1 // run 2 3 // Copyright 2009 The Go Authors. All rights reserved. 4 // Use of this source code is governed by a BSD-style 5 // license that can be found in the LICENSE file. 6 7 package main 8 9 import "os" 10 import "strconv" 11 12 type Test struct { 13 f float64 14 in string 15 out string 16 } 17 18 var tests = []Test{ 19 Test{123.5, "123.5", "123.5"}, 20 Test{456.7, "456.7", "456.7"}, 21 Test{1e23 + 8.5e6, "1e23+8.5e6", "1.0000000000000001e+23"}, 22 Test{100000000000000008388608, "100000000000000008388608", "1.0000000000000001e+23"}, 23 Test{1e23 + 8388609, "1e23+8388609", "1.0000000000000001e+23"}, 24 25 // "x" = the floating point value from converting the string x. 26 // These are exactly representable in 64-bit floating point: 27 // 1e23-8388608 28 // 1e23+8388608 29 // The former has an even mantissa, so "1e23" rounds to 1e23-8388608. 30 // If "1e23+8388608" is implemented as "1e23" + "8388608", 31 // that ends up computing 1e23-8388608 + 8388608 = 1e23, 32 // which rounds back to 1e23-8388608. 33 // The correct answer, of course, would be "1e23+8388608" = 1e23+8388608. 34 // This is not going to be correct until 6g has multiprecision floating point. 35 // A simpler case is "1e23+1", which should also round to 1e23+8388608. 36 Test{1e23 + 8.388608e6, "1e23+8.388608e6", "1.0000000000000001e+23"}, 37 Test{1e23 + 1, "1e23+1", "1.0000000000000001e+23"}, 38 } 39 40 func main() { 41 ok := true 42 for i := 0; i < len(tests); i++ { 43 t := tests[i] 44 v := strconv.FormatFloat(t.f, 'g', -1, 64) 45 if v != t.out { 46 println("Bad float64 const:", t.in, "want", t.out, "got", v) 47 x, err := strconv.ParseFloat(t.out, 64) 48 if err != nil { 49 println("bug120: strconv.Atof64", t.out) 50 panic("fail") 51 } 52 println("\twant exact:", strconv.FormatFloat(x, 'g', 1000, 64)) 53 println("\tgot exact: ", strconv.FormatFloat(t.f, 'g', 1000, 64)) 54 ok = false 55 } 56 } 57 if !ok { 58 os.Exit(1) 59 } 60 } 61
