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

package engine

import (
	"errors"
	"slices"
	"testing"
)

// TestAlphabetTableEnglish pins the English table against the solver ruleset: 26 letters,
// contiguous indices, the concrete lower-case characters the solver emits and the standard
// tile values. This is the real parity check the UI no longer carries (Stage 13).
func TestAlphabetTableEnglish(t *testing.T) {
	tab, err := AlphabetTable(VariantEnglish)
	if err != nil {
		t.Fatalf("AlphabetTable(english): %v", err)
	}
	if len(tab) != 26 {
		t.Fatalf("size = %d, want 26", len(tab))
	}
	for i, e := range tab {
		if int(e.Index) != i {
			t.Errorf("entry %d has Index %d, want %d (index must equal position)", i, e.Index, i)
		}
	}
	// a=index0/value1, q=index16/value10, z=index25/value10.
	if tab[0].Letter != "a" || tab[0].Value != 1 {
		t.Errorf("entry 0 = %q/%d, want a/1", tab[0].Letter, tab[0].Value)
	}
	if tab[16].Letter != "q" || tab[16].Value != 10 {
		t.Errorf("entry 16 = %q/%d, want q/10", tab[16].Letter, tab[16].Value)
	}
	if tab[25].Letter != "z" || tab[25].Value != 10 {
		t.Errorf("entry 25 = %q/%d, want z/10", tab[25].Letter, tab[25].Value)
	}
}

// TestAlphabetTableRussianVariants pins both Russian variants: they share the 33-letter
// alphabet but differ in tile values — most visibly ё (index 6), worth 3 in Russian
// Scrabble and 0 in Эрудит.
func TestAlphabetTableRussianVariants(t *testing.T) {
	ru, err := AlphabetTable(VariantRussianScrabble)
	if err != nil {
		t.Fatalf("AlphabetTable(russian_scrabble): %v", err)
	}
	er, err := AlphabetTable(VariantErudit)
	if err != nil {
		t.Fatalf("AlphabetTable(erudit): %v", err)
	}
	if len(ru) != 33 || len(er) != 33 {
		t.Fatalf("sizes = %d/%d, want 33/33", len(ru), len(er))
	}
	if ru[0].Letter != "а" || ru[0].Value != 1 {
		t.Errorf("russian entry 0 = %q/%d, want а/1", ru[0].Letter, ru[0].Value)
	}
	if ru[6].Letter != "ё" || ru[6].Value != 3 {
		t.Errorf("russian ё (entry 6) = %q/%d, want ё/3", ru[6].Letter, ru[6].Value)
	}
	if er[6].Letter != "ё" || er[6].Value != 0 {
		t.Errorf("erudit ё (entry 6) = %q/%d, want ё/0", er[6].Letter, er[6].Value)
	}
	if ru[32].Letter != "я" || er[32].Letter != "я" {
		t.Errorf("last letter = %q/%q, want я/я", ru[32].Letter, er[32].Letter)
	}
}

// TestAlphabetTableUnknownVariant rejects a variant outside the catalogue.
func TestAlphabetTableUnknownVariant(t *testing.T) {
	if _, err := AlphabetTable(Variant(99)); !errors.Is(err, ErrUnknownVariant) {
		t.Fatalf("got %v, want ErrUnknownVariant", err)
	}
}

// TestRackCodecRoundTrip pins the rack/exchange index codec the edge uses: EncodeRack maps
// concrete letters (with "?" for a blank) to indices (BlankIndex for the blank) and
// DecodeTiles inverts it. EncodeRack is case-insensitive so it accepts the lower-case
// Hand form and an upper-case letter alike.
func TestRackCodecRoundTrip(t *testing.T) {
	letters := []string{"c", "a", "t", "?"}
	idx, err := EncodeRack(VariantEnglish, letters)
	if err != nil {
		t.Fatalf("EncodeRack: %v", err)
	}
	if want := []int{2, 0, 19, BlankIndex}; !slices.Equal(idx, want) {
		t.Fatalf("EncodeRack = %v, want %v", idx, want)
	}
	back, err := DecodeTiles(VariantEnglish, idx)
	if err != nil {
		t.Fatalf("DecodeTiles: %v", err)
	}
	if !slices.Equal(back, letters) {
		t.Fatalf("DecodeTiles = %v, want %v", back, letters)
	}
	if up, err := EncodeRack(VariantEnglish, []string{"C"}); err != nil || !slices.Equal(up, []int{2}) {
		t.Errorf("EncodeRack upper-case = %v,%v; want [2],nil", up, err)
	}
}

// TestDecodeWordAndBounds covers the word-check decode and the out-of-range guard.
func TestDecodeWordAndBounds(t *testing.T) {
	w, err := DecodeWord(VariantEnglish, []int{2, 0, 19})
	if err != nil || w != "cat" {
		t.Fatalf("DecodeWord = %q,%v; want cat,nil", w, err)
	}
	if _, err := LetterForIndex(VariantEnglish, 26); !errors.Is(err, ErrIllegalPlay) {
		t.Errorf("out-of-range index: got %v, want ErrIllegalPlay", err)
	}
	if _, err := DecodeWord(VariantEnglish, []int{BlankIndex}); !errors.Is(err, ErrIllegalPlay) {
		t.Errorf("blank in word: got %v, want ErrIllegalPlay", err)
	}
}