core: fixed a slight bug.

The bug involves the incorrect logic for `core::num::flt2dec::decoder`.
This makes some numbers in the form of 2^n missing one final digits,
which breaks the bijectivity criterion. The regression tests have been
added, and f32 exhaustive test is rerun to get the updated result.

src/libcore/num/flt2dec/decoder.rs

@@ -75,7 +75,7 @@ pub fn decode<T: DecodableFloat>(v: T) -> (/*negative?*/ bool, FullDecoded) {
         FpCategory::Infinite => FullDecoded::Infinite,
         FpCategory::Zero => FullDecoded::Zero,
         FpCategory::Subnormal => {
-            // (mant - 2, exp) -- (mant, exp) -- (mant + 2, exp)
+            // neighbors: (mant - 2, exp) -- (mant, exp) -- (mant + 2, exp)
             // Float::integer_decode always preserves the exponent,
             // so the mantissa is scaled for subnormals.
             FullDecoded::Finite(Decoded { mant: mant, minus: 1, plus: 1,
@@ -83,13 +83,13 @@ pub fn decode<T: DecodableFloat>(v: T) -> (/*negative?*/ bool, FullDecoded) {
         }
         FpCategory::Normal => {
             let minnorm = <T as DecodableFloat>::min_pos_norm_value().integer_decode();
-            if mant == minnorm.0 && exp == minnorm.1 {
-                // (maxmant, exp - 1) -- (minnormmant, exp) -- (minnormmant + 1, exp)
+            if mant == minnorm.0 {
+                // neighbors: (maxmant, exp - 1) -- (minnormmant, exp) -- (minnormmant + 1, exp)
                 // where maxmant = minnormmant * 2 - 1
-                FullDecoded::Finite(Decoded { mant: mant << 1, minus: 1, plus: 2,
-                                              exp: exp - 1, inclusive: even })
+                FullDecoded::Finite(Decoded { mant: mant << 2, minus: 1, plus: 2,
+                                              exp: exp - 2, inclusive: even })
             } else {
-                // (mant - 1, exp) -- (mant, exp) -- (mant + 1, exp)
+                // neighbors: (mant - 1, exp) -- (mant, exp) -- (mant + 1, exp)
                 FullDecoded::Finite(Decoded { mant: mant << 1, minus: 1, plus: 1,
                                               exp: exp - 1, inclusive: even })
             }

src/libcoretest/num/flt2dec/mod.rs

@@ -216,6 +216,13 @@ pub fn f32_shortest_sanity_test<F>(mut f: F) where F: FnMut(&Decoded, &mut [u8])
     // 10^18 * 0.314159231156617216
     check_shortest!(f(3.141592e17f32) => b"3141592", 18);
 
+    // regression test for decoders
+    // 10^8 * 0.3355443
+    // 10^8 * 0.33554432
+    // 10^8 * 0.33554436
+    let twoto25: f32 = StdFloat::ldexp(1.0, 25);
+    check_shortest!(f(twoto25) => b"33554432", 8);
+
     // 10^39 * 0.340282326356119256160033759537265639424
     // 10^39 * 0.34028234663852885981170418348451692544
     // 10^39 * 0.340282366920938463463374607431768211456
@@ -308,6 +315,13 @@ pub fn f64_shortest_sanity_test<F>(mut f: F) where F: FnMut(&Decoded, &mut [u8])
     // 10^18 * 0.314159200000000064
     check_shortest!(f(3.141592e17f64) => b"3141592", 18);
 
+    // regression test for decoders
+    // 10^20 * 0.18446744073709549568
+    // 10^20 * 0.18446744073709551616
+    // 10^20 * 0.18446744073709555712
+    let twoto64: f64 = StdFloat::ldexp(1.0, 64);
+    check_shortest!(f(twoto64) => b"18446744073709552", 20);
+
     // pathological case: high = 10^23 (exact). tie breaking should always prefer that.
     // 10^24 * 0.099999999999999974834176
     // 10^24 * 0.099999999999999991611392
@@ -492,7 +506,7 @@ pub fn f32_exhaustive_equivalence_test<F, G>(f: F, g: G, k: usize)
     // so why not simply testing all of them?
     //
     // this is of course very stressful (and thus should be behind an `#[ignore]` attribute),
-    // but with `-O3 -C lto` this only takes about two hours or so.
+    // but with `-C opt-level=3 -C lto` this only takes about an hour or so.
 
     // iterate from 0x0000_0001 to 0x7f7f_ffff, i.e. all finite ranges
     let (npassed, nignored) = iterate("f32_exhaustive_equivalence_test",
@@ -500,7 +514,7 @@ pub fn f32_exhaustive_equivalence_test<F, G>(f: F, g: G, k: usize)
         let x: f32 = unsafe {mem::transmute(i as u32 + 1)};
         decode_finite(x)
     });
-    assert_eq!((npassed, nignored), (2121451879, 17643160));
+    assert_eq!((npassed, nignored), (2121451881, 17643158));
 }
 
 fn to_string_with_parts<F>(mut f: F) -> String

src/libcoretest/num/flt2dec/strategy/grisu.rs

@@ -60,7 +60,7 @@ fn shortest_f32_exhaustive_equivalence_test() {
     //
     // this reports the progress and the number of f32 values returned `None`.
     // with `--nocapture` (and plenty of time and appropriate rustc flags), this should print:
-    // `done, ignored=17643160 passed=2121451879 failed=0`.
+    // `done, ignored=17643158 passed=2121451881 failed=0`.
 
     use core::num::flt2dec::strategy::dragon::format_shortest as fallback;
     f32_exhaustive_equivalence_test(format_shortest_opt, fallback, MAX_SIG_DIGITS);