//
//  Copyright 2006-2019 WebPKI.org (http://webpki.org).
//
//  Licensed under the Apache License, Version 2.0 (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS,
//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//  See the License for the specific language governing permissions and
//  limitations under the License.
//
 
// This package converts numbers in IEEE-754 double precision into the
// format specified for JSON in EcmaScript Version 6 and forward.
// The core application for this is canonicalization:
// https://tools.ietf.org/html/draft-rundgren-json-canonicalization-scheme-02

package jsoncanonicalizer

import (
    "errors"
    "math"
    "strconv"
    "strings"
)

const invalidPattern uint64 = 0x7ff0000000000000

func NumberToJSON(ieeeF64 float64) (res string, err error) {
    ieeeU64 := math.Float64bits(ieeeF64)

    // Special case: NaN and Infinity are invalid in JSON
    if (ieeeU64 & invalidPattern) == invalidPattern {
        return "null", errors.New("Invalid JSON number: " + strconv.FormatUint(ieeeU64, 16))
    }

    // Special case: eliminate "-0" as mandated by the ES6-JSON/JCS specifications
    if ieeeF64 == 0 {  // Right, this line takes both -0 and 0
        return "0", nil
    }

    // Deal with the sign separately
    var sign string = ""
    if ieeeF64 < 0 {
        ieeeF64 =-ieeeF64
        sign = "-"
    }

    // ES6 has a unique "g" format
    var format byte = 'e'
    if ieeeF64 < 1e+21 && ieeeF64 >= 1e-6 {
        format = 'f'
    }

    // The following should do the trick:
    es6Formatted := strconv.FormatFloat(ieeeF64, format, -1, 64)

    // Minor cleanup
    exponent := strings.IndexByte(es6Formatted, 'e')
    if exponent > 0 {
        // Go outputs "1e+09" which must be rewritten as "1e+9"
        if es6Formatted[exponent + 2] == '0' {
            es6Formatted = es6Formatted[:exponent + 2] + es6Formatted[exponent + 3:]
        }
    }
    return sign + es6Formatted, nil
}