fix #18702(fix parseutils.parseFloat) (#18703) [backport:1.0]

* fix #18702
* Apply suggestions from code review

(cherry picked from commit 901c5de)

lib/system/strmantle.nim

@@ -240,15 +240,16 @@ proc nimParseBiggestFloat(s: string, number: var BiggestFloat,
   var ti = 0
   let maxlen = t.high - "e+000".len # reserve enough space for exponent
 
-  result = i - start
+  let endPos = i
+  result = endPos - start
   i = start
   # re-parse without error checking, any error should be handled by the code above.
-  if i < s.len and s[i] == '.': i.inc
-  while i < s.len and s[i] in {'0'..'9','+','-'}:
+  if i < endPos and s[i] == '.': i.inc
+  while i < endPos and s[i] in {'0'..'9','+','-'}:
     if ti < maxlen:
       t[ti] = s[i]; inc(ti)
     inc(i)
-    while i < s.len and s[i] in {'.', '_'}: # skip underscore and decimal point
+    while i < endPos and s[i] in {'.', '_'}: # skip underscore and decimal point
       inc(i)
 
   # insert exponent

tests/stdlib/tparseutils.nim

@@ -1,5 +1,5 @@
-import parseutils
-import sequtils
+import std/[parseutils, sequtils, sugar]
+
 
 let input = "$test{}  $this is ${an{  example}}  "
 let expected = @[(ikVar, "test"), (ikStr, "{}  "), (ikVar, "this"),
@@ -41,3 +41,12 @@ doAssert value == 1_000_000
 var i64Value: int64
 discard parseBiggestInt("9223372036854775807", i64Value)
 doAssert i64Value == 9223372036854775807
+
+block:
+  var f: float
+  let res = collect:
+    for x in ["9.123456789012345+","11.123456789012345+","9.123456789012345-","8.123456789012345+","9.12345678901234-","9.123456789012345"]:
+      (parseFloat(x, f, 0), $f)
+  doAssert res == @[(17, "9.123456789012344"), (18, "11.123456789012344"),
+                    (17, "9.123456789012344"), (17, "8.123456789012344"),
+                    (16, "9.12345678901234"), (17, "9.123456789012344")]