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

@@ -245,15 +245,16 @@ 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

@@ -0,0 +1,43 @@
+import parseutils, sequtils, sugar
+
+
+let input = "$test{}  $this is ${an{  example}}  "
+let expected = @[(ikVar, "test"), (ikStr, "{}  "), (ikVar, "this"),
+                  (ikStr, " is "), (ikExpr, "an{  example}"), (ikStr, "  ")]
+doAssert toSeq(interpolatedFragments(input)) == expected
+
+var value = 0
+discard parseHex("0x38", value)
+doAssert value == 56
+
+value = -1
+doAssert(parseSaturatedNatural("848", value) == 3)
+doAssert value == 848
+
+value = -1
+discard parseSaturatedNatural("84899999999999999999324234243143142342135435342532453", value)
+doAssert value == high(int)
+
+value = -1
+discard parseSaturatedNatural("9223372036854775808", value)
+doAssert value == high(int)
+
+value = -1
+discard parseSaturatedNatural("9223372036854775807", value)
+doAssert value == high(int)
+
+value = -1
+discard parseSaturatedNatural("18446744073709551616", value)
+doAssert value == high(int)
+
+value = -1
+discard parseSaturatedNatural("18446744073709551615", value)
+doAssert value == high(int)
+
+value = -1
+doAssert(parseSaturatedNatural("1_000_000", value) == 9)
+doAssert value == 1_000_000
+
+var i64Value: int64
+discard parseBiggestInt("9223372036854775807", i64Value)
+doAssert i64Value == 9223372036854775807