v0.14.0 web replay UI, My games, CLI replay

internal/client/display/fen.go

@@ -0,0 +1,50 @@
+package display
+
+import (
+	"fmt"
+	"strings"
+)
+
+// BoardFromFEN renders a FEN's piece placement in the server's ASCII board
+// format (files on the first and last lines, ranks at both ends of a row,
+// "." for an empty square), so RenderBoard colors it the same way.
+func BoardFromFEN(fen string) (string, error) {
+	fields := strings.Fields(fen)
+	if len(fields) == 0 {
+		return "", fmt.Errorf("empty FEN")
+	}
+	rows := strings.Split(fields[0], "/")
+	if len(rows) != 8 {
+		return "", fmt.Errorf("FEN has %d ranks, want 8", len(rows))
+	}
+	var b strings.Builder
+	b.WriteString("  a b c d e f g h\n")
+	for i, row := range rows {
+		rank := byte('8' - i)
+		b.WriteByte(rank)
+		b.WriteByte(' ')
+		files := 0
+		for _, c := range row {
+			switch {
+			case c >= '1' && c <= '8':
+				for n := 0; n < int(c-'0'); n++ {
+					b.WriteString(". ")
+				}
+				files += int(c - '0')
+			case strings.ContainsRune("pnbrqkPNBRQK", c):
+				b.WriteRune(c)
+				b.WriteByte(' ')
+				files++
+			default:
+				return "", fmt.Errorf("invalid FEN character %q", c)
+			}
+		}
+		if files != 8 {
+			return "", fmt.Errorf("rank %c has %d files, want 8", rank, files)
+		}
+		b.WriteByte(rank)
+		b.WriteByte('\n')
+	}
+	b.WriteString("  a b c d e f g h")
+	return b.String(), nil
+}

internal/client/display/fen_test.go

@@ -0,0 +1,28 @@
+package display
+
+import "testing"
+
+func TestBoardFromFEN(t *testing.T) {
+	got, err := BoardFromFEN("rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "  a b c d e f g h\n" +
+		"8 r n b q k b n r 8\n" +
+		"7 p p p p p p p p 7\n" +
+		"6 . . . . . . . . 6\n" +
+		"5 . . . . . . . . 5\n" +
+		"4 . . . . P . . . 4\n" +
+		"3 . . . . . . . . 3\n" +
+		"2 P P P P . P P P 2\n" +
+		"1 R N B Q K B N R 1\n" +
+		"  a b c d e f g h"
+	if got != want {
+		t.Errorf("board:\n%s\nwant:\n%s", got, want)
+	}
+	for _, bad := range []string{"", "8/8/8/8/8/8/8 w - - 0 1", "9/8/8/8/8/8/8/8 w - - 0 1", "x7/8/8/8/8/8/8/8 w - - 0 1"} {
+		if _, err := BoardFromFEN(bad); err == nil {
+			t.Errorf("BoardFromFEN(%q) accepted", bad)
+		}
+	}
+}