
commit
c5b6b5c7c1
@ -160,11 +160,11 @@ String: a sequence of arbitrary bytes.
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Bytes stored within a computer do not have a "bit order", since
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they are always treated as a unit. However, a byte considered as
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an integer between 0 and 255 does have a most- and least-
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significant bit, and since we write numbers with the most-
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significant digit on the left, we also write bytes with the most-
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significant bit on the left. In the diagrams below, we number the
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bits of a byte so that bit 0 is the least-significant bit, i.e.,
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an integer between 0 and 255 does have a most- and
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least-significant bit, and since we write numbers with the
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most-significant digit on the left, we also write bytes with the
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most-significant bit on the left. In the diagrams below, we number
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the bits of a byte so that bit 0 is the least-significant bit, i.e.,
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the bits are numbered:
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.nf
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@ -209,15 +209,15 @@ compressed byte sequence:
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starting with the least-significant bit of the data
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element. These are referred to here as integer values
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and are considered unsigned.
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* Prefix codes are packed starting with the most-
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significant bit of the code.
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* Prefix codes are packed starting with the most-significant
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bit of the code.
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.fi
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In other words, if one were to print out the compressed data as
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a sequence of bytes, starting with the first byte at the
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*right* margin and proceeding to the *left*, with the most-
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significant bit of each byte on the left as usual, one would be
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able to parse the result from right to left, with fixed-width
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*right* margin and proceeding to the *left*, with the
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most-significant bit of each byte on the left as usual, one would
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be able to parse the result from right to left, with fixed-width
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elements in the correct MSB-to-LSB order and prefix codes in
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bit-reversed order (i.e., with the first bit of the code in the
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relative LSB position).
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@ -225,8 +225,8 @@ relative LSB position).
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.ti 0
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2. Compressed representation overview
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A compressed data set consists of a header and a series of meta-
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blocks. Each meta-block decompresses to a sequence of 0
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A compressed data set consists of a header and a series of
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meta-blocks. Each meta-block decompresses to a sequence of 0
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to 16,777,216 (16 MiB) uncompressed bytes. The final uncompressed data is
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the concatenation of the uncompressed sequences from each meta-block.
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@ -259,30 +259,37 @@ duplicated is two, but the last command in the meta-block is permitted to have
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only literals and no pointer to a string to duplicate.
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Each command in the compressed data is represented using three categories
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of prefix codes: one set of prefix codes are for the literal sequence
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lengths (also referred to as literal insertion lengths) and backward
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of prefix codes:
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1) One set of prefix codes are for the literal sequence lengths
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(also referred to as literal insertion lengths) and backward
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copy lengths (that is, a single code word represents two lengths,
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one of the literal sequence and one of the backward copy), a separate
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set of prefix codes are for literals, and a third set of prefix codes are for
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distances. The prefix code descriptions for each meta-block appear in a compact
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form just before the compressed data in the meta-block header. The insert and
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copy length and distance prefix codes may be followed by extra bits that are
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added to the base values determined by the codes. The number of extra bits is
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determined by the code.
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one of the literal sequence and one of the backward copy).
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2) One set of prefix codes are for literals.
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3) One set of prefix codes are for distances.
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The prefix code descriptions for each meta-block appear in a compact
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form just before the compressed data in the meta-block header.
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The insert-and-copy length and distance prefix codes may be followed by
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extra bits that are added to the base values determined by the codes.
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The number of extra bits is determined by the code.
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One meta-block command then appears as a sequence of prefix codes:
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Insert and copy length, literal, literal, ..., literal, distance
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Insert-and-copy length, literal, literal, ..., literal, distance
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where the insert and copy defines the number of literals that immediately
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follow and the copy length, and the distance defines how far back to go
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for the copy, used in combination with the copy length. The resulting
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where the insert-and-copy defines an insertion length and a copy length.
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The insertion length determines the number of literals that immediately
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follow. The distance defines how far back to go for the copy and the
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copy length determine the number of bytes to copy. The resulting
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uncompressed data is the sequence of bytes:
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literal, literal, ..., literal, copy, copy, ..., copy
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where the number of literal bytes and copy bytes are determined by the
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insert and copy length code. (The number of bytes copied for a static
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insert-and-copy length code. (The number of bytes copied for a static
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dictionary entry can vary from the copy length.)
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The last command in the meta-block may end with the last literal if the
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@ -301,14 +308,13 @@ a new block type and block count is read from the stream immediately
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preceding the next element of that category, which will use the new
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block type.
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The insert and copy block type directly determines which prefix code to
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use for the next insert and copy element. For the literal and distance
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The insert-and-copy block type directly determines which prefix code to
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use for the next insert-and-copy element. For the literal and distance
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elements, the respective block type is used in combination with other
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context information to determine which prefix code to use for the next
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element.
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Consider the following
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example:
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Consider the following example:
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(IaC0, L0, L1, L2, D0)(IaC1, D1)(IaC2, L3, L4, D2)(IaC3, L5, D3)
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@ -341,7 +347,7 @@ meta-block is then:
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IaC0 L0 L1 LBlockSwitch(1, 3) L2 D0 IaC1 DBlockSwitch(1, 3) D1
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IaCBlockSwitch(1, 2) IaC2 L3 L4 D2 IaC3 LBlockSwitch(0, 1) L5 D3
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where *BlockSwitch(t, n) switches to block type t for a count of n elements.
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where xBlockSwitch(t, n) switches to block type t for a count of n elements.
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Note that in this example DBlockSwitch(1, 3) immediately precedes the
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next required distance D1. It does not follow the last distance of
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the previous block, D0. Whenever an element of a category is needed,
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@ -349,14 +355,14 @@ and the block count for that category has reached zero, then a new
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block type and count is read from the stream just before reading that next
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element.
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The block switch commands for the first blocks of each category are not part
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The block-switch commands for the first blocks of each category are not part
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of the meta-block compressed data. Instead the first block type
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is defined to be 0, and the first block count for each category is
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encoded in the meta-block header. The prefix codes for the block types and counts, a total of
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six prefix codes over the three categories, are defined in a compact form in the meta-block
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header.
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Each category of value (insert-and-copy lengths, literals and distances)
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Each category of value (insert-and-copy lengths, literals, and distances)
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can be encoded with any prefix code from a collection of prefix
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codes belonging to the same category appearing in the meta-block header. The
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particular prefix code used can depend on two factors: the block
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@ -366,10 +372,9 @@ the uncompressed data, and in the case of distances, the context is the copy
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length from the same command. For insert-and-copy lengths, no context
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is used and the prefix code depends only on the block type. In the case
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of literals and distances, the context is mapped to a context ID in
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the range 0..63 for literals and 0..3 for distances and the matrix
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of the prefix code indices for each block type and context ID,
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called the context map, is encoded in a compact form in the meta-
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block header.
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the range 0..63 for literals and 0..3 for distances. The matrix
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of the prefix code indexes for each block type and context ID,
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called the context map, is encoded in a compact form in the meta-block header.
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For example, the prefix code to use to decode L2 depends on the
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block type (1), and the literal context ID determined by the two uncompressed
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@ -379,8 +384,8 @@ type (0), and the distance context ID determined by the copy length decoded
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from IaC0. The prefix code to use to decode IaC3 depends only on the block
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type (1).
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In addition to the parts listed above (prefix code for insert-
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and-copy lengths, literals, distances, block types and block counts
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In addition to the parts listed above (prefix code for insert-and-copy
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lengths, literals, distances, block types and block counts,
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and the context map), the meta-block header contains the number of
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uncompressed bytes coded in the meta-block and two additional parameters used in
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the representation of match distances: the number of postfix bits and
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@ -577,7 +582,7 @@ Prefix codes are used for different purposes in the brotli
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format, and each purpose has a different alphabet size. For
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literal codes the alphabet size is 256. For insert-and-copy
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length codes the alphabet size is 704. For block count codes,
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the alphabet size is 26. For distance codes, block type codes and
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the alphabet size is 26. For distance codes, block type codes, and
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the prefix codes used in compressing the context map, the
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alphabet size is dynamic and is based on parameters defined in
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later sections. The following table summarizes the alphabet sizes
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@ -585,25 +590,25 @@ for the various prefix codes and the sections where they are defined.
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.nf
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.KS
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+-----------------+------------------------+-------------+
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+-----------------+-------------------------+------------+
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| Prefix code | Alphabet size | Definition |
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+-----------------+------------------------+-------------+
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+-----------------+-------------------------+------------+
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| literal | 256 | |
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+-----------------+------------------------+-------------+
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+-----------------+-------------------------+------------+
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| distance | 16 + NDIRECT + | Section 4. |
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| | (48 << NPOSTFIX) | |
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+-----------------+------------------------+-------------+
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+-----------------+-------------------------+------------+
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| insert-and-copy | 704 | Section 5. |
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| length | | |
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+-----------------+------------------------+-------------+
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+-----------------+-------------------------+------------+
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| block count | 26 | Section 6. |
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+-----------------+------------------------+-------------+
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+-----------------+-------------------------+------------+
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| block type | NBLTYPESx + 2, | Section 6. |
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| | (where x is I, L or D) | |
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+-----------------+------------------------+-------------+
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| | (where x is I, L, or D) | |
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+-----------------+-------------------------+------------+
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| context map | NTREESx + RLEMAXx | Section 7. |
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| | (where x is L or D) | |
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+-----------------+------------------------+-------------+
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+-----------------+-------------------------+------------+
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.KE
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.fi
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@ -615,8 +620,8 @@ prefix code distinguishes between simple and complex prefix
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codes. If this value is 1, then a simple prefix code follows.
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Otherwise the value indicates the number of leading zeros.
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A simple prefix code can have only up to four symbols with non-
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zero code length. The format of the simple prefix code is as
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A simple prefix code can have only up to four symbols with
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non-zero code length. The format of the simple prefix code is as
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follows:
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.nf
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@ -657,7 +662,7 @@ follows:
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prefix code.
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* if NSYM = 4, the code lengths (in order of symbols decoded)
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depend on the tree-select bit: 2, 2, 2, 2, (tree-select bit 0)
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depend on the tree-select bit: 2, 2, 2, 2 (tree-select bit 0),
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or 1, 2, 3, 3 (tree-select bit 1).
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.fi
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@ -724,11 +729,10 @@ in the compressed data, where the bits are parsed from right to left):
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.KE
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.fi
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We can now define the format of the complex prefix code as
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follows:
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We can now define the format of the complex prefix code as follows:
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.nf
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2 bits: HSKIP, values of 0, 2 or 3 represent the respective
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2 bits: HSKIP, values of 0, 2, or 3 represent the respective
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number of skipped code lengths. The skipped lengths
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are taken to be zero. (An HSKIP of 1 indicates a
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Simple prefix code.)
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@ -742,7 +746,7 @@ follows:
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is for symbol 4.
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The code lengths of code length symbols are between 0 and
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5 and they are represented with 2 - 4 bits according to
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5, and they are represented with 2 - 4 bits according to
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the variable length code above. A code length of 0 means
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the corresponding code length symbol is not used.
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@ -805,7 +809,7 @@ the number of direct distance codes, denoted by NDIRECT (0..120). Both of
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these parameters are encoded in the meta-block header. We will also
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use the following derived parameter:
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POSTFIX_MASK = ((1 << NPOSTFIX) - 1)
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POSTFIX_MASK = (1 << NPOSTFIX) - 1
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The first 16 distance symbols are special symbols that reference
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past distances as follows:
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@ -830,10 +834,10 @@ past distances as follows:
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.fi
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The ring buffer of four last distances is initialized by the values
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16, 15, 11 and 4 (i.e. the fourth-to-last is set to 16, the third-to-last
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to 15, the second-to-last to 11 and the last distance to 4) at the
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beginning of the *stream* (as opposed to the beginning of the meta-
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block) and it is not reset at meta-block boundaries. When a distance
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16, 15, 11, and 4 (i.e. the fourth-to-last is set to 16, the third-to-last
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to 15, the second-to-last to 11, and the last distance to 4) at the
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beginning of the *stream* (as opposed to the beginning of the meta-block)
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and it is not reset at meta-block boundaries. When a distance
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symbol 0 appears, the distance it represents (i.e. the last distance
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in the sequence of distances) is not pushed to the ring buffer of
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last distances, in other words, the expression "(second, third,
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@ -865,7 +869,7 @@ Given a distance symbol "dcode" (>= 16 + NDIRECT), and extra bits
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.nf
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hcode = (dcode - NDIRECT - 16) >> NPOSTFIX
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lcode = (dcode - NDIRECT - 16) & POSTFIX_MASK
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offset = ((2 + (hcode & 1)) << ndistbits) - 4;
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offset = ((2 + (hcode & 1)) << ndistbits) - 4
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distance = ((offset + dextra) << NPOSTFIX) + lcode + NDIRECT + 1
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.fi
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@ -881,7 +885,7 @@ of a meta-block is represented with the following triplet:
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<insert-and-copy length code, insert extra bits, copy extra bits>
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The insert-and-copy length code, the insert extra bits and the copy
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The insert-and-copy length code, the insert extra bits, and the copy
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extra bits are encoded back-to-back, the insert-and-copy length code
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is encoded using a prefix code over the insert-and-copy length code
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alphabet, while the extra bits values are encoded as fixed-width
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@ -999,7 +1003,7 @@ are initialized to 1 and 0, respectively, at the end of the
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meta-block header.
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Since the first block type of each block category is 0, the block
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type of the first block switch command is not encoded in
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type of the first block-switch command is not encoded in
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the compressed data. Instead the block count for each category
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that has more than one type is encoded in the meta-block header.
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@ -1009,13 +1013,13 @@ count down to exactly zero at the end of the meta-block.
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The number of different block types in each block category, denoted
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by NBLTYPESL, NBLTYPESI, and NBLTYPESD for literals, insert-and-copy
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lengths and distances, respectively, is encoded in the meta-block
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lengths, and distances, respectively, is encoded in the meta-block
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header, and it must equal to the largest block type plus one in that
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block category. In other words, the set of literal, insert-and-copy
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length and distance block types must be [0..NBLTYPESL-1],
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length, and distance block types must be [0..NBLTYPESL-1],
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[0..NBLTYPESI-1], and [0..NBLTYPESD-1], respectively. From this it
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follows that the alphabet size of literal, insert-and-copy length and
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distance block type codes is NBLTYPESL + 2, NBLTYPESI + 2 and
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follows that the alphabet size of literal, insert-and-copy length, and
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distance block type codes is NBLTYPESL + 2, NBLTYPESI + 2, and
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NBLTYPESD + 2, respectively.
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Each block count in the compressed data is represented with a pair
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@ -1044,7 +1048,7 @@ number of extra bits and the range of block counts are as follows:
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.KE
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.fi
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The first block switch command of each block category is special in
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The first block-switch command of each block category is special in
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the sense that it is encoded in the meta-block header, and as
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described earlier the block type code is omitted, since it is an
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implicit zero.
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@ -1072,10 +1076,10 @@ p1 and p2 are initialized to zero.
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There are four methods, called context modes, to compute the
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Context ID:
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.nf
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* LSB6, where the Context ID is the value of six least
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significant bits of p1,
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* MSB6, where the Context ID is the value of six most
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significant bits of p1,
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* LSB6, where the Context ID is the value of six
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least-significant bits of p1,
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* MSB6, where the Context ID is the value of six
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most-significant bits of p1,
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* UTF8, where the Context ID is a complex function of p1, p2,
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optimized for text compression, and
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* Signed, where Context ID is a complex function of p1, p2,
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@ -1466,7 +1470,7 @@ the following:
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fill bits are not zero, then the stream should be
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rejected as invalid)
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2 bits: MNIBBLES, # of nibbles to represent the uncompressed
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length, encoded with the following fixed length code:
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length, encoded with the following fixed-length code:
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Value Bit Pattern
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----- -----------
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@ -1563,7 +1567,7 @@ the following:
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2 bits: NPOSTFIX, parameter used in the distance coding
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4 bits: four most significant bits of NDIRECT, to get the
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4 bits: four most-significant bits of NDIRECT, to get the
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actual value of the parameter NDIRECT, left-shift
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this four bit number by NPOSTFIX bits
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@ -1773,7 +1777,7 @@ specification, where non-conformant compressed data sequences should be discarde
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A possible attack against a system containing a decompressor
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implementation (e.g. a web browser) is to exploit a buffer
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overflow caused by an invalid compressed data. Therefore decompressor
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implementations should perform bound-checking for each memory access
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implementations should perform bounds-checking for each memory access
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that result from values decoded from the compressed stream.
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.ti 0
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@ -5678,8 +5682,8 @@ length is 122,784 bytes and the zlib CRC-32 of the byte sequence is
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9ce0a58de0a49ee0a4bee0a4aae0a4a8e0a495e0a4bee0a4b0e0a58de0a4b0e0
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a4b5e0a4bee0a488e0a4b8e0a495e0a58de0a4b0e0a4bfe0a4afe0a4a4e0a4be
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The number of words for each length is given by the following bit-
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depth array:
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The number of words for each length is given by the following
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bit-depth array:
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NDBITS := 0, 0, 0, 0, 10, 10, 11, 11, 10, 10,
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10, 10, 10, 9, 9, 8, 7, 7, 8, 7,
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Reference in New Issue
Block a user