45 lines
1.5 KiB
JavaScript
45 lines
1.5 KiB
JavaScript
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'use strict';
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var GetIntrinsic = require('get-intrinsic');
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var $pow = GetIntrinsic('%Math.pow%');
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module.exports = function bytesAsFloat64(rawBytes) {
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// return new Float64Array(new Uint8Array(rawBytes).buffer)[0];
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/*
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Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary64 value.
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If value is an IEEE 754-2008 binary64 NaN value, return the NaN Number value.
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Return the Number value that corresponds to value.
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*/
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var sign = rawBytes[7] & 0x80 ? -1 : 1; // first bit
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var exponent = ((rawBytes[7] & 0x7F) << 4) // 7 bits from index 7
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| ((rawBytes[6] & 0xF0) >> 4); // 4 bits from index 6
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var mantissa = ((rawBytes[6] & 0x0F) * 0x1000000000000) // 4 bits from index 6
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+ (rawBytes[5] * 0x10000000000) // 8 bits from index 5
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+ (rawBytes[4] * 0x100000000) // 8 bits from index 4
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+ (rawBytes[3] * 0x1000000) // 8 bits from index 3
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+ (rawBytes[2] * 0x10000) // 8 bits from index 2
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+ (rawBytes[1] * 0x100) // 8 bits from index 1
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+ rawBytes[0]; // 8 bits from index 0
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if (exponent === 0 && mantissa === 0) {
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return sign * 0;
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}
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if (exponent === 0x7FF && mantissa !== 0) {
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return NaN;
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}
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if (exponent === 0x7FF && mantissa === 0) {
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return sign * Infinity;
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}
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exponent -= 1023; // subtract the bias
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// Handle subnormal numbers
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if (exponent === -1023) {
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return sign * mantissa * 5e-324; // $pow(2, -1022 - 52)
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}
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return sign * (1 + (mantissa / 0x10000000000000)) * $pow(2, exponent);
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};
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