Add replacement frexp() for 5.3 without math.frexp() available

Author: ToxicFrog

frexp.lua

@@ -0,0 +1,24 @@
+-- math.frexp() replacement for Lua 5.3 when compiled without LUA_COMPAT_MATHLIB.
+-- The C approach is just to type-pun the float, but we can't do that here short
+-- of stupid loadstring() tricks, which would be both architecture and version
+-- dependent and a maintenance headache at best. So instead we use math.
+
+local abs,floor,log = math.abs,math.floor,math.log
+local log2 = log(2)
+
+return function(x)
+  if x == 0 then return 0.0,0.0 end
+  local e = floor(log(abs(x)) / log2)
+  if e > 0 then
+    -- Why not x / 2^e? Because for large-but-still-legal values of e this
+    -- ends up rounding to inf and the wheels come off.
+    x = x * 2^-e
+  else
+    x = x / 2^e
+  end
+  -- Normalize to the range [0.5,1)
+  if abs(x) >= 1.0 then
+    x,e = x/2,e+1
+  end
+  return x,e
+end

io/f.lua

@@ -3,8 +3,7 @@
 local struct = require "vstruct"
 local io   = require "vstruct.io"
 local unpack = table.unpack or unpack
-local frexp = assert(math.frexp,
-  "math.frexp missing -- please recompile lua with LUA_COMPAT_MATHLIB enabled")
+local frexp = math.frexp or require "vstruct.frexp"
 
 local sizes = {
   [4] = {1,  8, 23};

test/frexp.lua

@@ -0,0 +1,76 @@
+-- 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 frexp = require "vstruct.frexp"
+
+-- No point in running these tests if there isn't a builtin frexp to compare to.
+if not math.frexp then return end
+
+test.group "frexp()"
+
+local function test_frexp(x)
+  local m,e = math.frexp(x)
+  if m >= 1 or m <= -1 then
+    -- At least on my machine, frexp() sometimes returns 1 as the mantissa, in
+    -- violation as the spec. Correct this so that we can more properly compare
+    -- things.
+    m,e = m/2, e+1
+  end
+  local m2,e2 = frexp(x)
+  local ok = m == m2 and e == e2
+  test.record("frexp equivalence", ok, x,
+    "(builtin) %f,%f != %f,%f (lua)", m, e, m2, e2)
+end
+
+local inf = math.huge
+local z = 0.0
+local nz = -z
+local doubles = {
+  0.0, 1.0, 2.0,
+  1/2, 1/4, 1/8,
+  4.9406564584124654418e-324,
+  7.4169128616906696301e-309,
+  1.483382572338133926e-308,
+  2.225073858507200889e-308,
+  2.2250738585072013831e-308,
+  2.2250738585072018772e-308,
+  2.9667651446762683461e-308,
+  3.7084564308453353091e-308,
+  4.4501477170144022721e-308,
+  8.9884656743115795386e+307,
+  8.9884656743115815345e+307,
+  1.1984620899082105386e+308,
+  1.4980776123852631234e+308,
+  1.7976931348623157081e+308,
+  4.9406564584124654418e-324,
+  2.225073858507200889e-308,
+  2.2250738585072013831e-308,
+  1.7976931348623157081e+308,
+  0.0,
+  nz,
+  -4.9406564584124654418e-324,
+  -7.4169128616906696301e-309,
+  -1.483382572338133926e-308,
+  -2.225073858507200889e-308,
+  -2.2250738585072013831e-308,
+  -2.2250738585072018772e-308,
+  -2.9667651446762683461e-308,
+  -3.7084564308453353091e-308,
+  -4.4501477170144022721e-308,
+  -8.9884656743115795386e+307,
+  -8.9884656743115815345e+307,
+  -1.1984620899082105386e+308,
+  -1.4980776123852631234e+308,
+  -1.7976931348623157081e+308,
+  -4.9406564584124654418e-324,
+  -2.225073858507200889e-308,
+  -2.2250738585072013831e-308,
+  -1.7976931348623157081e+308,
+  nz,
+}
+
+for _,double in ipairs(doubles) do
+  test_frexp(double)
+end

test/regression.lua

@@ -25,26 +25,3 @@ T("p passes invalid value to string.char and crashes",
   "> p2,2",
   x"FD 00", -192.098910,
   x"FD 00", -192.00)
-
--- math.frexp() replacement for Lua 5.3 when compiled without LUA_COMPAT_MATHLIB.
--- Work in progress; fails on very large/small numbers because 2^e ends up being
--- inf/0 in the final calculation of (x/2^e).
-local abs,floor,log = math.abs,math.floor,math.log
-local log2 = log(2)
-local function frexp(x)
-  if x == 0 then return 0, 0 end
-  local e = floor(log(abs(x)) / log2 + 1)
-  return x / 2 ^ e, e
-end
-
-local function test_frexp(x)
-  local m,e = math.frexp(x)
-  local m2,e2 = frexp(x)
-  test.record("frexp equivalence", m == m2 and e == e2, x,
-    "(builtin) %f,%f != %f,%f (lua)", m, e, m2, e2)
-end
-
--- test_frexp(1.7976931348623157081e+308)
--- test_frexp(-1.7976931348623157081e+308)
--- test_frexp(1.7976931348623157081e-308)
--- test_frexp(-1.7976931348623157081e-308)

test/struct-test-gen.lua

@@ -8,7 +8,7 @@ local CONF = {
   MAX_DEPTH = 5,     -- Limit on nesting depth of test formats.
   COMPOUND_LENGTH = 2,   -- # formats in a sequence.
   SEED = false,    -- Seed for generating tests.
-  
+
   NROF_TESTS = 2^10,  -- number of random test cases to run
 }
 
@@ -77,8 +77,8 @@ local test = require "vstruct.test.common"
 local record = test.record
 local __EOG__ = __EOG__
 
-local abs,ceil,floor,fmod,frexp,log = math.abs, math.ceil, math.floor, math.fmod, math.frexp, math.log
-
+local abs,ceil,floor,fmod = math.abs, math.ceil, math.floor, math.fmod
+local frexp = math.frexp or require "vstruct.frexp"
 local char_ = string.char
 
 local function char(x) return char_(x % 256) end
@@ -136,7 +136,7 @@ function dump.number(x)
   if e == 0 then return ("%.0f"):format(m) end
 
   local op = e < 0 and "/" or "*"
-  e = math.abs(e)
+  e = abs(e)
   local exp = e > 1 and "2^"..e or "2"
   return ("%.0f%s%s"):format(m, op, exp)
 end