lib/standard: fix `Float::to_precision`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --git a/lib/standard/string.nit b/lib/standard/string.nit
index e3d9d70..f78c0ac 100644 (file)
--- a/lib/standard/string.nit
+++ b/lib/standard/string.nit
@@ -1799,13 +1799,15 @@ redef class Float
                return str
        end
 
-       # `self` representation with `nb` digits after the '.'.
+       # `String` representation of `self` with the given number of `decimals`
        #
-       #     assert 12.345.to_precision(1) == "12.3"
-       #     assert 12.345.to_precision(2) == "12.35"
-       #     assert 12.345.to_precision(3) == "12.345"
-       #     assert 12.345.to_precision(4) == "12.3450"
-       fun to_precision(nb: Int): String
+       #     assert 12.345.to_precision(0)    == "12"
+       #     assert 12.345.to_precision(3)    == "12.345"
+       #     assert (-12.345).to_precision(3) == "-12.345"
+       #     assert (-0.123).to_precision(3)  == "-0.123"
+       #     assert 0.999.to_precision(2)     == "1.00"
+       #     assert 0.999.to_precision(4)     == "0.9990"
+       fun to_precision(decimals: Int): String
        do
                if is_nan then return "nan"
 
@@ -1816,25 +1818,34 @@ redef class Float
                        return  "-inf"
                end
 
-               if nb == 0 then return self.to_i.to_s
+               if decimals == 0 then return self.to_i.to_s
                var f = self
-               for i in [0..nb[ do f = f * 10.0
+               for i in [0..decimals[ do f = f * 10.0
                if self > 0.0 then
                        f = f + 0.5
                else
                        f = f - 0.5
                end
                var i = f.to_i
-               if i == 0 then return "0.0"
-               var s = i.to_s
+               if i == 0 then return "0." + "0"*decimals
+
+               # Prepare both parts of the float, before and after the "."
+               var s = i.abs.to_s
                var sl = s.length
-               if sl > nb then
-                       var p1 = s.substring(0, s.length-nb)
-                       var p2 = s.substring(s.length-nb, nb)
-                       return p1 + "." + p2
+               var p1
+               var p2
+               if sl > decimals then
+                       # Has something before the "."
+                       p1 = s.substring(0, sl-decimals)
+                       p2 = s.substring(sl-decimals, decimals)
                else
-                       return "0." + ("0"*(nb-sl)) + s
+                       p1 = "0"
+                       p2 = "0"*(decimals-sl) + s
                end
+
+               if i < 0 then p1 = "-" + p1
+
+               return p1 + "." + p2
        end
 
        # `self` representation with `nb` digits after the '.'.