use dragonbox algorithm; alternative to #18008 (#18139)

* use dragonbox algorithm; alternative to #18008
* removed unsafe code

changelog.md

@@ -76,6 +76,10 @@
 - `json` and `jsonutils` now serialize NaN, Inf, -Inf as strings, so that
   `%[NaN, -Inf]` is the string `["nan","-inf"]` instead of `[nan,-inf]` which was invalid json.
 
+- `system.addFloat` now uses the "Dragonbox" algorithm, which ensures correct roundtrips of floating point
+  numbers, that the minimum length representation of a floating point number is used and correct rounding.
+  Use `-d:nimLegacyAddFloat` for a transition period.
+
 - `strformat` is now part of `include std/prelude`.
 
 - Deprecated `proc reversed*[T](a: openArray[T], first: Natural, last: int): seq[T]` in `std/algorithm`.

compiler/condsyms.nim

@@ -134,3 +134,5 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasUnifiedTuple")
   defineSymbol("nimHasIterable")
   defineSymbol("nimHasTypeofVoid")
+
+  defineSymbol("nimHasDragonBox")

lib/system/formatfloat.nim

@@ -7,6 +7,19 @@
 #    distribution, for details about the copyright.
 #
 
+when not defined(nimLegacyAddFloat) and not defined(nimscript) and
+    not defined(js) and defined(nimHasDragonBox):
+  import dragonbox
+
+  proc writeFloatToBuffer*(buf: var array[65, char]; value: BiggestFloat): int =
+    ## This is the implementation to format floats.
+    ##
+    ## returns the amount of bytes written to `buf` not counting the
+    ## terminating '\0' character.
+    result = toChars(buf, value, forceTrailingDotZero=true)
+    buf[result] = '\0'
+
+else:
   proc c_sprintf(buf, frmt: cstring): cint {.header: "<stdio.h>",
                                       importc: "sprintf", varargs, noSideEffect.}
 
@@ -17,18 +30,10 @@ proc writeToBuffer(buf: var array[65, char]; value: cstring) =
       inc i
 
   proc writeFloatToBuffer*(buf: var array[65, char]; value: BiggestFloat): int =
-  ## This is the implementation to format floats in the Nim
-  ## programming language. The specific format for floating point
-  ## numbers is not specified in the Nim programming language and
-  ## might change slightly in the future, but at least wherever you
-  ## format a float, it should be consistent.
+    ## This is the implementation to format floats.
     ##
     ## returns the amount of bytes written to `buf` not counting the
     ## terminating '\0' character.
-  ##
-  ## * `buf` - A buffer to write into. The buffer does not need to be
-  ##           initialized and it will be overridden.
-  ##
     var n: int = c_sprintf(addr buf, "%.16g", value)
     var hasDot = false
     for i in 0..n-1:

tests/float/tfloat6.nim

@@ -1,10 +1,10 @@
 discard """
   output: '''
-1e-06 : 1e-06
-1e-06 : 1e-06
+0.000001 : 0.000001
+0.000001 : 0.000001
 0.001 : 0.001
-1e-06 : 1e-06
-1e-06 : 1e-06
+0.000001 : 0.000001
+0.000001 : 0.000001
 10.000001 : 10.000001
 100.000001 : 100.000001
 '''