javascript实现数据结构: 串的块链存储表示

和线性表的链式存储结构相类似，也可采用链式方式存储串值。由于串结构的特殊性--结构中的每个数据元素是一个字符，则用链表存储串值时，存在一个“结点大小”的问题，即每个结点可以存放一个字符，也可以存放多个字符。

下面是结点大小为4（即每个结点存放4个字符）的链表:

head --> (a) --> (b) --> (c) --> ... --> (i)

当结点大小大于1时，由于串长不一定是结点大小的整倍数，则链表中的最后一个结点不一定全被串值占满，此时通常补上“#”或其它非串值字符。

为了便于进行串的操作，当以链表存储串值时，除头指针外还可附设一个尾指针指示链表中的最后一个结点，并给出当前串的长度，称如此定义的串存储结构为块链结构。

由于一般情况下，对串进行操作时，只需要从头向尾顺序扫描即可，则对串值不必建立双向链表。设尾指针的目的是为了便于进行连接操作，但应注意连接时需处理第一个串尾的无效字符。

在链式存储方式中，结点大小的选择和顺序存储方式的格式选择一样都很重要，它直接影响到串处理的效率。如果串很长，这要求我们考虑串值的存储密度：

存储密度 = 串值所占的存储位 / 实际分配的存储位

串值的链式存储结构对某些串操作，如连接操作等有一定方便之处，但总的来说不如另外两种存储结构灵活，它占用存储量大且操作复杂。

结构图：

实现代码：

function Chunk(chunkSize) {
        this.chunkSize = chunkSize || 4;
        this.ch = [];
        for (var i = 0; i < this.chunkSize; i++) {
            this.ch[i] = '#';
        }
        // type: Chunk
        this.next = null;
    }

    exports.LString = LString;
    function LString(chunkSize) {
        // type Chunk
        this.head = null;
        // type: chunk
        this.tail = null;
        // 串的当前长度
        this.length = 0;
        this.chunkSize = chunkSize || 4;
    }

    LString.prototype = {
        // 将字符串转换成LString类型
        strAssign: function (chars) {
            this.head = this.tail = new Chunk(this.chunkSize);
            this.length = chars.length;

            var current = this.head;
            for (var i = 0, len = chars.length; i < len; i++) {
                current.ch[i % this.chunkSize] = chars[i];
                if (i + 1 < len && (i + 1) % this.chunkSize === 0) {
                    current.next = new Chunk();
                    current = current.next;
                }
            }

            this.tail = current;
        },
        // 字符串对比
        // TODO 是否去掉chunkSize的对比
        strCompare: function (tLString) {
            var current = this.head;
            var curT = tLString.head;

            if (this.length !== tLString.length) return false;

            while (current) {
                for (var i = 0; i < this.chunkSize; i++) {
                    if (current.ch[i] !== curT.ch[i]) return false;
                }

                current = current.next;
                curT = curT.next;
            }

            return true;
        },
        clearString: function () {
            this.head = this.tail = null;
            this.length = 0;
        },
        concat: function (tLSting) {
            if (!tLSting.length) return;

            var ret = new LString(this.chunkSize);

            if (this.head === null) {
                copyString(ret, tLSting);
            } else {
                ret.head = ret.tail = new Chunk(this.chunkSize);
                copyString(ret, this);

                var index = ret.tail.ch.indexOf('#');
                if (index === -1) {
                    copyString(ret, tLSting);
                } else {
                    copyString(ret, tLSting, ret.tail, tLSting.head, index);
                }
            }

            return ret;
        },
        substring: function (pos, len) {
            pos = ~~pos || 0;
            len = ~~len || this.length;
            if (pos < 0 || pos > this.length - 1 || len < 0 || len > this.length - pos)
                throw new Error('unexpected parameter');

            var sub = new LString(this.chunkSize);
            var current = findPosChunk(this, pos);
            var curS = sub.head = new Chunk(this.chunkSize);
            var i = 0;
            sub.length = len;

            outerloop: while (current) {
                for (var j = 0, size = this.chunkSize; j < size; j++) {
                    if (i === len) {
                        break outerloop;
                    } else {
                        curS.ch[j] = current.ch[(i + pos) % this.chunkSize];
                        i++;
                        if ((i + pos) % this.chunkSize === 0) {
                            current = current.next;
                        }
                        if (i % this.chunkSize === 0 && (current.ch[i] || current.next)) {
                            curS.next = new Chunk(this.chunkSize);
                            curS = curS.next;
                        }
                    }
                }
            }

            return sub;
        },
        toString: function () {
            var current = this.head;

            if (current === null) return '';

            var str = '';
            while (current) {
                for (var i = 0, len = this.chunkSize; i < len; i++) {
                    var ch = current.ch[i];
                    if (ch === '#') {
                        return str;
                    } else {
                        str += current.ch[i];
                    }
                }
                current = current.next;
            }

            return str;
        }
    };

    function findPosChunk(lString, pos) {
        var current = lString.head;
        while (current) {
            for (var i = 0, len = lString.chunkSize; i < len; i++) {
                if (pos-- === 0) return current;
            }
            current = current.next;
        }
    }

    function copyString(destination, target, curD, currT, offset) {
        offset = offset || 0;
        currT = currT || target.head;
        curD = curD || destination.head;
        var k = 0;

        while (currT) {
            for (var i = 0, len = target.chunkSize; i < len; i++, k++) {
                var j = k % curD.chunkSize + offset;
                curD.ch[j % curD.chunkSize] = currT.ch[i];

                if ((j + 1) % curD.chunkSize === 0 && (currT.ch[i + 1] || currT.next)) {
                    curD.next = new Chunk(destination.chunkSize);
                    curD = curD.next;
                }
            }

            currT = currT.next;
        }

        destination.tail = curD;
        destination.length += target.length;
    }

    var a = new LString();
    var b = new LString();
    var c = new LString();

    a.strAssign('abcdefg');
    console.log(a + '');
    b.strAssign('hijklmno');
    console.log(b + '');
    c.strAssign('abcdefg');
    console.log(a.strCompare(b));
    console.log(a.strCompare(c));
    var t = a.concat(b);
    console.log(t + '');
    t = t.substring(2, 5);
    console.log(t + '');

单元测试代码：

describe('LString tests', function(){
    var a = new LString(5);
    var b = new LString(4);
    var c = new LString(5);
    var t;

    it('should assign string', function(){
        a.strAssign('abcdefg');
        expect(a + '').toBe('abcdefg');

        b.strAssign('hijklmno');
        expect(b + '').toBe('hijklmno');

        c.strAssign('abcdefg');
        expect(c + '').toBe('abcdefg');
    });

    it('should compare', function(){
        expect(a.strCompare(b)).toBe(false);
        expect(a.strCompare(c)).toBe(true);
    });

    it('should concat', function(){
        t = a.concat(b);
        expect(t + '').toBe('abcdefghijklmno');
    });

    it('should substring', function(){
        t = t.substring(2, 5);
        expect(t + '').toBe('cdefg');
    });
});