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133 lines
5.7 KiB
133 lines
5.7 KiB
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Copied from the old Etherpad. Found in /infrastructure/ace/
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Goals:
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- no unicode (for efficient escaping, sightliness)
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- efficient operations for ACE and collab (attributed text, etc.)
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- good for time-slider
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- good for API
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- line-ending aware
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X more coherent (deleting or styling text merging with insertion)
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- server-side syntax highlighting?
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- unify author map with attribute pool
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- unify attributed text with changeset rep
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- not: reversible
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- force final newline of document to be preserved
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- Unicode bad:
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- ugly (hard to read)
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- more complex to parse
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- harder to store and transmit correctly
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- doesn't save all that much space anyway
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- blows up in size when string-escaped
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- embarrassing for API
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# Attributes:
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An "attribute" is a (key,value) pair such as (author,abc123456) or
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(bold,true). Sometimes an attribute is treated as an instruction to
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add that attribute, in which case an empty value means to remove it.
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So (bold,) removes the "bold" attribute. Attributes are interned and
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given numeric IDs, so the number "6" could represent "(bold,true)",
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for example. This mapping is stored in an attribute "pool" which may
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be shared by multiple changesets.
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Entries in the pool must be unique, so that attributes can be compared
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by their IDs. Attribute names cannot contain commas.
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A changeset looks something like the following:
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Z:5g>1|5=2p=v*4*5+1$x
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With the corresponding pool containing these entries:
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...
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4 -> (author,1059348573)
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5 -> (bold,true)
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...
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This changeset, together with the pool, represents inserting
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a bold letter "x" into the middle of a line. The string consists of:
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- a letter Z (the "magic character" and format version identifier)
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- a series of opcodes (punctuation) and numeric values in base 36 (the
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alphanumerics)
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- a dollar sign ($)
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- a string of characters used by insertion operations (the "char bank")
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If we separate out the operations and convert the numbers to base 10, we get:
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Z :196 >1 |5=97 =31 *4 *5 +1 $"x"
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Here are descriptions of the operations, where capital letters are variables:
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":N" : Source text has length N (must be first op)
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">N" : Final text is N (positive) characters longer than source text (must be second op)
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"<N" : Final text is N (positive) characters shorter than source text (must be second op)
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">0" : Final text is same length as source text
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"+N" : Insert N characters from the bank, none of them newlines
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"-N" : Skip over (delete) N characters from the source text, none of them newlines
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"=N" : Keep N characters from the source text, none of them newlines
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"|L+N" : Insert N characters from the source text, containing L newlines. The last
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character inserted MUST be a newline, but not the (new) document's final newline.
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"|L-N" : Delete N characters from the source text, containing L newlines. The last
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character inserted MUST be a newline, but not the (old) document's final newline.
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"|L=N" : Keep N characters from the source text, containing L newlines. The last character
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kept MUST be a newline, and the final newline of the document is allowed.
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"*I" : Apply attribute I from the pool to the following +, =, |+, or |= command.
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In other words, any number of * ops can come before a +, =, or | but not
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between a | and the corresponding + or =.
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If +, text is inserted having this attribute. If =, text is kept but with
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the attribute applied as an attribute addition or removal.
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Consecutive attributes must be sorted lexically by (key,value) with key
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and value taken as strings. It's illegal to have duplicate keys
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for (key,value) pairs that apply to the same text. It's illegal to
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have an empty value for a key in the case of an insertion (+), the
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pair should just be omitted.
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Characters from the source text that aren't accounted for are assumed to be kept
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with the same attributes.
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Additional Constraints:
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- Consecutive +, -, and = ops of the same type that could be combined are not allowed.
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Whether combination is possible depends on the attributes of the ops and whether
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each is multiline or not. For example, two multiline deletions can never be
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consecutive, nor can any insertion come after a non-multiline insertion with the
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same attributes.
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- "No-op" ops are not allowed, such as deleting 0 characters. However, attribute
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applications that don't have any effect are allowed.
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- Characters at the end of the source text cannot be explicitly kept with no changes;
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if the change doesn't affect the last N characters, those "keep" ops must be left off.
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- In any consecutive sequence of insertions (+) and deletions (-) with no keeps (=),
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the deletions must come before the insertions.
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- The document text before and after will always end with a newline. This policy avoids
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a lot of special-casing of the end of the document. If a final newline is
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always added when importing text and removed when exporting text, then the
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changeset representation can be used to process text files that may or may not
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have a final newline.
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Attribution string:
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An "attribution string" is a series of inserts with no deletions or keeps.
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For example, "*3+8|1+5" describes the attributes of a string of length 13,
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where the first 8 chars have attribute 3 and the next 5 chars have no
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attributes, with the last of these 5 chars being a newline. Constraints
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apply similar to those affecting changesets, but the restriction about
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the final newline of the new document being added doesn't apply.
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Attributes in an attribution string cannot be empty, like "(bold,)", they should
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instead be absent.
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-------
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Considerations:
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- composing changesets/attributions with different pools
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- generalizing "applyToAttribution" to make "mutateAttributionLines" and "compose"
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