parseBiggestFloat is now builtin

lib/system/jssys.nim

@@ -639,3 +639,87 @@ proc addChar(x: string, c: char) {.compilerproc, asmNoStackFrame.} =
   """
 
 {.pop.}
+
+proc tenToThePowerOf(b: int): BiggestFloat =
+  var b = b
+  var a = 10.0
+  result = 1.0
+  while true:
+    if (b and 1) == 1:
+      result *= a
+    b = b shr 1
+    if b == 0: break
+    a *= a
+
+const
+  IdentChars = {'a'..'z', 'A'..'Z', '0'..'9', '_'}
+
+# XXX use JS's native way here
+proc nimParseBiggestFloat(s: string, number: var BiggestFloat, start = 0): int {.
+                          compilerProc.} =
+  var
+    esign = 1.0
+    sign = 1.0
+    i = start
+    exponent: int
+    flags: int
+  number = 0.0
+  if s[i] == '+': inc(i)
+  elif s[i] == '-':
+    sign = -1.0
+    inc(i)
+  if s[i] == 'N' or s[i] == 'n':
+    if s[i+1] == 'A' or s[i+1] == 'a':
+      if s[i+2] == 'N' or s[i+2] == 'n':
+        if s[i+3] notin IdentChars:
+          number = NaN
+          return i+3 - start
+    return 0
+  if s[i] == 'I' or s[i] == 'i':
+    if s[i+1] == 'N' or s[i+1] == 'n':
+      if s[i+2] == 'F' or s[i+2] == 'f':
+        if s[i+3] notin IdentChars: 
+          number = Inf*sign
+          return i+3 - start
+    return 0
+  while s[i] in {'0'..'9'}:
+    # Read integer part
+    flags = flags or 1
+    number = number * 10.0 + toFloat(ord(s[i]) - ord('0'))
+    inc(i)
+    while s[i] == '_': inc(i)
+  # Decimal?
+  if s[i] == '.':
+    var hd = 1.0
+    inc(i)
+    while s[i] in {'0'..'9'}:
+      # Read fractional part
+      flags = flags or 2
+      number = number * 10.0 + toFloat(ord(s[i]) - ord('0'))
+      hd = hd * 10.0
+      inc(i)
+      while s[i] == '_': inc(i)
+    number = number / hd # this complicated way preserves precision
+  # Again, read integer and fractional part
+  if flags == 0: return 0
+  # Exponent?
+  if s[i] in {'e', 'E'}:
+    inc(i)
+    if s[i] == '+':
+      inc(i)
+    elif s[i] == '-':
+      esign = -1.0
+      inc(i)
+    if s[i] notin {'0'..'9'}:
+      return 0
+    while s[i] in {'0'..'9'}:
+      exponent = exponent * 10 + ord(s[i]) - ord('0')
+      inc(i)
+      while s[i] == '_': inc(i)
+  # Calculate Exponent
+  let hd = tenToThePowerOf(exponent)
+  if esign > 0.0: number = number * hd
+  else:           number = number / hd
+  # evaluate sign
+  number = number * sign
+  result = i - start