add base & pack

base.go

@@ -0,0 +1,50 @@
+package unibase2n
+
+import (
+	"errors"
+)
+
+// Base has an encoding buffer thus should not be copied.
+//    total size: 8 bytes
+type Base struct {
+	off uint16  // starting offset
+	til uint16  // remianing indicator starting offset
+	bit uint8   // 2^bit, max is 15 (32768)
+	pos uint8   // bitwise buffer position
+	buf [2]byte // en/decoding buffer
+}
+
+var (
+	ErrInvalidBitSize   = errors.New("bit size >= 16 or == 0")
+	ErrOffsetOverflow   = errors.New("offset overflow")
+	ErrTailOverflow     = errors.New("tail overflow")
+	ErrTailInCodingArea = errors.New("tail in coding area")
+)
+
+// NewBase generates a new base2n config
+func NewBase(off, til uint16, bit uint8) (*Base, error) {
+	if bit >= 16 || bit == 0 {
+		return nil, ErrInvalidBitSize
+	}
+	offe := uint32(off) + 1<<bit // [off, offe)
+	if offe > 0x10000 {
+		return nil, ErrOffsetOverflow
+	}
+	tile := uint32(til) // [til, tile)
+	if bit%2 == 0 {
+		tile += uint32(bit / 2)
+	} else {
+		tile += uint32(bit)
+	}
+	if tile > 0x10000 {
+		return nil, ErrTailOverflow
+	}
+	if tile > uint32(off) || uint32(til) < offe {
+		return nil, ErrTailInCodingArea
+	}
+	return &Base{
+		off: off,
+		til: til,
+		bit: bit,
+	}, nil
+}