feat(*): code identifyMarkedArea & use it in automaton

- will now allow counting backwards from the lastIndex
- will never yield empty string as entity's value

Author: sanjayaksaxena

src/automaton.js

@@ -32,7 +32,8 @@
 
 /* eslint-disable no-console */
 /* eslint-disable guard-for-in */
-var composePatterns = require( './compose-patterns.js' );
+const composePatterns = require( './compose-patterns.js' );
+const identifyMarkedArea = require( './identify-marked-area.js' );
 
 const eosTokenN = 2070000;
 const eosTokenX = '$%^EoS^%$';
@@ -172,7 +173,7 @@ var simpleFSM = function ( cache, token2Ignore ) {
     if ( mark ) {
       // Update last element of `mark` to simplifies computations during fsm
       // execution. Update must happen as a deep copy & not directly!
-      markedStates[ state ] = [ mark[ 0 ], ( length - mark[ 1 ] - 1 ) ];
+      markedStates[ state ] = identifyMarkedArea( mark, length );
     }
 
     if ( customProperty !== undefined ) {
@@ -261,8 +262,14 @@ var simpleFSM = function ( cache, token2Ignore ) {
 
     var customProperty = customPropertyAtStates[ m0 ];
     if ( mark ) {
+      // `match[ 1 ]` will now point to the index of the token where
+      // the entity should end because `match[ 0 ]` is pointing to the index
+      // of the token of detected entity's start and by adding `mark[ 1 ]`
+      // i.e. the `lastIndex` to it, we get the required value.
+      match[ 1 ] = match[ 0 ] + mark[ 1 ];
+      // For `match[ 0 ]`, simply adding `mark[ 0 ]` i.e. `firstIndex` yields
+      // the desried value.
       match[ 0 ] += mark[ 0 ];
-      match[ 1 ] -= mark[ 1 ];
     }
 
     // Removed `customProperty !== undefined &&` check while coding pos experiment

src/identify-marked-area.js

@@ -0,0 +1,17 @@
+const identifyMarkedArea = function ( mark, length ) {
+  // Length Minus 1.
+  const lm1 = length - 1;
+  let [ firstIndex, lastIndex ] = mark;
+
+  if ( firstIndex < 0 ) firstIndex += length;
+  firstIndex = Math.max( firstIndex, 0 );
+  if ( firstIndex > lm1 ) firstIndex = 0;
+
+  if ( lastIndex < 0 ) lastIndex += length;
+  lastIndex = Math.min( lastIndex, lm1 );
+  if ( lastIndex < firstIndex ) lastIndex = lm1;
+
+  return [ firstIndex, lastIndex ];
+}; // identifyMarkedArea()
+
+module.exports = identifyMarkedArea;