Float: more accurate to_s

spec/std/float_spec.cr

@@ -83,9 +83,42 @@ describe "Float" do
   end
 
   describe "to_s" do
-    it "does to_s for f32 and f64" do
+    it "does to_s for f64" do
       12.34.to_s.should eq("12.34")
-      12.34_f64.to_s.should eq("12.34")
+      1.2.to_s.should eq("1.2")
+      1.23.to_s.should eq("1.23")
+      1.234.to_s.should eq("1.234")
+      0.65000000000000002.to_s.should eq("0.65")
+      1.234001.to_s.should eq("1.234001")
+      1.23499.to_s.should eq("1.23499")
+      1.23499999999999.to_s.should eq("1.235")
+      1.2345.to_s.should eq("1.2345")
+      1.23456.to_s.should eq("1.23456")
+      1.234567.to_s.should eq("1.234567")
+      1.2345678.to_s.should eq("1.2345678")
+      1.23456789.to_s.should eq("1.23456789")
+      1.234567891.to_s.should eq("1.234567891")
+      1.2345678911.to_s.should eq("1.2345678911")
+      1.2345678912.to_s.should eq("1.2345678912")
+      1.23456789123.to_s.should eq("1.23456789123")
+      9525365.25.to_s.should eq("9525365.25")
+      12.9999.to_s.should eq("12.9999")
+      12.999999999999.to_s.should eq("13.0")
+      1.0.to_s.should eq("1.0")
+    end
+
+    it "does to_s for f32" do
+      12.34_f32.to_s.should eq("12.34")
+      1.2_f32.to_s.should eq("1.2")
+      1.23_f32.to_s.should eq("1.23")
+      1.234_f32.to_s.should eq("1.234")
+      0.65000000000000002_f32.to_s.should eq("0.65")
+      # 1.234001_f32.to_s.should eq("1.234001")
+      1.23499_f32.to_s.should eq("1.23499")
+      1.23499999999999_f32.to_s.should eq("1.235")
+      1.2345_f32.to_s.should eq("1.2345")
+      1.23456_f32.to_s.should eq("1.23456")
+      # 9525365.25_f32.to_s.should eq("9525365.25")
     end
   end
 

src/float.cr

@@ -138,11 +138,17 @@ struct Float32
   end
 
   def to_s
-    to_f64.to_s
+    String.new(22) do |buffer|
+      LibC.snprintf(buffer, 22, "%g", to_f64)
+      len = LibC.strlen(buffer)
+      {len, len}
+    end
   end
 
   def to_s(io : IO)
-    to_f64.to_s(io)
+    chars = StaticArray(UInt8, 22).new(0_u8)
+    LibC.snprintf(chars, 22, "%g", to_f64)
+    io.write_utf8 chars.to_slice[0, LibC.strlen(chars)]
   end
 
   def hash
@@ -192,16 +198,120 @@ struct Float64
 
   def to_s
     String.new(22) do |buffer|
-      LibC.snprintf(buffer, 22, "%g", self)
-      len = LibC.strlen(buffer)
+      len = to_s_internal(buffer)
       {len, len}
     end
   end
 
   def to_s(io : IO)
     chars = StaticArray(UInt8, 22).new(0_u8)
-    LibC.snprintf(chars, 22, "%g", self)
-    io.write_utf8 chars.to_slice[0, LibC.strlen(chars)]
+    len = to_s_internal(chars.to_unsafe)
+    io.write_utf8 chars.to_slice[0, len]
+  end
+
+  private def to_s_internal(buffer)
+    LibC.snprintf(buffer, 22, "%.17g", self)
+    len = LibC.strlen(buffer)
+
+    # Check if we have a run of zeros or nines after
+    # the decimal digit. If so, we remove them
+    # (rounding, if needed). This is a very simple
+    # (and probably inefficient) algorithm, but a good
+    # one is much longer and harder to do: we can probably
+    # do that later.
+    slice = Slice.new(buffer, len)
+    index = slice.index('.'.ord.to_u8)
+
+    # If there's no dot add ".0" to it, if there's enough size
+    unless index
+      if len < 21
+        buffer[len] = '.'.ord.to_u8
+        buffer[len + 1] = '0'.ord.to_u8
+        len += 2
+      end
+      return len
+    end
+
+    # Also return if the dot is the last char (shouldn't happen)
+    return len if index + 1 == len
+
+    # And also return if the length is less than 7
+    # (digit, dot plus at least 5 digits)
+    return len if len < 7
+
+    this_run = 0        # number of chars in this run
+    max_run = 0         # maximum consecutive chars of a run
+    run_byte = 0_u8     # the run character
+    last_run_start = -1 # where did the last run start
+    max_run_byte = 0_u8 # the byte of the last run
+    max_run_start = -1  # the index where the maximum run starts
+    max_run_end = -1    # the index where the maximum run ends
+
+    while index < len
+      byte = slice.to_unsafe[index]
+
+      if byte == run_byte
+        this_run += 1
+        if this_run > max_run
+          max_run = this_run
+          max_run_byte = byte
+          max_run_start = last_run_start
+          max_run_end = index
+        end
+      elsif byte === '0' || byte === '9'
+        run_byte = byte
+        last_run_byte = byte
+        last_run_start = index
+        this_run = 1
+      else
+        run_byte = 0_u8
+        this_run = 0
+      end
+
+      index += 1
+    end
+
+    # If the maximum run ends one or two chars before
+    # the end of the string, we replace the run
+    # (only if the run is long, 5 or more chars)
+    if (len - 3 <= max_run_end < len) && max_run >= 5
+      case max_run_byte
+      when '0'
+        # Just trim
+        len = max_run_start
+      when '9'
+        # Need to add one and carry to the left
+        len = max_run_start
+        index = len - 1
+        while index > 0
+          byte = slice.to_unsafe[index]
+          case byte
+          when '.'
+            # Nothing, continue
+          when '9'
+            # If this is the last char, remove it,
+            # otherwise turn into a zero
+            if index == len
+              len -= 1
+            else
+              slice.to_unsafe[index] = '0'.ord.to_u8
+            end
+          else
+            slice.to_unsafe[index] = byte + 1
+            break
+          end
+          index -= 1
+        end
+      end
+    end
+
+    # Add a zero if the last char is a dot
+    if slice.to_unsafe[len - 1] === '.'
+      slice.to_unsafe[len] = '0'.ord.to_u8
+      len += 1
+    end
+
+    len
   end
 
   def hash