Refactor file entry checksum calculation.

Move actual algorithm implementation into add_checksum_byte() function.
Avoid utf16_length() call: we already have valid file name length.
Avoid extra data copying: take it from the name buffer instead of making
a temporary file name entry.

diff --git a/libexfat/utils.c b/libexfat/utils.c
index 0d3b809..e0523e0 100644 (file)
--- a/libexfat/utils.c
+++ b/libexfat/utils.c
@@ -52,6 +52,20 @@ void exfat_get_name(const struct exfat_node* node, char* buffer, size_t n)
                exfat_bug("failed to convert name to UTF-8");
 }
 
+static uint16_t add_checksum_byte(uint16_t sum, uint8_t byte)
+{
+       return ((sum << 15) | (sum >> 1)) + byte;
+}
+
+static uint16_t add_checksum_bytes(uint16_t sum, const void* buffer, size_t n)
+{
+       int i;
+
+       for (i = 0; i < n; i++)
+               sum = add_checksum_byte(sum, ((const uint8_t*) buffer)[i]);
+       return sum;
+}
+
 uint16_t exfat_start_checksum(const struct exfat_entry_meta1* entry)
 {
        uint16_t sum = 0;
@@ -59,35 +73,30 @@ uint16_t exfat_start_checksum(const struct exfat_entry_meta1* entry)
 
        for (i = 0; i < sizeof(struct exfat_entry); i++)
                if (i != 2 && i != 3) /* skip checksum field itself */
-                       sum = ((sum << 15) | (sum >> 1)) + ((const uint8_t*) entry)[i];
+                       sum = add_checksum_byte(sum, ((const uint8_t*) entry)[i]);
        return sum;
 }
 
 uint16_t exfat_add_checksum(const void* entry, uint16_t sum)
 {
-       int i;
-
-       for (i = 0; i < sizeof(struct exfat_entry); i++)
-               sum = ((sum << 15) | (sum >> 1)) + ((const uint8_t*) entry)[i];
-       return sum;
+       return add_checksum_bytes(sum, entry, sizeof(struct exfat_entry));
 }
 
 le16_t exfat_calc_checksum(const struct exfat_entry_meta1* meta1,
                const struct exfat_entry_meta2* meta2, const le16_t* name)
 {
        uint16_t checksum;
-       const int name_entries = DIV_ROUND_UP(utf16_length(name), EXFAT_ENAME_MAX);
+       const int name_entries = DIV_ROUND_UP(meta2->name_length, EXFAT_ENAME_MAX);
        int i;
 
        checksum = exfat_start_checksum(meta1);
        checksum = exfat_add_checksum(meta2, checksum);
        for (i = 0; i < name_entries; i++)
        {
-               struct exfat_entry_name name_entry = {EXFAT_ENTRY_FILE_NAME, 0};
-               memcpy(name_entry.name, name + i * EXFAT_ENAME_MAX,
-                               MIN(EXFAT_ENAME_MAX, EXFAT_NAME_MAX - i * EXFAT_ENAME_MAX) *
-                               sizeof(le16_t));
-               checksum = exfat_add_checksum(&name_entry, checksum);
+               checksum = add_checksum_byte(checksum, EXFAT_ENTRY_FILE_NAME);
+               checksum = add_checksum_byte(checksum, 0);
+               checksum = add_checksum_bytes(checksum, name + i * EXFAT_ENAME_MAX,
+                               EXFAT_ENAME_MAX * sizeof(le16_t));
        }
        return cpu_to_le16(checksum);
 }