PHP_FUNCTION(str_pad)

{

    /* Input arguments */

    zend_string *input;                /* Input string 输入字符串*/

    zend_long pad_length;            /* Length to pad to 填充到多长.*/

    /* Helper variables */

    size_t num_pad_chars;        /* Number of padding characters (total - input size) 要填充进去的字符个数*/

    char *pad_str = " "; /* Pointer to padding string */

    size_t pad_str_len = 1; // 填充字符的个数

    zend_long   pad_type_val = STR_PAD_RIGHT; /* The padding type value 填充类型，左填充、右填充、左右填充。默认右填充*/

    size_t       i, left_pad=0, right_pad=0;

    zend_string *result = NULL;    /* Resulting string 返回值*/

    if (zend_parse_parameters(ZEND_NUM_ARGS(), "Sl|sl", &input, &pad_length, &pad_str, &pad_str_len, &pad_type_val) == FAILURE) {

        return;

    }

    /* If resulting string turns out to be shorter than input string,

       we simply copy the input and return. */

    /* 如果pad_length（参数2）小于等于输入字符串的长度，则返回原始的输入字符串。*/

    if (pad_length < 0  || (size_t)pad_length <= ZSTR_LEN(input)) {

        RETURN_STRINGL(ZSTR_VAL(input), ZSTR_LEN(input));

    }

    /* 填充字符串长度为0，如：str_pad("abc", 10, "")，则Warning级别的错误。

       填充字符串的默认长度为1，即str_pad("abc", 10)，的情况下pad_str_len=1。*/

    if (pad_str_len == 0) {

        php_error_docref(NULL, E_WARNING, "Padding string cannot be empty");

        return;

    }

    /*pad_type只能为STR_PAD_LEFT, STR_PAD_RIGHT, or STR_PAD_BOTH 或 0, 1, 2*/

    if (pad_type_val < STR_PAD_LEFT || pad_type_val > STR_PAD_BOTH) {

        php_error_docref(NULL, E_WARNING, "Padding type has to be STR_PAD_LEFT, STR_PAD_RIGHT, or STR_PAD_BOTH");

        return;

    }

    num_pad_chars = pad_length - ZSTR_LEN(input); // 需要被填充进去的字符的数量

    if (num_pad_chars >= INT_MAX) { // num_pad_chars的最大值是2147483647。#define INT_MAX 2147483647

        php_error_docref(NULL, E_WARNING, "Padding length is too long");

        return;

    }

    result = zend_string_safe_alloc(1, ZSTR_LEN(input), num_pad_chars, 0);

    ZSTR_LEN(result) = 0;

    /* We need to figure out the left/right padding lengths. */

    switch (pad_type_val) {

        case STR_PAD_RIGHT:

            left_pad = 0;

            right_pad = num_pad_chars;

            break;

        case STR_PAD_LEFT:

            left_pad = num_pad_chars;

            right_pad = 0;

            break;

        // 左填充数量小于右，left_pad <= right_pad

        case STR_PAD_BOTH:

            left_pad = num_pad_chars / 2;

            right_pad = num_pad_chars - left_pad;

            break;

    }

    /* First we pad on the left. */

    /* 左填充：循环添加字符 */

    for (i = 0; i < left_pad; i++)

        ZSTR_VAL(result)[ZSTR_LEN(result)++] = pad_str[i % pad_str_len];

    /* Then we copy the input string. */

    /* 左填充完成后，附加输入字符串 */

    memcpy(ZSTR_VAL(result) + ZSTR_LEN(result), ZSTR_VAL(input), ZSTR_LEN(input));

    ZSTR_LEN(result) += ZSTR_LEN(input);

    /* Finally, we pad on the right. */

    /* 右填充：循环添加字符串 */

    for (i = 0; i < right_pad; i++)

        ZSTR_VAL(result)[ZSTR_LEN(result)++] = pad_str[i % pad_str_len];

    // 添加字符串结束标志'\0'

    ZSTR_VAL(result)[ZSTR_LEN(result)] = '\0';

    // 返回新字符串

    RETURN_NEW_STR(result);

}