finish EncodeLen

len.go

@@ -0,0 +1,54 @@
+package unibase2n
+
+import "math/bits"
+
+// EncodeLen calculate encode length
+//    offset will be appended as til+offset at last
+func (bs Base) EncodeLen(in int) (out, offset int) {
+	if in <= 0 {
+		return 0, 0
+	}
+	if bits.OnesCount8(bs.bit) == 1 { // 2的幂永无offset (1/2/4/8)
+		return in * (16 >> "\x00\x00\x01\x00\x02\x00\x00\x00\x03"[bs.bit]), 0
+	}
+	if bs.bit == 6 { // 特判 (6) 3*8=4*6
+		out = in / 3 * 4 * 2
+		offset = in % 3
+		if offset == 0 {
+			return
+		}
+		// 8 = 6+2, 16 = 6+6+4, 再加结尾指示字符
+		out += (offset + 1 + 1) * 2
+		return
+	}
+	if bs.bit < 8 { // (3/5/7)
+		out = in / int(bs.bit) * 8 * 2
+		offset = in % int(bs.bit)
+		if offset == 0 {
+			return
+		}
+		remain := offset * 8
+		out += (remain/int(bs.bit) + 1 + 1) * 2
+		return
+	}
+	if bs.bit%2 == 0 { // bit 大于8且可被2整除 (10/12/14)
+		hb := int(bs.bit) >> 1
+		out = in / hb * 8 * 2
+		offset = in % hb
+		if offset == 0 {
+			return
+		}
+		remain := offset * 8
+		out += (remain/int(bs.bit) + 1 + 1) * 2
+		return
+	}
+	// (9/11/13/15)
+	out = in / int(bs.bit) * 16 * 2
+	offset = in % int(bs.bit)
+	if offset == 0 {
+		return
+	}
+	remain := offset * 8
+	out += (remain/int(bs.bit) + 1 + 1) * 2
+	return
+}