172 lines
4.9 KiB
Markdown
172 lines
4.9 KiB
Markdown
# Quick notes about binary CMap format (bcmap)
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The format is designed to package some information from the CMap files located at external/cmap. Please notice for size optimization reasons, the original information blocks can be changed (split or joined) and items in the blocks can be swapped.
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The data stored in binary format in network byte order (big-endian).
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# Data primitives
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The following primitives used during encoding of the file:
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- byte (B) – a byte, bits are numbered from 0 (less significant) to 7 (most significant)
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- bytes block (B[n]) – a sequence of n bytes
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- unsigned number (UN) – the number is encoded as sequence of bytes, bit 7 is flag to continue decoding the byte, bits 6-0 store number information, e.g. bytes 0x818407 will represent 16903 (0x4207). Limited to the 32 bit.
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- signed number (SN) – the number is encoded as sequence of bytes, as UN, however shall be transformed before encoding: if n < 0, the n shall be encoded as (-2*n-1) using UN encoding, other n shall be encoded as (2*n) using UN encoding. So the lowest bit of the number indicates the sign of the initial number
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- unsigned fixed number (UB[n]) – similar to the UN, but it represents an unsigned number that is stored in B[n]
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- signed fixed number (SB[n]) – similar to the SN, but it represents a signed number that is stored in B[n]
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- string (S) – the string is encoded as sequence of bytes. First comes length is characters encoded as UN, when UTF16 characters encoded as UN.
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# File structure
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The first byte is a header:
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- bits 2-1 – indicate a CMapType. Valid values are 1 and 2
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- bit 0 – indicate WMode. Valid values are 0 and 1.
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Then records follow. The records starts from the record header encoded as B, where bits 7-5 indicate record type (see description of other bits below):
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- 0 – codespacerange
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- 1 – notdefrange
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- 2 – cidchar
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- 3 – cidrange
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- 4 – bfchar
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- 5 – bfrange
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- 6 – reserved
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- 7 – metadata
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## Metadata record
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The metadata record header bit 4-0 contain id of the metadata:
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- 0 – comment, body of the record is encoded comment string (S)
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- 1 – UseCMap, body of the record is usecmap id string (S)
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## Data records
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The records that have types 0 – 5, have the following fields in the header:
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- bit 4 – indicate the char or start/end entries are stored in a sequence in this block
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- bits 3-0 – contain length of the data size minus 1 in this block (dataSize)
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The amount of entries encoded as UN follows the header. The items records follow (see below).
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### codespacerange (0)
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Represents the following CMap block:
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n begincodespacerange
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<start> <end>
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endcodespacerange
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First record format is:
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- start as B[dataSize]
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- endDelta as UB[dataSize], end is calculated as (start + endDelta)
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Next record format is:
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- startDelta as UB[dataSize], start = end + startDelta
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- endDelta as UB[dataSize], end = start + endDelta
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### notdefrange (1)
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Represents the following CMap block:
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n beginnotdefrange
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<start> <end> code
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endnotdefrange
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First record format is:
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- start as B[dataSize]
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- endDelta as UB[dataSize], end is calculated as (start + endDelta)
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- code as UN
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Next record format is:
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- startDelta as UB[dataSize], start = end + startDelta
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- endDelta as UB[dataSize], end = start + endDelta
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- code as UN
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### cidchar (2)
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Represents the following CMap block:
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n begincidchar
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<char> code
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endcidchar
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First record format is:
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- char as B[dataSize]
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- code as UN
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Next record format is:
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- if sequence = 0, charDelta as UB[dataSize], char = char + charDelta + 1
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- if sequence = 1, char = char + 1
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- codeDelta as SN, code = code + codeDelta
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### cidrange (3)
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Represents the following CMap block:
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n begincidrange
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<start> <end> code
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endcidrange
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First record format is:
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- start as B[dataSize]
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- endDelta as UN[dataSize], end is calculated as (start + endDelta)
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- code as UN
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Next record format is:
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- if sequence = 0, startDelta as UB[dataSize], start = end + startDelta + 1
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- if sequence = 1, start = end + 1
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- endDelta as UN[dataSize], end = start + endDelta
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- code as UN
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### bfchar (4)
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Represents the following CMap block:
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n beginbfchar
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<char> <code>
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endbfchar
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First record format is:
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- char as B[ucs2Size], where ucs2Size = 2 (here and below)
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- code as B[dataSize]
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Next record format is:
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- if sequence = 0, charDelta as UN[ucs2Size], char = charDelta + charDelta + 1
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- if sequence = 1, char = char + 1
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- codeDelta as SB[dataSize], code = code + codeDelta
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### bfrange (5)
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Represents the following CMap block:
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n beginbfrange
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<start> <end> <code>
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endbfrange
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First record format is:
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- start as B[ucs2Size]
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- endDelta as UB[ucs2Size], end is calculated as (start + endDelta)
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- code as B[dataSize]
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Next record format is:
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- if sequence = 0, startDelta as UB[ucs2Size], start = end + startDelta + 1
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- if sequence = 1, start = end + 1
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- endDelta as UB[ucs2Size], end = start + endDelta
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- code as B[dataSize]
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