port to Lua; small safety catch in Perl version

decode.lua

@@ -0,0 +1,14 @@
+for i = 1, #arg do
+	local rf = assert(arg[i])
+	local rfh = assert(io.open(rf, 'rb'))
+
+	local wf = rf:gsub("%.[sS][pP][dD]$", ".txt", 1)
+	assert(rf ~= wf)
+	local wfh = assert(io.open(wf, 'w'))
+
+	assert(rfh:seek("set", 1029))
+	for pair in rfh:lines(16) do
+		wl, A = string.unpack("<dd", pair)
+		wfh:write(wl, "\t", A, "\n")
+	end
+end

decode.pl

@@ -8,6 +8,7 @@
 	open my $rfh, "<:raw", $src;
 
 	(my $dest = $src) =~ s/\.spd$/.txt/;
+	die "$src doesn't look like spd file" if $src eq $dest;
 	open my $wfh, ">", $dest;
 
 	# 1024 looks like a fine header size, but there is