Add fromNumber

Author: sharkdp

docs/Data/BigInt.md

@@ -29,6 +29,14 @@ fromInt :: Int -> BigInt
 
 Convert an integer to a BigInt.
 
+#### `fromNumber`
+
+``` purescript
+fromNumber :: Number -> BigInt
+```
+
+Convert a Number to a BigInt. The fractional part is truncated.
+
 #### `toNumber`
 
 ``` purescript

src/Data/BigInt.js

@@ -19,6 +19,15 @@ exports["fromBase'"] = function(just) {
 
 exports.fromInt = bigInt;
 
+function truncate(n) {
+  if (n > 0) return Math.floor(n);
+  return Math.ceil(n);
+}
+
+exports.fromNumber = function(x) {
+  return bigInt(truncate(x));
+};
+
 exports.toBase = function(base) {
   return function (x) {
     return x.toString(base);

src/Data/BigInt.purs

@@ -4,6 +4,7 @@ module Data.BigInt
   , fromString
   , fromBase
   , fromInt
+  , fromNumber
   , toString
   , toBase
   , abs
@@ -38,6 +39,9 @@ foreign import fromBase' :: forall a. (a -> Maybe a)
 -- | Convert an integer to a BigInt.
 foreign import fromInt :: Int -> BigInt
 
+-- | Convert a Number to a BigInt. The fractional part is truncated.
+foreign import fromNumber :: Number -> BigInt
+
 -- | Converts a BigInt to a Number. Loses precision for numbers which are too
 -- | large.
 foreign import toNumber :: BigInt -> Number

test/Main.purs

@@ -5,7 +5,8 @@ import Control.Monad.Eff.Console (CONSOLE, log)
 import Control.Monad.Eff.Exception (EXCEPTION)
 import Control.Monad.Eff.Random (RANDOM)
 import Data.Array (filter, range)
-import Data.BigInt (BigInt, abs, fromInt, prime, pow, odd, even, fromString, toNumber, fromBase, toBase, toString, not, or, xor, and, shl, shr)
+import Data.BigInt (BigInt, abs, fromInt, fromNumber, prime, pow, odd, even, fromString, toNumber,
+                    fromBase, toBase, toString, not, or, xor, and, shl, shr)
 import Data.Foldable (fold)
 import Data.Int as Int
 import Data.Maybe (Maybe(..), fromMaybe)
@@ -85,6 +86,14 @@ main = do
   assert $ (toBase 16 <$> fromString "255") == Just "ff"
   assert $ toString (fromInt 12345) == "12345"
 
+  log "Converting from Number to BigInt"
+  assert $ fromNumber 0.0 == zero
+  assert $ fromNumber 3.4 == three
+  assert $ fromNumber (-3.9) == -three
+  assert $ fromNumber 1.0e7 == fromInt 10000000
+  assert $ Just (fromNumber 1.0e47) == fromString "1e47"
+  quickCheck (\x -> fromInt x == fromNumber (Int.toNumber x))
+
   log "Conversions between String, Int and BigInt should not loose precision"
   quickCheck (\n -> fromString (show n) == Just (fromInt n))
   quickCheck (\n -> Int.toNumber n == toNumber (fromInt n))