Fix float-parsing logic

With the float string representation fixed in Nim devel (See
nim-lang/Nim#18139), that uncovered a bug in
the logic for parsing float TOML values.

For e.g. to parse "foo = 0.123", internally, 0.1 + 0.02 + 0.003 was
done which would evaluate to 0.12300000000000001. Earlier
float stringification bug caused that value to print out as "0.123"
instead of "0.12300000000000001".

This is now fixed by using parseutils.parsefloat, which parses the
"0.123" float value as 0.123 and not 0.12300000000000001.

Fixes NimParsers#45.

src/parsetoml.nim

@@ -36,6 +36,8 @@ import streams
 import strutils
 import tables
 import unicode
+from parseutils import parseFloat
+
 export tables
 
 when (NimMajor, NimMinor, NimPatch) < (1, 4, 0):
@@ -271,9 +273,9 @@ proc parseEncoding(state: var ParserState): TomlValueRef =
 proc parseDecimalPart(state: var ParserState): float64 =
   var
     nextChar: char
-    invPowerOfTen = 10.0
     firstPos = true
     wasUnderscore = false
+    decimalPartStr = "0."
 
   result = 0.0
   while true:
@@ -293,11 +295,11 @@ proc parseDecimalPart(state: var ParserState): float64 =
         raise newTomlError(state, "decimal part empty")
       break
 
-    result = result + (int(nextChar) - int('0')).float / invPowerOfTen
-    invPowerOfTen *= 10
+    decimalPartStr.add(nextChar)
 
     firstPos = false
-
+  doAssert decimalPartStr.len > 2 # decimalPartStr shouldn't still be "0." at this point
+  discard parseutils.parseFloat(decimalPartStr, result)
 
 proc stringDelimiter(kind: StringType): char {.inline, noSideEffect.} =
   result = (case kind

tests/test1.nim

@@ -95,3 +95,7 @@ file_name = "test.txt"
     tomlRef.setEmptyTableVal()
     check:
       tomlRef.kind == TomlValueKind.Table
+
+suite "bug fixes":
+  test "issue-45":
+    check $("some_float = 0.123".parseString()["some_float"].getFloat()) == "0.123"