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- # sourcemap.litcoffee

- ¶

  Source maps allow JavaScript runtimes to match running JavaScript back to the original source code that corresponds to it. This can be minified JavaScript, but in our case, we’re concerned with mapping pretty-printed JavaScript back to CoffeeScript.

  In order to produce maps, we must keep track of positions (line number, column number) that originated every node in the syntax tree, and be able to generate a map file — which is a compact, VLQ-encoded representation of the JSON serialization of this information — to write out alongside the generated JavaScript.

  ## LineMap

- ¶

  A LineMap object keeps track of information about original line and column positions for a single line of output JavaScript code. SourceMaps are implemented in terms of LineMaps.

      class LineMap
        constructor: (@line) ->
          @columns = []

        add: (column, [sourceLine, sourceColumn], options={}) ->
          return if @columns[column] and options.noReplace
          @columns[column] = {line: @line, column, sourceLine, sourceColumn}

        sourceLocation: (column) ->
          column-- until (mapping = @columns[column]) or (column <= 0)
          mapping and [mapping.sourceLine, mapping.sourceColumn]

- ¶

  ## SourceMap

- ¶

  Maps locations in a single generated JavaScript file back to locations in the original CoffeeScript source file.

  This is intentionally agnostic towards how a source map might be represented on disk. Once the compiler is ready to produce a “v3”-style source map, we can walk through the arrays of line and column buffer to produce it.

      class SourceMap
        constructor: ->
          @lines = []

- ¶

  Adds a mapping to this SourceMap. `sourceLocation` and `generatedLocation` are both `[line, column]` arrays. If `options.noReplace` is true, then if there is already a mapping for the specified `line` and `column`, this will have no effect.

        add: (sourceLocation, generatedLocation, options = {}) ->
          [line, column] = generatedLocation
          lineMap = (@lines[line] or= new LineMap(line))
          lineMap.add column, sourceLocation, options

- ¶

  Look up the original position of a given `line` and `column` in the generated code.

        sourceLocation: ([line, column]) ->
          line-- until (lineMap = @lines[line]) or (line <= 0)
          lineMap and lineMap.sourceLocation column

- ¶

  ## V3 SourceMap Generation

- ¶

  Builds up a V3 source map, returning the generated JSON as a string. `options.sourceRoot` may be used to specify the sourceRoot written to the source map. Also, `options.sourceFiles` and `options.generatedFile` may be passed to set “sources” and “file”, respectively.

        generate: (options = {}, code = null) ->
          writingline       = 0
          lastColumn        = 0
          lastSourceLine    = 0
          lastSourceColumn  = 0
          needComma         = no
          buffer            = ""

          for lineMap, lineNumber in @lines when lineMap
            for mapping in lineMap.columns when mapping
              while writingline < mapping.line
                lastColumn = 0
                needComma = no
                buffer += ";"
                writingline++

- ¶

  Write a comma if we’ve already written a segment on this line.

              if needComma
                buffer += ","
                needComma = no

- ¶

  Write the next segment. Segments can be 1, 4, or 5 values. If just one, then it is a generated column which doesn’t match anything in the source code.

  The starting column in the generated source, relative to any previous recorded column for the current line:

              buffer += @encodeVlq mapping.column - lastColumn
              lastColumn = mapping.column

- ¶

  The index into the list of sources:

              buffer += @encodeVlq 0

- ¶

  The starting line in the original source, relative to the previous source line.

              buffer += @encodeVlq mapping.sourceLine - lastSourceLine
              lastSourceLine = mapping.sourceLine

- ¶

  The starting column in the original source, relative to the previous column.

              buffer += @encodeVlq mapping.sourceColumn - lastSourceColumn
              lastSourceColumn = mapping.sourceColumn
              needComma = yes

- ¶

  Produce the canonical JSON object format for a “v3” source map.

          v3 =
            version:    3
            file:       options.generatedFile or ''
            sourceRoot: options.sourceRoot or ''
            sources:    options.sourceFiles or ['']
            names:      []
            mappings:   buffer

          v3.sourcesContent = [code] if options.inline

          JSON.stringify v3, null, 2

- ¶

  ## Base64 VLQ Encoding

- ¶

  Note that SourceMap VLQ encoding is “backwards”. MIDI-style VLQ encoding puts the most-significant-bit (MSB) from the original value into the MSB of the VLQ encoded value (see Wikipedia). SourceMap VLQ does things the other way around, with the least significat four bits of the original value encoded into the first byte of the VLQ encoded value.

        VLQ_SHIFT            = 5
        VLQ_CONTINUATION_BIT = 1 << VLQ_SHIFT             # 0010 0000
        VLQ_VALUE_MASK       = VLQ_CONTINUATION_BIT - 1   # 0001 1111

        encodeVlq: (value) ->
          answer = ''

- ¶

  Least significant bit represents the sign.

          signBit = if value < 0 then 1 else 0

- ¶

  The next bits are the actual value.

          valueToEncode = (Math.abs(value) << 1) + signBit

- ¶

  Make sure we encode at least one character, even if valueToEncode is 0.

          while valueToEncode or not answer
            nextChunk = valueToEncode & VLQ_VALUE_MASK
            valueToEncode = valueToEncode >> VLQ_SHIFT
            nextChunk |= VLQ_CONTINUATION_BIT if valueToEncode
            answer += @encodeBase64 nextChunk

          answer

- ¶

  ## Regular Base64 Encoding

- ¶

        BASE64_CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

        encodeBase64: (value) ->
          BASE64_CHARS[value] or throw new Error "Cannot Base64 encode value: #{value}"

- ¶

  Our API for source maps is just the `SourceMap` class.

      module.exports = SourceMap