Add regression tests for B#21

Three tests are added, checking both the reported bug (certain values to
format p will crash vstruct) and the underlying bug that causes it
(fractional values to formats i and u are not truncated properly).

Author: ToxicFrog

vstruct/test.lua

@@ -32,6 +32,7 @@ require "vstruct.test.basic"
 require "vstruct.test.fp-bigendian"
 require "vstruct.test.fp-littleendian"
 require "vstruct.test.error"
+require "vstruct.test.regression"
 
 if arg and #arg > 0 then
   require "vstruct.test.struct-test-gen"

vstruct/test/regression.lua

@@ -0,0 +1,27 @@
+-- regression tests for vstruct
+-- tests for fixed bugs go here to make sure they don't recur
+-- tests are named after the bug they trigger, not the intended behaviour
+
+local test = require "vstruct.test.common"
+
+local x = test.x
+local T = test.autotest
+
+test.group "regression tests"
+
+-- B#21
+-- this was found as an error in packing fixed point values, but is actually an underlying issue
+-- with how u and i handle non-integer inputs
+-- in short: don't assume string.char() truncates, because it doesn't.
+T("i rounds instead of truncating",
+  "> i2",
+  x"FF FF", -1.5,
+  x"FF FF", -1)
+T("u rounds instead of truncating",
+  "> u2",
+  x"00 01", 1.5,
+  x"00 01", 1)
+T("p passes invalid value to string.char and crashes",
+  "> p2,2",
+  x"FD 00", -192.098910,
+  x"FD 00", -192.00)