# DAWG vs GADDAG — self-play stress test

> **Note.** The GADDAG generator has since been **removed** from the codebase — the DAWG
> is the sole move generator. This document is kept as the record of the comparison that
> justified that choice. `cmd/stress` now benchmarks the DAWG alone.

The two move generators were compared by playing greedy AI-vs-AI games on the same
dictionary with the same seeds, measuring speed and memory. Reproduce with:

```
go run ./cmd/builddict          # build testdata/sowpods.{dawg,gaddag}
go run ./cmd/stress -games 200
```

## Setup

- Dictionary: English **SOWPODS**, 267,752 words (2–15 letters).
- Board: standard 15×15; greedy player (highest-scoring move), seeded RNG.
- 200 games per generator, identical seeds; mode = both orientations.
- Machine: this dev container (Go 1.26, 12 cores; single-threaded run).

## Results (200 games)

Includes the **deterministic cross-set optimization** (one arc enumeration via
`completers()` for one-sided squares instead of probing every letter); both one-sided
cases are deterministic for the GADDAG, only the right-extension is for the DAWG.

| metric | DAWG | GADDAG |
|---|---:|---:|
| structure size | **732.5 KB** | 5.12 MB |
| games / turns / plays | 200 / 4783 / 4769 | 200 / 4783 / 4769 |
| moves generated | 3,880,236 | 3,880,236 |
| generation time | **23.68 s** | 25.98 s |
| µs / move-generation call | **4951** | 5431 |
| moves generated / sec | **163,863** | 149,383 |
| arcs traversed | **261.8 M** | 276.0 M |
| arcs / move generated | **67.5** | 71.1 |
| heap allocated | 16.79 GB | 16.79 GB |
| GC cycles | 5624 | 5605 |
| avg final game score | 849.3 | 849.3 |

`GADDAG vs DAWG: 1.10× generation time, 7.16× structure size, 1.00× heap.`
Peak process RSS (both structures mapped): ~42 MB.

The optimization cut the GADDAG's cross-set arcs (285.5 M → 276.0 M) and narrowed the
arc gap (1.08× → 1.05×), but the verdict is unchanged: **the GADDAG is still ~10 %
slower, 7× larger and traverses slightly more arcs.** End-to-end time is dominated by
the shared per-move scoring (`Evaluate`, ~3.9 M calls), not by graph search, so the
search-algorithm difference barely moves the total — and what difference remains favours
the narrower, smaller DAWG.

## Interpretation

- **Correctness.** Both generators produce the *identical* set of moves and scores at
  every position (identical turns/plays/moves/score above, and the Stage-8 mutual oracle
  agreeing on 119 positions over 5 games). They differ only in how they search.
- **Speed.** The DAWG is ~10 % faster end-to-end and traverses ~8 % **fewer** arcs.
- **The GADDAG does not win here, contrary to Gordon's paper.** Two reasons:
  1. We use the **final-flag GADDAG** (the minimized DAWG of `REV(x)◊y`, completion via
     accepting states) so that `dafsa` can be used essentially unmodified. This variant
     lacks Gordon's *letter-sets-on-arcs* compression, so it is both ~7× larger and has
     wider nodes — it traverses *more* arcs, not fewer, erasing the theoretical edge.
  2. The workload is a **scoring solver**: every generated move is scored (cross-words,
     premiums, bonus) by the shared `Evaluate`. That shared per-move cost dominates, so
     the search-algorithm difference is small — and what remains favours the simpler,
     narrower DAWG.
- **Memory.** The GADDAG structure is 7.16× larger (5.12 MB vs 732 KB). Per-move heap
  allocation is identical (dominated by shared scoring), and overall RSS is modest.

## Decision

**Use the DAWG (Appel-Jacobson) generator as the production default.** For this library
(a move generator that scores and ranks every play) it is smaller (7×), at least as fast,
and simpler to operate: it needs no separator symbol or custom alphabet, `dafsa`'s
`Save`/`Load` work unchanged, and it requires the fewest `dafsa` additions (only the
shared low-level traversal API).

The **GADDAG generator is kept and remains selectable** (`scrabble.NewGADDAGGenerator`),
both as an alternative and as a continuing correctness oracle for the DAWG.

### Caveats / what could change the picture

- A **letter-set GADDAG** (Gordon's true compressed form) plus **incremental cross-set
  maintenance** would shrink the GADDAG and cut its arc count; it might then beat the DAWG
  on raw move generation. This was not pursued: the DAWG already meets the goal, and the
  7× size gap is decisive for a scoring-solver workload where generation is not the
  bottleneck. It remains a documented future optimization.
