如何利用js编码函数encodeURIComponent进行源码解析

function URIAddEncodedOctetToBuffer(octet, result, index) {
 result[index++] = 37; // Char code of '%'.
 result[index++] = hexCharCodeArray[octet >> 4];
 result[index++] = hexCharCodeArray[octet & 0x0F]; return index;
}function URIEncodeOctets(octets, result, index) {
 if (hexCharCodeArray === 0) { // 0-F
 hexCharCodeArray = [48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 65, 66, 67, 68, 69, 70];
 }
 index = URIAddEncodedOctetToBuffer(octets[0], result, index); if (octets[1]) index = URIAddEncodedOctetToBuffer(octets[1], result, index); if (octets[2]) index = URIAddEncodedOctetToBuffer(octets[2], result, index); if (octets[3]) index = URIAddEncodedOctetToBuffer(octets[3], result, index); return index;
}// 对unicode中除了代理对外的字符编码function URIEncodeSingle(cc, result, index) {
 var x = (cc >> 12) & 0xF; var y = (cc >> 6) & 63; var z = cc & 63; // unicode最多值是三个字节
 var octets = new InternalArray(3); // ascii码
 if (cc <= 0x007F) {
 octets[0] = cc;
 } else if (cc <= 0x07FF) {
 octets[0] = y + 192;
 octets[1] = z + 128;
 } else {
 octets[0] = x + 224;
 octets[1] = y + 128;
 octets[2] = z + 128;
 } return URIEncodeOctets(octets, result, index);
}function URIEncodePair(cc1 , cc2, result, index) {
 var u = ((cc1 >> 6) & 0xF) + 1; var w = (cc1 >> 2) & 0xF; var x = cc1 & 3; var y = (cc2 >> 6) & 0xF; var z = cc2 & 63; var octets = new InternalArray(4);
 octets[0] = (u >> 2) + 240;
 octets[1] = (((u & 3) << 4) | w) + 128;
 octets[2] = ((x << 4) | y) + 128;
 octets[3] = z + 128; return URIEncodeOctets(octets, result, index);
}// ECMA-262, section 15.1.3function Encode(uri, unescape) {
 uri = TO_STRING(uri); var uriLength = uri.length; var array = new InternalArray(uriLength); var index = 0; for (var k = 0; k < uriLength; k++) { var cc1 = %_StringCharCodeAt(uri, k); if (unescape(cc1)) { array[index++] = cc1;
 } else { if (cc1 >= 0xDC00 && cc1 <= 0xDFFF) throw MakeURIError(); // 非高代理项
 if (cc1 < 0xD800 || cc1 > 0xDBFF) {
 index = URIEncodeSingle(cc1, array, index);
 } else { // 高代理项
 k++; if (k == uriLength) throw MakeURIError(); var cc2 = %_StringCharCodeAt(uri, k); // 不是合法的低代理项
 if (cc2 < 0xDC00 || cc2 > 0xDFFF) throw MakeURIError();
 index = URIEncodePair(cc1, cc2, array, index);
 }
 }
 } var result = %NewString(array.length, NEW_ONE_BYTE_STRING); for (var i = 0; i < array.length; i++) {
 %_OneByteSeqStringSetChar(i, array[i], result);
 } return result;
}// ECMA-262 - 15.1.3.4function URIEncodeComponent(component) {
 var unescapePredicate = function(cc) {
 if (isAlphaNumeric(cc)) return true; // !
 if (cc == 33) return true; // '()*
 if (39 <= cc && cc <= 42) return true; // -.
 if (45 <= cc && cc <= 46) return true; // _
 if (cc == 95) return true; // ~
 if (cc == 126) return true; return false;
 }; return Encode(component, unescapePredicate);
}

URIEncodeComponent实现过程为，直接调用Encode函数进行编码，首先利用unescapePredicate 函数跳过一些特殊的字符，这些字符不需要进行编码，具体看代码。基本工程为，逐个字符遍历，如果当前字符为低代理项，则报错（cc1 >= 0xDC00 && cc1 <= 0xDFFF）；因为低代理项必须跟在高代理项后面。接着判断如果当前字节不是高代理项(cc1 < 0xD800 || cc1 > 0xDBFF)，则直接调用URIEncodeSingle函数进行编码，如果当前字符编码为高代理项，则取下一位，如果下一位超过了字符串长度，则报错，如果下一位不是低代理项则报错，因为高代理项后面必须跟着低代理项，如果下一位是低代理项，则调用URIEncodePair函数对代理对进行编码。Encode函数主要是判断，字符的是不是不需要编码的字符，是不是代理对，是不是一般的字符三种情形。
URIEncodeSingle函数基本流程是对一个字符进行某种运算，最后调用URIEncodeOctets函数按字节进行编码。unicode码最大为三个字节，所以URIEncodeSingle函数里定义了数组长度为3。URIEncodePair函数基本流程为，对高低代理项进行某种运算，高低代理项一共是四个字节，然后调用URIEncodeOctets函数对四个字节进行编码。