diff --git a/.npmignore b/.npmignore
index dfb9347..ea9bfd0 100644
--- a/.npmignore
+++ b/.npmignore
@@ -1,3 +1,2 @@
 src/
-test.json
-build.sh
+test/
diff --git a/src/pert-cli.coffee b/src/pert-cli.coffee
index 6ffc38c..848d7da 100755
--- a/src/pert-cli.coffee
+++ b/src/pert-cli.coffee
@@ -2,7 +2,7 @@
 chalk = require 'chalk'
 cli = require 'commander'
 fs = require 'fs'
-pert = require '../lib/pert.js'
+Pert = require '../lib/pert.js'
 
 ex = [{id: 0, depends: [], duration: 2}, { id: 1, depends: [0], duration: 3},{id: 2, depends: [0,1], duration: 4}]
 
@@ -17,9 +17,10 @@ cli
   .command 'example'
   .description 'show an example of the JSON data format'
   .action ->
+    pert = new Pert ex, cli.verbose
     didSomething = yes
     console.log chalk.bold.green('Before:'), ex
-    console.log chalk.bold.green('After calculations:'), pert.calculate ex
+    console.log chalk.bold.green('After calculations:'), pert.calculate()
     console.log chalk.green 'Tip:',chalk.bold 'optional fields can be freely included in the input data'
 
 cli
@@ -32,8 +33,8 @@ cli
     fs.readFile file, (error,content) ->
       if error then err error
       else
-        if options.json then console.log JSON.stringify (pert.calculate JSON.parse(content))
-        else console.log pert.calculate JSON.parse(content)
+        pert = new Pert JSON.parse(content), cli.verbose
+        console.log pert.calculate options
 
 cli.parse process.argv
 
diff --git a/src/pert.coffee b/src/pert.coffee
index c5f8b74..f443192 100755
--- a/src/pert.coffee
+++ b/src/pert.coffee
@@ -1,7 +1,9 @@
 chalk = require 'chalk'
 fs = require 'fs'
 
-pert = 
+module.exports = class Pert
+  constructor: (@list, @verbose) ->
+
   log: (x...) -> if @verbose then console.log chalk.bold("Pert:"), x...
   err: (x...) -> console.log chalk.bold (chalk.red "Pert:"), x...
 
@@ -9,44 +11,41 @@ pert =
   maxa: (l) -> return Math.max.apply null, l
 
   # Find the activity with given id
-  toActivity: (id,list) ->
-    if !list? then pert.err "list is",list
+  toActivity: (id) =>
+    if !@list? then @err "list is", @list
     item = {}
-    list.forEach (x) -> if x.id is id then item = x
+    @list.forEach (x) -> if x.id is id then item = x
     return item
 
-  # Find the item 
-  calculateEndDay: (item,list) ->
+  # Compute the item's end day 
+  calculateEndDay: (item) =>
     if !item.startDay?
-      pert.log "calculating start day of",item.id
-      item.startDay = pert.calculateStartDay item, list
-    pert.log "start day of",item.id,"is",item.startDay
+      @log "calculating start day of",item.id
+      item.startDay = @calculateStartDay item
+    @log "start day of",item.id,"is",item.startDay
     item.endDay = item.startDay + item.duration
-    pert.log "end day of",item.id,"is",item.endDay
+    @log "end day of",item.id,"is",item.endDay
     return item.endDay
 
   # Find out which day the activity starts
-  calculateStartDay: (item,list) ->
+  calculateStartDay: (item) =>
     if !item.depends? or item.depends.length is 0 then return item.startDay = 0
-    item.startDay = pert.maxa item.depends.map((x) -> pert.toActivity x,list).map((x) -> pert.calculateEndDay x, list)
-    pert.log "start day of",item.id,"is",item.startDay
+    item.startDay = @maxa item.depends.map(@toActivity).map @calculateEndDay
+    @log "start day of",item.id,"is",item.startDay
     # write max delay time to each depend
-    item.depends.forEach (x) ->
-      pert.log "checking permittedDelay to dependency", x, "of", item
-      i = pert.toActivity x, list
+    item.depends.forEach (x) =>
+      @log "checking permittedDelay to dependency", x, "of", item
+      i = @toActivity x
       if !i.permittedDelay?
-        i.permittedDelay = item.startDay - pert.calculateEndDay i, list
-        pert.log "written permittedDelay to dependency", x, "of", item, "as", i.permittedDelay
-      else pert.log "aborting permittedDelay: already calculated"
-      pert.log "permitted delay of",x,"is",i.permittedDelay
+        i.permittedDelay = item.startDay - @calculateEndDay i
+        @log "written permittedDelay to dependency", x, "of", item, "as", i.permittedDelay
+      else @log "aborting permittedDelay: already calculated"
+      @log "permitted delay of",x,"is",i.permittedDelay
     return item.startDay
 
   # Find out which activity has the highest id
-  highestID: (list) -> return pert.maxa(list.map (x) -> x.id)
+  highestID: => return @maxa(@list.map (x) -> x.id)
 
-  calculate: (list,verbose) ->
-    pert.verbose = verbose
-    pert.calculateEndDay (pert.toActivity pert.highestID(list), list), list
-    return list
-
-module.exports = pert
+  calculate: (options) ->
+    @calculateEndDay @toActivity @highestID()
+    if options?.json then JSON.stringify @list else @list