adding indentation support

.gitignore

@@ -1,3 +1,4 @@
 browser/*
 examples/*.js
 node_modules/*
+.vscode

README.md

@@ -503,9 +503,142 @@ environments:
   * Safari
   * Opera
 
+Fork changes
+------------
+
+This fork is designed for experimenting with indentation based grammars, bases on this [issue comment](https://github.com/pegjs/pegjs/issues/217#issuecomment-286595368) on that topic.
+
+It adds the following to `src/parser.pegjs`
+
+```js
+{
+  const OPS_TO_PREFIXED_TYPES = {
++:    "++": "increment_match",
++:    "--": "decrement_match"
+
+PrefixedOperator
++:  / "++"
++:  / "--"
+
+SuffixedOperator
++:  / "+" !"+"
+```
+
+Which enabled out grammar to support `++Indent` and `--Indent`
+
+- `++Indent` tells the parser to increase the minimum amount of matches required for `Indent`
+- `--Indent` tells the parser to decrease the minimum amount of matches required for `Indent`
+
+A simple indentation based grammar can then be expressed as:
+
+```js
+Start
+  = Statements
+
+Statements
+  = Statement*
+
+Statement
+  = Indent* @(S / I)
+
+S
+  = "S" EOS {
+      return "S";
+    }
+
+I
+  = "I" EOL ++Indent @Statements --Indent
+  / "I" EOS { return []; }
+
+Indent "indent"
+  = "\t"
+ / !__ "  "
+
+__ "white space"
+ = " \t"
+ / " "
+
+EOS
+  = EOL
+  / EOF
+
+EOL
+  = "\n"
+
+EOF
+  = !.
+```
+
+More details can be found [here](https://gist.github.com/dmajda/04002578dd41ae8190fc)
+
+Intentation based PEG.js Grammar
+--------------------------------
+
+Describes a simple indentation-based language. A program in this language is
+a possibly empty list of the following statements:
+
+* S (simple) Consists of the letter `S`.
+* I (indent)
+
+Consists of the letter `I`, optionally followed by a newline and a list
+of statements indented by one indentation level (2 spaces) relative to
+the I statement itself.
+
+Statements are terminated by a newline or `EOF`.
+
+Example: `indentation/samples/simple-indent.js.txt`
+
+```bash
+  I
+    S
+    I
+      S
+    S
+```
+
+The grammar needs to be compiled without caching.
+
+To generate an indentation parser, try:
+
+```bash
+$ gulp parser
+$ ll -a lib/
+```
+
+`ll -a` should let you see if the timestamps were updated, and thus new files generated by the compiler.
+
+Now try generating an *indentation parser*
+
+```bash
+$ bin/./pegjs -o indentation/parsers/indent-parser.js indentation/grammars/simple-indent.pegjs`
+```
+
+This should (hopefully) generate a parser in `indentation/parsers/indent-parser.js`
+I sadly get this *error*, but getting close ;)
+
+```bash
+Cannot read property 'apply' of undefined
+```
+
 Development
 -----------
 
+To contribute or experiment with your own patches and tweaks:
+- make changes in the `/src` folder
+- add examples to test your changes in `/examples` and tests in `/test`
+
+To generate a new PEG parser, run `gulp parser` which generates `.js` files in `/lib`
+The binary `pegjs` can be found in `/bin` and uses `lib/peg` to execute.
+
+`let peg = require("../lib/peg")`
+
+Testing
+-------
+
+Tests can be found in `/test` and are written in chai.
+
+Resources
+---------
   * [Project website](https://pegjs.org/)
   * [Wiki](https://github.com/pegjs/pegjs/wiki)
   * [Source code](https://github.com/pegjs/pegjs)

indentation/grammars/simple-indent.pegjs

@@ -0,0 +1,35 @@
+Start
+  = Statements
+
+Statements
+  = Statement*
+
+Statement
+  = Indent* statement:(S / I) { return statement; }
+
+S
+  = "S" EOS {
+      return "S";
+    }
+
+I
+  = "I" EOL ++Indent statements:Statements --Indent { return statements; }
+  / "I" EOS { return []; }
+
+Indent "indent"
+  = "\t"
+ / !__ "  "
+
+__ "white space"
+ = " \t"
+ / " "
+
+EOS
+  = EOL
+  / EOF
+
+EOL
+  = "\n"
+
+EOF
+  = !.

indentation/samples/simple-indent.js.txt

@@ -0,0 +1,5 @@
+  I
+    S
+    I
+      S
+    S