Initial QSfera import

Server/vendor/github.com/segmentio/ksuid/base62.go

@@ -0,0 +1,202 @@
+package ksuid
+
+import (
+	"encoding/binary"
+	"errors"
+)
+
+const (
+	// lexographic ordering (based on Unicode table) is 0-9A-Za-z
+	base62Characters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+	zeroString       = "000000000000000000000000000"
+	offsetUppercase  = 10
+	offsetLowercase  = 36
+)
+
+var (
+	errShortBuffer = errors.New("the output buffer is too small to hold to decoded value")
+)
+
+// Converts a base 62 byte into the number value that it represents.
+func base62Value(digit byte) byte {
+	switch {
+	case digit >= '0' && digit <= '9':
+		return digit - '0'
+	case digit >= 'A' && digit <= 'Z':
+		return offsetUppercase + (digit - 'A')
+	default:
+		return offsetLowercase + (digit - 'a')
+	}
+}
+
+// This function encodes the base 62 representation of the src KSUID in binary
+// form into dst.
+//
+// In order to support a couple of optimizations the function assumes that src
+// is 20 bytes long and dst is 27 bytes long.
+//
+// Any unused bytes in dst will be set to the padding '0' byte.
+func fastEncodeBase62(dst []byte, src []byte) {
+	const srcBase = 4294967296
+	const dstBase = 62
+
+	// Split src into 5 4-byte words, this is where most of the efficiency comes
+	// from because this is a O(N^2) algorithm, and we make N = N / 4 by working
+	// on 32 bits at a time.
+	parts := [5]uint32{
+		binary.BigEndian.Uint32(src[0:4]),
+		binary.BigEndian.Uint32(src[4:8]),
+		binary.BigEndian.Uint32(src[8:12]),
+		binary.BigEndian.Uint32(src[12:16]),
+		binary.BigEndian.Uint32(src[16:20]),
+	}
+
+	n := len(dst)
+	bp := parts[:]
+	bq := [5]uint32{}
+
+	for len(bp) != 0 {
+		quotient := bq[:0]
+		remainder := uint64(0)
+
+		for _, c := range bp {
+			value := uint64(c) + uint64(remainder)*srcBase
+			digit := value / dstBase
+			remainder = value % dstBase
+
+			if len(quotient) != 0 || digit != 0 {
+				quotient = append(quotient, uint32(digit))
+			}
+		}
+
+		// Writes at the end of the destination buffer because we computed the
+		// lowest bits first.
+		n--
+		dst[n] = base62Characters[remainder]
+		bp = quotient
+	}
+
+	// Add padding at the head of the destination buffer for all bytes that were
+	// not set.
+	copy(dst[:n], zeroString)
+}
+
+// This function appends the base 62 representation of the KSUID in src to dst,
+// and returns the extended byte slice.
+// The result is left-padded with '0' bytes to always append 27 bytes to the
+// destination buffer.
+func fastAppendEncodeBase62(dst []byte, src []byte) []byte {
+	dst = reserve(dst, stringEncodedLength)
+	n := len(dst)
+	fastEncodeBase62(dst[n:n+stringEncodedLength], src)
+	return dst[:n+stringEncodedLength]
+}
+
+// This function decodes the base 62 representation of the src KSUID to the
+// binary form into dst.
+//
+// In order to support a couple of optimizations the function assumes that src
+// is 27 bytes long and dst is 20 bytes long.
+//
+// Any unused bytes in dst will be set to zero.
+func fastDecodeBase62(dst []byte, src []byte) error {
+	const srcBase = 62
+	const dstBase = 4294967296
+
+	// This line helps BCE (Bounds Check Elimination).
+	// It may be safely removed.
+	_ = src[26]
+
+	parts := [27]byte{
+		base62Value(src[0]),
+		base62Value(src[1]),
+		base62Value(src[2]),
+		base62Value(src[3]),
+		base62Value(src[4]),
+		base62Value(src[5]),
+		base62Value(src[6]),
+		base62Value(src[7]),
+		base62Value(src[8]),
+		base62Value(src[9]),
+
+		base62Value(src[10]),
+		base62Value(src[11]),
+		base62Value(src[12]),
+		base62Value(src[13]),
+		base62Value(src[14]),
+		base62Value(src[15]),
+		base62Value(src[16]),
+		base62Value(src[17]),
+		base62Value(src[18]),
+		base62Value(src[19]),
+
+		base62Value(src[20]),
+		base62Value(src[21]),
+		base62Value(src[22]),
+		base62Value(src[23]),
+		base62Value(src[24]),
+		base62Value(src[25]),
+		base62Value(src[26]),
+	}
+
+	n := len(dst)
+	bp := parts[:]
+	bq := [stringEncodedLength]byte{}
+
+	for len(bp) > 0 {
+		quotient := bq[:0]
+		remainder := uint64(0)
+
+		for _, c := range bp {
+			value := uint64(c) + uint64(remainder)*srcBase
+			digit := value / dstBase
+			remainder = value % dstBase
+
+			if len(quotient) != 0 || digit != 0 {
+				quotient = append(quotient, byte(digit))
+			}
+		}
+
+		if n < 4 {
+			return errShortBuffer
+		}
+
+		dst[n-4] = byte(remainder >> 24)
+		dst[n-3] = byte(remainder >> 16)
+		dst[n-2] = byte(remainder >> 8)
+		dst[n-1] = byte(remainder)
+		n -= 4
+		bp = quotient
+	}
+
+	var zero [20]byte
+	copy(dst[:n], zero[:])
+	return nil
+}
+
+// This function appends the base 62 decoded version of src into dst.
+func fastAppendDecodeBase62(dst []byte, src []byte) []byte {
+	dst = reserve(dst, byteLength)
+	n := len(dst)
+	fastDecodeBase62(dst[n:n+byteLength], src)
+	return dst[:n+byteLength]
+}
+
+// Ensures that at least nbytes are available in the remaining capacity of the
+// destination slice, if not, a new copy is made and returned by the function.
+func reserve(dst []byte, nbytes int) []byte {
+	c := cap(dst)
+	n := len(dst)
+
+	if avail := c - n; avail < nbytes {
+		c *= 2
+		if (c - n) < nbytes {
+			c = n + nbytes
+		}
+		b := make([]byte, n, c)
+		copy(b, dst)
+		dst = b
+	}
+
+	return dst
+}