Motivation / Background

This Pull Request has been created because I want Faker to be faster and use less memory in DB seeds and automated tests.

This PR was generated using Claude Code with Fable 5. The change has been reviewed by me and tested by me.

Replace the sub-then-gsub double scan with a single gsub whose block tracks whether the leading (non-zero) digit has been emitted, and look digits up in a frozen DIGITS table instead of calling Integer#to_s for every replaced character.

Additional information

Benchmark (Ruby 3.4.9, arm64-darwin25, benchmark-ips):

require 'benchmark/ips'
require 'faker'

Benchmark.ips do |x|
  x.config(warmup: 2, time: 8)
  x.report("numerify('###-###-####')") { Faker::Base.numerify('###-###-####') }
  x.report("numerify(40 '#'s)") { Faker::Base.numerify('#' * 40) }
  x.report('PhoneNumber.phone_number') { Faker::PhoneNumber.phone_number }
end

And a memory benchmark of the change to show the difference in memory usage.

• Ruby 3.4.9 (arm64-darwin25)
• memory_profiler 1.1.0
• Comparison is against main at 9b078038, with only this change applied.

require 'memory_profiler'
require 'faker'

BENCHES = {
  "numerify('###-###-####')" => -> { Faker::Base.numerify('###-###-####') },
  "numerify(leading_zero: true)" => -> { Faker::Base.numerify('##########', leading_zero: true) },
  'PhoneNumber.phone_number' => -> { Faker::PhoneNumber.phone_number }
}.freeze

# Warm up I18n translation caches so they don't count against the first bench
BENCHES.each_value(&:call)

N = 1000
BENCHES.each do |name, blk|
  report = MemoryProfiler.report { N.times { blk.call } }
  puts format('%-30s per call: %8.1f objects / %10.1f bytes',
              name,
              report.total_allocated.fdiv(N),
              report.total_allocated_memsize.fdiv(N))
end

Results (per call, averaged over 1000 calls)

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug, refactor something, or add a feature.
• Tests and Rubocop are passing before submitting your proposed changes.

If you're proposing a new generator or locale:

• Double-check the existing generators documentation to make sure the new generator you want to add doesn't already exist.
• You've reviewed and followed the Contributing guidelines.