util.format: A string.format wrapper that gracefully handles invalid arguments

1 files changed, 74 insertions, 0 deletions

diff --git a/util/format.lua b/util/format.lua
new file mode 100644
index 00000000..5f2b12be
--- /dev/null
+++ b/util/format.lua
@@ -0,0 +1,74 @@
+--
+-- A string.format wrapper that gracefully handles invalid arguments
+--
+
+local tostring = tostring;
+local select = select;
+local assert = assert;
+local unpack = unpack;
+local type = type;
+
+local function format(format, ...)
+	local args, args_length = { ... }, select('#', ...);
+
+	-- format specifier spec:
+	-- 1. Start: '%%'
+	-- 2. Flags: '[%-%+ #0]'
+	-- 3. Width: '%d?%d?'
+	-- 4. Precision: '%.?%d?%d?'
+	-- 5. Option: '[cdiouxXaAeEfgGqs%%]'
+	--
+	-- The options c, d, E, e, f, g, G, i, o, u, X, and x all expect a number as argument, whereas q and s expect a string.
+	-- This function does not accept string values containing embedded zeros, except as arguments to the q option.
+	-- a and A are only in Lua 5.2+
+
+
+	-- process each format specifier
+	local i = 0;
+	format = format:gsub("%%[^cdiouxXaAeEfgGqs%%]*[cdiouxXaAeEfgGqs%%]", function(spec)
+		if spec ~= "%%" then
+			i = i + 1;
+			local arg = args[i];
+			if arg == nil then -- special handling for nil
+				arg = "<nil>"
+				args[i] = "<nil>";
+			end
+
+			local option = spec:sub(-1);
+			if option == "q" or option == "s" then -- arg should be string
+				args[i] = tostring(arg);
+			elseif type(arg) ~= "number" then -- arg isn't number as expected?
+				args[i] = tostring(arg);
+				spec = "[%s]";
+			end
+		end
+		return spec;
+	end);
+
+	-- process extra args
+	while i < args_length do
+		i = i + 1;
+		local arg = args[i];
+		if arg == nil then
+			args[i] = "<nil>";
+		else
+			args[i] = tostring(arg);
+		end
+		format = format .. " [%s]"
+	end
+
+	return format:format(unpack(args));
+end
+
+local function test()
+	assert(format("%s", "hello") == "hello");
+	assert(format("%s") == "<nil>");
+	assert(format("%s", true) == "true");
+	assert(format("%d", true) == "[true]");
+	assert(format("%%", true) == "% [true]");
+end
+
+return {
+	format = format;
+	test = test;
+};