Rename tgamma to gamma (#7929)

* Rename tgamma to gamma

* set the deprecating version  0.19.0

* update changelog and use description in deprecated pragma

changelog.md

@@ -87,6 +87,7 @@
 - Added the parameter ``val`` for the ``CritBitTree[int].inc`` proc.
 - An exception raised from ``test`` block of ``unittest`` now shows its type in
   the error message
+- The proc ``tgamma`` was renamed to ``gamma``. ``tgamma`` is deprecated.
 
 ### Language additions
 

lib/pure/math.nim

@@ -280,12 +280,18 @@ when not defined(JS): # C
   proc erfc*(x: float64): float64 {.importc: "erfc", header: "<math.h>".}
     ## The complementary error function
 
+  proc gamma*(x: float32): float32 {.importc: "tgammaf", header: "<math.h>".}
+  proc gamma*(x: float64): float64 {.importc: "tgamma", header: "<math.h>".}
+    ## The gamma function
+  proc tgamma*(x: float32): float32
+    {.deprecated: "use gamma instead", importc: "tgammaf", header: "<math.h>".}
+  proc tgamma*(x: float64): float64
+    {.deprecated: "use gamma instead", importc: "tgamma", header: "<math.h>".}
+    ## The gamma function
+    ## **Deprecated since version 0.19.0**: Use ``gamma`` instead.
   proc lgamma*(x: float32): float32 {.importc: "lgammaf", header: "<math.h>".}
   proc lgamma*(x: float64): float64 {.importc: "lgamma", header: "<math.h>".}
     ## Natural log of the gamma function
-  proc tgamma*(x: float32): float32 {.importc: "tgammaf", header: "<math.h>".}
-  proc tgamma*(x: float64): float64 {.importc: "tgamma", header: "<math.h>".}
-    ## The gamma function
 
   proc floor*(x: float32): float32 {.importc: "floorf", header: "<math.h>".}
   proc floor*(x: float64): float64 {.importc: "floor", header: "<math.h>".}
@@ -557,6 +563,7 @@ when isMainModule and not defined(JS):
     return sqrt(num)
 
   # check gamma function
+  assert(gamma(5.0) == 24.0) # 4!
   assert($tgamma(5.0) == $24.0) # 4!
   assert(lgamma(1.0) == 0.0) # ln(1.0) == 0.0
   assert(erf(6.0) > erf(5.0))