StringView::Pool — amortized-allocation view factory

[tobi]

What

StringView::Pool pre-allocates a batch of StringView objects tied to a single backing string. pool.view(offset, length) returns a pre-built view with just two long writes — zero Ruby object allocation in steady state.

Why

When parsing a large buffer, you extract dozens of substrings per parse call. Each StringView.new allocates one Ruby object (~235ns). In a hot loop processing thousands of messages, this dominates:

10,000 messages × 20 fields × 235ns = 47ms just in allocation overhead

With a pool, each .view() is ~13ns (18x faster) and allocates nothing.

The looped parser pattern

pool = StringView::Pool.new(buffer)

records.each do |record_offset, field_offsets|
  name  = pool.view(record_offset, name_len)       # ~13ns, 0 alloc
  value = pool.view(record_offset + sep, value_len) # ~13ns, 0 alloc
  process(name, value)
  pool.reset!  # rewind cursor — views get reused next iteration
end

After the first iteration, the pool has enough capacity and every subsequent .view() call is zero-allocation. The pool stabilizes at the high-water mark of views needed per iteration.

Performance

StringView.new:       ~235ns/view   (4.3M views/s)
Pool.view (grow):     ~170ns/view   (5.8M views/s)  — 1.4x faster
Pool.view (reuse):     ~13ns/view  (77M views/s)     — 18x faster

GC safety

Every view in the pool is a real Ruby object managed by the GC. The pool holds them in a Ruby Array (GC-visible). Each view holds a strong reference to the backing string via rb_gc_mark_movable. Compaction-safe via dcompact callbacks. No tricks, no unsafe pointers, no finalizers.

API

pool = StringView::Pool.new(string)      # create pool, pre-allocate 32 views
view = pool.view(byte_offset, byte_len)  # return next pre-allocated view (~13ns)
pool.reset!                               # rewind cursor, reuse views
pool.size                                 # views handed out since last reset
pool.capacity                             # total pre-allocated slots
pool.backing                              # the frozen backing string

Growth: starts at 32 slots, doubles when exhausted (32 → 64 → 128 → …).

View lifetime after reset!

After pool.reset!, previously returned views are still valid Ruby objects, but their offset/length will be overwritten by the next .view() call that reuses that slot. If you need a view to outlive a reset:

• Call .to_s to materialize it into a String before resetting
• Use StringView.new for long-lived views
• Don't call reset! (let the pool grow — views stay valid forever)

Tests

285 tests, 643 assertions, 0 failures. Includes:

• Parser loop pattern tests with zero-allocation verification
• Exponential growth tests
• GC safety tests
• Allocation counting: 0 per .view() when pre-warmed vs 1 per StringView.new