py/objfloat: Add implementation of high-quality float hashing.

Disabled by default.

py/objfloat.c

@@ -59,6 +59,55 @@ const mp_obj_float_t mp_const_float_pi_obj = {{&mp_type_float}, M_PI};
 
 #endif
 
+#if MICROPY_FLOAT_HIGH_QUALITY_HASH
+// must return actual integer value if it fits in mp_int_t
+mp_int_t mp_float_hash(mp_float_t src) {
+#if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_DOUBLE
+typedef uint64_t mp_float_uint_t;
+#elif MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
+typedef uint32_t mp_float_uint_t;
+#endif
+    union {
+        mp_float_t f;
+        #if MP_ENDIANNESS_LITTLE
+        struct { mp_float_uint_t frc:MP_FLOAT_FRAC_BITS, exp:MP_FLOAT_EXP_BITS, sgn:1; } p;
+        #else
+        struct { mp_float_uint_t sgn:1, exp:MP_FLOAT_EXP_BITS, frc:MP_FLOAT_FRAC_BITS; } p;
+        #endif
+        mp_float_uint_t i;
+    } u = {.f = src};
+
+    mp_int_t val;
+    const int adj_exp = (int)u.p.exp - MP_FLOAT_EXP_BIAS;
+    if (adj_exp < 0) {
+        // value < 1; must be sure to handle 0.0 correctly (ie return 0)
+        val = u.i;
+    } else {
+        // if adj_exp is max then: u.p.frc==0 indicates inf, else NaN
+        // else: 1 <= value
+        mp_float_uint_t frc = u.p.frc | ((mp_float_uint_t)1 << MP_FLOAT_FRAC_BITS);
+
+        if (adj_exp <= MP_FLOAT_FRAC_BITS) {
+            // number may have a fraction; xor the integer part with the fractional part
+            val = (frc >> (MP_FLOAT_FRAC_BITS - adj_exp))
+                ^ (frc & ((1 << (MP_FLOAT_FRAC_BITS - adj_exp)) - 1));
+        } else if ((unsigned int)adj_exp < BITS_PER_BYTE * sizeof(mp_int_t) - 1) {
+            // the number is a (big) whole integer and will fit in val's signed-width
+            val = (mp_int_t)frc << (adj_exp - MP_FLOAT_FRAC_BITS);
+        } else {
+            // integer part will overflow val's width so just use what bits we can
+            val = frc;
+        }
+    }
+
+    if (u.p.sgn) {
+        val = -val;
+    }
+
+    return val;
+}
+#endif
+
 STATIC void float_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t kind) {
     (void)kind;
     mp_float_t o_val = mp_obj_float_get(o_in);