scrabble-game/backend/internal/engine/decode.go

package engine

import (
	"fmt"

	"scrabble-solver/board"
	"scrabble-solver/rules"
	"scrabble-solver/scrabble"
)

// ActionKind classifies a turn in the move log.
type ActionKind uint8

const (
	// ActionPlay is a tile placement forming one or more words.
	ActionPlay ActionKind = iota
	// ActionPass is a forfeited turn.
	ActionPass
	// ActionExchange swaps tiles with the bag.
	ActionExchange
	// ActionResign abandons the game.
	ActionResign
	// ActionTimeout is the auto-resignation a missed turn becomes; recorded by
	// the game domain in a later stage, never produced by the engine itself.
	ActionTimeout
)

// String renders the action kind for logs and GCG export.
func (a ActionKind) String() string {
	switch a {
	case ActionPlay:
		return "play"
	case ActionPass:
		return "pass"
	case ActionExchange:
		return "exchange"
	case ActionResign:
		return "resign"
	case ActionTimeout:
		return "timeout"
	}
	return "unknown"
}

// TileRecord is a single placed tile decoded to a concrete letter, so the move
// log is independent of any dictionary and of the solver's internal encoding.
type TileRecord struct {
	Row, Col int
	// Letter is the concrete character placed; for a blank, the letter it stands
	// for.
	Letter string
	// Blank reports whether the tile was placed from a blank (and so scored 0).
	Blank bool
}

// MoveRecord is one turn in the dictionary-independent history. A play carries
// the placed tiles, the words it formed and its score; a pass or resignation
// carries only the action; an exchange carries the number of tiles swapped. The
// game domain adds timestamps and persistence around these values.
type MoveRecord struct {
	Player int
	Action ActionKind
	Tiles  []TileRecord // ActionPlay only
	Words  []string     // ActionPlay only: the main word first, then cross words
	Count  int          // ActionExchange only: number of tiles swapped
	Score  int          // points scored this turn (0 for non-plays)
	Total  int          // the player's running total after this turn
}

// recordPlay decodes a scored move into a dictionary-independent MoveRecord for
// the given player.
func (g *Game) recordPlay(player int, m scrabble.Move) MoveRecord {
	tiles := make([]TileRecord, len(m.Tiles))
	for i, p := range m.Tiles {
		tiles[i] = TileRecord{Row: p.Row, Col: p.Col, Letter: g.letter(p.Letter), Blank: p.Blank}
	}
	words := make([]string, 0, 1+len(m.Cross))
	words = append(words, g.word(m.Main))
	for _, cw := range m.Cross {
		words = append(words, g.word(cw))
	}
	return MoveRecord{
		Player: player,
		Action: ActionPlay,
		Tiles:  tiles,
		Words:  words,
		Score:  m.Score,
		Total:  g.scores[player],
	}
}

// letter decodes one alphabet index to its concrete character via the ruleset's
// alphabet. A malformed index yields the empty string.
func (g *Game) letter(idx byte) string {
	s, err := g.rules.Alphabet.Character(idx)
	if err != nil {
		return ""
	}
	return s
}

// word decodes a solver word's letters to a concrete string via the ruleset's
// alphabet. A malformed word yields the empty string.
func (g *Game) word(w scrabble.Word) string {
	s, err := g.rules.Alphabet.Decode(w.Letters)
	if err != nil {
		return ""
	}
	return s
}

// ReplayBoard reconstructs the board the play records produce, on an empty board
// for ruleset rs, using only the alphabet and never a dictionary: each recorded
// letter is re-indexed and the placements are applied. Non-play records are
// ignored. It realises the history invariant in docs/ARCHITECTURE.md §9.1 — an
// archived game replays from decoded values plus its variant metadata alone.
func ReplayBoard(rs *rules.Ruleset, records []MoveRecord) (*board.Board, error) {
	b := board.New(rs.Rows, rs.Cols)
	for _, rec := range records {
		if rec.Action != ActionPlay {
			continue
		}
		placements := make([]scrabble.Placement, len(rec.Tiles))
		for i, t := range rec.Tiles {
			idx, err := rs.Alphabet.Index(t.Letter)
			if err != nil {
				return nil, fmt.Errorf("engine: replay letter %q at (%d,%d): %w", t.Letter, t.Row, t.Col, err)
			}
			placements[i] = scrabble.Placement{Row: t.Row, Col: t.Col, Letter: idx, Blank: t.Blank}
		}
		scrabble.Apply(b, scrabble.Move{Tiles: placements})
	}
	return b, nil
}