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# rewriter.coffee

#

The CoffeeScript language has a good deal of optional syntax, implicit syntax, and shorthand syntax. This can greatly complicate a grammar and bloat the resulting parse table. Instead of making the parser handle it all, we take a series of passes over the token stream, using this Rewriter to convert shorthand into the unambiguous long form, add implicit indentation and parentheses, balance incorrect nestings, and generally clean things up.

#

Set up exported variables for both Node.js and the browser.

    if process?
      {helpers} = require('./helpers')
    else
      this.exports = this
      helpers      = this.helpers

#

Import the helpers we need.

    {include} = helpers

#

The Rewriter class is used by the Lexer, directly against its internal array of tokens.

    exports.Rewriter = class Rewriter

#

Helpful snippet for debugging: puts (t[0] + '/' + t[1] for t in @tokens).join ' '

#

Rewrite the token stream in multiple passes, one logical filter at a time. This could certainly be changed into a single pass through the stream, with a big ol' efficient switch, but it's much nicer to work with like this. The order of these passes matters -- indentation must be corrected before implicit parentheses can be wrapped around blocks of code.

      rewrite: (tokens) ->
        @tokens = tokens
        @adjustComments()
        @removeLeadingNewlines()
        @removeMidExpressionNewlines()
        @closeOpenCalls()
        @closeOpenIndexes()
        @addImplicitIndentation()
        @tagPostfixConditionals()
        @addImplicitBraces()
        @addImplicitParentheses()
        @ensureBalance BALANCED_PAIRS
        @rewriteClosingParens()
        @tokens

#

Rewrite the token stream, looking one token ahead and behind. Allow the return value of the block to tell us how many tokens to move forwards (or backwards) in the stream, to make sure we don't miss anything as tokens are inserted and removed, and the stream changes length under our feet.

      scanTokens: (block) ->
        i = 0
        loop
          break unless @tokens[i]
          move = block.call this, @tokens[i], i
          i += move
        true

      detectEnd: (i, condition, action) ->
        levels = 0
        loop
          token = @tokens[i]
          return action.call this, token, i     if levels is 0 and condition.call this, token, i
          return action.call this, token, i - 1 if not token or levels < 0
          levels += 1 if include EXPRESSION_START, token[0]
          levels -= 1 if include EXPRESSION_END, token[0]
          i += 1
        i - 1

#

Massage newlines and indentations so that comments don't have to be correctly indented, or appear on a line of their own.

      adjustComments: ->
        @scanTokens (token, i) ->
          return 1 unless token[0] is 'HERECOMMENT'
          [before, prev, post, after] = [@tokens[i - 2], @tokens[i - 1], @tokens[i + 1], @tokens[i + 2]]
          if after and after[0] is 'INDENT'
            @tokens.splice i + 2, 1
            if before and before[0] is 'OUTDENT' and post and prev[0] is post[0] is 'TERMINATOR'
              @tokens.splice i - 2, 1
            else
              @tokens.splice i, 0, after
          else if prev and prev[0] not in ['TERMINATOR', 'INDENT', 'OUTDENT']
            if post and post[0] is 'TERMINATOR' and after and after[0] is 'OUTDENT'
              @tokens.splice(i + 2, 0, @tokens.splice(i, 2)...)
              if @tokens[i + 2][0] isnt 'TERMINATOR'
                @tokens.splice i + 2, 0, ['TERMINATOR', "\n", prev[2]]
            else
              @tokens.splice i, 0, ['TERMINATOR', "\n", prev[2]]
            return 2
          return 1

#

Leading newlines would introduce an ambiguity in the grammar, so we dispatch them here.

      removeLeadingNewlines: ->
        @tokens.shift() while @tokens[0] and @tokens[0][0] is 'TERMINATOR'

#

Some blocks occur in the middle of expressions -- when we're expecting this, remove their trailing newlines.

      removeMidExpressionNewlines: ->
        @scanTokens (token, i) ->
          return 1 unless include(EXPRESSION_CLOSE, @tag(i + 1)) and token[0] is 'TERMINATOR'
          @tokens.splice i, 1
          return 0

#

The lexer has tagged the opening parenthesis of a method call. Match it with its paired close. We have the mis-nested outdent case included here for calls that close on the same line, just before their outdent.

      closeOpenCalls: ->
        @scanTokens (token, i) ->
          if token[0] is 'CALL_START'
            condition = (token, i) ->
              (token[0] in [')', 'CALL_END']) or (token[0] is 'OUTDENT' and @tokens[i - 1][0] is ')')
            action = (token, i) ->
              idx = if token[0] is 'OUTDENT' then i - 1 else i
              @tokens[idx][0] = 'CALL_END'
            @detectEnd i + 1, condition, action
          return 1

#

The lexer has tagged the opening parenthesis of an indexing operation call. Match it with its paired close.

      closeOpenIndexes: ->
        @scanTokens (token, i) ->
          if token[0] is 'INDEX_START'
            condition = (token, i) -> token[0] in [']', 'INDEX_END']
            action    = (token, i) -> token[0] = 'INDEX_END'
            @detectEnd i + 1, condition, action
          return 1

#

Object literals may be written with implicit braces, for simple cases. Insert the missing braces here, so that the parser doesn't have to.

      addImplicitBraces: ->
        stack = []
        @scanTokens (token, i) ->
          if include EXPRESSION_START, token[0]
            stack.push(if (token[0] is 'INDENT' and (@tag(i - 1) is '{')) then '{' else token[0])
          if include EXPRESSION_END, token[0]
            stack.pop()
          last = stack[stack.length - 1]
          if token[0] is ':' and (not last or last[0] isnt '{')
            stack.push '{'
            idx = if @tag(i - 2) is '@' then i - 2 else i - 1
            tok = ['{', '{', token[2]]
            tok.generated = yes
            @tokens.splice idx, 0, tok
            condition = (token, i) ->
              [one, two, three] = @tokens.slice(i + 1, i + 4)
              return false if 'HERECOMMENT' in [@tag(i + 1), @tag(i - 1)]
              ((token[0] in ['TERMINATOR', 'OUTDENT']) and not ((two and two[0] is ':') or (one and one[0] is '@' and three and three[0] is ':'))) or
                (token[0] is ',' and one and (one[0] not in ['IDENTIFIER', 'STRING', '@', 'TERMINATOR', 'OUTDENT']))
            action = (token, i) ->
              @tokens.splice i, 0, ['}', '}', token[2]]
            @detectEnd i + 2, condition, action
            return 2
          return 1

#

Methods may be optionally called without parentheses, for simple cases. Insert the implicit parentheses here, so that the parser doesn't have to deal with them.

      addImplicitParentheses: ->
        classLine    = no
        @scanTokens (token, i) ->
          classLine  = yes if token[0] is 'CLASS'
          prev       = @tokens[i - 1]
          next       = @tokens[i + 1]
          idx        = 1
          callObject = not classLine and token[0] is 'INDENT' and next and next.generated and next[0] is '{' and prev and include(IMPLICIT_FUNC, prev[0])
          idx        = 2 if callObject
          seenSingle = no
          classLine  = no  if include(LINEBREAKS, token[0])
          token.call = yes if prev and not prev.spaced and token[0] is '?'
          if prev and (prev.spaced and (include(IMPLICIT_FUNC, prev[0]) or prev.call) and include(IMPLICIT_CALL, token[0]) and
              not (token[0] is 'UNARY' and (@tag(i + 1) in ['IN', 'OF', 'INSTANCEOF']))) or callObject
            @tokens.splice i, 0, ['CALL_START', '(', token[2]]
            condition = (token, i) ->
              return yes if not seenSingle and token.fromThen
              seenSingle = yes if token[0] in ['IF', 'ELSE', 'UNLESS', '->', '=>']
              (not token.generated and @tokens[i - 1][0] isnt ',' and include(IMPLICIT_END, token[0]) and
                not (token[0] is 'INDENT' and (include(IMPLICIT_BLOCK, @tag(i - 1)) or @tag(i - 2) is 'CLASS' or @tag(i + 1) is '{'))) or
                token[0] is 'PROPERTY_ACCESS' and @tag(i - 1) is 'OUTDENT'
            action = (token, i) ->
              idx = if token[0] is 'OUTDENT' then i + 1 else i
              @tokens.splice idx, 0, ['CALL_END', ')', token[2]]
            @detectEnd i + idx, condition, action
            prev[0] = 'FUNC_EXIST' if prev[0] is '?'
            return 2
          return 1

#

Because our grammar is LALR(1), it can't handle some single-line expressions that lack ending delimiters. The Rewriter adds the implicit blocks, so it doesn't need to. ')' can close a single-line block, but we need to make sure it's balanced.

      addImplicitIndentation: ->
        @scanTokens (token, i) ->
          if token[0] is 'ELSE' and @tag(i - 1) isnt 'OUTDENT'
            @tokens.splice i, 0, @indentation(token)...
            return 2
          if token[0] is 'CATCH' and
              (@tag(i + 2) is 'TERMINATOR' or @tag(i + 2) is 'FINALLY')
            @tokens.splice i + 2, 0, @indentation(token)...
            return 4
          if include(SINGLE_LINERS, token[0]) and @tag(i + 1) isnt 'INDENT' and
              not (token[0] is 'ELSE' and @tag(i + 1) is 'IF')
            starter = token[0]
            [indent, outdent] = @indentation token
            indent.fromThen   = true if starter is 'THEN'
            indent.generated  = outdent.generated = true
            @tokens.splice i + 1, 0, indent
            condition = (token, i) ->
              (include(SINGLE_CLOSERS, token[0]) and token[1] isnt ';') and
                not (token[0] is 'ELSE' and starter not in ['IF', 'THEN'])
            action = (token, i) ->
              idx = if @tokens[i - 1][0] is ',' then i - 1 else i
              @tokens.splice idx, 0, outdent
            @detectEnd i + 2, condition, action
            @tokens.splice i, 1 if token[0] is 'THEN'
            return 2
          return 1

#

Tag postfix conditionals as such, so that we can parse them with a different precedence.

      tagPostfixConditionals: ->
        @scanTokens (token, i) ->
          if token[0] in ['IF', 'UNLESS']
            original  = token
            condition = (token, i) ->
              token[0] in ['TERMINATOR', 'INDENT']
            action    = (token, i) ->
              original[0] = 'POST_' + original[0] if token[0] isnt 'INDENT'
            @detectEnd i + 1, condition, action
            return 1
          return 1

#

Ensure that all listed pairs of tokens are correctly balanced throughout the course of the token stream.

      ensureBalance: (pairs) ->
        levels   = {}
        openLine = {}
        @scanTokens (token, i) ->
          for pair in pairs
            [open, close] = pair
            levels[open] or= 0
            if token[0] is open
              openLine[open] = token[2] if levels[open] == 0
              levels[open] += 1
            levels[open] -= 1 if token[0] is close
            throw new Error("too many #{token[1]} on line #{token[2] + 1}") if levels[open] < 0
          return 1
        unclosed = key for key, value of levels when value > 0
        if unclosed.length
          open = unclosed[0]
          line = openLine[open] + 1
          throw new Error "unclosed #{open} on line #{line}"

#

We'd like to support syntax like this:

    el.click((event) ->
      el.hide())

In order to accomplish this, move outdents that follow closing parens inwards, safely. The steps to accomplish this are:

1. Check that all paired tokens are balanced and in order.
2. Rewrite the stream with a stack: if you see an `EXPRESSION_START`, add it to the stack. If you see an `EXPRESSION_END`, pop the stack and replace it with the inverse of what we've just popped.
3. Keep track of "debt" for tokens that we manufacture, to make sure we end up balanced in the end.
4. Be careful not to alter array or parentheses delimiters with overzealous rewriting.

      rewriteClosingParens: ->
        stack = []
        debt  = {}
        (debt[key] = 0) for key, val of INVERSES
        @scanTokens (token, i) ->
          tag = token[0]
          inv = INVERSES[token[0]]
          if include EXPRESSION_START, tag
            stack.push token
            return 1
          else if include EXPRESSION_END, tag
            if debt[inv] > 0
              debt[inv] -= 1
              @tokens.splice i, 1
              return 0
            else
              match = stack.pop()
              mtag  = match[0]
              oppos = INVERSES[mtag]
              return 1 if tag is oppos
              debt[mtag] += 1
              val = [oppos, if mtag is 'INDENT' then match[1] else oppos]
              if @tokens[i + 2]?[0] is mtag
                @tokens.splice i + 3, 0, val
                stack.push(match)
              else
                @tokens.splice i, 0, val
              return 1
          else
            return 1

#

Generate the indentation tokens, based on another token on the same line.

      indentation: (token) ->
        [['INDENT', 2, token[2]], ['OUTDENT', 2, token[2]]]

#

Look up a tag by token index.

      tag: (i) ->
        @tokens[i] and @tokens[i][0]

#

## Constants

#

List of the token pairs that must be balanced.

    BALANCED_PAIRS = [['(', ')'], ['[', ']'], ['{', '}'], ['INDENT', 'OUTDENT'],
      ['PARAM_START', 'PARAM_END'], ['CALL_START', 'CALL_END'], ['INDEX_START', 'INDEX_END']]

#

The inverse mappings of `BALANCED_PAIRS` we're trying to fix up, so we can look things up from either end.

    INVERSES = {}
    for pair in BALANCED_PAIRS
      INVERSES[pair[0]] = pair[1]
      INVERSES[pair[1]] = pair[0]

#

The tokens that signal the start of a balanced pair.

    EXPRESSION_START = pair[0] for pair in BALANCED_PAIRS

#

The tokens that signal the end of a balanced pair.

    EXPRESSION_END   = pair[1] for pair in BALANCED_PAIRS

#

Tokens that indicate the close of a clause of an expression.

    EXPRESSION_CLOSE = ['CATCH', 'WHEN', 'ELSE', 'FINALLY'].concat EXPRESSION_END

#

Tokens that, if followed by an `IMPLICIT_CALL`, indicate a function invocation.

    IMPLICIT_FUNC    = ['IDENTIFIER', 'SUPER', ')', 'CALL_END', ']', 'INDEX_END', '@', 'THIS']

#

If preceded by an `IMPLICIT_FUNC`, indicates a function invocation.

    IMPLICIT_CALL    = [
      'IDENTIFIER', 'NUMBER', 'STRING', 'JS', 'REGEX', 'NEW', 'PARAM_START', 'CLASS',
      'IF', 'UNLESS', 'TRY', 'SWITCH', 'THIS', 'NULL', 'UNARY'
      'TRUE', 'FALSE', 'YES', 'NO', 'ON', 'OFF',
      '@', '->', '=>', '[', '(', '{'
    ]

#

Tokens indicating that the implicit call must enclose a block of expressions.

    IMPLICIT_BLOCK   = ['->', '=>', '{', '[', ',']

#

Tokens that always mark the end of an implicit call for single-liners.

    IMPLICIT_END     = ['POST_IF', 'POST_UNLESS', 'FOR', 'WHILE', 'UNTIL', 'LOOP', 'TERMINATOR', 'INDENT']

#

Single-line flavors of block expressions that have unclosed endings. The grammar can't disambiguate them, so we insert the implicit indentation.

    SINGLE_LINERS    = ['ELSE', "->", "=>", 'TRY', 'FINALLY', 'THEN']
    SINGLE_CLOSERS   = ['TERMINATOR', 'CATCH', 'FINALLY', 'ELSE', 'OUTDENT', 'LEADING_WHEN']

#

Tokens that end a line.

    LINEBREAKS       = ['TERMINATOR', 'INDENT', 'OUTDENT']