diff --git a/.gitignore b/.gitignore
index 204ade5..65b85ce 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 node_modules/
+test/
 bin/
diff --git a/.npmignore b/.npmignore
index 9781602..dfb9347 100644
--- a/.npmignore
+++ b/.npmignore
@@ -1,2 +1,3 @@
 src/
+test.json
 build.sh
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..8f1f389
--- /dev/null
+++ b/README.md
@@ -0,0 +1,68 @@
+# Pert
+
+Pert is a command line tool built to assist in the process of creating pert diagrams and calculating permitted delays.
+
+## Installation
+
+Make sure you have `node` and `npm` installed and working
+
+__From git__
+
+1. clone this repo and run `build.sh`
+2. run `bin/pert`
+
+__From npm__
+
+1. `npm install -g pert`
+2. run `pert` in your shell
+
+## Usage
+
+    Usage: pert loads activity data from JSON and computes the possible activity delays
+
+    Commands:
+
+      example                       show an example of the JSON data format
+      calculate|c [options] <file>  calculate data on given JSON document
+
+    Options:
+
+      -h, --help     output usage information
+      -V, --version  output the version number
+      --verbose      be verbose (for debugging)
+
+This is the help information for the `pert calculate` command:
+
+    Usage: calculate|c [options] <file>
+
+    calculate data on given JSON document
+
+    Options:
+
+      -h, --help  output usage information
+      -j, --json  output json data
+
+## Data format
+
+This is a valid input document:
+
+```json
+[
+  {"id": 0, "duration": 3},
+  {"id": 1, "duration": 1},
+  {"id": 2, "duration": 2, "depends": [0]},
+  {"id": 3, "duration": 5, "depends": [1]},
+  {"id": 4, "duration": 5, "depends": [1]},
+  {"id": 5, "duration": 2, "depends": [2,3,4]}
+]
+```
+
+And this is the output of the `calculate` command on the previous document using the `--json` flag:
+
+```json
+[{"id":0,"duration":3,"startDay":0,"endDay":3,"permittedDelay":0},{"id":1,"duration":1,"startDay":0,"endDay":1,"permittedDelay":0},{"id":2,"duration":2,"depends":[0],"startDay":3,"endDay":5,"permittedDelay":1},{"id":3,"duration":5,"depends":[1],"startDay":1,"endDay":6,"permittedDelay":0},{"id":4,"duration":5,"depends":[1],"startDay":1,"endDay":6,"permittedDelay":0},{"id":5,"duration":2,"depends":[2,3,4],"startDay":6,"endDay":8}]
+```
+
+## License
+
+MIT
diff --git a/package.json b/package.json
index 1f3c760..b87a59b 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "pert",
-  "version": "0.0.1",
+  "version": "0.1.0",
   "description": "pert diagram calculator",
   "main": "pert.coffee",
   "bin": {
diff --git a/src/pert.coffee b/src/pert.coffee
index 94c7588..4719945 100755
--- a/src/pert.coffee
+++ b/src/pert.coffee
@@ -1,9 +1,9 @@
 #!/usr/bin/env coffee
 chalk = require 'chalk'
+cli = require 'commander'
+fs = require 'fs'
 
-verbose = no
-
-log = (x...) -> if verbose then console.log chalk.bold("Log:"), x...
+log = (x...) -> if cli.verbose then console.log chalk.bold("Log:"), x...
 err = (x...) -> console.log chalk.bold (chalk.red "Error:"), x...
 
 # Returns the highest number in an array of numbers
@@ -28,16 +28,19 @@ calculateEndDay = (item,list) ->
 
 # Find out which day the activity starts
 calculateStartDay = (item,list) ->
-  if item.depends.length is 0 then return item.startDay = 0
-  max = maxa item.depends.map((x) -> toActivity x,list).map((x) -> calculateEndDay x, list)
-  log "start day of",item.id,"is",max
+  if !item.depends? or item.depends.length is 0 then return item.startDay = 0
+  item.startDay = maxa item.depends.map((x) -> toActivity x,list).map((x) -> calculateEndDay x, list)
+  log "start day of",item.id,"is",item.startDay
   # write max delay time to each depend
   item.depends.forEach (x) ->
-    log "writing permittedDelay to dependency", x, "of", item
+    log "checking permittedDelay to dependency", x, "of", item
     i = toActivity x, list
-    i.permittedDelay = max - calculateEndDay i, list
+    if !i.permittedDelay?
+      i.permittedDelay = item.startDay - calculateEndDay i, list
+      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 = max
+  return item.startDay
 
 # Find out which activity has the highest id
 highestID = (list) -> return maxa(list.map (x) -> x.id)
@@ -47,4 +50,36 @@ calculate = (list) ->
   return list
 
 ex = [{id: 0, depends: [], duration: 2}, { id: 1, depends: [0], duration: 3},{id: 2, depends: [0,1], duration: 4}]
-console.log calculate ex
+
+cli
+  .version '0.1'
+  .usage 'loads activity data from JSON and computes the possible activity delays'
+  .option '--verbose', 'be verbose (for debugging)'
+
+didSomething = no
+
+cli
+  .command 'example'
+  .description 'show an example of the JSON data format'
+  .action ->
+    didSomething = yes
+    console.log chalk.bold.green('Before:'), ex
+    console.log chalk.bold.green('After calculations:'), calculate ex
+    console.log chalk.green 'Tip:',chalk.bold 'optional fields can be freely included in the input data'
+
+cli
+  .command 'calculate <file>'
+  .description 'calculate data on given JSON document'
+  .alias 'c'
+  .option '-j, --json', 'output json data'
+  .action (file,options) ->
+    didSomething = yes
+    fs.readFile file, (error,content) ->
+      if error then err error
+      else
+        if options.json then console.log JSON.stringify (calculate JSON.parse(content))
+        else console.log calculate JSON.parse(content)
+
+cli.parse process.argv
+
+if !didSomething then console.log chalk.green('Tip:'), 'see', chalk.bold(cli.name()+' --help')