ruby 38 lines · 5 steps

Parsing fixed-width records in Ruby

A data-driven parser turns fixed-width text lines into typed hashes using a declarative field table.

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Intermediate parsing data-driven-design type-coercion enumerable modules
1module FixedWidthParser
2 FIELDS = [
3 { name: :record_type, range: 0...2 },
4 { name: :account_id, range: 2...12, type: :integer },
5 { name: :last_name, range: 12...42, type: :string },
6 { name: :first_name, range: 42...62, type: :string },
7 { name: :balance, range: 62...74, type: :cents },
8 { name: :opened_on, range: 74...82, type: :date },
9 { name: :active, range: 82...83, type: :flag }
10 ].freeze
11 
12 module_function
13 
14 def parse(io)
15 io.each_line.filter_map do |line|
16 next if line.strip.empty?
17 parse_line(line.chomp)
18 end
19 end
20 
21 def parse_line(line)
22 FIELDS.each_with_object({}) do |field, record|
23 raw = line[field[:range]].to_s
24 record[field[:name]] = cast(raw, field[:type])
25 end
26 end
27 
28 def cast(raw, type)
29 case type
30 when :integer then Integer(raw.strip, 10)
31 when :string then raw.strip
32 when :cents then Integer(raw.strip, 10) / 100.0
33 when :date then Date.strptime(raw.strip, "%Y%m%d")
34 when :flag then raw.strip == "Y"
35 else raw
36 end
37 end
38end
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Describing structure as data lets one small engine handle every field without branching per column.
  2. 2Ruby ranges over strings slice fixed-width columns cleanly, and filter_map drops blanks in one pass.
  3. 3Centralizing type coercion in a case statement keeps parsing logic in one place and easy to extend.

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