new(module): hashes

Author: litlighilit

src/float_utils/hashes.nim

@@ -0,0 +1,56 @@
+
+import std/hashes; export Hash
+from ./isX import isfinite, isinf, isnan
+from std/math import frexp
+
+const PyHASH_BITS* = when sizeof(pointer) >= 8: 61 else: 31
+const PyHASH_MODULUS* = (1 shl PyHASH_BITS) - 1
+
+const PyHASH_INF* = 314159
+  
+type
+  Py_uhash_t = uint
+static: assert sizeof(Py_uhash_t) == sizeof(Hash)
+
+
+proc Py_HashDouble*(v: float): Hash =
+  ## `_Py_HashDouble`
+  if not isfinite(v):
+    if isinf(v):
+      return if v > 0.0: PyHASH_INF else: -PyHASH_INF
+    else:
+      raise newException(ValueError, "cannot hash NaN")
+  
+  var (m, e) = frexp(v)
+
+  var sign = true
+  if m < 0.0:
+    sign = false
+    m = -m
+
+  var x: Py_uhash_t = 0
+  var y: Py_uhash_t
+
+  while m != 0.0:
+    x = ((x shl 28) and PyHASH_MODULUS) or (x shr (PyHASH_BITS - 28))
+    m *= 268435456.0 # 2**28
+    e -= 28
+    y = Py_uhash_t(m)
+    m -= float(y)
+    x += y
+    if x >= PyHASH_MODULUS:
+      x -= PyHASH_MODULUS
+
+  # adjust for the exponent; first reduce it modulo PyHASH_BITS
+  e = if e >= 0:
+    e mod PyHASH_BITS
+  else:
+    PyHASH_BITS - 1 - ((-1 - e) mod PyHASH_BITS)
+  x = ((x shl e) and PyHASH_MODULUS) or (x shr (PyHASH_BITS - e))
+
+  x = x * Py_uhash_t(sign)
+  if x == Py_uhash_t.high:
+    x = cast[Py_uhash_t](-2)
+  return Hash x
+
+proc hash*(v: float): Hash = Py_HashDouble(v)