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CSV based Ruby decision tables.


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CSV Decision

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CSV based Ruby decision tables

csv_decision is a RubyGem for CSV based decision tables. It accepts decision tables implemented as a CSV file, which can then be used to execute complex conditional logic against an input hash, producing a decision as an output hash.

Why use csv_decision?

Typical "business logic" is notoriously illogical - full of corner cases and one-off exceptions. A decision table can express data-based decisions in a way that comes naturally to subject matter experts, who typically use spreadsheet models. Business logic can be encapsulated in a table, avoiding the need for tortuous conditional expressions.

This gem and the examples below take inspiration from rufus/decision. (That gem is no longer maintained and CSV Decision has better decision-time performance, at the expense of slower table parse times and more memory -- see benchmarks/rufus_decision.rb.)


To get started, just add csv_decision to your Gemfile, and then run bundle:

gem 'csv_decision'

or simply

gem install csv_decision

Simple example

This table considers two input conditions: topic and region, labeled in:. Certain combinations yield an output value for team_member, labeled out:.

in:topic | in:region  | out:team_member
sports   | Europe     | Alice
sports   |            | Bob
finance  | America    | Charlie
finance  | Europe     | Donald
finance  |            | Ernest
politics | Asia       | Fujio
politics | America    | Gilbert
politics |            | Henry
         |            | Zach

When the topic is finance and the region is Europe the team member Donald is selected. This is a "first match" decision table in that as soon as a match is made execution stops and a single output row (hash) is returned.

The ordering of rows matters. Ernest, who is in charge of finance for the rest of the world, except for America and Europe, must come after his colleagues Charlie and Donald. Zach has been placed last, catching all the input combos not matching any other row.

Here's the example as code:

 # Valid CSV string
 data = <<~DATA
   in :topic, in :region,  out :team_member
   sports,    Europe,      Alice
   sports,    ,            Bob
   finance,   America,     Charlie
   finance,   Europe,      Donald
   finance,   ,            Ernest
   politics,  Asia,        Fujio
   politics,  America,     Gilbert
   politics,  ,            Henry
   ,          ,            Zach

 table = CSVDecision.parse(data)
 table.decide(topic: 'finance', region: 'Europe') #=> { team_member: 'Donald' }
 table.decide(topic: 'sports', region: nil) #=> { team_member: 'Bob' }
 table.decide(topic: 'culture', region: 'America') #=> { team_member: 'Zach' }

An empty in: cell means "matches any value".

Note that all column header names are symbolized, so it's actually more accurate to write in :topic; however spaces before and after the : do not matter.

If you have cloned this gem's git repo, then the example can also be run by loading the table from a CSV file:

table = CSVDecision.parse(Pathname('spec/data/valid/simple_example.csv'))

We can also load this same table using the option: first_match: false, which means that all matching rows will be accumulated into an array of hashes.

table = CSVDecision.parse(data, first_match: false)
table.decide(topic: 'finance', region: 'Europe') #=> { team_member: %w[Donald Ernest Zach] }

For more examples see spec/csv_decision/table_spec.rb. Complete documentation of all table parameters is in the code - see lib/csv_decision/parse.rb and lib/csv_decision/table.rb.

CSV Decision features

  • Either returns the first matching row as a hash (default), or accumulates all matches as an array of hashes (i.e., parse option first_match: false or CSV file option accumulate).
  • Fast decision-time performance (see benchmarks folder). Automatically indexes all constants-only columns that do not contain any empty strings.
  • In addition to strings, can match basic Ruby constants (e.g., =nil), regular expressions (e.g., =~ on|off), comparisons (e.g., > 100.0 ) and Ruby-style ranges (e.g., 1..10)
  • Can compare an input column versus another input hash key - e.g., > :column.
  • Any cell starting with # is treated as a comment, and comments may appear anywhere in the table.
  • Column symbol expressions or Ruby methods (0-arity) may be used in input columns for matching - e.g., :column.zero? or :column == 0.
  • May also use Ruby methods in output columns - e.g., :column.length.
  • Accepts data as a file, CSV string or an array of arrays.

Constants other than strings

Although csv_decision is string oriented, it does recognise other types of constant present in the input hash. Specifically, the following classes are recognized: Integer, BigDecimal, NilClass, TrueClass and FalseClass.

This is accomplished by prefixing the value with one of the operators =, == or :=. (The syntax is intentionally lax.)

For example:

   data = <<~DATA
     in :constant, out :value
     :=nil,        :=nil
     ==false,      ==false
     =true,        =true
     = 0,          = 0
     :=100.0,      :=100.0
 table = CSVDecision.parse(data)
 table.decide(constant: nil) # returns value: nil      
 table.decide(constant: 0) # returns value: 0        
 table.decide(constant: BigDecimal('100.0')) # returns value: BigDecimal('100.0')       

Column header symbols

All input and output column names are symbolized, and those symbols may be used to form simple expressions that refer to values in the input hash.

For example:

   data = <<~DATA
     in :node, in :parent, out :top?
     ,         == :node,   yes
     ,         ,           no
   table = CSVDecision.parse(data)
   table.decide(node: 0, parent: 0) # returns top?: 'yes'
   table.decide(node: 1, parent: 0) # returns top?: 'no'

Note that there is no need to include an input column for :node in the decision table - it just needs to be present in the input hash. The expression, == :node should be read as :parent == :node. It can also be shortened to just :node, so the above decision table may be simplified to:

   data = <<~DATA
     in :parent, out :top?
        :node,   yes
     ,           no

These comparison operators are also supported: !=, >, >=, <, <=. In addition, you can also apply a Ruby 0-arity method - e.g., .present? or .nil?. Negation is also supported - e.g., !.nil?. Note that .nil? can also be written as := nil?, and !.nil? as := !nil?, depending on preference.

For more simple examples see spec/csv_decision/examples_spec.rb.

Input guard conditions

Sometimes it's more convenient to write guard expressions in a single column specialized for that purpose. For example:

data = <<~DATA
  in :country, guard:,          out :ID, out :ID_type, out :len
  US,          :CUSIP.present?, :CUSIP,  CUSIP,        :ID.length
  GB,          :SEDOL.present?, :SEDOL,  SEDOL,        :ID.length
  ,            :ISIN.present?,  :ISIN,   ISIN,         :ID.length
  ,            :SEDOL.present?, :SEDOL,  SEDOL,        :ID.length
  ,            :CUSIP.present?, :CUSIP,  CUSIP,        :ID.length
  ,            ,                := nil,  := nil,       := nil

table = CSVDecision.parse(data)
table.decide(country: 'US',  CUSIP: '123456789') #=> { ID: '123456789', ID_type: 'CUSIP', len: 9 }
table.decide(country: 'EU',  CUSIP: '123456789', ISIN:'123456789012') 
  #=> { ID: '123456789012', ID_type: 'ISIN', len: 12 }

Input guard: columns may be anonymous, and must contain non-constant expressions. In addition to 0-arity Ruby methods, the following comparison operators are allowed: ==, !=, >, >=, < and <=. Also, regular expressions are supported - i.e., =~ and !~.

Output if conditions

In some situations it is useful to apply filter conditions after all the output columns have been derived. For example:

data = <<~DATA
  in :country, guard:,          out :ID, out :ID_type, out :len,   if:
  US,          :CUSIP.present?, :CUSIP,  CUSIP8,       :ID.length, :len == 8
  US,          :CUSIP.present?, :CUSIP,  CUSIP9,       :ID.length, :len == 9
  US,          :CUSIP.present?, :CUSIP,  DUMMY,        :ID.length,
  ,            :ISIN.present?,  :ISIN,   ISIN,         :ID.length, :len == 12
  ,            :ISIN.present?,  :ISIN,   DUMMY,        :ID.length,
  ,            :CUSIP.present?, :CUSIP,  DUMMY,        :ID.length,

table = CSVDecision.parse(data)
table.decide(country: 'US',  CUSIP: '123456789') #=> {ID: '123456789', ID_type: 'CUSIP9', len: 9}
table.decide(CUSIP: '12345678', ISIN:'1234567890') #=> {ID: '1234567890', ID_type: 'DUMMY', len: 10}

Output if: columns may be anonymous, and must contain non-constant expressions. In addition to 0-arity Ruby methods, the following comparison operators are allowed: ==, !=, >, >=, < and <=. Also, regular expressions are supported - i.e., =~ and !~.

Input set columns

If you wish to set default values in the input hash, you can use a set column rather than an in column. The data row beneath the header is used to specify the default expression. There are three variations: set (unconditional default) set/nil?(set if nil? true) and set/blank? (set if blank? true). Note that the decide! method will mutate the input hash.

data = <<~DATA
  set/nil? :country, guard:,          set: class,    out :PAID, out: len,     if:
  US,                ,                :class.upcase,
  US,                :CUSIP.present?, != PRIVATE,    :CUSIP,    :PAID.length, :len == 9
  !=US,              :ISIN.present?,  != PRIVATE,    :ISIN,     :PAID.length, :len == 12
  US,                :CUSIP.present?, PRIVATE,       :CUSIP,    :PAID.length,
  !=US,              :ISIN.present?,  PRIVATE,       :ISIN,     :PAID.length,

table = CSVDecision.parse(data)
table.decide(CUSIP: '1234567890', class: 'Private') #=> {PAID: '1234567890', len: 10}
table.decide(ISIN: '123456789012', country: 'GB', class: 'private') #=> {PAID: '123456789012', len: 12}

Input path columns

For hashes that contain sub-hashes, it's possible to specify a path for the purposes of matching. (Arrays are currently not supported.)

data = <<~DATA
  path:,   path:,    out :value
  header,  ,         :source_name
  header,  metrics,  :service_name
  payload, ,         :amount
  payload, ref_data, :account_id
table = CSVDecision.parse(data, first_match: false)

input = {
  header: { 
    id: 1, type_cd: 'BUY', source_name: 'Client', client_name: 'AAPL',
    metrics: { service_name: 'Trading', receive_time: '12:00' } 
  payload: { 
    tran_id: 9, amount: '100.00',
    ref_data: { account_id: '5010', type_id: 'BUYL' } 

table.decide(input) #=> { value: %w[Client Trading 100.00 5010] }


csv_decision includes thorough RSpec tests:

# Execute within a clone of the csv_decision Git repository:
bundle install

Planned features

csv_decision is still a work in progress, and will be enhanced to support the following features:

  • Supply a pre-defined library of functions that may be called within input columns to implement custom matching logic, or from the output columns to formulate the final decision.
  • Built-in lookup functions evaluate other decision tables to implement guard conditions, or supply output values.
  • Available functions may be extended with a user-supplied library of Ruby methods for custom logic.
  • Output columns may construct interpolated strings containing references to column symbols.

Reasons for the limitations of column expressions

The simple column expressions allowed by csv_decision are purposely limited for reasons of understandability and maintainability. The whole point of this gem is to make decision rules easier to express and comprehend as declarative, tabular logic. While Ruby makes it easy to execute arbitrary code embedded within a CSV file, this could easily result in hard to debug logic that also poses safety risks.


See CHANGELOG.md for a list of changes.


CSV Decision © 2017-2018 by Brett Vickers. CSV Decision is licensed under the MIT license. Please see the LICENSE document for more information.