array: add ringbuf

Change-Id: Ifb59b1b2d6b173652c17833b8b6a021d449ecd2a

Author: qieguo2016

go/basic/array/ringbuf.go

@@ -0,0 +1,202 @@
+/*
+	ring buff，an implement of cache
+*/
+
+package array
+
+import (
+	"errors"
+	"io"
+)
+
+var ErrOutOfRange = errors.New("out of range")
+
+
+// ring buffer底层是一个数组，使用单调递增的左右下标来标识虚拟的存储空间，写到数组末尾之后再从头部开始写
+type RingBuf struct {
+	begin int64 // 虚拟存储空间左边界
+	end   int64 // 虚拟存储空间右边界
+	data  []byte
+	index int  // 数组可写下标，另外一种常见的优化是控制数组长度为2的n次方，然后用位操作代替取模
+}
+
+func NewRingBuf(size int) (rb RingBuf) {
+	rb.data = make([]byte, size)
+	rb.begin = 0
+	rb.end = 0
+	rb.index = 0
+	return
+}
+
+func (rb *RingBuf) Size() int64 {
+	return int64(len(rb.data))
+}
+
+func (rb *RingBuf) Begin() int64 {
+	return rb.begin
+}
+
+func (rb *RingBuf) End() int64 {
+	return rb.end
+}
+
+// 从尾部写入，写满数组后覆盖写头部，达到ring效果
+func (rb *RingBuf) Write(p []byte) (n int, err error) {
+	if len(p) > len(rb.data) {
+		err = ErrOutOfRange
+		return
+	}
+	for n < len(p) {
+		written := copy(rb.data[rb.index:], p[n:])
+		rb.end += int64(written)
+		n += written
+		rb.index += written
+		if rb.index >= len(rb.data) {  // 回到头部
+			rb.index -= len(rb.data)
+		}
+	}
+	if int(rb.end-rb.begin) > len(rb.data) {
+		rb.begin = rb.end - int64(len(rb.data))
+	}
+	return
+}
+
+// 从指定位置写入，写满数组后覆盖写头部，达到ring效果
+func (rb *RingBuf) WriteAt(p []byte, offset int64) (n int, err error) {
+	if offset+int64(len(p)) > rb.end || offset < rb.begin {
+		err = ErrOutOfRange
+		return
+	}
+	var writeOff int
+	if rb.end-rb.begin < int64(len(rb.data)) {
+		writeOff = int(offset - rb.begin)
+	} else {
+		writeOff = rb.index + int(offset-rb.begin)
+	}
+	if writeOff > len(rb.data) {
+		writeOff -= len(rb.data)
+	}
+	writeEnd := writeOff + int(rb.end-offset)
+	if writeEnd <= len(rb.data) {
+		n = copy(rb.data[writeOff:writeEnd], p)
+	} else {
+		n = copy(rb.data[writeOff:], p)
+		if n < len(p) {
+			n += copy(rb.data[:writeEnd-len(rb.data)], p[n:])
+		}
+	}
+	return
+}
+
+func (rb *RingBuf) ReadAt(p []byte, offset int64) (n int, err error) {
+	if offset > rb.end || offset < rb.begin {
+		err = ErrOutOfRange
+		return
+	}
+	var readOff int
+	if rb.end-rb.begin < int64(len(rb.data)) {
+		readOff = int(offset - rb.begin)
+	} else {
+		readOff = rb.index + int(offset-rb.begin)
+	}
+	if readOff >= len(rb.data) {
+		readOff -= len(rb.data)
+	}
+	readEnd := readOff + int(rb.end-offset)
+	if readEnd <= len(rb.data) {
+		n = copy(p, rb.data[readOff:readEnd])
+	} else {
+		n = copy(p, rb.data[readOff:])
+		if n < len(p) {
+			n += copy(p[n:], rb.data[:readEnd-len(rb.data)])
+		}
+	}
+	if n < len(p) {
+		err = io.EOF
+	}
+	return
+}
+
+func (rb *RingBuf) Evacuate(offset int64, length int) (newOff int64) {
+	if offset+int64(length) > rb.end || offset < rb.begin {
+		return -1
+	}
+	var readOff int
+	if rb.end-rb.begin < int64(len(rb.data)) {
+		readOff = int(offset - rb.begin)
+	} else {
+		readOff = rb.index + int(offset-rb.begin)
+	}
+	if readOff >= len(rb.data) {
+		readOff -= len(rb.data)
+	}
+
+	if readOff == rb.index {
+		// no copy evacuate
+		rb.index += length
+		if rb.index >= len(rb.data) {
+			rb.index -= len(rb.data)
+		}
+	} else if readOff < rb.index {
+		var n = copy(rb.data[rb.index:], rb.data[readOff:readOff+length])
+		rb.index += n
+		if rb.index == len(rb.data) {
+			rb.index = copy(rb.data, rb.data[readOff+n:readOff+length])
+		}
+	} else {
+		var readEnd = readOff + length
+		var n int
+		if readEnd <= len(rb.data) {
+			n = copy(rb.data[rb.index:], rb.data[readOff:readEnd])
+			rb.index += n
+		} else {
+			n = copy(rb.data[rb.index:], rb.data[readOff:])
+			rb.index += n
+			var tail = length - n
+			n = copy(rb.data[rb.index:], rb.data[:tail])
+			rb.index += n
+			if rb.index == len(rb.data) {
+				rb.index = copy(rb.data, rb.data[n:tail])
+			}
+		}
+	}
+	newOff = rb.end
+	rb.end += int64(length)
+	if rb.begin < rb.end-int64(len(rb.data)) {
+		rb.begin = rb.end - int64(len(rb.data))
+	}
+	return
+}
+
+func (rb *RingBuf) Resize(newSize int) {
+	if len(rb.data) == newSize {
+		return
+	}
+	newData := make([]byte, newSize)
+	var offset int
+	if rb.end-rb.begin == int64(len(rb.data)) {
+		offset = rb.index
+	}
+	if int(rb.end-rb.begin) > newSize {
+		discard := int(rb.end-rb.begin) - newSize
+		offset = (offset + discard) % len(rb.data)
+		rb.begin = rb.end - int64(newSize)
+	}
+	n := copy(newData, rb.data[offset:])
+	if n < newSize {
+		copy(newData[n:], rb.data[:offset])
+	}
+	rb.data = newData
+	rb.index = 0
+}
+
+func (rb *RingBuf) Skip(length int64) {
+	rb.end += length
+	rb.index += int(length)
+	for rb.index >= len(rb.data) {
+		rb.index -= len(rb.data)
+	}
+	if int(rb.end-rb.begin) > len(rb.data) {
+		rb.begin = rb.end - int64(len(rb.data))
+	}
+}