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

package engine

import (
	"fmt"
	"strings"
)

// AlphabetEntry is one letter of a variant's alphabet: its alphabet-index byte, the
// concrete character and its tile point value. It is the dictionary-independent display
// table the edge sends to the client, produced from the variant's solver
// ruleset (its alphabet and value table) and so pinned by the solver version, not by any
// dictionary.
type AlphabetEntry struct {
	// Index is the alphabet-index byte the wire uses for this letter (0..Size-1).
	Index byte
	// Letter is the concrete character, in the case the solver ruleset emits (lower).
	Letter string
	// Value is the tile's point score.
	Value int
}

// BlankIndex is the wire sentinel for a blank tile inside an alphabet-index sequence (a
// rack or an exchange list). It is out of range of every offered variant's alphabet (the
// largest has 33 letters), so it never collides with a real letter index. A placed blank
// instead travels as an ordinary tile carrying its designated letter's index alongside a
// separate blank flag. The constant is untyped so it serves both byte (FlatBuffers ubyte)
// and int (the gateway/backend JSON edge) call sites.
const BlankIndex = 0xFF

// variantCodec is the cached per-variant alphabet data backing the wire helpers: the
// ordered display table and a case-insensitive letter→index lookup. Both are derived once
// from the solver ruleset (see variantCodecs).
type variantCodec struct {
	table         []AlphabetEntry
	letterToIndex map[string]byte
}

// variantCodecs holds one codec per offered variant, built once at package load from each
// ruleset's alphabet and value table. The rulesets are needed only here (not per request),
// so the hot path never rebuilds them.
var variantCodecs = buildVariantCodecs()

func buildVariantCodecs() map[Variant]*variantCodec {
	m := make(map[Variant]*variantCodec, len(Variants()))
	for _, v := range Variants() {
		rs, ok := v.ruleset()
		if !ok {
			continue
		}
		size := rs.Alphabet.Size()
		table := make([]AlphabetEntry, size)
		lut := make(map[string]byte, size)
		for i := range size {
			ch, err := rs.Alphabet.Character(byte(i))
			if err != nil {
				// An offered variant's alphabet never yields a bad index; skip defensively.
				continue
			}
			table[i] = AlphabetEntry{Index: byte(i), Letter: ch, Value: rs.Values[i]}
			lut[strings.ToLower(ch)] = byte(i)
		}
		m[v] = &variantCodec{table: table, letterToIndex: lut}
	}
	return m
}

// AlphabetTable returns a copy of variant's full alphabet as an ordered (index, letter,
// value) table, or ErrUnknownVariant. Entry i has Index i, so the slice doubles as an
// index→(letter, value) lookup. It needs no dictionary — the data comes from the solver
// ruleset alone — so it is safe to build for any offered variant and is the same table the
// client caches for display while live play exchanges bare indices.
func AlphabetTable(v Variant) ([]AlphabetEntry, error) {
	c, ok := variantCodecs[v]
	if !ok {
		return nil, fmt.Errorf("%w: %d", ErrUnknownVariant, v)
	}
	out := make([]AlphabetEntry, len(c.table))
	copy(out, c.table)
	return out, nil
}

// LetterForIndex maps one alphabet index to its concrete letter for variant. It is the
// wire-decode primitive for a placed tile (a blank carries its designated letter's index).
// An out-of-range index is an illegal play.
func LetterForIndex(v Variant, idx int) (string, error) {
	c, ok := variantCodecs[v]
	if !ok {
		return "", fmt.Errorf("%w: %d", ErrUnknownVariant, v)
	}
	if idx < 0 || idx >= len(c.table) {
		return "", fmt.Errorf("%w: alphabet index %d for %s", ErrIllegalPlay, idx, v)
	}
	return c.table[idx].Letter, nil
}

// EncodeRack maps a decoded rack (the Game.Hand form: concrete letters with "?" for an
// undesignated blank) to wire alphabet indices, using BlankIndex for each blank. It backs
// the per-player state view, whose rack the client renders via the cached table.
func EncodeRack(v Variant, letters []string) ([]int, error) {
	c, ok := variantCodecs[v]
	if !ok {
		return nil, fmt.Errorf("%w: %d", ErrUnknownVariant, v)
	}
	out := make([]int, len(letters))
	for i, l := range letters {
		if l == blankLetter {
			out[i] = BlankIndex
			continue
		}
		idx, ok := c.letterToIndex[strings.ToLower(l)]
		if !ok {
			return nil, fmt.Errorf("%w: rack letter %q for %s", ErrTilesNotOnRack, l, v)
		}
		out[i] = int(idx)
	}
	return out, nil
}

// DecodeTiles maps a wire rack/exchange index list back to the decoded letter form ("?"
// for a blank, BlankIndex), for handing to the existing letter-based exchange path.
func DecodeTiles(v Variant, idx []int) ([]string, error) {
	out := make([]string, len(idx))
	for i, x := range idx {
		if x == BlankIndex {
			out[i] = blankLetter
			continue
		}
		l, err := LetterForIndex(v, x)
		if err != nil {
			return nil, fmt.Errorf("%w (exchange)", err)
		}
		out[i] = l
	}
	return out, nil
}

// DecodeWord maps a sequence of alphabet indices to a concrete word (word-check carries no
// blanks). The client constrains input to the variant's alphabet, so every index is a real
// letter.
func DecodeWord(v Variant, idx []int) (string, error) {
	var sb strings.Builder
	for _, x := range idx {
		l, err := LetterForIndex(v, x)
		if err != nil {
			return "", fmt.Errorf("%w (word check)", err)
		}
		sb.WriteString(l)
	}
	return sb.String(), nil
}