Implement session creation

pkg/codeGenerator/codeGenerator.go

@@ -0,0 +1,67 @@
+package codeGenerator
+
+import (
+	"crypto/rand"
+)
+
+// some symbols are missing to prevent ambiguities
+var alphabet = [...]byte{
+	'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', 'M', 'N', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
+	'1', '2', '3', '4', '5', '6', '7', '8', '9',
+}
+
+// Generate a code, that can be guessed after 256^randomLength attempts on average.
+// randomLength can be at most 8 (= 64b of randomness)
+// See GenerateCode for more details
+func GenerateRandomCode(incremental uint64, randomLength uint8) ([]byte, error) {
+	var buf [8]byte
+	if _, err := rand.Read(buf[:randomLength]); err != nil {
+		return nil, err
+	}
+	var random uint64
+	for i := uint8(0); i < randomLength; i++ {
+		random |= uint64(buf[i]) << i * 8
+	}
+	return GenerateCode(incremental, random), nil
+}
+
+// Generate a code (a textual identifier) that is compact and easy for humans to deal with.
+// It exists for all possible values of the arguments, but the smaller they are, the shorter the code.
+// The code is guaranteed to be unique, if at least one of the arguments is unique.
+//
+// The typical usage is to use easy to predict yet easy to make unique value for incremental
+// (to ensure uniqueness) and a random value for random to make the code unpredictable.
+//
+// Beware the code is not guaranteed to be parsable back to incremental and random arguments.
+func GenerateCode(incremental uint64, random uint64) []byte {
+	var incrementalNumerals = countNumerals(incremental)
+	var randomNumerals = countNumerals(random)
+
+	var res = make([]byte, incrementalNumerals+randomNumerals)
+	genCode(random, res)
+	genCode(incremental, res[:incrementalNumerals])
+	return res
+}
+
+// Count the bytes needed to stored encoded x
+func countNumerals(x uint64) (numerals uint64) {
+	for i := x; i > 0; i /= uint64(len(alphabet)) {
+		numerals++
+	}
+	if numerals == 0 {
+		numerals = 1
+	}
+	return
+}
+
+// Write encoded x to res from its end
+func genCode(x uint64, res []byte) {
+	if x == 0 {
+		res[len(res)-1] = alphabet[0]
+		return
+	}
+	for i := len(res) - 1; x > 0 && i >= 0; i-- {
+		res[i] = alphabet[x%uint64(len(alphabet))]
+		x /= uint64(len(alphabet))
+	}
+}