Port Float16 conversion from Rust

spec/rfc_spec.cr

@@ -4,13 +4,13 @@ require "./spec_helper"
 
 tests = [
   # Disabled as half-precision floats are not supported:
-  # { %(0.0), "f9 00 00" },
-  # { %(-0.0), "f9 80 00" },
-  # { %(1.0), "f9 3c 00" },
-  # { %(1.5), "f9 3e 00" },
-  # { %(65504.0), "f9 7b ff" },
-  # { %(0.00006103515625), "f9 04 00" },
-  # { %(-4.0), "f9 c4 00" },
+  { %(0.0), "f9 00 00" },
+  { %(-0.0), "f9 80 00" },
+  { %(1.0), "f9 3c 00" },
+  { %(1.5), "f9 3e 00" },
+  { %(65504.0), "f9 7b ff" },
+  # { %(0.00006103515625), "f9 04 00" }, TODO: Something about the presentation
+  { %(-4.0), "f9 c4 00" },
   # { %(5.960464477539063e-8), "f9 00 01" },
   # { %(Infinity), "f9 7c 00" },
   # { %(NaN), "f9 7e 00" },

src/cbor/lexer.cr

@@ -84,7 +84,7 @@ class CBOR::Lexer
     when 0xe0..0xf8
       consume_simple_value(read_size(byte - 0xe0))
     when 0xf9
-      raise ParseError.new("Half-precision floating point numbers are not supported")
+      Token::FloatT.new(value: Float32.new(read(UInt16)))
     when 0xfa
       Token::FloatT.new(value: read(Float32))
     when 0xfb

src/cbor/type/float_16.cr

@@ -0,0 +1,44 @@
+# Returns a Float32 by reading the 16 bit as a IEEE 754 half-precision floating
+# point (Float16).
+def Float32.new(i : UInt16)
+  # Check for signed zero
+  if i & 0x7FFF_u16 == 0
+    return (i.unsafe_as(UInt32) << 16).unsafe_as(Float32)
+  end
+
+  half_sign = (i & 0x8000_u16).unsafe_as(UInt32)
+  half_exp = (i & 0x7C00_u16).unsafe_as(UInt32)
+  half_man = (i & 0x03FF_u16).unsafe_as(UInt32)
+
+  # Check for an infinity or NaN when all exponent bits set
+  if half_exp == 0x7C00_u32
+    # Check for signed infinity if mantissa is zero
+    if half_man == 0
+      return ((half_sign << 16) | 0x7F80_0000_u32).unsafe_as(Float32)
+    else
+      # NaN, keep current mantissa but also set most significiant mantissa bit
+      return ((half_sign << 16) | 0x7FC0_0000_u32 | (half_man << 13)).unsafe_as(Float32)
+    end
+  end
+
+  # Calculate single-precision components with adjusted exponent
+  sign = half_sign << 16
+  # Unbias exponent
+  unbiased_exp = ((half_exp.unsafe_as(Int32)) >> 10) - 15
+
+  # Check for subnormals, which will be normalized by adjusting exponent
+  if half_exp == 0
+    # Calculate how much to adjust the exponent by
+    e = half_man.unsafe_as(UInt16).leading_zeros_count - 6
+
+    # Rebias and adjust exponent
+    exp = (127 - 15 - e) << 23
+    man = (half_man << (14 + e)) & 0x7F_FF_FF_u32
+    return (sign | exp | man).unsafe_as(Float32)
+  end
+
+  # Rebias exponent for a normalized normal
+  exp = (unbiased_exp + 127).unsafe_as(UInt32) << 23
+  man = (half_man & 0x03FF_u32) << 13
+  (sign | exp | man).unsafe_as(Float32)
+end