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
+}