Implement Scrabble move generator (DAWG) with English and Russian rules

A Go library that returns every legal play ranked by score and scores or
validates plays, using the Appel-Jacobson DAWG algorithm over
github.com/iliadenisov/dafsa v1.1.0.

- DAWG move generation (across / down / both), full tournament scoring with a
  per-tile breakdown; public Solver: GenerateMoves (ranked), ScorePlay,
  ValidatePlay.
- Rulesets: English Scrabble, Russian Scrabble, Эрудит (parameterizable Ruleset).
- cmd/builddict (build the DAWG from the dictionaries submodule), cmd/stress
  (self-play benchmark), selfplay engine; brute-force test oracle.
- A GADDAG was implemented, benchmarked and removed (the DAWG was smaller and
  faster for a scoring solver); see RESULTS.md and ALGORITHM.md.

cmd/builddict/main.go

@@ -0,0 +1,67 @@
+// Command builddict converts a word list into a serialized DAWG, cached under testdata
+// for the tests and the benchmark. By default it reads the English SOWPODS list from
+// the dictionaries submodule.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/iliadenisov/alphabet"
+
+	"scrabble-solver/internal/dictdawg"
+	"scrabble-solver/internal/wordlist"
+)
+
+func main() {
+	dict := flag.String("dict", "dictionaries/english/sowpods.txt", "word list file (one word per line)")
+	out := flag.String("out", "testdata", "output directory")
+	name := flag.String("name", "sowpods", "base name for the output file")
+	minLen := flag.Int("min", 2, "minimum word length")
+	maxLen := flag.Int("max", 15, "maximum word length")
+	flag.Parse()
+
+	idx := alphabet.Latin()
+
+	t0 := time.Now()
+	words, err := wordlist.Read(*dict, idx, *minLen, *maxLen)
+	if err != nil {
+		log.Fatalf("read %s: %v", *dict, err)
+	}
+	fmt.Printf("loaded %d words from %s in %s\n", len(words), *dict, time.Since(t0).Round(time.Millisecond))
+
+	if err := os.MkdirAll(*out, 0o755); err != nil {
+		log.Fatal(err)
+	}
+
+	t := time.Now()
+	f, err := dictdawg.Build(idx, words)
+	if err != nil {
+		log.Fatalf("build dawg: %v", err)
+	}
+	path := filepath.Join(*out, *name+".dawg")
+	if err := dictdawg.Save(f, path); err != nil {
+		log.Fatalf("save: %v", err)
+	}
+	size := int64(0)
+	if fi, err := os.Stat(path); err == nil {
+		size = fi.Size()
+	}
+	fmt.Printf("DAWG %d nodes, %s, built+saved in %s -> %s\n",
+		f.NumNodes(), humanBytes(size), time.Since(t).Round(time.Millisecond), path)
+}
+
+func humanBytes(n int64) string {
+	switch {
+	case n >= 1<<20:
+		return fmt.Sprintf("%.2f MB", float64(n)/(1<<20))
+	case n >= 1<<10:
+		return fmt.Sprintf("%.1f KB", float64(n)/(1<<10))
+	default:
+		return fmt.Sprintf("%d B", n)
+	}
+}